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6.0 Hot start idle problem

C

catman78

New member
Hi all,

Trying to get handle on issue.
2004 6.0 MPI, 350 hrs.

Cold starts are fine.
When engine is warm on start the idle will hunt between 400-1100 rpms.
Throttle body is clean.
IAC was clean.
Injectors were bench flowed and are good.
Fuel filter and water separator only have 20 hours on them.

I can get it to settle in to normal idle If I put her into gear than back to neutral while running.
You can see at end of video linked.

Any suggestions?

https://youtu.be/YtRDsw773AM
 
Have you cleaned all of the connectors on the engine? Pull all the connections apart and clean with electrical cleaner and put back together with dielectric grease. Intermittent faults are normally wiring related. Sand all grounds metal to metal and look for any bad wires cracked, bare or burnt. Once you are sure the wiring is good then you can diagnose individual components if necessary.
 
kimcrwbr1 said:
Have you cleaned all of the connectors on the engine? Pull all the connections apart and clean with electrical cleaner and put back together with dielectric grease. Intermittent faults are normally wiring related. Sand all grounds metal to metal and look for any bad wires cracked, bare or burnt. Once you are sure the wiring is good then you can diagnose individual components if necessary.
Click to expand...


Thanks,
Issue is repeatable as it only happens with hot motor. I will check wires.
 
What do you guys think of this idea.
Small crack in intake manifold?
My engine hatch sits real close to top of motor.

The intakes are plastic on GM Vortecs.

Intake when warm expands allowing air to bypass IAC.
I found this on LS1 Vette forum.

Several things can contribute to a surging idle. Once surging begins, the ecm can't do much to stop it, and it will likely make things worse.

As the rpm oscillates up and down, the calculated load changes inversely with rpm. As the load and rpm change, the spark advance, fuel and IAC position begin to oscillate as well, since they are a function of rpm and load. This is likey to amplify the rpm oscillation making the surge worse.
 
Possible....but why will the idle stabilize with the in-and-out-of gear exercise, per the video...

I'd be inclined to get a scanner and see if what the computer 'sees' is normal.....similar to kim's thought - I wouldn't be surprised to find an intermittent connection....
 
makomark said:
Possible....but why will the idle stabilize with the in-and-out-of gear exercise, per the video...

I'd be inclined to get a scanner and see if what the computer 'sees' is normal.....similar to kim's thought - I wouldn't be surprised to find an intermittent connection....
Click to expand...


So when engine is warm, there could be a connection issue, something expands, moves, etc.?

Yes I agree that I can get it to stabilize which is odd.
I can also stabilize idle by bumping up the throttle to around 1k and then slowly bringing it back down to normal position. Once there idle stay solid.

Also, I get no check engine lights.
 
catman78 said:
.....Also, I get no check engine lights.
Click to expand...

The Light only turns on when the computer senses something "out of bounds"; sensors can lose their accuracy and still be "in bounds" and cause all kinds of "driveability concerns"....the classic example is in most every Delphi manual I have: the ECT (coolant temp sensor) can read 'cold' and cause the too much fuel to be delivered by the injectors (sensor says 60 degrees and coolant is actually 150 deg)...and no check engine light comes on....

Would also be good to ensure the throttle body mounting nuts are secure as well as all of the sensors....
 
makomark said:
The Light only turns on when the computer senses something "out of bounds"; sensors can lose their accuracy and still be "in bounds" and cause all kinds of "driveability concerns"....the classic example is in most every Delphi manual I have: the ECT (coolant temp sensor) can read 'cold' and cause the too much fuel to be delivered by the injectors (sensor says 60 degrees and coolant is actually 150 deg)...and no check engine light comes on....

Would also be good to ensure the throttle body mounting nuts are secure as well as all of the sensors....
Click to expand...

Does this model have an O2 sensor?
Would be great to see what the fuel rail pressure and O2 readings are during the roughness. For that matter, the timing and coolant temp sensor output too!
 
