Topic: High idle at start.

[SDWE61988]

I have noticed as it gets colder outside, that the jeep's RPM's rev higher when I first start it in the morning.  I was doing this last year before I had the piston skirts crack (leading to my total engine reubuild).  When it is cold, Below 25 deg, when the engine first starts it rev's up to about 1700 rpm, depending how cold it is outside it may acually climb up over 2000rpms or more if I don't shut it down and wait a minute or so.  This cannot be good on a cold engine. As the engine warms up the idle will settle down to about 750-800.   I have a 4.0l throttle body on the engine and have had it for years.  I am thinking the the cold air's density may be why it wants to rev harder until the Idle air control motor can restrict it.  I have adjusted the set screw on the butterfly valve and that seemed to help some (found that from a search on here).  I am not sure, but I think the idle air control motor is from the 2.5 TB.  My question is If I puchase a 4.0 Idle air motor, will the harness attach to it.  I am not sure how much difference there is between the motors.  I would try and swap the TB back to the 2.5 one, however that is long gone.  This has been an issue for a while now, and I have have done some troubleshooting, but I have not figured out the root cause of this issue.  I know many on here have performed who has performed the 4.0 TB swap, so any help would be appreciated.

http://napaonline.com/Catalog/CatalogItemDetail.aspx?R=CRB21855_0336307564
http://napaonline.com/Catalog/CatalogItemDetail.aspx?R=CRB21856_0169845636

[sharpxmen]

it will rev higher with the 4.0L IAC. it's normal to have a higher idle at start when colder outside but it sounds like its too high, i would maybe look and see if the IAC valve is not clogged with oil or gunk and prevents it from moving until it's warm enough so it wont stick.

[SDWE61988]

It is pretty clean in there.  I have swapped out the IAC with a new one from Napa, back in june and that one did not work any better.  The pintle on that over extended and got stuck.  I ended up putting the old one back in.  maybe i should try a third one at this point.  The last time I swapped them out the header ended up glowing hot as the  IAC motor calibrated itself.  That is probably how the last header ended up with a large crack in the welds at the collector. I assumed that is how the IAC's adjusted themselves when they were first connected.  Maybe I have two bad IAC's

[Jeffy]

You have a 4.0L TB but your not sure if you have a 2.5L IAC?  4.0L and 2.5L IAC's aren't interchangeable.  Did you swap the 2.5L IAC housing onto the TB?

1700 rpm isn't going to hurt the engine even when cold.  There's no load and you're not reving that high.

[SDWE61988]

Yes, That was a question if they were interchangeable or not.  So I must have used to 2.5 IAC then, because I did not make any changes to the wiring.    I am not sure about the housing.  What is the best way to ID the housing?   I performed a test in the spring when I replace the IAC the first time to see if there was a vacuum leak somewhere that could be causing this.  I started the vehicle,  the then blocked the TB with my hand to starve it of air.  the jeep stalled immediately.  So I don't think there is a vacuum leak.

 1700 is not to bad, (that is what it does the rest of the year) but when it creeps up to 2000-2500when it gets real cold, I get paranoid.  That is when I first noticed the load ticking noise last year before I tore into the engine to find the piston skirts in the oil pan.  That may have been coincidnece, but I don't want to find out the hard way. 

[Jeffy]

Yes, That was a question if they were interchangeable or not.  So I must have used to 2.5 IAC then, because I did not make any changes to the wiring.    I am not sure about the housing.  What is the best way to ID the housing?   I performed a test in the spring when I replace the IAC the first time to see if there was a vacuum leak somewhere that could be causing this.  I started the vehicle,  the then blocked the TB with my hand to starve it of air.  the jeep stalled immediately.  So I don't think there is a vacuum leak.

 1700 is not to bad, (that is what it does the rest of the year) but when it creeps up to 2000-2500when it gets real cold, I get paranoid.  That is when I first noticed the load ticking noise last year before I tore into the engine to find the piston skirts in the oil pan.  That may have been coincidnece, but I don't want to find out the hard way. 

The IAC housing will say 4.0L or 2.5L.  Unless you changed it, it's a 4.0L.  Which means your IAC is a 4.0L one.  IIRC, the plunger on the 2.5L won't seal on a 4.0L IAC housing.

[Bounty Hunter]

Jeffy is correct, the 2.5L IAC plunger head is too small a diameter to close the larger hole in the 4.0L IAC housing.  The 4.0L IAC housing has a larger hole to allow more air at idle for the larger motor.

[sharpxmen]

you need to keep the 2.5L housing and IAC motor - just to clarify as it sounded like you plan to put the 4.0 on.

[Bounty Hunter]

I can send you the 2.5L IAC housing if you need one.  They're generally attached with tamper-proof torx bolts so you may need a new socket if you don't have one already.  The IAC harness is the same for '91-'95 YJ's.

[grumpygy]

Isn't this discussed in the FAQ on changing the TB out.  Cause when I 1st changed mine, the Idle was way High.  Did the change around and its right where it should be now.

[Jeffy]

Isn't this discussed in the FAQ on changing the TB out.  Cause when I 1st changed mine, the Idle was way High.  Did the change around and its right where it should be now.

Yes.

[SDWE61988]

Thanks for the responses.  I will have to verify if the housing is a 4.0 or 2.5 tonight.   

[jfrabat]

There is also an allen bolt that adjusts the throttle plate closed position; it is easier to adjust if you take out the TB, as the allen is on the bottom of the TB.  It is unlikely, but it COULD be that the bolt is leaving a gap on the plate, which gives you the high throttle.  Just be sure when you adust it that the plate does not hit the TB neck, as it can cause damage and can make the plate stick in the closed position (making you have to tap the throttle to unstick it) - DO NOT ASK WHY I KNOW THIS!

[bestos]

had same problem when i did my 4.0 TB 2 weekends ago.  had to match 2.5L IAC with 2.5L IAC housing and life was good.  when u compare the 4.0 housing to the 2.5, you will see the difference in how they look inside.