Need Help...idle problems

Okay I'm getting a little frustrated with the ol'87 Comanche. She has a 4.0 and MT with 171k on her. She was running like a champ until I put an overbored TB I got from artsound on ebay, then it all went to pot. She was idling way to high, 1500-2000 tried adjusting no good. I took it to a local shop and they tried their hand, got it down to 1000-1500 but it would sometimes idle all over the place. So, I put a new TPS and IAC for good measure, still acting wacky. So it's off to the Jeep Dealer to see the wizard, with my fingers crossed. They adjusted the TPS and diagnosed a faulty o2 sensor, and wanted $300 plus to fix. Luckily with info garnered from this site I knew this vehicle had an unperformed recall for the o2 so I escape for around $82. Now I'm feeling pretty good about myself as I fire up the ol' girl... idle around 700... great. So off to the homestead... I'm up to speed around 45 for a stretch when I have to stop for a light. My smile fades... she is now idling at 1000-1200... AHHHHHHH. So that's where I'm at at inital startup great around 700. After a drive and a stop back to 1000-1200. I am at a real loss what next, vacum lines? Anyone have any ideas or suggestions? I hate to go back to the dealer as I'm probably out of cheap luck there. Any help will be greatly appreciated. Thanx,

Garrett

Does the problem go away if you put the original throttle body back on?

Could be loose intake manifold bolts, just a though.

katarn

Make sure you have the MAP vacuum line plugged back in and check the rest of the vacuum lines. Vacuum leaks are the major cause of high idle. The IAC could be giving you problems. Disconnect the battery for about 10 minutes and start up again. The computer will do a reset and start trying to adjust the IAC. It may take a few starts but hopefully the IAC can compensate.

I disconnected the battery for about 30m and tried again still same result, except this time I left her running while I was trying to see if I could find a vacum leak... after a few minutes not sure how long she idled back down below 1000 rpm on her own... any thoughts.

Garrett

By the way after she idled down on her own, I drove her again and she idled back up again.

Garrett

check the wire harness junction block above the brake booster. I have seen a few times in the past where dirt and such collects there and causes a slight short between the TPS wires where they enter the block and give the 'puter false readings.
wash it out with brake cleaner

gapirate72 said:

By the way after she idled down on her own, I drove her again and she idled back up again.

Garrett

Click to expand...

One workaround you can use the next time it idles "normal" unplug the IAC connector. This will keep your idle down all of the time because the ECU is no longer moving the IAC. Side effects - you'll have to keep it running when cold and the AC might bog it down if equipped.

You're not alone regarding this problem with Renix 4.0's. Latch relay supplies power to the ECU for a few seconds after shutdown in order to park the IAC in the open position for the next restart. Trouble is, on the next restart, it doesn't move back in to control idle air flow.

Some people claim TPS is the culprit but I haven't been able to confirm this by unplugging my TPS during normal idle. The idle just stays the same when I do that.

The IAC wiring carries a significant current and if just one of those paths fail, the IAC will stick in whatever position it's currently in. I'd check/qualify all the EFI wiring connections very carefully and remember what I said about unplugging the IAC when it's acting normally.

Does the problem go away if you put the original throttle body back on?

Does it with both TB,s

I unplugged the IAC and started it up, she did not like that at all.... idle shot up to 3000 and stayed there, tried it twice same result. Plugged it back up and restarted idle normal then back up to around 1100 after a drive....man this is frustrating. What about the Map sensor?

gapirate72 said:

I unplugged the IAC and started it up, she did not like that at all.... idle shot up to 3000 and stayed there, tried it twice same result. Plugged it back up and restarted idle normal then back up to around 1100 after a drive....man this is frustrating. What about the Map sensor?

Click to expand...

You misunderstood what I meant. Unplug IAC WHILE it's idling normally. Connected, shutting engine down will park IAC in the "start" position and if you unplug of course it will race on restart because it's already open and can't close.

Unplugging it while it's in a "stable" state and still RUNNING will keep the IAC in that position. If not, you got something seriously wrong.

I overlooked the "other TB" comment. What TPS are you using? Is the IAC the same on the newer TB? As questioned, what happens when you put the old one back on?

I'm also leaning towards a vacuum leak but try unpluggin IAC while it's running, warm and idling normally.

Yeah I realized I misread you, I went back out and started it and then unplugged the IAC, then drove it and once again it idled high after the drive. When I switched the TB's I orginally had both the old Tps and IAC on the new one, could not get it adjusted right so bought a new of each and tried that no luck either way, I'm thinking its got to be a vacum leak somewhere... finding it is going to be the problem... I may try and reset the ECU again just for kicks and cross my fingers.

Tried the IAC again, she will idle about 1500 after driven with it uplugged after normal idle. I plugged it back in and let her idle for a few minutes and as usual she idles back down on her own. That makes me think its not a vacum line... seems that would make it idle high always. Maybe the ECU is wacking out.

Did you check the intake manifold bolts? Which is a vacuum leak.