Erratic idle (1 Viewer)

[fumanchu]

I've been trying to diagnose an erratic idle problem while in "D" only. Does not do it in "P". The idle is not cyclic or rhythmic in nature but will jump up by 100-250 rpm at idle only. It will do it randomly when sitting at a light for a few minutes it may jump up and down a few times, sometimes not at all. Never does it until fully warm either. I know cyclic pulsing usually points to a vacuum leak, but this is totally random when it occurs. I have no CEL from this. Also checked the codes anyway.....nothing.

Recently had the manifold off for starter contacts, was not doing this prior to that. Cleaned the throttle body and dielectric greased all clip electric connectors at the time.

Truck drives great under power, it's just the idle problem in Drive.


Any ideas? I'm thinking TPS or throttle body problems, but don't want to throw parts at it.

Rig is a '99 w/ 175k in very nice condition.

Thanks in advance.

Shawn

 

[yeti in ga]

Sounds like something I have also .. For me it seems to almost go away when I have the AC off but does do it more when hot and stopped at a traffic light. I have tried lots of things but have never figured out what it is and that was over 10k miles ago.

 

[unklwedy]

Possible ground strap loose after repair ?

 

[Bismarck]

Sounds stupid but have you cleaned the MAF?

 

[fumanchu]

Update.... Cleaned MAF (probably never done, looked fine BTW) and disconnected battery for 30 min. Obviously the truck is working out its fuel trims but at this point the erratic idle behavior in Drive is gone. With another 30 or so restarts we will see if it stays gone after the truck settles into the proper long term behavior.

 

[Jackson Sanders]

My truck has this same problem, haven't really had time to dig into it yet though.

In for the results!

 

[CurbsEnthusiasm]

Mine only does it with the AC on. Not sure if it's the fan clutch but if I keep the AC on max it will not do it. The moment it's off max every 45 sec or so I get a 150 rpm jump for a second.

 

[kennyao30]

this happened to me, like exactly the same symptoms..... turned out to be a coil pack about to fail. took a new coil back and systematically went through all coils replacing one coil at a time with the new one. if the symptom didn't improve, i put the old coil back in and went on to the next coil pack

 

[fumanchu]

Its back! the erratic idle that is. I figured it would be.

this happened to me, like exactly the same symptoms..... turned out to be a coil pack about to fail. took a new coil back and systematically went through all coils replacing one coil at a time with the new one. if the symptom didn't improve, i put the old coil back in and went on to the next coil pack

Did you ever have a CEL? It really acts like this is happening, like its dropping a cylinder here and there. Why only in Drive though?

All the coils look original too.

What is everyone's idle rpm in Drive? Not that the factory tach is accurate, but just curious. Mine can be as high as 800 warm and as low as maybe 550. It's never really consistent.

 

[kennyao30]

Its back! the erratic idle that is. I figured it would be.



Did you ever have a CEL? It really acts like this is happening, like its dropping a cylinder here and there. Why only in Drive though?

All the coils look original too.

What is everyone's idle rpm in Drive? Not that the factory tach is accurate, but just curious. Mine can be as high as 800 warm and as low as maybe 550. It's never really consistent.

never a CEL, and as far as why only in drive.... your guess is as good as mine, it could be the amount of load on the engine. i put a lot of money into trying to find the problem ( replaced tps, idle control motor, searched for vac leaks, and a few other things) none of those things worked, until i went through replacing coil packs systematically.

 

[fumanchu]

I thought I'd follow up on this thread. This weekend I replaced the TPS sensor and set it by feel and ear. Truck runs fantastic.
No erratic surging anymore, plus as a side.... the D idle vibration that many of us experience is also gone. Very smooth idle at a tick over 600 rpm in Drive.

The TPS is cheap. I paid 75 dollars for it. May want to consider this component as part of the maintenance plan like you would coils.

 

[Jackson Sanders]

Fumanchu, is this repair still holding up as far as the D idle vibration is concerned?

 

[fumanchu]

I wrote a long reply, and its was lost ..... anyway. D vibe is definitely reduced to almost nothing. The TPS was the culprit.

 

[Jackson Sanders]

Thanks, I'm going to give it a shot soon as nothing else recommended on the forums has helped so far. I'll let you all know when I get around to it.

 

[dy9213]

I went through this exact same thing last month. It was running fine then I cleaned throttle body, MAF, and seafoamed causing it to throw the same symptoms.

I did the usual checks and didn't get anywhere. So I took it to the shop and they had to reset throttle position. I got the instructions from them.

1. Remove airbox
2. Turn on ignition while pushing the throttle plate closed with hand.
3. Reinstall airbox
4. Run the engine and let idle for 10min with no load (AC, heavy electric draw, etc)

Lo and behold, this fixed it for me. Worth a shot.

 

[SanDiegoCruiser]

I had the exact same D rough idle. Just replaced my APS and calibrated it with the Scan gage and it appears to have fixed my issue. No codes were even thrown so it took some hunting to figure this out.