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Ok, still having a high idle issue.......

YELLAHEEP

YELLAHEEP

NAXJA Forum User
Location
Littleton, Colorado
'96 4.0L auto.

Replaced the TPS and the IAC with used units from a good running engine. I've solved the 2000 rpm issue it had before, but I can't seem to figure out why it still idles/won't quickly return to idle.

Sometimes I have to push the gas pedal down to get it to start - not everytime. When it's running - cold or warm - the idle generally sits right at 1000 rpm, on rare occasions, down to 850-900 rpm. When I drive or revv the engine, it speeds up fine, but the rpms take a long time to come down. Kinda annoying in traffic because then it pushes forward and I'm using the brake more. At about 20-30 mph, it's like having cruise control - it won't come down from 1500 rpm for several minutes.

Any ideas? I'm probably gonna bite the bullet and go buy new IAC and TPS units. Any chance the MAP is bad?

Thanks in advance!
 
Yeah, definitely not a mechanical problem. The throttle body, cables, springs all return to the idle/closed position. It seems like something is controlling the IAC to make it slowly move.

I can hear the IAC buzz as it moves to come down from RPM's above 1500. What controls the IAC? Maybe I have a bad ECU?

I've cleaned the carbon from the IAC - should be moving fine, just seems like either the IAC has bad mechanical movement, or it's getting a signal causing it to slowly move.
 
Big_Al said:
Just a thought...Did you disconnect the battery to reset the computer after installing the new parts?...Al
Click to expand...


No. I've never heard of doing that unless you need to clear ECU codes. I have a code scanner so I've been addressing the codes when they've come up.

Can't hurt to try it though..... :dunno:
 
Reason I mentioned it was that My Plunger fell off the end of my IAC and I took it off, put it back together and when I cranked it my idle was at 3500.:looney: So disconnected the battery, let it set for 5 minuntes, restarted and all went back to nornal. HTH...Al
 
Big_Al said:
Reason I mentioned it was that My Plunger fell off the end of my IAC and I took it off, put it back together and when I cranked it my idle was at 3500.:looney: So disconnected the battery, let it set for 5 minuntes, restarted and all went back to nornal. HTH...Al
Click to expand...

After you do this take it out for a 30+ minute drive or it will only learn the idle parameters. I did that and had a dead spot in my throttle for like 3 years, disconnected the battery when I replaced the alternator and drove once I did the replacment, dead spot went away.
 
X2, but If disconnecting the battery and doing the relearn procdure RichP suggested does not solve the problem I would try a new IAC next, and do the battery disconnect / relearn again.

Might check the electrical connection to the IAC first as well. It definately sounds like an IAC issue, but just for the hell of it make sure the TPS is not loose and check the TPS gorund wire (ECU side) when the idle gets high. If it has more than about 0.05 volts from the TPS ground wire to the battery negative post it could be a ground problem, common on the Renix years, but I dont think it is common on the later years.
 
If the throttle is shut and the engine RPM is still elevated, you've got a vacuum leak. It's the only way the engine can get the necessary air to continue to run at higher speeds. That said, the IAC is nothing more that a controled vacuum leak to regulate the engine speed, so I'd disconnect the battery, clean all the air passages in the throttle body, and reinstall the IAC.
Take it out for a good drive and see if it's better. If not, check EVERY vacuum line that is ported to the manifold and check EVERY vacuum device that's controled by those lines (MAP, HVAC, cruise control, brake booster, etc) to ensure they hold vacuum. You'll find it smoewhere in there 90% of the time, but as stated earlier, don't overlook the intake bolts as well-- they are a pain to check, but if one or two are loose (I'll bet you find at least one) you can get a big enough leak to cause fits!
 
Something that is rarely mentioned is the source of the "Vacuum leak", whether its the IAC or somewhere else can be detected by covering the top corner IAC port on top of the throttle body with a very flat metal plate (don't use your hand or anything flexible unless you want it sucked into the engine!:eek:) to see if the engine dies or keeps running. If it dies, the problem is in the IAC or IAC port, like trash, carbon, etc.. If it continues to run you have an intake leak elsewhere, vacuum hose, IM gasket, Throttle body gasket....
 
Ecomike said:
Something that is rarely mentioned is the source of the "Vacuum leak", whether its the IAC or somewhere else can be detected by covering the top corner IAC port on top of the throttle body with a very flat metal plate (don't use your hand or anything flexible unless you want it sucked into the engine!:eek:) to see if the engine dies or keeps running. If it dies, the problem is in the IAC or IAC port, like trash, carbon, etc.. If it continues to run you have an intake leak elsewhere, vacuum hose, IM gasket, Throttle body gasket....
Click to expand...
And that you can find using a simple spray bottle of water and mist, you will hear the idle change when you find the spot.. My son's TJ started doing it, I'll be hunting that problem down tomorrow.:doh: What I'm wondering is whether the IAC is cleanable or not or is just one of those you have to replace.
 
hanaxj87 said:
Check all your intake bolts.

Nick
Click to expand...

I'm with Nick. I had a goofy high idle problem on my 96 a while back. Ended up being a slightly loose intake manifold.
 
I went through the same thing on my 89, IAC, TPS, ECU, intake manifold, vac lines, finally broke down and swapped the throttle body and I'll be damn if it didn't fix it.
 
Ecomike said:
Something that is rarely mentioned is the source of the "Vacuum leak", whether its the IAC or somewhere else can be detected by covering the top corner IAC port on top of the throttle body with a very flat metal plate
Click to expand...
my costco card served that function for about 6 months, it's a very good suggestion, you might try it yella. Even a business card will work until it gets wet.
 
What size are the bolts in the 2.5L engine for the intake manifold, got one missing so thats probably the high idle, at any rate it's a starting point. The other bolt I can see that is in there look like a 'opps, too long, lets use 4 washers as spacers' kind of fix. And of course it's gotta be the $#$#$ back one thats missing.
 
RichP said:
What size are the bolts in the 2.5L engine for the intake manifold, got one missing so thats probably the high idle, at any rate it's a starting point. The other bolt I can see that is in there look like a 'opps, too long, lets use 4 washers as spacers' kind of fix. And of course it's gotta be the $#$#$ back one thats missing.
Click to expand...

Your asking us? LOL.:roflmao:Not even sure 5-90 knows that one. But there are a few other 2.5 L guys running around here.

Just had a thought, they may be the same as the 4.0. Isn't the 2.5 a 4.0 with just 4 cylinders?
 
Ecomike said:
Your asking us? LOL.:roflmao:Not even sure 5-90 knows that one. But there are a few other 2.5 L guys running around here.

Just had a thought, they may be the same as the 4.0. Isn't the 2.5 a 4.0 with just 4 cylinders?
Click to expand...

Pretty much and I'm NOT taking one out of my 4.0 just to check size, my karma is pretty good but I have no intention of hanging my butt out for it to paddle. :D
 
RichP said:
Pretty much and I'm NOT taking one out of my 4.0 just to check size, my karma is pretty good but I have no intention of hanging my butt out for it to paddle. :D
Click to expand...

I posted the 4.0 manifold bolt size recently (like 8 to 12 weeks ago) in the thread about head gasket changing and Manifold gaskets. Long blow by blow discussion with Muadib (sp is wrong, but that is how it is pronounced) and 5-90.

Try a seach of Ecomike and "Head gasket" in this forum. It should be late in the thread. I am thinking they are 8 mm by 1.5 or 1.25.

Oh, and you can't have near as much fun if you don't "hang your butt out there" once in a while!:)
 
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