🔧 Technical 2003 Tahoe Z71 5.3 Misfire Issue - Need Help!

[slowchevy]

2003 Tahoe Z71 5.3 w/ flex, 300k miles.

Runs great when cold, when it warms up it begins missing on all 8 steady rising counts on a scanner.

Did plugs, wires (NGK iridium OEM equivalent, acdelco OEM equivalent wires)

No change.

Crank position relearn no change.

Tried cleaning chassis grounds where the coil pack and injector circuits ground, no luck.

Everything else seems right. Decided to drop it off at a shop and pay for diag since anything more seemed to be more than my area of expertise. He cleaned a few more grounds. No changes. My truck has been at that shop for over 3 weeks now.

They can't figure it out.

Anyone got any ideas? I miss driving this big dumb thing. This is the first actual problem I've had with this truck that I couldn't figure out.

Click to read more...

 

[Jimy Bilmo]

Alldata shows a few things for that thing:


DTCP0300

Symptom Descriptor

Random/Multiple Cylinder Misfire Detected

Verified Fixes

Fix Description
Fuel Pump Wiring Harness

Probable Causes:
Crankcase Ventilation System for Vacuum Leaks
Exhaust System Has Restrictions
Fuel InjectorFuel for Contamination
Ignition Coil
Powertrain Control Module (PCM) Grounds Corrosion
Spark Plug
Throttle Body and Intake Manifold Vacuum Leaks
Vacuum Hoses and Seals Improper Connections
Vacuum Hoses and Seals Restrictions
Vacuum Hoses and Seals Splits

 

[Jimy Bilmo]

Sounds like it's time for a smoke test.

 

[sktchy]

Yeah I'm gonna go with it sounds like a vacuum leak or possibly a fuel pressure thing.

 

[Mook]

SRT41320

 

[slowchevy]

Yeah I'm gonna go with it sounds like a vacuum leak or possibly a fuel pressure thing.

Fuel pressure is good 3psi less than spec 42psi.

 

[sktchy]

Fuel pressure is good 3psi less than spec 42psi.

That sounds low to me and is that with vacuum to the regulator blocked off or no?. I don't really like seeing em with less than 43.5

 

[sktchy]

The pcv will cause a vac leak and oil consumption, intake gaskets are notorious for leaking (do the knock sensors and harness while your there or you'll have a light in a week) EVAP canister could be plugged, purge valve could be leaking. There's just a lot to look at and I guess I'd really want more scantool data than just a random misfires code to look at before firing the cannon at things.

 

[slowchevy]

That sounds low to me and is that with vacuum to the regulator blocked off or no?. I don't really like seeing em with less than 43.5

My buddy that owns the shop said he was happy with it. Original fuel filter on truck he said if he thinks it's gonna cause issues he'll use the one I left in the glove box. I did a new delphi pump a few years ago. I don't know how he measured it. Tempted to pull it out and bring it elsewhere if he can't figure it out in the next week. This is a bigger shop that's been doing business for a decade and a half in my area in Kenosha.

 

[slowchevy]

The pcv will cause a vac leak and oil consumption, intake gaskets are notorious for leaking (do the knock sensors and harness while your there or you'll have a light in a week) EVAP canister could be plugged, purge valve could be leaking. There's just a lot to look at and I guess I'd really want more scantool data than just a random misfires code to look at before firing the cannon at things.

I did the intake gaskets when I did both knock sensors / the harness a few years ago. Used metal gaskets. Could evap canister cause a misfire when up to op temp? I do have a code for that but it's been on for literally 30k mi. Purge valve is newer.

 

[sktchy]

For what it's worth it could even be a dirty mass air flow sensor.

 

[sktchy]

I did the intake gaskets when I did both knock sensors / the harness a few years ago. Used metal gaskets. Could evap canister cause a misfire when up to op temp? I do have a code for that but it's been on for literally 30k mi. Purge valve is newer.

If you have an evap code you have a vacuum leak so yes.

 

[sktchy]

Could even be 02s or plugged cats though.

 

[slowchevy]

Could even be 02s or plugged cats though.

All of these maybes was why I dropped it at a shop lol

 

[CMNTMXR57]

If you have an evap code you have a vacuum leak so yes.

I was gonna go here. Any accompanying P0420 code? The thing that seems to tip me this way is that when it's "COLD" as in open loop operation, it's ignoring a multitude of sensor inputs and more or less running in a speed density mode, so it doesn't care, once it goes closed loop, then bam, it misfires. As mentioned, MAF, but check all air intake post-MAF. Check Exhaust for leaks, bad HO2S's (I would be checking these to monitor switch rates and mV's during those switching points). Catalyst would be the last thing I look at/replace.

 

[slowchevy]

I was gonna go here. Any accompanying P0420 code? The thing that seems to tip me this way is that when it's "COLD" as in open loop operation, it's ignoring a multitude of sensor inputs and more or less running in a speed density mode, so it doesn't care, once it goes closed loop, then bam, it misfires. As mentioned, MAF, but check all air intake post-MAF. Check Exhaust for leaks, bad HO2S's (I would be checking these to monitor switch rates and mV's during those switching points). Catalyst would be the last thing I look at/replace.

No catalyst threshold codes. They don't get abnormally hot either.

 

[CMNTMXR57]

P0420 isn't a heat code. It's just a measure of the catalytic converters efficiency of converting unspent fuel and carbon monoxide into CO2 and H20. Either the HO2S's are not working properly or the converter has truly lost it's ability to do this conversion.

 

[sktchy]

P0420 isn't a heat code. It's just a measure of the catalytic converters efficiency of converting unspent fuel and carbon monoxide into CO2 and H20. Either the HO2S's are not working properly or the converter has truly lost it's ability to do this conversion.

Thank you for adding some depth to what I was trying to get at.

Basically what we're trying to say is look into fuel trims and such, try unplugging the maf and see if it stops.

 

[slowchevy]

Thank you for adding some depth to what I was trying to get at.

Basically what we're trying to say is look into fuel trims and such, try unplugging the maf and see if it stops.

Haven't thought of that. If they let me come pick it up in the next few days I may do that. If I am looking at fuel trims what should I exactly look for? Kinda new digging this deep into things.

 

[sktchy]

Haven't thought of that. If they let me come pick it up in the next few days I may do that. If I am looking at fuel trims what should I exactly look for? Kinda new digging this deep into things.

I'm guessing your going to see high positive fuel trim numbers indicating a lean condition but I madly be wrong. Long terms follow what the short terms are doing. Negative numbers mean it has too much fuel. Maf and map data could also be helpful here. Another thing you might do is check the plugs to see if it'll point you in any certain direction as far as fueling goes. But, don't rely too heavily as misfires will indeed throw off the air fuel ratio too so you need to boil it down be ruling things out and testing things.

 

[SaturdaysGS]

You say it’s a flex fuel model..
Has the fuel composition % been reset? And/or is it pegged at a high % and you’re not running e85, or vice versa?
Possible bad flex fuel sensor?

 

[Turbocharged400sbc]

3800 swap it

 

[boostedguy05]

If you have it at a shop, and they can't figure it out, then it's probably not a very good shop, and I'd take it somewhere else.