Engine will not idle, it stumbles and surges and quickly stalls

I forgot to send your pictures of manifold area close to the #3 or#4 Injectors - sending now.

Great job so far. Just to be clear, what was the fuel pressure when this is happening?

The fact that you have a positive 25 on the fuel trim shows that this is extremely lean when this is happening, which is going to cause the engine to stall.

So based on everything that you have done so far, we need to do a couple more basic items to find out if we have a larger issue.

Start with checking the compression and let's see if we have a low cylinder.

https://www.2carpros.com/articles/how-to-test-engine-compression

Next, we need to hook a vacuum gauge to the engine and see if we have an internal vacuum leak if you didn't find an external leak.

As for this bolt, if you have no vacuum leak there then that may be an issue, but it is not causing this issue.

Let me know what you find with this, and we can go from there.

Thanks so much for your reply!
I forgot to mention that this pickup truck is a 1996 SE Model - Changeover model built in 02/96 so many parts are different than models built prior and conforms to California, EPA and US emissions. Also, ECU does not throw any codes and there is no check engine light unless I unplug a MAF sensor, IACV, etc., and then it will come on and throw a code and or codes like P0100, 110,171,300,301,304,505. I clear the codes and they do not come back.
My code reader test of I/M Monitors as follows:
Misfire - okay Fuel sys - okay Catalyst Mon - INC, HTD Catalyst - N/A, Evap Sys Mon - INC, Sec Air Sys - N/A, A/c Ref Mon - N/A, OS Senor Mon -INC, EGR/VVT SYS Mon - INC.

Code reader during run monitoring shows OBD2 Sta = CA and VIN # states not available for this model along with the other readings.

Tests completed
1) Fuel Pressure (trying to maintain Idle due to surge, stumbling and trying to stall out) was at or a little above 34.
2) Few seconds after Ign off it read 43
3) Fuel Pressure Regulator Test - I was somewhat confused with instructions but using a hand pump type vacuum tool as I recall the fuel pressure did decrease as I pumped the tool as I was to keep it from stalling out. This old man may have to redo the test - need 4 hands on this issue -LOL.

Question 1) do you thing the IAC-AAC valve could be defective even though soaked and sprayed cleaned with carb/injector cleaner and then later with seafoam?
Question 2) ST FTRM Readings lock in at +25 at and above above 1,000 to 1,400 RPMs but come down below - could 02 sensors be defective even though I get readings
Question 3) The PCV was very hard to get to, but it did manage to remove and replace the valve (Had to remove tire, oil filter to get to it). IT did have fluid in the hose as I disconnected it but was not able to check the other end of the hose for cracks etc..
Question 4) Where do i hook a Vacuum gauge to the engine for this test?
Question 5) Could the Coil in the Distibutor be weak or bad distributor
Question 6) ECU Reset or replace as last resort

I will try to do a compression test in a couple of days and get back to you.
Thanks,
LWRoos

This is great info and I think we will get into all this in more depth. However, I think we need to confirm compression first just because of the fuel trim that you are seeing when you accelerate.

If the compression checks out, then we will get into the fuel pressure that you are seeing including the regulator test that you performed.

However, if compression is not okay, then none of that matters.

Tests completed
Fuel Pressure
1) Fuel Pressure (trying to maintain Idle due to surge, stumbling and trying to stall out) was at or a little above 34.
2) Few seconds after Ignition off it read 43.

Fuel Pressure Regulator
The fuel pressure did decrease as I pumped the Vacuum tool connected to the Regulator and engine vac plugged off.

Vacuum Test
Using a T connector, I connected the vacuum tested to line at base of throttle body area and disconnected the MAF sensor to maintain RPMs at about 1500 to 1600 instead of fighting to keep it from stalling out. Read at a steady 19.

