P0300 code why can we fix it?

Good morning.

The smoke could be normal. If one of the valves were open and the smoke went into a cylinder, it would go by the rings and into the combustion chamber. Then it would account for the smoke at the oil dip stick tube.

Another possibility is that your gasket may be leaking directly into the valley and showing at the dipstick tube.

It sounds like your friends home made system has a lot of pressure. It should be very low pressure going into the system. It should be around 1 to 2 lbs. of pressure only.

If it took two or three minutes, it sounds like you are good. If it was the gasket, it would have shown right away.

Roy

Thanks Roy

I am 100% confident the gasket is not leaking at valley, or anywhere. I was concerned oil getting past rings/pistons was not possible unless a problem existed. The intake valves somewhere on a cylinder logically would be open and as you explained it worked its way through to crankcase/oil chambers.

A follow up question?

This started with a P0300 code. I have replaced gaskets, hoses, injectors, fuel regulator (psi 42 idle), fuel line from regulator, vacuum is 19 inhg, replaced cam sensor, map, purge valve, cleaned maf, cleaned valve covers/gaskets, confirmed all valves rocking/moving (not lifter). NOID test on injectors pass, spark test on coils pass except did see it miss on #1 briefly (see video), swapped bank1 / bank2 coil packs so #8 coil moved to #1 and this also tested/moved the coil connector wiring to primary connector. Dry compression testing shows 179 to 186 psi across #2-8 cylinders. #1 tested 160 psi. Wet test on #1 showed 160 also, but if I double crank cycles from 5 to 10 it reached 180 psi. Also after only 2 cycles it reached 151 psi. If I remove coil connector on 2-8 idle gets worse, on #1 it does not change. I sprayed seafoam injector cleaner into throttle but still has rough idle. I never see codes P0301-308, nor any other code. LTFT is 8% on bank 1, 17% on bank 2. STFT are +/- 3% on both banks. I plan to do a pressure check at upstream O2 sensors but think exhaust pressure clogged cat) would effect more cylinders? I am down to the coil wires from primary pack connector to ECU could have intermittent loss of signal for #1 coil? Or a bad ECU? Or is it most likely the lower compression on #1 cylinder indicates enough valve leakage (although 160) to misfire? If the attached video does indicate coil misfires then that would be separate and unrelated to cylinder pressure right?

Please provide ideas or thoughts?

Okay, if you have a 5.3 engine the pressure should be 50-60 lbs.

40 is too low and that will cause all kinds of issues and is the most common cause for the 300 code.

What engine do you have? I checked for the 5.3 since I saw number 8 coil.

Roy

I think it is because it is the E-85 Flex Z engine? Early on I found some blogs and videos with a 2004 5.3L Z Flex fuel and information provided was 48-52 psi. On video this was pump on but not cranked. On video when cranked and idle psi dropped 8 psi to 42. My Tahoe is 50 not cranked and 42 idle. In a blog I cannot locate a person referenced a manual that indicated 55-60 for V6 and others, but for E-85 fuel 5.3 it was 48-52. Can you help me confirm and if I am good what is your next suggestion?

The spec is 50-60 engine running.

Roy

345-414 kPa (50-60 psi)

Here is another source. I forgot to mention that when idle and I remove the fuel line from regulator it jumps from 42 to 50. I did replace the fuel filter early on at same time I replaced regulator. It appears my fuel pressure for Z engine is okay. Do you agree? My luck is it is not an obvious and easy fix. On other video a fix was replacing main coil wiring primary connector. Question: Do you think a leak down test on #1 should be performed? Or is 160 psi good regardless of it being 12% lower than other cylinders okay?

Roy I know you are the ASE MASTER EXPERT but that is wrong. The specs for a Flex Fuel 5.3 Z is 42 psi when idle / 48-52 without flow. This video below proves it after this guy put in a brand new fuel pump his new pressure at idle is 42 psi.

Either you are not ASE or you want to get rid of me. Oh well thanks for the dedicated support!!

I just joined ALLDAYdiy for $15.00 a year subscription. Under my vehicle specs, fuel pressure, it lists below. Per below it lists Z engine as (48-54 psi). That is for KEY ON, Engine OFF. Thus the idle pressure should be (40-46 psi) with engine running. (Or 8 psi less when running). My readings on my truck are 50psi KEY ON, Engine Off and 42psi running IDLE.

