02 Tahoe Heavy Misfire P0300

[D M 3]

I just picked up an 02 Tahoe 5.3. Previous owner tried to fix the issue but failed. The truck is running rich and only has a P0300 code. Its stumbles at start up and gradually gets worse and cuts off. Cats have been changed, coils, Intake Gaskets, O2 Sensors, Fuel Pressure Reg. ,Map Sensor. Fuel Pressure is around 55 psi. Any suggestions on what else to try would be appreciated.

 

[Madgrty]

Does it by any chance ever say "ENGINE POWER REDUCED" on the cluster? Do all cylinders have compression? Mass air flow sensor? Possibly throttle body?

 

[#1taho]

In my 2000 I had bad wiring at a few injectors. After I fixed it it's been good. I had the same issue. But there is a few problems other problems it could be also. I would go over what the other guy did in case he missed something.

 

[Z28TWITCH]

Couple of questions. Any smoke out the exhaust? Any coolant in the oil? What do you fuel trims look like? Any vacuum leaks? Multiple reasons for it to run like that. If the p/o has done all that work and still didnt fix it, its either deeper in or really simple...funny how things work like that. Myself, Id backtrack p/o fixes and make sure they're done correctly, start with vacuum lines and intake leaks. Check for coolant in the oil or sweet smelling steam coming out the exhaust. How many miles on the truck?

 

[D M 3]

Thanks. No reduced engine power, havent checked compression yet. I was thinking maybe wiring issue. Yes it smokes out the exhaust but its only fuel coming out not coolant. Checked it for vacuum leaks with starting fluid no change, no coolant in oil or oil in coolant. Truck has 150k on it and seems to not have been driven rough.

 

[Tonyrodz]

Does it by any chance ever say "ENGINE POWER REDUCED" on the cluster? Do all cylinders have compression? Mass air flow sensor? Possibly throttle body?

Just curious, what difference would it be if it said engine power reduced? Would that narrow it down even more to what the cause could be?

 

[Madgrty]

Our throttle bodies will sometimes go bad I know, since they are electronic. It'll come up on the cluster saying "Reduced engine power" mine did run rough and you could floor it and never make it to second gear. Check engine light was on but I never checked the code. I just swapped my LS throttle body on and all was good again. Not sure if maybe yours was having that same problem

 

[D M 3]

By the way i dont think my throttle body is electronic it has cables.

 

[Madgrty]

Does it look like this one? I thought GM only used the cable TB for the 5.3 in the express vans

 

[D M 3]

View attachment 70082 Does it look like this one? I thought GM only used the cable TB for the 5.3 in the express vans

No Definitley not. Idk why but its mechanical.

 

[Madgrty]

I wonder if P/O got a donor motor out of an express van... I'll have to look it up again but I'm 99% positive that the were all drive by wire (electronic) not cable driven

 

[D M 3]

Ill look at it in the morning. Gonna check some things on it.

 

[Madgrty]

Alright I was wrong, from 1999-2002? Maybe 03 but I believe if you either hadn't got traction control or cruise control it came with a cable TB. If you had those options then it came with the electronic TB.

 

[Tonyrodz]

My 03 is dbw.

 

[retiredsparky]

I am surprised that temp sensors were not changed- Intake air and coolant temp sensors can affect the fuel mixture and are not real expensive. If the PCM thinks the engine is real cold, the mixture will be extra rich.

Have you checked the spark plugs to see if any of them are blackened to see which cylinders are running rich?

You could have injectors that are not shutting off properly. Or injectors whose spray patterns are not optimal any more. If the fuel doesn't atomize correctly, some will not ignite properly. Remove them and have checked and cleaned at an injection shop.

Has the compression been checked? Low compression could cause incomplete combustion.

You didn't say that the plugs and wires or air cleaner had been changed. These are pretty common maintenance items.

I suppose a last resort would be to find a used or new PCM or have the PCM flashed at an independent shop or a dealer.
Larry

 

[D M 3]

Ordered a rebuilt ECU. I think this is the issue after speaking with previous owner. Hopefully it works will let u guys know. Thanks

 

[Tonyrodz]

Hopefully that's the issue and you'll be done with it. What made you come to that conclusion?