Compression Test
CYL 1 = 155
CYL 2 = 150
CYL 3 =168
CYL 4 =109

Notes About Cylinder 4: My rental compression tester had a short extension and was very hard to connect so i don't know if it had a great seal but let's assume it did.
This truck is my nephew's. Don't know much about its history before he asked for help. When I was removing the Spark Plugs, I noticed #4 plug did not seem torque down as tight the others like too easy to break away as removing. I think this one and possibly the second one had been installed cross threaded before. While installing #4 plug it had sort of a wiggle to it at first and as all of the plugs it seemed like it took more than normal turning of plugs before they bottomed out. I was able to torque them all at specs 14 -20 lbs.

I will check to see if #4 loosens up after it is able to idle. Any suggestions? should I use some bolt tight or the like if it is an issue.

OBD2 Readings
Disconnected the MAF sensor and took some readings as it idled smoothly in Park but at about 1,500 - 1,600 RPMs and in drive it will idle about 938 RPMs smoothly - when I drive it it spits, jerks etc., at about 2,500 RPMs.

MAF Disconnected P0100 and check engine light on readings all under stopped conditions.

MIL Status ON = both in Park and Drive
Fuel Sys 1 = Closed then changes to open1 both in Park and Drive
Fuel Sys 2 = N/A both in Park and Drive
Cal Load % = 28.2 to 40.8 in Park and 42.0 in Drive
Coolant (F) = 185-189 in Park and 192-194 in Drive
ST FTRM 1% = 1..6 to 25.0 (mostly 25.0) in Park and 0 to 5.5 in Drive
LT FTRM 1% = 0 in Park and -2.3 in Drive
MAP (HG) = 8.0-8.3 in Park and 12.4-13.0 in Drive
Engine Speed = 1513-1638 in Park and 938-983 in Drive
IGN ADV = 15.0-16.0 in Park and 10.0 in Drive
Veh Speed = 0 for both
IAT (F) = 129-142 in Park and 136-140 in Drive
MAF (LB/M) = .03 in both Park and Drive
ABSLT TPS % = 0 in both Park and Drive
O2S11 (V) = .72-.93 in Park and .77-.94 in Drive
ST FTRM 11% = -7.8 to -25.0 (mostly 25.0 in Park and -5.5 to .83 in Drive
O2S21 (V) = .87-.94 in Park and .04 i Drive
OBD2 Stat = CA for both - I guess this means it is a California Emissions type Vehicle

Yesterday after my last message I decided to go ahead and install a new IACV-AAC in case the valve motor wasn't working. I disconnected the battery for a couple of hours before install.

It did not help much - little to none but it did try to idle a little longer at first - LOL.

Then I drove for about 25 miles - mixed city and highway driving - hoping the ECU would make adjustments to correct but to no avail.

The engine light did not come on but as I mentioned earlier it never does unless I unplug something.

This morning I remembered to check for codes and P0171 was stored and of course no engine check light so I do not understand why the engine light does not appear for this 1996 truck when a code is stored unless I unplug a sensor or something.

P0171 Freeze Frame Info I discovered this morning from my driving yesterday after installing the IACV:
Fuel Sys 1 = CLSD
Fuel Sys 2 = N/A
Calc Load % = 31.0
Coolant (F) = 187
ST FTRM 1% = 0
LT FTRM 1 % = 0
MAP (HG) = 16.2
ENG Speed = 2300
VEN Speed = 41

Thanks Kenny.
Regards Larry

So, we are going to need to dig deeper into this. This is a lot of info that we need to start narrowing down possible causes.

Try clearing the codes and unplug the o2 sensors and see if the engine will idle. Basically, if the o2 sensors are not reading correctly then it is going to give false info and that is going to cause the PCM to overcompensate in the opposite direction.

You may need to let it try and idle for a few minutes before it smooths out.

Hi Kenny,

Thanks for your reply!

I will disconnect the O2 sensors Tomorrow and get back to you.

I forgot to mention while driving after installing the IACV I noticed the Temp Guage was not working on the instrument panel (only time it happened).

While inspecting under the hood over the last 3 weeks of messing with this truck i was unplugging different connections and checking for a good contact and I noticed probably a coolant sensor electrical connection at the sensor was loose on the sensor after unplugging it (you could actually swivel it some left to right).

