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gmoss98
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Name : Glenn Moss
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PostSubject: Fluctuating idle   Wed Oct 26, 2016 3:40 pm

I'm having an odd problem with my 98 Riviera. 3800 Series II SC.
On cold starts it starts OK and runs fine until the temperature gets to about 180 degrees or so. When coming to a stop (throttle in idle position) the rpm fluctuate between 300 and 1200. At times it will stall out. After it reaches full operating temp it's fine again.
I have replaced the ECT and checked the TPS for a nice steady signal and it looks ok. I suspect the EGR and have not checked the voltage on it yet.
Any thoughts on this?
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Abaddon
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PostSubject: Re: Fluctuating idle   Thu Oct 27, 2016 1:09 pm

EGR or possible the Idle Air Control motor. It's more likely the IAC, as the EGR is active during heavy accel and engine load.
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albertj
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PostSubject: Re: Fluctuating idle   Fri Oct 28, 2016 5:17 pm

Abaddon wrote:
EGR or possible the Idle Air Control motor. It's more likely the IAC, as the EGR is active during heavy accel and engine load.


+ on the IAC but while you are at it remove and clean the throttle body and clean out the air passages - since you are going to be in there already. And be sure especially that you clean out the air passage to the IAC.


Last edited by albertj on Mon Oct 31, 2016 11:36 am; edited 1 time in total
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gmoss98
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PostSubject: Re: Fluctuating idle   Fri Oct 28, 2016 10:19 pm

I've checked the voltage on the EGR, it's showing zero on the 12 volt terminal. Looks like I have work to do under the dash. Any advice on replacing the pcm? Can I reprogram with a Tech II or an HP tuner?
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Abaddon
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PostSubject: Re: Fluctuating idle   Sat Oct 29, 2016 10:24 am

Um. Which terminal did you check voltage at? If you measured "0" anywhere, you were probably checking the ground, or "high" or "low" control. The only voltage that can be monitored at the EGR is the 5V reference circuit. Other than that, you check the wiring to the EGR with a Tech II (or comparable) and a test light....or both together. The only real way to test the circuits to see if they're functioning properly is to command the EGR in increments of 10 and watch the test light. Plus, you'd more than likely have an SES light with a code(s) if the EGR were bad. There are some instances that would NOT set a code, but it's rare.
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albertj
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PostSubject: Re: Fluctuating idle   Mon Oct 31, 2016 4:47 pm

gmoss98 wrote:
I've checked the voltage on the EGR, it's showing zero on the 12 volt terminal. Looks like I have work to do under the dash. Any advice on replacing the pcm? Can I reprogram with a Tech II or an HP tuner?

Did you remove and clean the throttle body and clean out the air passages? especially the air passage to the IAC.

Here's the thing: the throttle body will look clean from the air intake side (outside) behind the screen, but will likely be pretty nasty behind the flapper especially if it's never been removed for cleaning. Problem is that throttle body cleaner will not reach much of the problem area unless you *remove* the TB, and if you don't remove the TB all that happens is you drain the gunk further into the air intake where it causes other problems. At some point you have to remove the $*(&#( thing. You will need a TB gasket (less than $10) - the GM dealers tend not to have them but the NAPA/Advance Auto/etc. stores tend to have several on hand at a time.

As for the IAC air passage it can get enough gunk in it to render the IAC about useless. The PCM will try to meter the idle from feedback from sensors, but with the IAC gunked up the fuel/air mix will oscillate instead of settling as the PCM constantly resets the fuel-air mix to effectively overcompensate for the erroneous readings. At full op temp it's not so persnickety but what you will notice is that if you stop the car and let it idle at full op temp the idle will increase and decrease in a range of 30-50 or so RPM (not much but noticeable). When everything is clean the idle does not vary that much if at all under those conditions. Put another way, if your idle is not steady it is time to pull and clean the TB and the IAC passages. If the idle is very problematic you may need to replace the IAC.

Depending on prior maintenance on your car, the problem may be both an EGR dying and dirty throttle body/IAC. I have noticed that off brand EGRs do not last very long - about 20-30K miles - before needing replacement. They fit and function but not for long. Put it to you this way: On Rockauto.com the GM OE EGR is one of the most popular parts among the EGR choices for the 98 Riv even though almost *all* the other choices are cheaper.

Albertj
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