Fast idle at start up

[motorr50]

I have a 88 MJ 4.0L, a/t, 2wd with just under 60k original, it's been a great runner until this week. When I start her up the idle races, if I tap the pedal sometimes it drops. If that doesn't work I have to shut it off and turn the key to the one position for 5 to 10 seconds then start it. The second method gives me a slightly high idle for a few seconds but then drops to a normal idle.Could this be a linkage issue? Any help would be great!

Motorr50

[BLHTAZ]

It could be a linkage problem, but it's more likely the IAC. I would try removing it and cleaning it and the throttle body real good first, and if that doesn't fix it, then it may be time to replace it. :cheers:

[james750]

Also, when it races, try unplugging the TPS to see if it calms down, if it calms down right away, there is your problem, it would either need to be adjusted and/or replaced. As Brent said, its probably the IAC. It could also be a MAT, MAP, or a CTS.

[motorr50]

I'll check it all,Thanks for the help.

[BLHTAZ]

Yep...I just went through this not long ago myself, but mine would rev up to 3000+ RPM and not come down no matter what. I replaced the IAC, MAP, CTS TPS...all to no avail. My problem ended up being a bad ground connection on the battery harness that was my fault from when I replaced the harness... :doh:

 

I have a TPS that I bought for my truck and used it for about 15 - 20 minutes. It still looks new and is in the original box from Standard if you determine you need one and can wait a couple days to get it. I paid $114 for it, but will sell it for $80 shipped. :thumbsup:

[motorr50]

Thanks to Brent the problem is solved. The IAC was coated in a thick layer of carbon, cleaned it and the TB and it runs like a champ. Thanks for the expert tech. As far as the TPS when do they tend to fail?

Mike

[BLHTAZ]

As far as the TPS when do they tend to fail?

Mike

That's not anything anyone can really say. The one on our YJ failed at about 40K. I have seen 20 year old Jeeps that they have never failed on...so... :dunno:

 

Glad to hear it is working OK for you now though... :thumbsup:

[turbotyla]

Yep...I just went through this not long ago myself, but mine would rev up to 3000+ RPM and not come down no matter what. I replaced the IAC, MAP, CTS TPS...all to no avail. My problem ended up being a bad ground connection on the battery harness that was my fault from when I replaced the harness... :doh:

 

I have a TPS that I bought for my truck and used it for about 15 - 20 minutes. It still looks new and is in the original box from Standard if you determine you need one and can wait a couple days to get it. I paid $114 for it, but will sell it for $80 shipped. :thumbsup:

 

 

IAC, MAP, CTS TPS

My truck will not idle it just climbs to 2 to 3k and stays there will not come down. i need to replace these ? and i don't know what they are lol can i get an idiot version :) i know one is throttle position sensor. but it ran great until today when i started it? some help would be much appriciated

[BLHTAZ]

Not a problem...

 

IAC is the Idle Air Control that is mounted right beside the TPS on the front side of the throttle body. This picture shows the IAC...the TPS is just to the right of it.

 

 

MAP is the Manifold Absolute Pressure sensor and is mounted on the firewall slightly to the left of center and has a vacuum hose running to it from the base of the throttle body.

 

CTS is the Coolant Temperature Sensor and it is in the left side of the block underneath the manifolds.

 

The IAC is the most common culprit in this case and often times it can be solved by simply removing & cleaning it and the throtle body real good. If cleaning it doesn't work, then try replacing it.

[turbotyla]

ok i just went out and looked and it is plugged in. i have good grounds. but i have a third connector there that i can't find anything to plug into and i seem to be missing a fair amount of vacum hoses. anybody know where i could get these. or would a pick n pull junk yard be my best bet.

 

and is there any tech write up on how to remove uneeded vacuum hoses for emission and random junk. so i can get ride of all the random junk. my truck needs some tlc lol 500 junker but will be nice someday

 

 

AND!!!! my heater does not work. i drained all the water out today and added premixed antifreeze and it isnt taking that much. simple way to burp the air. i think thats why i get no heat. i don't even get the fan tho ?

[turbotyla]

Ok well i figured out it is vacuum leaks. Does anyone have a diagram To show where all the vacuum lines go. i have missing ones and it makes my truck hate me lol

[BLHTAZ]

I have posted a link to the Factory Service Manual in the DIY forum. Go over there and download the manual and start tracing :cheers:

[mjeff87]

the only five vaccuum lines I have hooked on mine are:

 

-CCV line from the back of the valve cover to the TB

-MAP sensor to the TB

-fuel pressure regulator (from the port on the front of the manifold)

-brake booster

-manifold vac to the vac resivoir canister, formerly behind the rear bumper but relocated to where the washer fluid resivoir used to be

 

Everything else is capped/removed. Runs great. But I don't have emmisions inspection to worry about, YMMV.

 

Jeff

[turbotyla]

yea i have no emmisions either. and i think i am going to do what you said and cap things off. thanks guys for the help

[BLHTAZ]

YMMV.

 

Jeff

You may want to go to the DIY forum and add that to the "Acronym" post at the top :idea: ;)

[carnuck]

No-one yet mentioned the stupid connector above the brake booster ('87 til very early '89) that I can't recall the name of. Try wiggling the connector and see if the idle drops.

[BLHTAZ]

No-one yet mentioned the stupid connector above the brake booster ('87 til very early '89) that I can't recall the name of. Try wiggling the connector and see if the idle drops.

I think you are talking about the C101 connector, and yes...sometimes that gets corrosion in it and can cause issues :cheers:

[turbotyla]

Well it was warm out side yesterday. So i went out side and checked all my vacuum leaks. played with it some. and then i saw a gap on the intake gasket and as soon as i touched it the idle dropped.

 

So i got some rtv out put it one ther and let it set up just to see if that is what it is. But the truck warmed up and idles high still.

BUT was dropped by about 500 rpms. So my conclusion is that is the problem but the intake gasket is failed else where also So this weekend i am parking it in a buddys garage and redoing the intake Gasket.

 

And i will take this time to replace my exon valdez power steering pump.