iac problems

my 88 xj is having a very high idle problem. about a week ago, i started it and it went strait up to 3000 rpm and stayed there, so i shut it off, restarted it and all was well, then about 3 days later, i stoped at dairy queen and at the drive thru when i shut it off it did it again, but i shut it down and restarted it 7 or 8 times and it kept doing it, then i finaly started it and put it in gear before it had a chance to get above 600 or so rpm and then just stood on the brakes the two blocks home. then i went to disney saterday and it didnt give me a problem at all, now today it has made up its mind to do it again. so its out at my girlfriends house where it will stay till i can fix it. i think the problem is the iac. i pulled it out and cleaned it off and put it back in but its still the same. if its running u can unhook it and it doesnt make a differnce at all. the butterfly in the tb is not stuck open and the little port that the iac is on is sucking some SERIOUS air thru it at idle.if u cover it about 3/4 of the way up then it will idle right around 600 rpm just fine. could it be anything other than the iac? i realy need to know cause im 17 and poor and cant afford to waste 80 or 100 on a sensor that is not the problem.

kolby

Does your '88 have the C101 connector? It's a big connector mounted on a firewall bracket near the brake booster. If you do have that connector, it's possible some of the contacts are making poor contact and causing this problem. I've heard that some of the '88s have it and some don't. Either the IAC motor isn't getting the proper control signal because of a bad connection or the device itself is bad. Since it works some of the time, I'd bet on a bad connection somewhere between it and the computer. My '87 had the same sort of problems until I cleaned all the contacts in the C101.

Do a search, you'll find a lot of info. It's a common problem and there are tales of much intrigue out there about it (Chrysler would probably try to tell you that you are just mistakenly stepping on your accelerator pedal :lecture: ). From what I hear it's rather predictable that your culprit will be a bad TPS.

Very shortly after getting my '87 4.0 manual XJ, I did a coolant flush & change and then a pressure wash of the engine. On restart, things seemed normal but over a few minutes time the idle speed crept up and was eventually stuck in the stratosphere and wouldn't go down. Didn't have good service info in the Haynes manual that came with the car so I checked the forum. Didn't hit the answer in initial searches so kept fiddling around and found out if I disconnected the TPS the idle would go back to normal and I could limp around (albeit with very poor throttle response - and there may be other problems with an automatic but I can give you a workaround if you need). That experiment seemed to prove the IAC was functional so I stopped looking at it. Subsequent searches of the forum based on TPS revealed more than a couple instances of failed TPS after washing the engine, and lots of other failures for other reasons, all manifesing themselves in that ultra high idling condition. I made a run to the local pick & pull and grabbed a sensor and a bag of other misc spare parts & trim pieces for the reasonable sum of ten bucks. Installed the (new to me) sensor in about five minutes, didn't even bother to adjust it :eyes: , and the problem was cured.

TPS!!! I had the very same problem, but I could fix it by adjusting the TPS. I still idle at 900 and I can´t adjust tps more to lower it. I would have to buy new sensor to get idle at 750rpm, but it has to wait because here in Finland a TPS costs about 400€ (1€=1.25$)...

okay so if i unplug my tps and its normal then its not the iac? is the c101 just a big conector going into another big conector? pretty sure mine has that. whats the best way to clean it?

kolby

C101 is that big connector with all the engine harnesses going into it, high on the firewall sort of above the brake master cylinder. You can pull it apart & see if it's corroded or dirty. Mine was clean, I don't think my old Jeep has ever been off road. Clean enough, I didn't bother trying to get it cleaner, but look for a spray electrical contact cleaner that won't hurt plastic, someone else on the forum could probably suggest a specific brand - I didn't need it so I didn't go looking for it. Then you could sort of weatherproof with dielectric silicone grease. For me at least, unplugging the TPS made the engine return to normal idle speed. I think that would prove the IAC is more or less functional and that an errant input from the TPS is what is taking it high - that sort of behavior could still be caused by bad connections anywhere between the TPS and the ECU. Swapping out the part (because I could get it so cheap) was the route I took, since all the easily accessible wiring and connectors looked good. :us:

:wave1: Wow $500.00 US for a TPS in Finland? Seems high but I sell Timberjack at my shop and a lot of the stuff for their wheeled equipment comes out of Finland and it is high priced here.