Misfire on Cylinders 1 & 8

[berkankaya]

Hello everyone,

I have been chasing a misfire issue in Cylinders 1 & 8. The vehicle is a 2003 Chevrolet Tahoe 5.3 Flex Fuel, 2WD with 173k miles. The only engine code is P0300.

So far, i have replaced spark plugs, wires, swapped coils (no change), replaced injectors, and fuel pressure regulator and fuel pressure regulator vacuum hose. I have about 44 PSI fuel pressure when running.

When driving engine feels smooth, I do not notice any loss of power. At idle it shakes, never stalls. I have checked misfire count while driving and at times cylinder 8 will not misfire. I do smell a bit of exhaust, especially with the A/C on. Exhaust bolt after the cat was loose when i bought the vehicle and i just tightened it and it started to run a lot better. I am not sure if there is still an exhaust leak. I have a scan tool that reads live data but unfortunately I do not understand what to make of the information in front of me.

Your help would be much appreciated.

Thanks!

 

[corvette744]

Compression test and or leak down test.

 

[SnowDrifter]

Adding to what 'Vette said: check fuel trims - could be first pointer to a vacuum leak. Or exhaust leak. Aside from being dangerous... Atmospheric air intruding into the system is going to throw the oxygen sensors for a loop (no pun intended)

could also swap injectors around and see if the issue follows I read right past that part don't mind me

 

[OR VietVet]

Exactly what @corvette744 and @SnowDrifter said. Should have been done first before throwing money at it.

He said he replaced the injectors.

By the way, welcome to the forum.

 

[berkankaya]

Thanks everyone. I just ordered a leak down tester. Should i bother testing all cylinders or just 1 & 8?

@SnowDrifter i will check fuel trims.

Also, there is an oil leak. I added a quart of oil about a week ago and today i got the warning to check oil. I suspect this leak to be a bad rear main seal. Would this cause a misfire?

Thanks

 

[SnowDrifter]

Damn I read right past that!!

Thanks everyone. I just ordered a leak down tester. Should i bother testing all cylinders or just 1 & 8?

@SnowDrifter i will check fuel trims.

Also, there is an oil leak. I added a quart of oil about a week ago and today i got the warning to check oil. I suspect this leak to be a bad rear main seal. Would this cause a misfire?

Thanks

Test all

Oil leak: Not directly. But if you say.... ran it out of oil and damaged the engine, then that could cause a misfire.

 

[OR VietVet]

Thanks everyone. I just ordered a leak down tester. Should i bother testing all cylinders or just 1 & 8?

@SnowDrifter i will check fuel trims.

Also, there is an oil leak. I added a quart of oil about a week ago and today i got the warning to check oil. I suspect this leak to be a bad rear main seal. Would this cause a misfire?

Thanks

Run the compression test on all cylinders with all plugs out of the engine and post pics of the plugs, numbered. The compression can be rented at auto parts stores too.

 

[berkankaya]

I never ran it out of oil but who knows if the previous owner did.

Thanks! I will make some time this weekend to work on the Tahoe. I will post the results.

 

[Fless]

Have the MAF and throttle body been cleaned?

 

[OR VietVet]

I never ran it out of oil but who knows if the previous owner did.

Thanks! I will make some time this weekend to work on the Tahoe. I will post the results.

Pics during the process helps too.

 

[Joseph Garcia]

Welcome to the Forum from NH.

Lots of knowledgeable folks here who freely share their knowledge, experiences, and perspectives. Knowledge is power.

I hope that you will become a participating member in the Forum's discussions.

Pics of the truck, please.

You are already receiving sage advice from the knowledgeable folks on this Forum.

 

[rockola1971]

Piston domes excessively covered in carbon? Run the engine with intake hose disconnected from throttle body. (It will throw a code and engine may run like crap). Take a spray bottle and mist water into intake. Give engine slight rev. +(500-1000rpm). Do this until you get substantially bored and then put intake hose back on and clear codes. Go take it for a spin on the highway and open it up a little. This will clear carbon off of valves, combustion chamber, piston crowns and maybe even a little out of ring grooves. The water mist will not hurt your engine. DO NOT DUMP WATER IN THE INTAKE.

 

[InterceptorF]

Hello everyone,

I have been chasing a misfire issue in Cylinders 1 & 8. The vehicle is a 2003 Chevrolet Tahoe 5.3 Flex Fuel, 2WD with 173k miles. The only engine code is P0300.

So far, i have replaced spark plugs, wires, swapped coils (no change), replaced injectors, and fuel pressure regulator and fuel pressure regulator vacuum hose. I have about 44 PSI fuel pressure when running.

When driving engine feels smooth, I do not notice any loss of power. At idle it shakes, never stalls. I have checked misfire count while driving and at times cylinder 8 will not misfire. I do smell a bit of exhaust, especially with the A/C on. Exhaust bolt after the cat was loose when i bought the vehicle and i just tightened it and it started to run a lot better. I am not sure if there is still an exhaust leak. I have a scan tool that reads live data but unfortunately I do not understand what to make of the information in front of me.

Your help would be much appreciated.

Thanks!

My 2001 Yukon with 280K miles was bucking and misfiring in cold weather at start - once warm it was fine. It was the intake gaskets that fixed mine after new /filter/pressure regulator and cleaning the injectors. It was a tough one to find.