So, I got out of the truck during the above test drive and checked the sensor and swiveled it a little left and the instrument panel gauge was working when I got back in the truck.

Would this coolant temperature sensor affect anything causing the PO171 code? The gauge has always worked as i keep a close eye on the gauges while working on a vehicle. This was the only time, and I figured while installing the IACV i must have rubbed against it causing it to lose connection - lol. It definitely needs to be replaced and put on my to do list.

Other than this the only noticeable items while inspecting the truck was a ground strap connected to the firewall was not connected to anything so I connected it to the head of the engine and the intake may be missing a bolt that i mentioned earlier.

I must mention that the truck was broken into, and the radio system was stolen. They cut the wires to the radio and cigarette lighter under the dash. Also, a variable on/off/dimmer dash light control switch was stolen or lost due to the theft. There are no vacuum lines as the heater and a/c controls above the radio area are electic controlled. My Nephew states the truck ran fine after the theft and this issue started much later.
Thanks,
Larry

It is possible that the ECT can affect air fuel ratio but not when it is cold. The PCM monitors the temperature only to determine when to start using o2 sensor issue. So, to rule this out, you can monitor the temperature in the PCM with a scan tool when this is acting up. Clearly the temperature should be ambient temperature when the engine is off and cold. Then it will warm up as it is starting. If it is not, then that is an issue.

Hi Kenny,

Clearing the codes using the scanner (I did not disconnect the battery) and unplug the o2 sensors Failed the test as the engine still will not idle. Pretty much the same once the rpm falls below 1000 - 1200 or so it starts stumbling, surging and quickly comes down and finally stalls out below 500 rpm.

I did get codes PO135 & PO141 then later code PO130 But not a code for 02 Sensor Circuit Malfunction (Bank 1 Sensor 2).

I even drove it after it reached 178 F temperature - mixed city and Highway then got more gas - lol - sure using gas pretty fast.

Check engine light came on after the 2nd or 3rd start cycle. Had to go to bathroom - LOL.

Scanner reading as follows:
Freeze frame for PO130
Calc Load% = 52.9
Coolant (F) = 75
ST FTRM 1% = 0
LT FTRM 1% = 0
Map (HG) =17.7

After Engine warm up trying to idle it before and after driving it as follows:
MIL Status = ON
Fuel Sys 1 = open1
Fuel Sys 2 = N/A
Cal Load % = varies 38.8 - 41.2
Coolant (F) = got it up to 180
ST FTRM 1% = 0
LT FTRM 1% = 0
MAP (HG) = varies 22.7 +/-
Engine Speed = tried to hold at steady 1100 at idle after driving
IGN ADV = varies with rpm10 - 19
Veh Speed = 0
IAT (F) = varies 104-120 +/-
MAF (LB/M) =.1 - 2.0 +/-
ABSLT TPS % = varies with rpm 3.5 - 5.1+/-
O2S11 (V) = .3
ST FTRM 11% = 0
O2S21 (V) = .3
OBD2 Stat = CA for both - I guess this means it is a California Emissions type Vehicle

Do you think it could be the coil system built-in to the distributor? I did notice a member with a similar issue stating he and another member changed their distributor correcting their issue.

You mentioned ECT what is the definition, so i can look for the part to see if it is the one i was speaking of.

Thanks,
Larry

PS: This truck is driving this old man nuts - LOL!

If it is still doing it with the o2 unplugged, then we need to put a vacuum gauge on the engine and see what the internal vacuum is doing.

Here is a chart that will help with this.

