I have to take my 91 245 to the Drive Clean emission tester soon, but I'm still plagued with a surging idle, 1000 to 1500 and back, and the little gauge I built to display O2 sensor voltage reads very low, except for a few seconds after I let of the gas, or when I disconnect the FPR and plug that vacuum line. In the latter case the gauge shows normal, if a bit slow, up-down voltage like I was used to having. I searched for a vacuum leak, cleaned the throttle body (twice, the first time was 6 months ago or so, and may well have started this whole episode), and replaced the FPR and O2 sensor.

If I disconnect the IAC, the idle drops to a nice idle, if a bit low, and I can reconnect it, and the idle stays down until I re-start. Throttle switch seems to work (disconnected and both IAC hoses, and TPS, rotated IAC so I could see the valve, re-started, and watched the valve slowly open when I shorted pins 1 and 2 at the TPS switch).

Problem is, as I see it, the valve never closes under normal conditions, hence my continuously lean burn. Drivability and fuel economy are good, and no knocking. Error codes are 221,231, and of course, 223. Is this a IAC problem, or is the computer stuffed? Any opinions? TIA

Marten

I'd agree with Art. I had a problem with my RPM's surging at stop lights and when I was in park. A friend sugested I check the gas lines. I checked the gas lines which were fine, but I noticed the intake manifold gasket had a couple gaping holes in it. i replaced the intake manifold gasket and the problem has gone away completely.

Hi Marten,

If you don't find an exhaust leak, I'll bet your intake manifold gasket is missing a piece. Either trouble takes a bit of sleuthing with a stethoscope (a piece of hose if you wash your ears later) and if the manifold gasket, you might see some difference in the insulator color on the plugs-- like blackened ones on those plugs where the manifold leak isn't. That's your O2 sensor at work trying to compensate.

Is the check engine light on? I had a somewhat similar problem on an '89 244 that was apparently caused by an exhaust system leak UPSTREAM of the cat and O2 sensor (rusted out cat flange connection on the header pipe). This was a TOUGH problem to diagnose.

Good luck!
--
Seven bricks, yep, six!

Thanks guys. I think the intake manifold is the more likely suspect. The exhaust is new including the down pipe. I'll do some more sleuthing and update this (long) weekend. Happy Passover and Easter for those of you so inclined.

Marten