In the past week, my 87 has started doing a little seek-n-find at idle. It's not super-bad, just bouncing between 600-700 RPM. My first thought is the IAC valve (I had trouble with that sticking years ago, so it might be due for a good cleaning, if not replacement). Is there a more likely culprit?

Another thing I noticed is that the dash mileage was surprisingly high... it was reading about 25 MPG. It normally shows about 21 (and calculating it manually shows it's really about 19). I filled up today and the *real* mileage for the last tank was about 18 MPG.

Are these symptoms possibly related? I don't *think* so, but life is full of surprises! I believe the MPG calc in the dash is based on injector pulse duration. Maybe the idle problem is throwing it off???

Cheers!

When it's a hunting idle problem, my first check is always the IAC circuit. Pull the TB off and clean the IAC valve as well as all of the passages. Be careful cleaning the IAC valve so it doesn't get damaged. You will probably find a bunch of carbon build up in there.

Once that is done, you can do the minimum idle reset procedure, although it's not really necessary if everything is idling fine.

As far as the dash mileage reading, I'm not sure on that. Injector pulses are one factor, not sure on what else is involved.

Matthew

Thanks Matatk. I finally found time to pull the throttle body off and get the IAC out and clean it up. It seems to be idling fine on first test. We'll see how it does this coming week.

I'm baffled by the sudden jump in the calculated gas mileage. Something is confusing the ECM, that's for sure. It's one thing to be off by about 10%, but now it's off by about 40%.

My guesses on the mpg would be just that....guesses. Have you changed anything else recently? Any stored codes? You can disconnect the battery to reset the ecm and see it that helps. Do you have a live scanner/laptop to watch it and see if it is running rich/lean? How new is your O2?

Yea, what Matthew said... The MPG thing could be showing a slow O2, and with only one on the engine, they should be replaced frequently - 30k miles or 2 years.

I haven't changed anything else recently. No codes, either.

I do have a laptop and cable that I can use to see how it's running (will add that to my list of things to do).

The O2 sensor is definitely NOT new. It might be 10 years old, it might be 20 years old! I've read about different types of O2 sensors over the years, is there an obvious best choice?

I do have a custom chip and a shop told me (years ago, while doing an emissions diagnosis) that it was running too rich at idle. I think I heard somewhere along the way that the O2 sensor is not involved at idle... is that correct?

Thanks!

In open loop the O2 is not involved yet, the ecm runs off of preprogrammed tables. Once in closed loop it uses sensor data.

O2 sensors? AC Delco are good, so are NTK/NGK and Denso.

Avoid Bosch!!!!

Quote:

THAT's good to know! I called one place this evening and Bosch is what they had. Normally, I would think they make pretty good parts.

I have personally had zero problems with bosch and that's what I have in my car now.

But NGK makes a good product.

Check rockauto.com for the best price.