Vacuum Test.
Using a T connector, I connected the vacuum tester to line at base of throttle body area and disconnected the MAF sensor to maintain RPMs at about 1,500 to 1,600 instead of fighting to keep it from stalling out. It read at a steady 19 as per one of my messages to you on 5/22. I must add to my earlier message that at the lower rpm the gauge was jumping all over the dial at lower rpm with the MAF connected. Maybe you could give me instructions on how and where to place the vacuum gauge if I did it wrong.
Sometimes I have sent you a couple of replies on the same day to try to give you additional info. Sorry if I am not communicating properly with you. Please review my messages as I also sent Fuel Pressure, fuel regulator and Compression test etc..
Thanks for your help!!!
Larry

Okay. So, we need to know what the needle is doing when it is jumping around. Can you get a recording of this? Just hook it up as you stated and get a recording of what the gauge is doing and what the RPM is. Even if the engine is running poorly the engine vacuum would be steady if the air intake and valves are running properly.

If the engine is running at higher RPM and the needle smooths out, then we need to revisit a vacuum leak.

However, let's start with a video of what the needle is doing when you see it bouncing around. Based on the action of the needle it will give us an idea as to what the issue is.

Thanks for your reply! I will attempt this as soon as I can get some help and send you a video.
Larry

Hey Kenny,

Last weekend I installed my aftermarket IACV and both wires tips are the color of Red just prior to the pin connectors so thinking it didn't matter I hooked it up. Now I wonder and don't know if I screwed up in case one of them is supposed to be a hot wire and the other one a ground wire or link to the ECU. My old unit was the same way except the color was black on both tips. Neither units have any markings on the wires or pin connectors. Of course, it didn't fix this idle issue but if it is hooked up wrong maybe the IACV is not functional???

Please advise me if i should switch them at the connectors.
This weekend with some help I will get you the video of Vacuum Test a different RPMs.
Thanks,
Larry

That is interesting. If the engine will not idle at all then that could be an issue. Clearly the IAC is responsible for keeping the engine idling.

However, when the IAC fails it normally doesn't cause a stumble. It just stalls as if the engine was turned off.

The way I understood this issue was that it would run poorly and then eventually stall out. IAC valves will fire the engine and as the RPMs come down, they just don't stop dropping and the engine stalls out.

So maybe getting a video of the gauge and of what the engine is doing and that will help.

My IACV wiring question was about hooking up both wires tips to the connector plug. I understand one should be power when IGN Key on and the other communicates with the ECU.

The new IACV I installed wires have red colored tips on both and my old IACV has Black colored tips on both wires. Neither have any kind of marks showing which is a positive or negative lead wire. So, I assumed it didn't matter and hooked it up. Now I am wondering if the valve is not working or working in reverse. I can't find any info on the internet nor from any sellers, so I thought I'd ask you.

This truck also has a Air Charge Temp Sensor (aka: Intake Air Temp, Air Cleaner Tempe) in the air filter compartment instead of being incorporated in the MAF sensor like newer autos. Claims are that both can cause stumble and surge and stalling. Very Confusing.

This issue is that the engine runs well at normal city and highway driving but at idle in any gear it stumbles and surges and quickly stalls out.

ST Fuel trim at +25 while driving but when at falling to idle fuel trims sometimes drop as low as +1.6 or so when it starts to stumble, surge and quickly stalls out.

I will send a video of the Vacuum gauge to you over the weekend.

Here's a pix of the IACV Valve.

Just to be clear, the only time you have an issue is when the engine is idling and in gear. It runs fine in neutral and idling.

If the engine idles in neutral, then the IAC is not the issue.

You are correct that one wire is the power feed which is a shared power wire so you can find this by starting the engine and check for power.

No, the problem is when in Park, Neutral and in Gear it stumbles, surges and stalls.

The problem I am having is that the two wires on the Old and New IACV have no markings as to which go to power #2 pin or ECU Signa #4, so I do not know if it is hooked up backwards or if it even matters - LOL. None of the aftermarket sellers seem to provide this information on the wiring of the valve.
Larry

Okay. It appears it does matter so let's switch them and see what happens.

There is a power wire, and the other is the ground but that goes to the PCM and the PCM controls the IAC via the ground. So, if you have them reversed then you are running the motor in the opposite direction.

Hi Kenny,

I switched the wires on the IACV and installed a new fuel pressure regulator but no difference at idle but seemed a little crisper on Highway driving.

