Intermittent drop in idle speed. No check engine light on.

[99Vortec]

When the engine has warmed up into closed loop mode I can immediately duplicate the problem by putting it into drive and turning the wheels all the way to the right or left.

The idle will fluctuate and the headlights will even dim just a bit as the idle drops. But it doesn't stall.

My first thoughts IAC Valve? (Passed resistance test on pins A-B and C-D. About 50 Ohms or so on both sets of pins.)

Worn distributor? (How much timing variance should I see at idle with laptop hooked up to OBDII port?)
(If I play around a bit with my laptop/scanner software, I might be able to record some footage of the timing variation I see.)

When the engine is in this condition there is a *very* slight hesitation off idle to acceleration. But no other power or driveability problems.

Thinking back on it, I did do one thing wrong in the days before this problem occured. I left my ignition key in the on position for a few hours. (Without the engine started. A jump start was *not* needed.) Can this damage the PCM?

That being said, since I have purchased it, the engine has always had a bit of a slight variation at idle, but nothing like this before.

Thanks for any ideas.

Mark

 

[OR VietVet]

Clean the throttle bore with carb cleaner at the throttle plate areas and the MAF sensor with MAF sensor cleaner from the parts store and follow directions. May just do the trick and even get rid of a slight hesitation from stop when accel if that is occurring, also.

 

[99Vortec]

They were both cleaned about 10K miles ago.

Can a Mass airflow Sensor fail and not throw a code?

I will check again though.

 

[OR VietVet]

Anything is possible but would likely set a code.

Ok, we have to get something straight. Go find what I said at the bottom of the Forum page at Site Comments & Feedback about Sharing Information.

If you in fact knew like you just said that the MAF and throttle bore were cleaned about 10k ago and you also know how they can effect the idle and throttle response and then you post about a problem of the same and then don't share that info of previous work, that torques me a bunch. Why not share what I had to recommend to you? Are you hiding that? I doubt it. Dammit tell us everything you know! C'mon man!!!!!!!!!!!!!

If you didn't know it could effect what your problem is then, my bad and I apologize, but if you do and are honest then dammit start sharing everything. Read it and understand that I got a lot of responses from members here that try to help that agree. You want help, then ok, we will give it but use your head and share everything.

By the way they may need to be cleaned again, especially if using that crap K&N oil smeared air filter. Check for too much play at the throttle plate bushings causing throttle plate slop when working. Use a mechanic's stethoscope to listen for a vacuum leak. How old are tune up parts? Share the info you miser! LOL!


Kids, what are ya gonna do?

 

[99Vortec]

"This is very frustrating to me. For some reason when a member posts with a problem for their rig it is like pulling teeth to get them to share the pertinent information in detail in chronological order. They post a problem and don't tell till later that--had looked at for problem at the dealer, had a friend look at it, they have thrown parts at it with no fix, what tests have been run, how long with the problem, if anything in particular makes the problem happen, year, make, model, engine size, transmission... You get the idea, Share the information in order of when it all happens and then we have a better chance of helping. I have had too many times where I don't get details without asking 20 friggin' questions! PITA!!! C'mon man!

I ran repair shops for 25 years and was in the field for 35 years. In the future if you want any of my opinions or help then start sharing what I am asking for. Ok, I have now stepped down off my soap box."

Ok, well put. I did not intend to offend.

Cap and rotor, plugs changed about 10K miles as well. Spider assembly upgraded.
Throttle position sensor replaced.

Transmission rebuilt and, transfercase upgraded to correct "Pump Rub" about 7K miles ago. Air filter replaced

Plug wires not replaced yet.


If I think of anything else I will mention.
*Edit One of the downstream O2 sensors failed and was replaced about 10K ago. I honestly can't remember if it was bank 1 or bank 2.
The rest of the O2 sensors are stock As far as I know.
*Edit* It was left bank.

But can the ignition key left on damage the PCM? And how much timing variance noted through a laptop OBD2 scanner is acceptable?

1999 GMC Yukon Denali 4wd 5.7
Oh yea, and about 176K miles.

 

[OR VietVet]

See my reply at the other thread. Also reread my original post of sharing info. I edited because of you and others.

 

[99Vortec]

Thank you. I will check for deflection at the bushings.

 

[OR VietVet]

Now we're talkin'! Get the basics out of the way first and if the problem gets fixed then great! Then we can get in to other deeper checks!

 

[99Vortec]

I'll let everyone know what I ultimately find to be the problem.
Thanks


*These PCM systems are tedious when a component is malfunctioning somewhere, yet no error is reported.*

(Mechanical wear, not electrical makes sense though.)

 

[OR VietVet]

Always, always, always post if the problem is solved and what was the problem if you know it. Then we all learn from that for the next people.