Do you still think I have low fuel pressure? If not can you reread my situation and provide more feedback?

Thanks

Below from ALLDATA:

Your Vehicle: 2004 Chevy Truck Tahoe 2WD V8-5.3L VIN Z Flex Fuel

Specifications

Fuel Pressure (Key ON, Engine OFF) 385-425 kPa (55-62 psi)

Fuel Pressure (vin Z) 335-375 kPa (48-54 psi)

Then your pressure looks correct.

Your long term trim on the one bank is high indicating a lean condition. That bank needs to be checked. It could be an injector.

Roy

Roy

I installed new injectors during gasket upgrade. All test NOID and I can hear them clicking with stethoscope. Should I still swap them around?

Pictures below are from freeze data. Trims on both banks above 10% with bank 2 being worse.

Since I have new injectors, new MAP, and intake is not leaking would this point to bad O2 sensors? I will get two new sensors in mail tomorrow. Should I replace both upstream, or a full bank?

Were you able to see my video I sent yesterday of the spark test on cylinder #1? Did the spark appear to be missing?

Injectors that I installed below. They were new and supposed to be factory spec tested.

Product description
Specification:
NEW! Performance 8 Pack Fuel Injector 25326903 FOR 2002-07 CHEVY FLEX FUEL 5.3L 12580426 FJ502 FJ737

Interchange Part Number: 50130 832-12114 832-11182 217-1626;
Manufacturer Part Number: 25326903 12580426;
Other Part Number: FJ502 FJ737;

Fitment For:
2007 Chevrolet Silverado 1500 Classic
CHEVROLET AVALANCHE 1500 2005 V8-5.3L Fuel Injector
CHEVROLET EXPRESS 1500 2007 V8-5.3L Fuel Injector
CHEVROLET SILVERADO 1500 2002-2006 V8-5.3L Fuel Injector
CHEVROLET SUBURBAN 1500 2002-2006 V8-5.3L Fuel Injector
CHEVROLET TAHOE 2002-2006 V8-5.3L Fuel Injector
GMC SAVANA 1500 2007 V8-5.3L Fuel Injector
GMC SIERRA 1500 2002-2006 V8-5.3L Fuel Injector
GMC YUKON 2002-2006 V8-5.3L Fuel Injector
GMC YUKON XL 1500 2002-2006 V8-5.3L Fuel Injector

Bank 2 is too high. You have a lean condition for bank 2 only.

No to the O2 sensor. It is doing its job.

No video came up from yesterday. Please resend.

Swapping injectors from left to right will verify injectors if the long term goes to bank .

Roy

Roy, I am trying to attach the spark test video again (cylinder 1). Hope it works.

It indicated it compressed the video and uploaded it. Hope you can see it.

Okay,

1. You cannot clean a mass air flow sensor. There is no cleaner that can get the burned carbon off the probe. If the sensor was bad, both banks would read the same. not one side high and one side low,

2. A vacuum leak definitely affects fuel trim. The fuel mapping will change when unmetered air enters the intake.

3. Your misfires are not enough fuel mixture, not too much so there is no additional unburned fuel/air mix.

Roy

I read in an article about fuel trims. Would this mean that if my MAF was bad only cleaned it) or there were a slight intake leak (none found) that high trims would not be caused by higher air intake so I got a new MAF that fixe it thank you.

Roy today I swapped the injectors. Again a refresh is I am having misfires on 1 and 8. Vacuum is 19 steady, all plugs and cables replaced, coils moved around with no changes, MAP replaced, CAM sensor replaced, Fuel Regulator replaced, fuel pressure 50 on / 42 idle (Z engine).

I swapped injectors as follows:

Before swapping cylinders 2-7 running good with no misfires. 1 & 8 misfiring.

1&3 swapped
2&4 swapped
5&7 swapped
6&8 swapped

After swapping injectors no change. Cylinders 1&8 still the problem.

Roy attached is freeze data screens from drive this morning where P0300 code set.

Roy

I found an article about restricted exhaust (cats) below. This seems what is happening to my FTs. Thoughts?

