annoying high idle

[Tre]

its a 88 xj with 4.0 aw4. lately i've been having this intermittent problem with a high idle.

sometimes it'll idle high. close to 3000rpm when starting. then drop back down.

other times and this is the most annoying is i'll be driving then notice that when i'm braking its still wanting to surge forward. so its idling high again but in gear. now if i stop and let it sit a few minutes if it does this i can start it up again and everything is fine.

wtf is the problem? i've searched a bit and saw some theory on the latch circuit and IAS.

but is this a IAS problem or a TPS problem?

[TORX]

My 88' was doing the same thing not too long ago. I started by checking all my vacuum connections for leaks and re-torquing my manifold bolts. I discovered many to be loose to the point I could unscrew them with my finger and one to be missing. So after making sure I didnt have any sort of a vacuum leak that would cause a high idle I unplugged the IAS and TPS with the battery disconnected and looked at them. They both had a bunch of oil residue on them so I blew them out and cleaned them up and plugged them back in. Started it up and went for a test drive... no high idle. Im not sure what the problem was but I found many possible causes.

TORX

[goodburbon]

Its a vac problem. your brakes are the giveaway, you may have a ruptured brake booster.

[Tre]

i don't think it has todo with the brakes. as i don't have to even touch the brakes for this to happen. if i'm cruising 35mph and take my foot off the gas its like i got cruise controll.

[katarn444]

Definately try this, very good advice TORX.

K

My 88' was doing the same thing not too long ago. I started by checking all my vacuum connections for leaks and re-torquing my manifold bolts. I discovered many to be loose to the point I could unscrew them with my finger and one to be missing. So after making sure I didnt have any sort of a vacuum leak that would cause a high idle I unplugged the IAS and TPS with the battery disconnected and looked at them. They both had a bunch of oil residue on them so I blew them out and cleaned them up and plugged them back in. Started it up and went for a test drive... no high idle. Im not sure what the problem was but I found many possible causes.

TORX

[goodburbon]

Follow Torx's suggestions first as they are good for your jeep even if you don't solve the problem.


I suggested a leaking brake booster diaphragm because you said it was acting fine until you hit the brakes.

[Tre]

Follow Torx's suggestions first as they are good for your jeep even if you don't solve the problem.


I suggested a leaking brake booster diaphragm because you said it was acting fine until you hit the brakes.
sorry, i made it sound like that :D it was late when i typed it up.

[aaronh]

Did anything work my 90 is doing the same thing

[kahaikapu]

my 87 had the same prob...i replaced my bad 02 and that took care of that.
just a thought

[falcon556]

Happened to my '87, it would go bananas then calm down till the next time.
One time while high revving, I started messing with the connectors on top of the intake manifold. Not completely disconnecting them but making the pins move.
One of them afected the idle immediately. I turned the engine off, disconnected all the connectors, sprayed some WD-40, connected and diconnected them a few times to clean the contact points, never had that problem again.

[Drew Manzella]

My 88 is doing something similar. After it is warmed up and I shut it off then restart it soon after (such as when stopping for gas) it will idle at 3000 and not come off high idle. I usually have to shut it off, cycle the key several times and even start it a few times to get it back down. Once it is down it stays down until I shut it off again. If allowed to cool it won't do it.

I replaced the IAC back in March and the problem went away until last month. I am going to try cleaning the IAC and TPS but I am pretty sure the IAC has failed again.

[jfiscus]

I had the same problems, it came from an out-of-position/sticky TPS sensor. You can loosen the sensor & move it around to bring the idle down. Spray some cleaner on it & hopefully your problem goes away.

[Drew Manzella]

I don't think it is the TPS for 2 reasons:

1. It isn't constant. It is only on startup after the vehicle is warm (And not every time)
2. Replacing the IAC motor fixed the problem entirely

I would think that if it is an out of adjustment TPS then it would do it ALL the time since the adjustment is a physical constant. I will nonetheless check the TPS tonight to make sure it is tight and functioning properly.

[Hypoid]

Definitely check the IAC and clean the tip if required. While it can go bad electricaly, I would expect a longer service life. Something I have learned here is that the IAC has a ten second reset cycle after shut-down, try counting before starting again. It won't hurt a thing to check the TPS, there have been Tech Bulletins linking it to the same problem.

I vote for cleaning all the connectors before anything else. My understanding is that the I/Os run at 0-5 Volts nominal, all well and good. The ECU is still looking for an analog signal, infinitely variable between 0 and 5 volts. If environmental changes (moisture, heat) cause a connection (or control for that matter) to develop resistance, the ECU won't compesate for the skewed signal, It will control based on the inputs. We've all heard it: Garbage in = garbage out.

What I don't see mentioned much here is the resistance to chassis ground. All of your measurements are made between the pins at the TPS. One source had me check voltage from the ground pin to chassis ground. Any potential over 1/10 volt was un-acceptable. I ran a second ground wire under the hood because of that.

[acrid]

x3

on the vacuum, mine was a loose throttle body, same issue.

new gasket and tightened to spec, problem solved.

[dizzymac]

Mine was not constant either, but it turned out to be the TPS.

[Saudade]

As others have said......

My '88 did the same. I cleaned the IAC bore and TB out and replaced the TPS. My idle problem vanished. At the time, I was chasing a shifting problem so I was glad it addressed my idle problem too.

[philip_g]

chasing a funky idle too.

start the car, idles 400-500 RPM
warms up, idles 1500 RPM
shut if off, restart, idles 1000 RPM

Go figure. I haven't swapped the TPS because it's close to in spec, but did clean the IAC and TB, just haven't gotten to checking the intake bolts.

[falcon556]

Gentlemen, I mentioned that I fixed the erratic idle problem my 87 had by simply
disconnecting, spraying and then connecting and disconnecting a few times all the connectors on top of the intake manifold.
Has anybody tried it? Did it work? Wouldn't it be nice if it fixed your problem without having to replace anything?
Nothing wrong with simple solutions that work.

[19CHEROKEE89]

x2 /\