So far from my research through internet articles and videos I began chasing parts which is not good due to very limited funds.

I have new parts in this order such as: New Air Filter, New PCV, New MAF, Ran it out of gas on purpose for fresh fuel due to it setting for a few months because my nephew had no money for repairs, New Fuel Filter, New IACV-AAC, New Distributor at a smoking deal, New Air Cleaner Temp Sensor ( aka Air Charge Temp/Intake Air Temp Sensor - LOL) and lastly New Fuel Pressure Regulator.

I had Midas Muffler check the Catalytic Converter, is okay.

I had it checked by an auto service company (my grandson in law father used to own the business and trained the current owner who has owned it 30 years +/- after grandson father passed on) to diagnosis it. They were extremely busy and had it on two occasions for 2 days and then 3 days over the last 3 weeks. He didn't charge me anything - he even installed the new Distributor and the ACT sensor for me but could not figure it out.

Although I think it probably needs a smoke test and a backflow test as i believe they only checked the vacuum on the EGR valve and didn't pull it for testing and checking to see if it is clogged???

I don't know what's left other than EGR full check out, O2 Sensors (but we disconnected, and it would not idle), MAP, Multifunction TPS, Throttle Body, Weak Fuel Pump at idle? (We tested the pressure at higher rpm - MAF disconnected, is okay), and all the other electrical controlled vacuum switching solenoid valves - swirl control valves - purge valves - one-way valves- etc., smog pump, ECU, etc.

Vacuum Tests Videos - one with MAF disconnected to warm engine up - read 19.5 - Then 2 videos trying to assist it from stalling out. Then 2 videos starting - not assisting idle. One after new fuel pressure regulator and driving 25 miles +/-. One next morning with cold start. 2 Pictures - vacuum Routing and engine/Catalyst info.

Hopefully, these videos will assist in Diagnosis - See Attached.

These are great videos. Take a look at this attachment. You have valve guides or intake valves leaking. Basically, the giveaway is when you start the engine, and the vacuum is low, but the needle is bouncing wildly. This needle should be steady and about 14-20 inches of mercury. The key thing is that the needle is steady and not moving.

Yours is bouncing between 12 and 2 inches which means the intake valves are not selling allowing the engine to build the proper amount of vacuum. This is what is causing the engine to show the fuel trims at full lean because there is more air coming in due to the intake valves not sealing.

The way to narrow in on this is to do what is called a running compression check.

I know you did a regular compression check and found that cylinder four was low and didn't feel confident about that test. However, if that is accurate then we need to start with cylinder four as that is the likely cause of this.

Hi Kenny!

Thanks so much for your assistance.

So, as I understand with the engine trying to idle but surging, stumbling and quick stalling out the vac test points an internal engine vac leak. Therefore, to a valve job due to bad valve guides and or valves not seating properly. Is that correct?

What threw me off is the engine seems to run normal at higher idle rpm and while driving at city and highway speeds but just won't idle at a stop without assistances due surging, stumbling and quick stalling. I cannot hold a steady idle rpm pressing the accelerator pedal below 1,000 -1,400 RPMs in or out of gear, LOL.

Also, the vacuum test with the MAF disconnected the engine seems normal and smooth but creates idle at around 1575 in Park and 938 in Drive the reading hold pretty steady at 19 -19.5.

Could a weak fuel pump also cause a similar issue? As I remember we had to try to maintain/assist idle when we did the test and do not remember disconnecting the MAF sensor to maintain speed.

Grasping at straws now -LOL.

So many Sensors, Regulators, Coils, etc., etc., point to this or similar issue.

I am sorry I didn't take the vacuum test properly the first time along with the fuel pressure and compression test. I can also see the importance of videos sent to you.

I did learn how to take videos on my phone - AirDrop them to my email - follow the link to AirDrop site - download to my computer and upload to you among all the other things I am learning from you.

Thanks so much!

Looking forward to hearing back from you.

Totally understand. A weak fuel pump can cause similar issues like surging and stalling but it will not cause the vacuum gauge to bounce like that. This is going to be an external vacuum leak exactly as you talked about.