Restricted exhaust. (Although no two converters fail exactly the same, if fuel trim goes high under acceleration on only 1 bank, suspect a restriction on the other. The MAF reads complete air intake, and then ECM initially distributes fuel evenly to both sides. If much less air is going through one side, there will be more than enough fuel for that restricted side and not enough fuel for the unrestricted side.)

how do i fix a misfire trouble code p0300

A PO300 is a multi cylinder misfire. If the vehicle has not been tuned up for some time, it may need done. You also should check for vacuum leaks and make sure fuel pressure is good.

This guide can help us fix it.

https://www.2carpros.com/articles/engine-misfires-or-runs-rough

and

https://www.2carpros.com/articles/how-to-tune-up-a-car-engine

Please run down this guide and report back.

engine falls flat under load and sounds like spark knock checked for codes and got code p0300 multipel misfires changed plug did not help

They are two different items. The knock sensor's job is to keep engine timing at it optimum position, just before you get valve ping. Here is a picture:

went to shop and ask about knock sensor mechnic put tester on and drove said knock sensor was working and that he thought maybe converter had came apart but no codes for o2 only codes we get are maf vaf the misfire p0101 andp0300

Have you tried cleaning the MAF?

put new one on that fixed it, thank you.

Engine started running really rough especially at idle. kicks up a code P0300 tends to stall out when you rev the motor and let it drop down. found a crack in the PCV valve connection tube replaced that. still runs rough. I've been running this program iobdwiz from scantool.net checking the fuel trims and stuff like that and find the fuel trims go double digit positive sometimes on the long-term. replaced the plugs wires, checked the coils they all seem good. injectors seem good sound. good 02 sensors voltage goes up and down. any suggestions of what I should be checking next?

Welcome to 2CarPros.

If the fuel trims are that high, something is allowing unmetered air into the engine. How did you check for vacuum leaks? I know you repaired the PCV, but did you check the intake? Also, are there any exhaust leaks pre catalytic converter? Is fuel pressure within spec?

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

I have one other thing I would like for you to review. It deals with issues specific to a random misfire. Read through it and see if it is helpful for you.

_______________________________________
2004 Chevy Truck Silverado 1500 4WD V8-5.3L VIN T
Important Information - PIP 3056
Vehicle ALL Diagnostic Trouble Codes ( DTC ) Testing and Inspection P Code Charts P0300 Important Information - PIP 3056
IMPORTANT INFORMATION - PIP 3056
Subject: 97-05 Vehicles with 4.8 5.3 5.7 and 6.0 Gen III V8 Engines and Misfires on One Bank - kw CEL diagnostics driveability DTC P0171 P0172 P0174 P0175 P0300 P0301 P0302 P0303 P0304 P0305 P0306 P0307 P0308 #PIP3056 - (07/12/2004)

Models: (00-05 Cadillac Escalade - C6 and K6) and (02-05 Chevrolet Avalanche - C1, C2 K1 and K2) and (99-05 Chevrolet Silverado - C1, C2 K1 and K2) and (00-05 Chevrolet Suburban - C1, C2, K1 and K2) and (00-05 Chevrolet Tahoe - C1 and K1) and (99-05 GMC Sierra C1, C2 K1 and K2) and (00-05 GMC Yukon - C1, C2, K1 and K2) and (03-05 Chevrolet Express G1, G2, G3, H1, H2 ) and (03-05 GMC Savana G1, G2, G3, H1, H2 ) and (04-05 Buick Rainier S1, T1 ) and (03-05 Chevrolet Trail Blazer EXT S1, T1 ) and (03-05 GMC Envoy XL XUV S1, T1 ) and (03-05 Hummer H2) and (04-05 Cadillac CTS-V DB) and (98-02 Chevrolet Camaro FB) and (97-04 Chevrolet Corvette YB) and (98-02 Pontiac Firebird FB) and (04-05 Pontiac GTO VB)

The following diagnosis might be helpful if the vehicle exhibits the described symptoms.

Condition/Concern:
Check Engine Light with a P0300 due to 2 - 4 cylinders misfiring on the same bank of the engine. DTCs P0171, P0172, P0174 or P0175 may also set for the misfiring bank.

Recommendation/Instructions:
If the published misfire diagnostic does not isolate the cause, perform the following suggestions as necessary:
- Perform a fuel injector balance test for all 8 cylinders. If a fuel injector concern exists, it is possible to misfuel an entire bank of the engine, causing multiple cylinders on the same bank to misfire even though the root cause is a single fuel injector.

