Engine idles, but backfires

HI:
Can you tell if the backfire is through the intake or exhaust? Also, do you know how many miles were on it? My concern's are timing or burnt valves. Do you have a timing light you could check the ignition timing with?

It sounds like you have a clogged catalytic converter here is a guide to help you confirm the issue.

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

Joe

i can and will check timing. As far as where it backfires, sometimes out intake...sometimes out exhaust. If you open the throttle rapidly, out the intake. If you open the throttle slowly and try to force the engine to accelerate, it runs very rough, almost like a misfire, then out the exhaust.
Thanks for your response.

Hi:
Also recheck the plug wires. Make sure you have them on right. This guide can help as well.

https://www.2carpros.com/articles/engine-backfires-while-running

Joe

Timing is about 6 to 8 before at idle and goes up to around 14 or so when accelerated to about 1800 to 2000 rpm. This doesnt sound so far advanced as to make it backfire, I am thinking a vacuum leak, so this is the direction I am going. Got anymore ideas?

I appreciate the input about the plug wires, but if they were wrong, it would run crappy or not at all. This thing idles beautiful...just cant push the gas.

Hi:
I'm going to speak with a Jeep specialest. I will get back with you or he will.

Joe

These are what to inspect first althow my manuals say timing is not ajustable it is ajusted through the pcm let us know what you have also check the position of #1 cylender put #1 at top dead center on compression pull distibutor cap see if rotor is at #1 position also check the cam and crank sensors they tell pcm when to inject fuel and when to spark also how high is rpm just after start with foot off peddle
1 Inspect Ignition Timing Incorrectly adjusted engine timing.
2 Inspect Timing Set Slipped timing chain or worn timing gear(s).
3 Inspect Distributor Cap Loose or cracked distributor cap
4 Inspect Rotor - Ignition Damaged, worn, or corroded ignition rotor.
5 Inspect Distributor Cap And Rotor Kit Loose, cracked, or worn distributor cap and ignition rotor.
6 Inspect Wireset Damaged, worn, or deteriorating spark plug or coil wire(s).
7 Inspect Spark Plug Fouled, damaged or broken spark plug(s).
8 Inspect Firing Order Spark plug wire(s) not connected in correct firing order sequence

Thanks for all the great advice. i worked on it a few hours this evening and this is what I found.

First. Went through timing again, all correct, on TDC rotor is at #1 dead on.

Second. The engine is a 97 and the truck is a 98. i compared the distributor that came with the engine (used engine) to the 98 out of the vehicle. The cam pick up that rotates is not clocked the same. So I put the 97 distributor in the vehicle. But this made no difference in how it ran.

Third. Timing at idle is about 8 degrees before TDC. Sound pretty correct. This did not change with either distributor, stays the same.

Fourth. Found forward O2 sensor connector very dirty, cob webbs and all sorts of stuff. Cleaned this out and THE INTAKE BACKFIRE IS GONE. Punch the gas and no backfire.

Now..at about fifteen hundred RPM it gets very rough will popping out the exhaust. Sounds like very rich mixture? Why at fifteen hundred RPM and not before?

Have any advice on what would make it pop out the exhaust and run very rough above 1500 RPM?

if 02 sensor looked that bad how do the plugs dist.cap rotor and wires look like

New plugs, rotor and dist cap.

After some more troubleshooting today.

It does not backfire out the intake at all.
It runs great until about 1400 - 1500 RPM, then it begins to miss, keep going and you get unburnt fuel popping out the exhaust.

Cant tell if the miss is from ignition or too much fuel. I think ignition. I changed coil and plug wires to some known good from a running vehicle. No change. Changed all plugs, no change.

it is and ignition timing issue on you need to check the crank shaft position sensor and the camshaft position sensor and the throdle posiotn sensor they tell when it should spark and fuel the crank shaft position sensor is on left side of bell houseing of trans

I have followed this post because my jeep is doing the same thing it ended up being a clogged cat converter FYI it cost me $560.00 for a new one all fixed I love this site.

I have a 1998 Jeep Cherokee with the 4.0L I6. It had a problem where intermittantly it surged and sometimes backfired on acceleration. The more the throttle was depressed the worse the symptom became.

When it first started doing this I discovered that the fuel pressure was sometimes as low as 20 PSI. I changed the fuel pump/filter assembly and the problem disappeared for about 2K miles.

It has now come back but only on moderate throttle. I have checked the fuel pressure at the fuel rail and it appears to always be approximately 50 PSI.

This time the symptom is slightly different. If full (or almost full) throttle is applied during acceleration it accelerates properly and will then continue to run fine under all conditions until the next time the engine idles for an extended period (like at a stop light) or ran at very low throttle for a while like when in 4WD on level but rough surfaces such as a field or if the engine has been turned off.

It appeared that maybe the IAC solenoid was sticking open. I cleaned the solenoid and the the IA port. Again the problem went away for a few days then returned. So I changed the IAC solenoid. This did not change the symptom.

