2003 Gmc Problem

[Kroynon]

Hello,

New here with a 2003 GMC Sierra SLT 6.0 4x4. Love my truck but has recently developed rough idle/surge at idle which sometimes goes away. Runs fine otherwise except for a couple instrument cluster glitches.

Scan codes-Lean condition bank1 and bank2. Random misfires on all cylinders.

Fuel pressure checks good. Cleaned throttle body and MAF. New air filter/fuel filter. No obvious vacuum leaks although it does act like there is one. I've read where a leaking intake manifold could cause this but on all cylinders and not all the time?

Is it time to start changing out sensors in hopes of a fix?

[txab]

You say your fuel pressure is good, what is the fuel pressure reading, key on, engine off?

[Kroynon]

You say your fuel pressure is good, what is the fuel pressure reading, key on, engine off?

 

55 psi

Any ideas?

Thinking I should have a smoke test done somewhere.

[ChevyTech77]

Leaking intake gaskets. Smoke or propane test will show it.

[txab]

Leaking intake gaskets. Smoke or propane test will show it.

+1

[Kroynon]

Thanks guys,

I've never had any luck with propane with the fan blowing air all over the engine compartment and I don't see how to get to the intake /head seal area anyway.

 

I'll get a smoke test done today and post my findings.

 

This was the first time I scanned my truck and there was something else I noticed and forgot to mention- Timing at idle was erratic bouncing from 13-20 deg. but would give a steady reading at rpm above 1200 or so. Is this normal, caused by the lean condition or possibly a crank sensor error?

[ChevyTech77]

Thanks guys,

I've never had any luck with propane with the fan blowing air all over the engine compartment and I don't see how to get to the intake /head seal area anyway.

 

I'll get a smoke test done today and post my findings.

 

This was the first time I scanned my truck and there was something else I noticed and forgot to mention- Timing at idle was erratic bouncing from 13-20 deg. but would give a steady reading at rpm above 1200 or so. Is this normal, caused by the lean condition or possibly a crank sensor error?

 

Timing jump can be from faulty knock sensors/knock sensor wiring issues.

 

Propane test can be done with a chunk of hose on the end of the torch tip to extend it under the intake.

[Kirkathon25]

check your crank position sensor, if the misfires are on 1 or 8 and the computer cannot tell you which specific ones are misfiring, but you can see it on an expensive scanner, mine had the exact problem and i left it with a mechanic for 10 hrs before he figured out it was my crank position sensor, easy fix 25 mins for me.

[Kroynon]

Timing jump can be from faulty knock sensors/knock sensor wiring issues.

 

Could a faulty knock sensor be causing my idle issues?

[ChevyTech77]

By fluctuating the timing, its possible.

 

I still think you have an intake leak or a leaky fuel pressure regulator though.

[Kroynon]

Fuel press holds at 55 key off. 51 while idling. I think I've ruled that out.

Tried the propane under and around the intake this morning with a hose attached and no change.

I'll see if I can get a smoke test done today just to be sure then go after the crank sensor I guess.

[ChevyTech77]

Fuel press holds at 55 key off. 51 while idling. I think I've ruled that out.

Tried the propane under and around the intake this morning with a hose attached and no change.

I'll see if I can get a smoke test done today just to be sure then go after the crank sensor I guess.

 

 

You have to watch fuel trims, with a scan tool, while doing a propane test. The engine tune usually

won't change, unless you have a huge leak.

 

A failed fuel pressure regulator may hold correct pressure but will dump fuel into its vacuum line.

 

Still might have a knock sensor or knock sensor wiring concern........

[Kroynon]

Both knock sensors test at 100.7 ohms and no codes.

Fuel reg. vacuum line is dry. Pulled the hose and left the key on and see no indication of leakage.

Thanks for all the help so far.

[8.1HD/ALLISON]

+1 on the intake gaskets, very common issue. Runs bad cold, smooths out after warmed up due to expansion sealing the intake back up.

[Kroynon]

+1 on the intake gaskets, very common issue. Runs bad cold, smooths out after warmed up due to expansion sealing the intake back up.

Mine actually seems worse after running on the highway. Maybe heat expansion is causing the manifold to lose it's seal?