If you have not done so, find the website for Pirate 4x4 and read the article on MEFI controllers. Then download the diagnostic manual. It has two pages on steps to trouble shoot Rough, Unstable or Incorrect Idle. If you have access to a TechMate scanner (or equivalent) this will enable you to sort it out in a hurry.

If not, the knowledge in the manual as to how things work will still help. It strikes me that the first place to look is the IAC valve. This is instructed by the controller to let more or less air into the engine in order to control the idle speed. Since the engine idle is unstable, it is likely that a) the controller is getting bad speed information and misdirecting the valve, or b) some external factor (air leak) is allowing varying amount of air into the manifold and the valve is chasing the resultant varying rpm trying to keep it constant, or c) the valve is not quickly (or perhaps linearly) responding to the instructions from the controller resulting in an unstable feedback loop of the directed valve position and the resultant rpm. I like c) as a first guess. Perhaps when the engine is surging and seeking a stable idle speed you could pull the plug on the IAC and see if the engine settles down to a stable rpm. If so, that would eliminate your possible variable leak and likely point to a faulty IAC.

I can't explain why the engine rpm becomes stable after going into gear, other than to note that when the engine sees a load the IAC will immediately open much wider than perviously to keep the engine from stalling. At that point perhaps the "sticky" portion of the response curve in the IAC no longer applies. When you take off the load, the valve closes down normally and (for a time) behaves. If the controller was sending bad instructions to the IAC (situation a)), then I do not think that the in gear/out of gear trick would work. So to me, it sounds like a faulty IAC valve.
 
CaboJohn said:
If you have not done so, find the website for Pirate 4x4 and read the article on MEFI controllers. Then download the diagnostic manual. It has two pages on steps to trouble shoot Rough, Unstable or Incorrect Idle. If you have access to a TechMate scanner (or equivalent) this will enable you to sort it out in a hurry.

If not, the knowledge in the manual as to how things work will still help. It strikes me that the first place to look is the IAC valve. This is instructed by the controller to let more or less air into the engine in order to control the idle speed. Since the engine idle is unstable, it is likely that a) the controller is getting bad speed information and misdirecting the valve, or b) some external factor (air leak) is allowing varying amount of air into the manifold and the valve is chasing the resultant varying rpm trying to keep it constant, or c) the valve is not quickly (or perhaps linearly) responding to the instructions from the controller resulting in an unstable feedback loop of the directed valve position and the resultant rpm. I like c) as a first guess. Perhaps when the engine is surging and seeking a stable idle speed you could pull the plug on the IAC and see if the engine settles down to a stable rpm. If so, that would eliminate your possible variable leak and likely point to a faulty IAC.

I can't explain why the engine rpm becomes stable after going into gear, other than to note that when the engine sees a load the IAC will immediately open much wider than perviously to keep the engine from stalling. At that point perhaps the "sticky" portion of the response curve in the IAC no longer applies. When you take off the load, the valve closes down normally and (for a time) behaves. If the controller was sending bad instructions to the IAC (situation a)), then I do not think that the in gear/out of gear trick would work. So to me, it sounds like a faulty IAC valve.
Click to expand...


I will give this a shot. Thanks for info
 
Replaced the IAC, did not resolve issue.
I sprayed the hell out of the intake manifold with carb cleaner, did not notice any change in RPM
Engine ground looks good.
 
catman78 said:
I can also stabilize idle by bumping up the throttle to around 1k and then slowly bringing it back down to normal position. Once there idle stay solid.
Click to expand...

I reread this post and said "maybe the TPS is dirty" and not providing a consistent output......and it could stabilize after the RPM is elevated to 1000 RPM....this is where the scan tool would come in handy.....you could always remove the connector and watch the resistance change with an ohm meter as the throttle is raised and lowered - any jump or jitter would indicate an issue....
 
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