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Amedee, Former ASE Master Tech
Category: Car
Satisfied Customers: 26033
Experience:  ASE certified tech advanced level specialist. Wisconsin certified emissions state inspector
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OK. 2000 GMC Sierra- 199,000 miles. P0300 random misfire code--(only

Customer Question

OK. 2000 GMC Sierra- 199,000 miles. P0300 random misfire code--(only cylinders 1,8,5,6 misfire) replaced #6 coil, new wires, plugs, injectors cleaned, new fuel pump, regulator. The truck runs fine, with a slight stumble on idle...BUT...I can't get an inspection sticker with this code!! I've tried everything I can think of to clear this code with NO luck!!!
Submitted: 8 years ago.
Category: Car
Expert:  Amedee replied 8 years ago.

How does it run?

Does it have plenty of power?

When does it misfire? Under a load or a idle?

Do you have access to a scanner?
Customer: replied 8 years ago.

It runs pretty well. The only problem I have (other than the misfire code)is that it will stumble mostly after a restart: Ex: runs fine when first started in morning, after a quick stop for cofee, it will stumble and hesitate with no power under load, until the gas pedal is released for a few seconds, then it clears out and runs great with good accell. This condition has gotten better since I repalced the fuel pump (original) and regulator.


I use a SnapOn scanner and after I clear the code the misfires will come back slowly at idle and under power. Here's the thing that's got us baffled (I got a few people helping on this problem)...ONLY cyl. 1-8-5-6 misfire. Every other one in the firing order and across and diag. in the banks!! the other 4 cyl never misfire. All coils show NO Faults on the scanner.


Another strange thing: I recently cleared the code after cleaning the injectors and not having the code reappear for about 30 miles, well the "service engine" light came on again after about 30 miles....BUT...the next day it was OFF (I didn't clear it),,,SO...I took a chance and had it tested at the local inspection shop but the P0300 code is PENDING, meaning it's still there without the light!! Still no inspection sticker!!

Expert:  Amedee replied 8 years ago.
Ok... Thanks for the detailed answer.

How is engine vacuum?

SO if you clear the codes and let it idle, it wont misfire until you accelerate... is that correct?
Expert:  Amedee replied 8 years ago.
I just want to cover the basics for this problem before we move on.

Did you know there are two TSB's for this problem?

Here they are.

Bulletin No.: 06-06-04-046

Date: September 12, 2006


Information on Engine Misfire MIL/SES Light Illuminated or Flashing DTC P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308, P0420 or P0430

1999-2007 Cadillac, Chevrolet, GMC Full-Size Pickup and/or Utility Trucks

with 4.8L, 5.3L, 5.7L, 6.0L or 6.2L VORTEC GEN III, GEN IV, V-8 Engine (VINs V, C, T, Z, B, 3, M, 0, J, R, U, N, Y, K, 8 - RPOs LR4, LY2, LM7, L59, L33, LC9, LH6, LMG, LY5, L31, LQ4, LQ9, L76, LY6, L92)

with Active Fuel Management(TM) and E85 Flex Fuel

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.

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).

Remove all rust from the ground terminal, the cylinder head and the retaining nut or bolt.

Position the main engine wiring harness ground terminal and install the nut or bolt.


Tighten the retaining nut or bolt to 16 N.m (12 lb ft).

Apply some type of electrical moisture sealant to protect the harness terminal from further corrosion.


File In Section: 06 - Engine/Propulsion System

Bulletin No.: 00-06-04-024

Date: June, 2000


Engine Misfire, Water in Fuel, Service Engine Soon Lamp illuminates, DTC P0300 Set ( Replace EVAP Canister Vent Hose Asm/Replace EVAP Canister If Necessary)

1999-2000 Chevrolet and GMC C/K Pickup Trucks (Silverado and Sierra) Built Prior to August 31, 1999

Some drivers may comment about the Service Engine Soon lamp illuminating. The Powertrain Control Module (PCM) may have stored a Diagnostic Trouble Code (DTC) P0300, Engine Misfire Detected. The engine may be exhibiting a misfire, possibly in the # XXXXX and # XXXXX cylinders. This condition may also affect the six cylinder models. This condition may be more prevalent in areas with high amounts of rainfall.

