420A Jeep TB - How to fix high idle

[BLACK'98DSM]

Well the engine idles about 1100-1200 rpm once warm. And 1300-ish when cold. When coasting it stays about 2k.

I've got the IAC adjusted as far out as it can possibly go and still fit in the casing. All I can do with it is tighten it up more, which raises the idle.

I've got the throttle plate adjusted about one thread away from sticking. I could probably close it another half turn, but I just don't see that idling me down to 800.

Tps is new. All vacuum systems have been deleted, so there's no leaks. Vacuum gauge reads about 13 once warm at idle. Is that low? It seems low compared to other engines I've seen.

Is it possible that my IAC is bad? I've got another one I can try to see if it works alright, just haven't gotten around to doing so.

I can't really think of any other reasons why it would be idling high. With no leaks and everything adjusted right, it should idle at the stock number, right?

 

[dsmcrzy]

When I still had a modded Jeep TB and was playing with the IAC settings in megasquirt, the only IAC I could get to work consistently was a
Standard Motor Products IAC valve. The part number is: AC151T. I did not swap the tip on it. I bolted it up and was good to go.

 

[silvreclips]

Your vacuum seems on the low side. My car idles around 18 to 19 in.hg. One way I've always tested for vacuum leaks it to cover up the throttle body with my hand while it's running. See if the car idles down as you restrict the air coming in, and it should die as soon as you cover the TB completely. If it stumbles and tried to run still you have a leak somewhere. If it dies quickly then you should be good to go.

Try getting the TB to close some more by adjusting the stop screw. You may need to loosen the throttle plate itself some and try to get the bore sealed better.

 

[BLACK'98DSM]

When I still had a modded Jeep TB and was playing with the IAC settings in megasquirt, the only IAC I could get to work consistently was a
Standard Motor Products IAC valve. The part number is: AC151T. I did not swap the tip on it. I bolted it up and was good to go.

Thank you. For some reason they sell it at Home Depot

 

[BLACK'98DSM]

Try getting the TB to close some more by adjusting the stop screw. You may need to loosen the throttle plate itself some and try to get the bore sealed better.

I'll see what I can do. But there's a very fine line when the throttle starts sticking.

 

[rabenne]

13 in-hg sounds really low. Are you sure you don't have a vacuum leak? Grab a can of carb cleaner and spray all gasket surfaces on the intake manifold/head?

 

[BLACK'98DSM]

13 in-hg sounds really low. Are you sure you don't have a vacuum leak? Grab a can of carb cleaner and spray all gasket surfaces on the intake manifold/head?

Literally all of the vacuum systems have been deleted. The only port I have open is the one for the gauge. Both of the intake manifold gaskets are new. The throttle body gasket is new...

I'll try covering the tb to see what happens... But I don't think I have any leaks.

 

[1990TSIAWDTALON]

13 seems low Sol. I would bet there is a leak somewhere. The methods described can help you track it down. My cammed up 4g idles at 13hg so I suspect something is still allowing air in besides the TB. Keep us posted, surely we can run this down.

 

[BLACK'98DSM]

I ordered the SMP AC151T. Even if that's not the main issue, it won't hurt to have a new part.

It's dark outside now, supposed to storm for next few days... Will check vacuum after installing IAC.

 

[BLACK'98DSM]

So to update...

I replaced the IAC and adjusted the new one as far out as it can possibly go. This helped the revving issue in neutral, but still idled about 1100. I tried closing the throttle plate more, I have it about a quarter turn away from sticking, idled down to about 950. I checked the tb gasket, and it's still sealed tight and my rtv is still there.

I'm yet to check those 2 IM gaskets... But if I check those, and they're good, what could be causing such a low vacuum? And how would I idle it down by about 200 rpm?

 

[EShield]

You have to have a vacuum leak somewhere, I would do the easiest test metho, spray carb cleaner or brake clean and just see if you have a leak. Then proceed from there.

 

[BLACK'98DSM]

A couple days ago I used a skim coat of rtv on both sides of the intake manifold gaskets. Also applied proper torque on all bolts. Everything should be back together in the next 2 weeks, so we'll see how it is then.