- Inspect O2 sensor connections on the misfiring bank for corrosion or water intrusion. If water intrusion is found on the right bank, it may be due to the AC Evaporator Condensation dripping onto the O2 sensor harness. If this condition is found, reposition and shield the harness to prevent a repeat concern and repair the connections.

- Check for excessive exhaust backpressure using the restricted exhaust diagnosis.

- Swap the Position 1 O2 sensors side to side to see if the misfires move to the other bank of the engine. If so, replace the O2 sensor.

Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.

NOTE: GM bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform these technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions, and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition.

___________________________________________

Let me know.

Joe

i was trying to fix the problem with the service engine soon light coming on and blinking and showing code p0300. i was attempting to try cleaning the ground (g103 i believe) and dropped the bolt somewhere and haven,t been able to find it. i was hoping someone would tell me what size it is so i can replace it and see if the problem is fixed.

Code 300 is for a random cylinder misfire. That's not a ground issue. Start with spark plugs and wires. You didn't list the engine size or mileage so I can only make generalizations. A flashing Check Engine light indicates the most serious of diagnostic fault codes. It means you're supposed to stop the engine right away because too much raw, unburned fuel is going into the exhaust system where it will burn in the catalytic converter and overheat it.

For the bolt, on cars and trucks they're often odd sizes compared to what you can buy at a hardware store. You're better off getting the right one from a salvage yard.

I looked through the Repair Question Forums and do not see anything specific that helps answer what might be wrong with my vehicle so now I am sending my question to you and very much hope that you may be able to help. I have a 2002 Chevy Silverado Pick-up Truck. V8, 6.0 liter engine, K2500. 4 Wheel Drive. Automatic. 54,000 miles. Check Engine Soon light is blinking. Code P0300 Random misfire. Light does not come on at all while driving around town. Only comes on when on the highway at over 60 mph. The light blinks for several minutes and then eventually goes out. Then several minutes later it comes back on blinking again. When the light comes on, the truck continues to run smoothly and there is no indication that something is wrong.

I brought the truck to be serviced. They replaced Intake manifold gasket with new design. (They said there is a bulletin out from GM that if the gasket is the color green then it could become defective. They said my truck had the green one.) The garage seemed to assume that because of this bulletin that this must be what was wrong. They also did an oil and filter change and changed all eight Spark Plugs. ( My truck had 100,000 mile spark plugs so I’m not sure why they needed to change the spark plugs now).

The next day, I went on the highway with the truck and once again the light was blinking. I called the garage and they said to bring it back and they would check every thing again. At this time they rechecked the connections to intake and cleaned engine grounds and also replaced the fuel filter. When I picked up the truck, they said everything should be fine now. I took it on the highway and again the light was still blinking. I called the garage again and they said to bring it back. When I picked up the truck they said they once again checked everything and it now definitely should be fine. I went on the highway and the light is blinking on again.

I’m very frustrated and aggravated. It cost me almost $540.00 and I am feeling that bringing my truck to this garage might have been a waste of time and money because they did not solve the engine light problem. I’m thinking that they are not following diagnostics thoroughly to solve the problem. The second time I brought the truck back, I felt like I was being brushed off. They don’t seem to be doing anything more about it. I don’t know what to do next.

Also, prior to having this service engine light problem, I had this vehicle at the dealer and was told the Engine Oil Cooler Lines are starting to leak. They said they would need to replace the hose. The garage that I brought the truck to for the engine light said that this would not affect the light coming on and it is a separate problem. True?

One other thing I can think of mentioning: When I first start driving the vehicle I can hear a engine tapping type of noise but again I think this might be a separate problem. (I told the garage this as well.)

I am hoping you could give me some insight on what might be wrong. I would really appreciate any suggestions. I’m really discouraged! Please help!

