Inconsistent idle
 

Hi folks.
96 LT1 A4 about to turn 90K completely stock except K&N. Bought last Christmas with 81,000 miles.

My idle is a bit strange. When I first start, it's fine. Yet after warmup, it's a crapshoot as to where it is.
Sometimes it is at 650, other times ~1000 or a bit higher.
Once it is at that RPM, it stays there until I shut the car off and restart it.

Example. Yesterday I drove home and the idle was completely fine.
This morning the idle is at 1000 at every stoplight. When I got to work it was still at 1000 (I was cruising the parking lot close to 20MPH with my foot off the pedal). I shut it off and then immediately restarted it and it was at 650.

I haven't noticed anything that makes it such. Doesn't matter if the A/C is running or not.
No codes and the engine runs great and gets on average ~22MPG.

One hiccup I had was driving along Saturday and - just out of the blue - the engine's RPM started to surge a bit up and down. Started when I stopped and took a right turn onto a road after accelerating. It did this the rest of the trip home. When I stopped at a stoplight and when I was home and put it in Park the idle did the same. Turned it off, restarted and all was fine.

I have the manuals. What's a good starting point for troubleshooting?
Thanks in advance.

Yesterday drive home. Fine.
Drive to TSC. Fine.
Drive to IGA. Fine.

Figures that once I decide to trace down the gremlin he goes and hides somewhere.
May have to feed it after midnight.

I don't have a lot of experience on C4's but I'd look at cleaning your throttle body...you can use TB Cleaner or Brake Kleen...either will work just fine...I make this a yearly task !!!

Thanks for the idea! I did do that (along with the MAF) about 3 months ago. I thought about doing it again as well as checking the voltage on the TPS.

May also be IAC related...don't know if you have ever checked your TPS voltage but with key ON and back probing the connector on your '96 you should have 5 volts on the gray wire, less than 100 Mv on sensor ground (black) and something less than 1-5 volts on the signal wire from IDLE to WOT with key ON...I don't have the exact numbers though...most TPS problems result in hesitation issues.

I believe the LT1 uses the same or very similar TB that the TPI cars use, I had a problem with the throttle shaft sticking just a little bit when I bought my Vette. Same symptom, idle would not drop down to 600 unless I blipped the throttle real quick. Didn't do it all the time, just instances where I was very light on the throttle, stop and go traffic mainly, or in the drive thru.

Jared

Thanks, Jared. I did try blipping the throttle several times with no change. Even tried a quick WOT hit and no change.

I really appreciate the voltages.
Yeah, IAC was also what I was trying to think of (I'm an old carburetor guy).
On a 5.0 Mustang I had I took that one off and cleaned it since it was gummed up.
Hopefully I can do the same. Will check service manual.

Well maybe we can teach an "old dog" new tricks...LOL !!...good informational video and if you have a test light it's easy to test the circuit...yes, you may have some carbon build up there !!

The 5 volt ref. and the sensor grounds numbers (less than 100 mv) are standard but you should never see 5 volts on the signal wire when testing !!

Not sure about 96 but assuming it is similar to my 91, I'd start with a scanner and see where the issue is. Is the ECM commanding the idle to change? Is the IAC count consistent and what it is about? What is the CTS temperature as seen by the ECM and an IR Thermometer?

Thanks for that video, C5 Diag! That's super easy to understand and I have a feeling that may be the issue.
Although, I did not have a chance this weekend to tear into it.

Good idea on the scanner, aklim. All my car friends were at car shows this weekend so perhaps I can bum one this week and give it a look.

I'll let y'all know how it goes. I really do appreciate the thoughts and ideas.

You can clean the throttle body and all the routing stuff before you spend your money.

But the IAC is a part that is really not possible to check, using voltage, counts or whatever to come up with a "valid conclusion" as to if it is good or bad.

I have cleaned and lubed them in the past and sometimes it actually fixes it but only for a short time. It's a good diagnostic tool to see if there is any major change. But I always end up replacing it eventually.

So my personal philosophy is if I think it is bad, I just replace it.