Renix 4.0 High Idle

[Christian]

I have an 87 Comanche 4.0 with an automatic transmission that idles very high. It has around 230 xxx miles on it. I've been trying to diagnose the high idle problem for a while now and I can't seem to figure it out. When I start it up when its cold in park or neutral, it idles at around 1000 rpms and will fluctuate down to 500 and then back up to 1000 rpms until it warms up. As the engine gets warm the rpms increase and will peak around 1800rpms usually but sometimes will make its way up to 2500rpms. When in drive at a stop it idles at 1000 to 1200rpms when warm. I have replaced the iac, tps, map sensor, I cleaned the throttle body, thoroughly went through every vacuum line I could find and found no leaks. It's not very consistant as some days it idles higher than usual. Sometimes it will idle at about 600rpms after reaching temprature but sounds like its misfiring. When this occurs I turn the engine off and start it back up and the misfire stops but will idle high like it usually does. I've noticed that it never reaches 210 degrees after warm up. It usually runs around 160 degrees. I've owned a few other jeeps with the 4.0 and they all ran at 210 after fully warmed up. I have not replaced the coolant temp sensor yet but I might try that next. Any information on what I should try next is very appreciated. Thank you for reading.

[cruiser54]

go to www.cruiser54.com and complete Tips 1 through 5 before replacing any more parts. 

[KellyQ]

My high idle was TPS sensor.  Apparently it had been replaced at some point and wasn’t a direct replacement and was not seated properly.  Removed, shaved a bit of the plastic ring off to get a more flush fit and idle dropped right back to normal.  

[Marine1Texas]

a few things that cause high idle are TPS that is not adjusted right, idle Air control valve that is not moving in and out all the way, then a air leaks. A air leak which could be in vacuum lines, seal around throttle body, and seal around intake manifold to the head. Use some spray carb cleaner to check for leaks on seals and possible vacuum line connections..

 

My 87 4.0 has never idled perfect since day one. when warm it idles about 750 to 850. it bounces around all the time between those rpms. If I click the a/c on it will idle 750 to 800 rpms when warm. I only run 3 vacuum lines on manifold, one to the ball canister, one for the pcv valve, and one for the fuel pressure regulator. Then the one on side of the throttle body to the map sensor. I don't have the line going to the egr vlave hooked up. I also have the EGR capped off.

 

I know a dirty or bad fuel injector can cause idle issues. Of course cap, rotor and spark plugs can affect as well 

 

As Cruiser pointed out above do the tips above. The Renix has crappy ground system and would benefit all the way around for the tips. I went over board on the wiring and helped out a few things.

[cruiser54]

Last November and he/she has only posted once............

[Eagle]

For anyone else with a high idle problem on a Renix, there's another possible cause. The throttle cable from the gas peddle doesn't go direct to the throttle body, it goes to a mechanical bellcrank assebly that has a pivot located down on the driver's side frame rail, below the master cylinder. From there the bellcrank has a rod that extends up to move the throttle plate. That pivot mechanism can get dirty and rusty. I've found that period lubrication with PB Blaster cures my fast idle problems when they show up.

[Marine1Texas]

This is how my 87 mj has idled since day one

https://vimeo.com/261183616