High Idle / None of the usual fixes seems to work

[buckwheat]

My 87 MJ 4.0L has yet another issue I can't seem to track down. Extremely high idle. In the 2500-3500 range.

 

So far I have tried without success:

Adjusting TPS to spec using Cruisers Renix Tips.

Cleaning IAC

Cleaning TB

Pulling a TPS out of another Jeep and adjusting it per Cruisers Tips. Has the same problem.

Pulling the wires to the TPS and running it without them. Has zero effect.

Changing out the IAC.

I accidentally started it with the MAP sensor tube unplugged from the TB. The idle was around 1500 and seemed to stay there. Not sure if that is at all related.

 

I plan on looking for a vacuum leak by spraying everything with throttle body cleaner when I can get a can, but I wanted to make sure I wasn't missing something simple in the mean time. I searched and read quite a few High Idle Threads, but none of those solutions seem to work for me.

 

Whats got me stumped is why would the high idle be there after pulling the TPS wires. Shouldn't that tell the ECU to use some default setting?

 

Thanks in advance for any help or advice. 

[buckwheat]

Sprayed Mass Flow Cleaner spray (highly flammable) around the intake and on all vacuum lines. No change to idle. 

Checked the top bolts that hold the intake to the block, and they seem to be very tight.

 

I'm running out of ideas quickly. Any thoughts would be appreciated.

[ggcnash]

Wish I could help just don't know much about renix system? It sounds like a severe vacuum leak I'll bump it for you good luck

[Knucklehead97]

Checked the vacuum bottle behind the bumper? Is it 4x4? If so, check the vacuum lines running to the vacuum valve on the front axle. Checked the vacuum lines on the passenger side of the engine? I had completely forgot about those at first and had a few that were cracked. They didn't fix my problem but ANY vacuum leak is a bug on Renix engines.

[ftpiercecracker1]

Take the air cleaner off the throttle body and completely cover it while the truck is running. If you're high idle persists you have to have a large vacuum leak somewhere else

 

If the truck tries to die or the idle reduces greatly your IAC valve is malfunctioning in some way or another. Either the valve itself is malfunctioning or it is simply receiving the wrong signal.

 

This is the same problem I'm having with my truck. Fortunately for me every once in awhile the truck will idle normaly. During one time when it was idling normally I disconnected the IAC valve and have yet to reconnect it because it has idled perfectly ever since.

[Knucklehead97]

Gotta love a Renix, eh?

[buckwheat]

Knucklehead: Thanks for those ideas, I will be sure to check out the vacuum canister and the CAD connections.

 

FPC: Its good to know that I have a possible temporary fix if I can get it to idle normally once more. I will also make sure its not an IAC problem, although having tried two different IAC's one would think the problem is elsewhere.

 

Keep the ideas coming if you got em. I will report back on these possible fixes as soon as I can get to them.

[Knucklehead97]

Did you remember to reset the throttle butterfly per Cruisers tips? Just a shot in the dark.

[buckwheat]

Alright, did some more work on it today. I looked up the vacuum diagram for the Renix 4.0 and noticed a lot of mistakes from the PO. Got those fixed. Then realized the EGR had a hole in the diaphragm and wasn't working. Got a spare one from a 90 Jeep Cherokee parts vehicle I have sitting around. Neither of those two things changed the idle at all. 

 

Then I cleaned one of the IAC motors and was a little more forceful about it. Not rough, just put more elbow into cleaning it. Swapped that in. Perfect idle, right at 600-700 rpm. Stayed that way as long as it was parked. As soon as I took it for a test drive the idle became erratic. Meaning that sometimes it would idle perfectly at 700 rpm. Sometimes it would idle too low and die out. Sometimes it would idle higher at 1500 rpm or 2000 rpm. There did not seem to be a rhyme or reason to it. 

 

I have ordered a new IAC from Orielleys. In the meantime I have a third IAC I can grab and put in the truck. Right now my thinking is that the last two IACs I have tried are both faulty to some degree. I know that mathematically the odds are slight, but they are both used, so it is possible. 

 

Anyone have any other thoughts or ideas for me to try?

[tugalo]

i am interested in whatever you find out because my 88 idles at 750-800 on start up but when it gets to operating tempt, the idle goes to between 1200-1500.  I have gone thru all the things you have and they made little difference.  The one thing i haven't tried is adjusting the idle stop. 

 

Keep chasing it, and keep us posted, i'm sure that others are fighting the same bug

[cruiser54]

Tip 14 been done?

[buckwheat]

Cruiser, I have not done Tip 14. What I did is turn the screw all the way out (meaning the butterfly was closed) then screwed it back in until I heard the engine rpms pick up very slightly. I will go back when I get a chance and do it your way.

 

The problem is mostly fixed right now. It seems that I had 2 bad IACs in a row. I put a third one in and my idle is mostly fixed.

 

Recently I have had problems with it stalling as I start in first gear, and with it jerking as I drive along at low rpms. By jerking I mean it feels like a carbed vehicle when the fuel line is partly clogged. It surges forward then stalls for a bit, then surges forward, then stalls, but in very rapid succession. So I think my next thing is to check fuel pressure and see where that is at. But it is much better than it was, and is back to being my DD.

 

Thanks for everyones suggestions. I wanted to make sure to post my solution to the problem in case anyone comes across this thread in the future.