This comes and goes. (92 Formula L98) For a couple weeks it'll idle fine, then for no apparent reason, I'll start it up, and will get a loping 5-600 rpm idle, and sometimes the car will even die if I don't get it up near 1000 rpm. There doesn't seem to be any rhyme or reason. If I take it out and drive it a couple miles, when I get back and turn it off, then restart it, it will idle fairly normal, in the 800-1000 range. I also don't have the issue so much when it's in gear, only when it's idling in neutral. Now I do have a code 32 issue sometimes, but have a new egr, and tonight I pinched the egr line shut while it was in low rpm idle, and it didn't make much difference, so I'm thinking it's not the egr hanging open, or the solenoid telling it to hang open. My catalytic converter is rattling, but I don't think that's related either. Eventually I'm going to pull it off and knock the guts out of it, which I can do where I live. But any clues on the low idle? And why does it come and go?

 

Oh yeah, the last round I had with this issue, I switched from 91 to 93 octane and it stopped. Still on 93, and it's cropping up again this week. No comprendo señor.

 

Update--if anyone cares. Went out to start it tonight, and purrs like a kitty at about 1000 rpm. I'm going to check for a hairline crack in the dist cap. We got rain here this week, and though the car is under a carport, could be humidity got in there and I burned it off when I'd drove it last night. I also removed the under hood insulation a few weeks back, and water could be dripping through the hood scoop joints onto the dist cap, even though the only time water could get in there is when I wash it, because I never take it out in the rain. Who knows.

 

I'm getting a code 23 now, which the book says is the mc solenoid. I don't know much about that, but I thought that was something on a carb? I have tpi, so whats the equivalent? But that would explain the rough/inconsistent idle, but now what? Is the equivalent the map sensor? This Haynes manual is almost useless for stuff like this because it tries to cover too many years, codes and equipment change over ten years, even on the same model. Code 23's can also be mat sensors, but I don't think my car has one of those does it? A 1992 L98? My map sensor could also be contributing to my code 32 issue if it's bad. Any help would be appreciated!!

 

Seeing as this thread is basically me talking to myself, I'll inform on-lookers that I think I figured it out. The IAT sensor is what I need to be looking for, and I see that the connector has come unplugged from under the plenum. That would explain the problem. It also explains why the problem was intermittent. The connector was on the way to falling off, but not all the way off. It's off totally now, which is why it kicked out a code. When I took the plenum off a few weeks ago, the little clip that holds the IAT connector in place broke off from old age/brittleness. I plugged it back in, but the lack of a clip has allowed it to work loose. I may have to replace the connector, or it may come loose again. Any further thoughts welcomed....