Diagnosis of the fuel system may reveal the presence of water, possibly in the fuel rail near the # XXXXX and # XXXXX fuel injector. Water entering through the evaporative emission (EVAP) canister vent solenoid may cause this condition. Because of this, an engine misfire may develop.

A new EVAP canister vent hose assembly, P/N 15759042, that includes the valve and the solenoid has been developed to correct this condition.
The new solenoid has a revised mounting bracket and a sealant has been applied to the top lid seal surface of the solenoid to address water intrusion. Use the procedure and the part numbers listed below.

If standard diagnostics do not lead to the resolution of the misfire, do the following steps:

1. Remove the fuel rail at the # XXXXX and # XXXXX fuel injector from the engine and drain the fuel into a suitable container. Inspect the drained fuel for the presence of water. Refer to the Fuel Rail Assembly Replacement procedure in the applicable Service Manual.

2. If the presence of water is detected in the fuel rail, remove the fuel tank and inspect the fuel for the presence of water. Refer to the Fuel Tank Replacement procedure in the applicable Service Manual.

3. If water is present in the fuel tank (other than from a water enriched fuel fill), replace the EVAP canister vent hose assembly. Refer to the EVAP Vent Valve Replacement procedure in the applicable Service Manual.

4. Disconnect the EVAP canister line connections and remove the canister. Inspect for the presence of water at the canister line connections and inside the canister. If water is present, replace the EVAP canister. Refer to the EVAP Canister Replacement procedure in the applicable Service Manual.

5. Flush and clean the fuel system. Refer to the Fuel System Cleaning procedure in the applicable Service Manual.

Parts Information

Parts are currently available from GMSPO.

Customer: replied 8 years ago.
Did you get my reply?
Expert:  Amedee replied 8 years ago.
Did you get mine?
Customer: replied 8 years ago.


I haven't checked the vacuum.

The code will come back if it's idling or running. But the light doesn't come on until many misfires. I've replaced the evap vent sol. and hose recently and fuel pump module, I didn't check for water in gas, but I use inj cleaner often and had the inj pressure cleaned last week.

The only cyl misfiring are 1-8-5-6 and the coils show OK.

Expert:  Amedee replied 8 years ago.

Have we scoped the crank sensor?


Did you say that you can actually feel the misfire?

Customer: replied 8 years ago.

Crank sensor replaced.

It has a rough idle, been that way since new. But when it's going down the road I can't feel the misfire. Misfires are about 25-32 per cyl. per 100 cycles. The light comes on when they get into the thousands range. Now the light shut off itself WTF..but the code is still there as PENDING!!!

AND...this is the thing that leaves everyone I talk to about this problem confused: ONLY cyl 1-8-5-6 misfire. The other 4 cyls never misfire.

I will check the ground wire more closely tomorrow..I've had a look at it before.



Expert:  Amedee replied 8 years ago.



This is very odd.


I thought you said it misfires when accelerating?


But if it idles rough, then maybe we have a vacuum leak? CHeck the freeze frame on your scanner and see what RPM the engine is at when the misfires occurs.


ALso, disconnect the EGR valve somehow and see if this helps at all.


In addition, just a a side note, if I remember correctly, I think GM has a update for a new crank sensor and shim kit just for this type of problem. It pulls the crank sensor away from the flywheel just enough so that there is no more contact between the two (if there was before).


I guess what I am try to get at is that IS THAT THE ENGINE ACTULLAY MISFIRING when the computer says it is, or is it just the computer that thinks it's misfiring? Thats why I was recommending to scope the crank sensor.