This is most likly the cause, and the fix is free:

This is a TSB also from Chevy:
Information on Engine Misfire MIL/SES Light Illuminated or Flashing DTC P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308, P0420 or P0430 TECHNICAL SERVICE BULLETIN Reference Number(s): 06-06-04-046, Date of Issue:  September 12, 2006 Related Ref Number(s): 06-06-04-046 ARTICLE BEGINNING

SERVICE INFORMATION If you encounter vehicles that exhibit the above conditions, refer to SI for the appropriate DTC(s) set. If no trouble is found, the cause may be due to an ECM ground terminal that has corroded with rust over time. Inspect the main engine wiring harness ground terminal (G103) for this condition. The wire terminal (G103) attaches either to the front or to the rear of the right side cylinder head, depending on the model year of the Full Size Pickup and/or Utility Trucks. If the ECM ground terminal has been found to be corroded, then follow the service procedure outlined in this bulletin to correct the corrosion issue. Fig. 1: Engine Wiring Harness Ground Terminal Location (2007 Full-Size Pickup and/or Utility Trucks) Courtesy of GENERAL MOTORS CORP. Fig. 2: Engine Wiring Harness Ground Terminal Location (1999-2006 Full Size Pickup and/or Utility Trucks) Courtesy of GENERAL MOTORS CORP. 1. Remove either the nut or bolt securing the main engine wiring harness ground terminal (G103) to the right cylinder head. Refer to the above illustration to determine where the ground is located on the vehicle (1). 2. Remove all rust from the ground terminal, the cylinder head and the retaining nut or bolt. 3. Position the main engine wiring harness ground terminal and install the nut or bolt. Tighten:: Tighten the retaining nut or bolt to 16 N.m (12 lb ft). 4. Apply some type of electrical moisture sealant to protect the harness terminal from further corrosion.

Hello merlin2021,
Thank you so much for your very prompt response. I really appreciate it. The garage had said that they cleaned engine grounds but I will double check the EMC ground terminal and will remove all rust if found. Hopefully, this is the problem. I will let you know if it worked.

Thanks so much!
macfin

It says multiple misfire detected. What can be the problem?

Hello,

Direct Trouble Code (DTC) P0300 can be set by a variety of conditions. In the diagrams down below I have included the factory diagnostic troubleshooting guide for DTC P0300 for your vehicle. Please go through this guide and get back to us with what you are able to find out.

Thanks,
Alex
2CarPros

Okay, I will check it out. thanks

Good evening.

Just to throw another idea at you, check the fuel pressure. That is the most common cause for this code. If the pressure is low, it will cause multiple misfires.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Roy

Code p0300.
engine is misfiring on #2 and #7 cylinder at idle.
So far I have checked compression 155 and 160
switched coil, plug wire, plug and injector.
Checked fuel pressure 55-60 psi.
Checked EGR.
Checked for plugged exhaust.
Changed both rear o2 sensors.
Compared all scanner data with another truck that is known to be operating properly
the only difference noted being bank#2 sensor#2 is staying down around 100-200 mv.
At 1 point there was a map sensor code.
I suspected a possible cracked or leaking upper intake.
but when I called the dealer they told me they had never sold one or even priced one before. I'm running out of things to check.
Does anyone have any more ideas?
Thanks in advance.

Hello !! Cyl #3 and #7 are in the same side.. not next to each other...

On the same side on the distributor cap... Did you replace the "cap" How about the spark plugs? When did you replace them? Are the correct "heat"? Did you check the pick-up coil? could be "missing" at low RPM`s Let me Know and I`ll be thinking about it.. BTW thanks a Lot for your donation!!

Hello and thanks for responding
The spark plugs are approx. 2 months old and installed by myself at the shop where I work.
I am also a tech
The 5.3 litre is d.i.s. "one coil per plug"
#2 cyl and #7 cyl are the problem cyls
I am aware that they are on opposite ends of the engine.
The only thing I have found is that they follow each other in the firing order but I can't see the crank sensor causing the problem on just the two cyls.
Any other input would be appreciated.

Replace the intake gaskets, not the intake. That should fix your problem. You were getting a MAP code because the engine is sucking unmetered air, more that what the computer thinks should be. Basically, your computer is confused. It is taking the readings from the TP and O2 sensors and says that the intake should be sucking a certain volume of air, based on MAF. When you have an intake leak, it produces a lower vacuum that what the computer thinks it should, making the computer think that the MAP is bad. This also explains the O2 sensor reading (which is lean). The engine is sucking more air than what the computer can compensate for. Hope this helps. If you need any more info, don't hesitate to ask.
Dave