So, as it stands, I have a new fuel pump and fuel filter, a new IAC solenoid and a new rear oxygen sensor (changed for another reason). I still have the problem.

I had the codes read and they didn't show anything.

Any thoughts?

The surging immediately made me think of fuel pump, but you've already addressed that. Ignition timing could also cause your symptoms, but if you have the Inline6 , no timing adjustments required. So, the first thing to look at that would cause BOTH problems is a VACUUM Leak (hose or intake manifold), get a diagram for hose routing and check hoses - might be one pasted iside your hood, dealer may give you a free copy, or get a Haynes repair manual for your model/year #50010 84-99 jeep cherokee for about $15.00 at a locl auto parts store. If you don't already have one, I recommend highly - lots of useful info. Chilton also makes a manual, but it's not as user-friendly although it contains info that the haynes may not cover as well. Hope this helps...

I bought a code scanner and even though it had scanned clean right after I had a new fuel pump installed it now showed codes for both O2 sensors. I knew that the front sensor was old so I replaced it.

Immediately the Jeep ran better but still surged under the same conditions described above. Sometimes it showed a O2 sensor heater code and sometimes it didn't.

To make a long story short. There is a fuse that is only used for the O2 sensors. This fuse was blown. I replaced the fuse and NO MORE SURGE.

A FUUUUUSE????? Just an inky-dinky FUSE? Man. I'll be filing this info away for future reference as to symptoms and causes - Thank you for responding! This is how we learn from each other! O2 sensors send a voltage to the computer based on oxygen content of exhaust gas, the computer adjusts the fuel/air mix. So, no O2 sensor input sends the computer into freaksville by sending zero volts instead of the 0 to 1 volts the computer usually sees...(just thinking out loud). My manual recommends replacing O2 sensor every 82,500 miles.
A fuse? DUH... Makes me want to do a Carlos Mencia - "I'm a DEE-DEE-DEE!"
Thank you for getting back!

Maybe I didn't give enough information.

The fuse is for the O2 heater circuits. So the O2 heaters didn't work. Anytime the computer called for open loop operation and ignored O2 sensor input the Jeep ran fine. These times include cold start and WOT.

The problem presented itself when the computer called for closed loop and the O2 sensors were too cold. These times include idle and part throttle after warm up. If the engine load is great enough the exhaust stream temperature brings the O2 sensors to acceptable temperature and things work great.

In my case, I was in very rough country off road and a bush caught the leads for the rear O2 sensor and tore them out. While they were flopping around the heater wire shorted and blew the fuse. This started a long string of errors on my part trying to resolve this. I don't regret changing any of the components. The miles on the Jeep dictate that they be changed anyway. It was sure frustrating though!

Thanks for the ideas. They were good thoughts. Glad I didn't have to do that too!

Thanks for getting back with a technician's - based response! I did some digging and found what causes closed loop and open loop modes, plus that the O2 sensor will operate in a programmed default mode/value if your fuse problem occurs... Not used to that level of detail - I'm an ex-submariner who used to work on communications encryption gear, and I appreciate hearing the details every once in a while! Lots of Luck!

Started with stalling cranking for long periods of time and wot to get started
once started would run rough somtimes fine.
then stall when going to take off and not starting again unless wot hard to get started.
new distributer cap wires plugs, tps, etc nothing worked.
i then took the injectors out with rail cleaned best i could, found carbon build up where injectors enter the manifold cleaned & now will start no problem idle perfectly hit the gas revs fine .
but when driving after 2nd gear shift engine starts to bog down looses power really bad & backfires through the intake manifold, i can w.o.t it and it clears up until trying to maintain road speeds.
i checked the fuel pressure with a gauge reads 49 psi @ idel & stays almost the same during driving, when i shut the engine of with key of the pressure almost instantly starts to fall.
drops to below 30psi in under 3 minutes, hanes manual says its the fuel pressure regulator, this model is equipped with a vacuume assited pressure regulator, could this be the reason for the backfire and loss of power very costly want to make sure this is what wrong before dumping another 100 into it
thanks jim

pressure is ok as far as what range it is. fule pressure should not drop to less than 24 psi with in 5 minutes if it does the pump module is bad which is in the tank. backfiring can be caused by leaking injectors , vacuum leak etc.

1997 Grand Cherokee with fuel problem. After warming up and on the road the engine will start to cut out. Sounds kinda like it's backfiring under the hood. I have changed the complete fuel pump module (twice), replaced the coil, rotor, regapped the plugs removed the fuel rail and drained and blown air through the fuel line. Nothing has helped. The only fault codes are for the secondary oxygen sensor. When the symptom occurs you can put the gas to the floor and it will shift down, take off and run good until I let off and then it starts to cut out. It does not start until operating temperature is reached. Also, when symptom starts it will not idle. Sometimes after a few minutes at idle it will clear up. I have inspected the fuel lines and there are no kinks or leaks. I also swapped the main fuel pump relay with the horn relay and nothing changed. Please help. Thanks!

Hi mhobby,

I would suggest checking for vacumn leakages at intake hoses and manifold area. The idling system needs to be checked for fault.