Customer: replied 8 years ago.

with the scanner on the engine. the 1-8-5-6 cylinders misfire at idle, no accell, acell, highway speeds, etc. The engine seems to run fine, I would drive this truck anywhere, no problems with accleration, etc. BUT........the ECU sees those 4 cylinders misfiring!!!

I just want to clear the code so that I can get an inspection sticker. I've driven around for 2 years with this light on and the truck runs fine. But now I'd like to have it inspected (I'm in MA.) so that I can sell it.

Would the things you said make just those 4 cylinders misfire....that's the BIG question?? Or are the cylinders NOT misfiring and the ECU has a glitch and thinks they are??

Expert:  Amedee replied 8 years ago.

Well thats what we've gotta figure out.


If those cylinders are misfiring at idle only, I would be looking for a vacuum leak. You might want to smoke the intake with a smoke machine.


Also, check engine vacuum to be sure that its nice and steady.


I don't know how you are going to pass this inspection by only clearing the computer and not fixing this problem unless you time it just right! I mean by clearing the codes on the way to state inspection. Hopefully the readiness monitors will reset enough so that you will pass.

Customer: replied 8 years ago.

those 4 cyl misfire all the time, idle and driving. I can drive the truck for 40 miles on the highway with the scanner hooked up and watch them misfire.

I can't clear the code and get a sticker because it comes back before the readiness is done. I went today for a sticker because the light wasn't ON...but there tester said that the P0300 code was PENDING!! meaning it's going to pop the light.

BTW: the light went out by itself this time without clearing the codes!!!????

It's driving me crazy.

Expert:  Amedee replied 8 years ago.

SO when the computer says its misfiring when you are driving on the freeway, can you feel it misfire?

Customer: replied 8 years ago.

NO...I don't feel it misfire. The scanner shows maybe 25-32 misfires per 100 cycles, which I guess is not enough to feel anything on the highway with the vehicles ineartia and all!!

The truck runs fine.

Expert:  Amedee replied 8 years ago.

Well this leads me to believe that we should be looking close at that crank sensor to verify that it is actually misfiring or not. A scope is the best way to tell.


I have heard on these motors that the valves stick and causes low compression at times on certain cylinders. But because this is happening at idle on your motor, leads me to go in a different direction. But a vacuum test should prove me wrong or right.

Customer: replied 8 years ago.
how do I scope the crank sensor?
Expert:  Amedee replied 8 years ago.
DO you have a scope?
Customer: replied 8 years ago.



Expert:  Amedee replied 8 years ago.

We then to scope it, you would need to get your hands on a scope, either borrow or buy a scope, or have someone scope it for you.


The wave form will need to be studied very close to determine if the engine is actually misfiring or not.

Expert:  Amedee replied 8 years ago.

But before we do that, lets check engine vacuum at idle.


Let me know!

Customer: replied 8 years ago.
Expert:  Amedee replied 8 years ago.



Did we get a chance to check engine vacuum?

Customer: replied 8 years ago.

No, I can find a vacuum gauge...

BUT,,I just had the scanner on it.

I cleared the code and at idle---NO MISFIRES (actually #6 cyl shows a couple)

If I race the engine in neutral to about 1,000rpm's the MISFIRES come in and the light comes ON.

I thought I could check the Cranksensor with the scanner, but the only thing I see close is Cam HI to LOW and Cam Low to HI---which I assume is just the opening and closing of the valves as they follow the cam??? Beause this valve just increases as the engine runs...Am I right about this??

What do you think?

I won't be around my computer until sometime Friday....have to go home from work now...



Expert:  Amedee replied 8 years ago.

The cam signal from low to high and high to low is new to me. This must just be how that particular scanner interprets the cam signal. I don't believe this is in relation to the valves because while some are closed, some are open. They are not all open or closed at one time. I believe there is a low and a high reference wire at the cam sensor and this is what the computer is reading.


Let me know when we can discuss this further. I will be back in front of my computer on Friday as well.


Happy Thanksgiving!