High idle, and high rpms at start? Bad gas mileage

I have a Jeep wagoneer 4.0 88' with a rusty's 3" suspension lift and 31"s with stock gears. When i start it up it sometimes revs up to 3k to 4k and wont drop back down until i shut it off and even if i restart it it does it again until the 4rth try. yea?? Now typically my idle rpms are 1k. I have noticed recently that my gas mileage is not great but i think i figured out that when im in traffic at school its about 13-14, and when im just in normal everyday traffic it is about 15mpg. I thought i would add that i disconnected my airfilter because i have blow-by gettin in the air filter and it was just better with out it. Any help would be appreciated. I dont even know where to start.

My '89 had a similar problem...still has the same problem on occasion. I think it's the crank position sensor...just haven't gotten around to changing it. As a "quick fix", I cleaned the connector for the CKS and filled with dielectric grease...so far, so good.

IAC Idle Air Cotroller, it's located in your throttle body, you can remove it and try to clean it with carb cleaner to make it last a "little" longer. but eventually you'll need to get a new one. run a search and you'll find much guidance here about the procedure.

yea i have already replaced the IAC a little while back. but it may be the crank position sensor, only because i have no idea where it is. How much does it run and is it easy to get to.

If a CPS fails it won't let your jeep start, and if it's failing, it'll run very poorly and die on you randomly. Doesn't sound like it to me.

Could be a vacuum leak..

Edit: Say what? You don't have an air filter? That's very bad for your engine. One way to stop blow-by is to clean or replace your two air lines running off of your valve cover. If they plug up, you'll get lots of blowby.
Your mileage seems decent for your combination and jeep age. I average 17 mpg stock with upcountry package, 5 speed, and 235's.

I get the same high idle at startup. I find that if you disconnect the battery and then reconnect it, it'll make it go away for awhile.

i have the same problem, only started to do the high idle on startup after i replaced the entire CCV system. IDK whats going on, i think i can hear a vaccum leak from the intake manifold somewhere but have no idea how to track it down. I haven't noticed any vacuum lines missing or unpluged. Seeing as i started to have this problem once i replaced my CCV system i would start there and see if there is an issue within.

Sounds like a vacuum leak. I traced mine down to loose manifold bolts.

Popping a cheap chrome chevy breather in the valve cover stopped it from back dumping oil down the air filter. Now it just drains all over the engine. Something like a can of Restore may slow down the crankcase pressure by sealing the old rings. More power!

Whenever I have had a high idle on my 1990 4.0, it has been a failing Throttle Position Sensor (TPS). I'm on my third since the jeep was new. You may also notice some gear shifts being mistimed, if you have an auto tranny. The TPS has to be adjusted on years up to 1990, if you replace it or move it.

try spurting some wd 40 around your intake manifold gasket.. had an old tj , went over everything iac, vac lines... new intake gasket fixed it..

bigboyslimjimbo said:

yea i have already replaced the IAC a little while back. but it may be the crank position sensor, only because i have no idea where it is. How much does it run and is it easy to get to.

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Definately not the CPS, it has nothing to do with idle speed.

While you might have a vacuum leak, it is not causing your current problem. A sticking IAC is a usual suspect and it can be cleaned with solvent and reoiled with a 10WT oil, but be gentle, do NOT force the "pentle" end part to move in and out (sort of piston, or valve seal end piece, rides on a rotary threaded shaft which is turned by a pulse motor) or you will permanently damage it.

The TPS can be going bad and can be periodically calling for a higher idle speed. It can be tested with a volt-ohm multimeter. Search this forum for "Throttle Position sensor test".

Another commoin cause of high, intermitent high idle is a bad engine, frame, battery negative series of grounds. A loose or dirty ground that varies by as little as 5 ohms will cause a variable high idle too! Clean all the ground connections, 2 at the rear firewall, sevral at the dipstick, passenger side of the block, one at the drivers side head, and the battery negative post. New ground wires help too. Two of these can be done at no parts cost. The poor ground problem affects the readings the ECU computer sees for the TPS value, and makes the ECU think the TPS is calling for more engine speed.

I'm having the same issue. I'll try that out. Thanks.

look up kj139 postings.. he fixed this exact issue eace:

90XJLtd said:

I'm having the same issue. I'll try that out. Thanks.

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For the whole story on fixing and debuging all these 87-90 model Renix problems with high idles, and bad gas mileage search for my old thread here using the search words "RenX files Ecomike" . It will give you blow by blow test methods and data not even in the FSM for testing the TPS, CTS, IAT or MAT, O2 sensor and so on.

I have a '94 that rev's what I think is way (3000rpm) too high but drops to ok in a few sec's. Can I lower that first rev. or just count myself lucky it drops to normal. That high rev. is just when there's no oil pumped up yet.

94xjjohn said:

I have a '94 that rev's what I think is way (3000rpm) too high but drops to ok in a few sec's. Can I lower that first rev. or just count myself lucky it drops to normal. That high rev. is just when there's no oil pumped up yet.

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You probably have an injector leaking fuel after engine shutdown, or before you crank it. Too much excess fuel at start up, causing the high rpms.

There are three threads addressing this same issure right now. What you describe I believe is most easily caused by a bad latch relay which prevents the ECU from setting the IAC to the starting "home" position.