The faulty O2 sensor can contribute to the problem.

[quote:58fec844eb="KHLow2008"]Hi mhobby,

I would suggest checking for vacumn leakages at intake hoses and manifold area. The idling system needs to be checked for fault.

The faulty O2 sensor can contribute to the problem.[/quote:58fec844eb]


Unplugged O2 sensor and problem went away. Knew sensor could effect mpg but did not know it could make a vehicle undriveable. Thanks!

Once in a while my Jeep will lag when I take off and backfire. I have good plugs, clean ir filter, just had an injector service a while ago. What do you think it is?

Could be the following below

Dirty fuel injectors (cleaning the injectors often fixes this).
Bad MAP (manifold absolute pressure) sensor
Bad TPS (throttle position) sensor
Bad or dirty MAF (mass airflow) sensor
Low fuel pressure (leaky fuel pressure regulator or weak fuel pump)
Vacuum leaks (intake manifold, vacuum hoses, throttle body, EGR valve)
Bad gasoline (fuel contaminated with water or too much alcohol)

Sometimes, what feels like a hesitation is actually ignition misfire rather than lean misfire. The causes of ignition misfire may include:

Dirty or worn spark plugs
Bad plug wires
Weak ignition coil
Wet plug wires

my 96 jeep will backfire and sputter with no exceleration on pedal untill floored, or i can turn ignition off then back on and it runs normal for a little while ,ive changed coil, pvc elbow, wires and plugs,distributer, and gas filter there is no cat converter

Do you have a check engine light?

Check the fuel pressure with a gauge if its within specs-if okay

Could be caused by one of the following below

Oxygen sensor.
Catalytic converter.
Fuel injectors dirty/sticking.
Mass airflow sensor/Airflow meter.
Throttle position sensor.
Crankshaft position sensor
Knock sensor
Manifold absolute pressure sensor.
EGR Valve
Fuel pressure regulator leaking or defective fuel pump.
Fuel contamination.
Foul/defective spark plugs.
Open spark plug wires.
Ignition coil/Coil packs defective.
Incorrect ignition timing.
Cap and rotor.

Note:If it doesn't apply disregard it and keep testing.

thanks for helping no the check engine light only comes on when starting no cat converter or oxygn sensor,i took it to orilys and computer said oxygen sensor and had another person test it and his said throttle position sensor,new cap aand rotor i drove it from toledo to indy and no problems , new fuel filter,coil plugs and wires,pvc valve its bad now but befor i could turn it off and on while driving and it would work right, or when it starts acting up i can floor it and it runs,it was ideling fine but now it stalls sometime and backfires

My RHD Jeep was imported from Japan. It had a rusty fuel tank, but still put new fuel pump/filter in. Had problems, so put on new fuel tank but used same fuel pump. Had difficult starting without cycling the key, but that problem worked out and vehicle starts fine. Now after several hours of running, it has started stalling and backfiring. Acts like fuel starvation. Mechanic said it sounds like vapor locking, but was not doing it the next day when it had not run for hours. The temperature was 70 degrees and I do not use ethanol. I cannot deal with this as a rural mail carrier and need some trouble shooting ideas for mechanic to try. Does it have to act up before he can diagnose?

well in order to prove it to a repair shop,it has to act up .when they inspect it. but. I suspect a clogged catalytic converter. also it could have something to do with fuel pressure, I have some directions for you to check fuel pressure, it pretty cut and dry. it should also determine if your fuel pressure regulator is faulty.

https://www.2carpros.com/how_to/how_to_check_fuel_pressure.htm

i have a 1997 jeep grand cherokee. it is hard to start sometimes and when i get it started it will die on me even going down the road. than sometimes it will backfire which then will blow my muffler open. i have had the downstream o2 sensor changed the ingnition coil, and computer changed in it and the check engine light still comes on the second day after i get it back from the shop. what else could be the problem?

We need to have that check engine light code/s before we start diagnosis it sounds like a fuel related problem.

when i took it to the auto store to have a check ran on it he said that it was the 02-1 senors 2 but i already had one of the o2 sensors changed, would a code show up saying fuel pump or something of that sort. how would i know?

Lets slap a fuel pressure gauge on it and take the reading with and without the pressure regulator and report back with them readings.

Replaced the cam sensor crank sensor TPS Map sensor fule pressure regulator and did a complete tuneup it sits at an idle perfectly but upon acceleration it sputters and backfires ... im out of ideas

Could be a worn camshaft, low fuel pressure, check fuel pressure first. Have you read the codes yet?
Turn the key on. off, on, off, on and read the flashes of the check engine light. Cod 23 would flash as 22 blinks, a very short pause and three blinks, if other codes are stord, the pause will be a bit longer between blinks, the last code will be 55 and is normal.
Post your findings.

Got code 12

Engine start and idles fine for a few minute idle drops and backfires out of the exhaust. Sometime stalls or clears out like nothing happen. Also happens as driving.

Have the fuel pressure, idle air control valve and throttle position sensor to include the oxygen sensor whether its switching.