Unfortunately, I think you are looking at valve guides and a valve job to correct this.

Hi, this is my second reply today so please review the one above this one.

Looking through my replies I noticed that I did not explain things very well at times - sorry.

The videos I sent of the vacuum test were done with the MAF connected except the first one I had the MAF disconnected so I could warm up the engine and the Vac reading was pretty steady at 19.5. I did not do a vacuum reading in Drive at idle while the MAF was disconnected - should have - but we know the idle RPMs hold steady at around 938 RPMs.
All other tests have the MAF connected.

The engine runs fine and seems smooth to me at city and highway speeds but when it is time to stop at a red light in or out of gear is when it will not settle into normal Idle rpm and starts to quickly stumble, surge and stall out.

ST FTRM readings confuse the issue for me because with the MAF Disconnected they are as much as - 25 and with the MAF Connected they are mostly +25 - LOL

Cylinder 4 Compression reading I got by cranking the engine at least 10 revs were 40 + lbs less than the others. The service shop readings were 30 + lbs less on #4. I think I cranked more revolutions than he did - LOL!
Thanks,
Larry

Okay. I think I have a pretty good understanding as to what is happening. You most likely have an intake valve issue on cylinder 4 if that is low on compression. It is most likely a guide issue causing the valve to hang open.

The reason it smooths out at higher RPMs is because the vacuum needs are lower at higher RPM. Basically, as the engine revs up the valves open faster and the vacuum drops.

Also, when the engine revs faster it can actually cause the valve to break loose and it will seal better. It will never be what it should, but it will be better at higher RPMs.

Also, the actual engine vacuum is not going to care if you have sensors plugged in or not. That is why the gauge bounces like it does.

Clearly, I could be wrong, but you have all the signs of a sticking valve.

Hi Kenny,

Aside from the low compression issue I think my idle issue may be related to the MAF sensor.

I drove the truck to a Nissan dealership for a free inspection as they were not very busy after the Memorial Day Holiday and would look at it right away.

I drove it there with the MAF disconnected just so it would idle at a stop. Within 30 minutes I was told my MAF was bad, and they would have to order the venturi as the MAF is built into it from the factory and the computer doesn't like the aftermarket MAF units. Parts suppliers use the Venturi Part number to market a new MAF - LOL.

So, I drove probably 10 t0 15 miles and a couple of hours later I decided I would return the Replacement MAF (this one was the 2nd new replacement I tried) and i would put the old one back in until I could order a different and more expensive Nissan/Hitachi MAF.

When I took it out of the Venturi the MAF had a liquid substance on it - clear maybe a little brown color to it and putting my pinky finger into the ventrui mounting hole the liquid was the same feel and look. Can't tell if it's oily or coolant type of fluid.

Even from me driving about 5 to 7 miles back home and letting it set for a couple of hours I would think that if the tech sprayed the MAF sensor thru the port atop of the ventrui it would have dried by then.

I put the original MAF back on and started the truck and it idled for about 25 minutes in Park and In Drive/Reverse. Idle would fluctuate at times. I drove it and it tried to stall a couple times at stop lights.

What do you think about the liquid on the MAF. I am wondering if it is a Venturi or complete Throttle Body or bad gasket between them.

Regards,
Larry

I think you have entered a rabbit hole. Sorry for being blunt but we have a hierarchy of issues. Meaning you always fix the worst issue first and then after that is repaired, move onto the next issue if it is still there.

Clearly if they are different systems then this does not apply. However, in this case, you are trying to repair a MAF sensor issue when the base engine is not running properly. You need to fix that first and then you can properly evaluate if you have a MAF issue.

Right now, it is clear that no matter what you do to the MAF, it is not fixing the issue.

This would be compared to if you had a broken finger and a heart attack at the same time. You should not address the broken finger first because if you do not survive the heart attack then the finger doesn't matter.

So, let's address the engine issue and then that will allow us to properly diagnose a MAF issue.