high idle after tuneup
Thread Starter
Newbie
Joined: Dec 2011
Posts: 14
Likes: 0
From: Salt Lake City, UT
Year: 1990
Model: Cherokee
Engine: 4.0L
whether is warm or cold, when first started the idle is good. after i drive it some, it elevates. has nothing to do with hot or cold, just whether i just started it or have driven it some
CF Veteran
Joined: Jan 2009
Posts: 1,964
Likes: 6
From: Baltimore, Maryland
Year: 1994 1993
Model: Cherokee
Engine: 4.0
Senior Member
Joined: Aug 2011
Posts: 760
Likes: 6
From: Landers, CA
Year: Several
Model: Cherokee
Engine: 4.0
Bikini or Parka time makes no difference.
A cold or restart it's not in closed loop for a moment. Once in closed loop it idles fast.
If you like it like that, don't find/fix the vacuum leak. You can spend hundreds of dollars buying sensors, it won't help with a leak.
A cold or restart it's not in closed loop for a moment. Once in closed loop it idles fast.
If you like it like that, don't find/fix the vacuum leak. You can spend hundreds of dollars buying sensors, it won't help with a leak.
Thread Starter
Newbie
Joined: Dec 2011
Posts: 14
Likes: 0
From: Salt Lake City, UT
Year: 1990
Model: Cherokee
Engine: 4.0L
High idle AFTER it warms up?
Sounds like a vacuum leak.
The O2 sensor senses it's too lean and adds fuel. Then the total mix is too much - runs faster.
Do the smoke test to find the leak.
Blow cigarette smoke through a hose into the intake manifold. Use a rag or tape to put over the top of the throttle body to keep the smoke in. Look for where it leaks out where it shouldn't.
Sounds like a vacuum leak.
The O2 sensor senses it's too lean and adds fuel. Then the total mix is too much - runs faster.
Do the smoke test to find the leak.
Blow cigarette smoke through a hose into the intake manifold. Use a rag or tape to put over the top of the throttle body to keep the smoke in. Look for where it leaks out where it shouldn't.
but i dont smoke, what ever will i do?!?!
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Joined: Aug 2011
Posts: 43,971
Likes: 1,578
From: Prescott, Az
Year: 1990
Model: Cherokee (XJ)
Engine: 4.0
Thread Starter
Newbie
Joined: Dec 2011
Posts: 14
Likes: 0
From: Salt Lake City, UT
Year: 1990
Model: Cherokee
Engine: 4.0L
here is another thing i noticed, when i start it, it doesnt just sit at one rpm and stay there. the idle increases a little bit for a second, then it will drop back down for a couple seconds, then it will increase for a second, then drop back down for a couple. it continues to do this. idk if that points to anything specific but just thought i would include it
::CF Moderator::
Joined: Aug 2011
Posts: 43,971
Likes: 1,578
From: Prescott, Az
Year: 1990
Model: Cherokee (XJ)
Engine: 4.0
Got a voltmeter? Check this out. I would also suggest doing this complete ground refreshing as poor grounds alter the signals from the sensors to the ECU resulting in odd problems. I have a TPS write-up if you need it. but, do the grounds first.
Renix Ground Refreshing
The Renix era XJs and MJs were built with an under-engineered grounding system for the engine/transmission electronics. One problem in particular involves the multiple ground connection at the engine dipstick tube stud. A poor ground here can cause a multitude of driveabililty issues, wasted time, and wasted money replacing unnecessary components.
The components grounding at the dipstick tube stud are:
Distributor Sync Sensor, TCU main ground, TCU "Shift Point Logic", Ignition control Module, Injectors, ECU main ground which other engine sensors ground through, Oxygen sensor, Knock Sensor, Cruise Control, and Transmission Sync signal. All extremely important stuff.
The factory was aware of the issues with this ground point and addressed it by suggesting the following:
Remove the nut holding the wire terminals to the stud. Verify that the stud is indeed tightened securely into the block. Scrape any and all paint from the stud’s mounting surface where the wires will attach. Must be clean, shiny and free of any oil, grease, or paint.
Inspect the wire terminals. Check to see that none of the terminals are crimped over wire insulation instead of bare wire. Be sure the crimps are tight. It wouldn’t hurt to re-crimp them just as a matter of course. Sand and polish the wire terminals until clean and shiny on both sides. Reinstall all the wires to the stud and tighten the nut down securely.
While you’re in that general area, locate the battery negative cable which is fastened to the engine block just forward of the dipstick stud. Remove the bolt, scrape the block to bare metal, clean and polish the cable terminal, and reattach securely.
Another area where the grounding system on Renix era Jeeps was lacking is the engine to chassis ground. There is a braided cable from the back of the cylinder head that also attaches to the driver’s side of the firewall. This cable is undersized for it’s intended use and subject to corrosion and poor connections at each end.
First off, remove the cable end from the firewall using a 15mm wrench or socket. Scrape the paint off down to bare metal and clean the wire terminal. Reattach securely.
Remove the other end of the cable from the rear of the head using a 3’4" socket. Clean all the oil, paint and crud from the stud. Clean the wire terminal of the cable and reattach securely.
A suggestion regarding the braided cable:
I prefer to add a #4 Gauge cable from the firewall to a bolt on the rear of the intake manifold, either to a heat shield bolt or fuel rail bolt. A cable about 18" long with a 3/8" lug on each end works great and you can get one at any parts store already made up. Napa has them as part number 781116.
A further improvement to the grounding system can be made using a #4 cable, about 10" long with 3/8" terminals at each end. Attach one end of this cable to the negative battery bolt and the other end under the closest 10mm headed bolt on the radiator support just forward of the battery. Napa part number 781115.
If you want to upgrade your grounds and battery cables in general, contact Jon at www.kelleyswip.com. He makes an incredible cable upgrade for a very reasonable price.
Revised 11-28-2011
Renix Ground Refreshing
The Renix era XJs and MJs were built with an under-engineered grounding system for the engine/transmission electronics. One problem in particular involves the multiple ground connection at the engine dipstick tube stud. A poor ground here can cause a multitude of driveabililty issues, wasted time, and wasted money replacing unnecessary components.
The components grounding at the dipstick tube stud are:
Distributor Sync Sensor, TCU main ground, TCU "Shift Point Logic", Ignition control Module, Injectors, ECU main ground which other engine sensors ground through, Oxygen sensor, Knock Sensor, Cruise Control, and Transmission Sync signal. All extremely important stuff.
The factory was aware of the issues with this ground point and addressed it by suggesting the following:
Remove the nut holding the wire terminals to the stud. Verify that the stud is indeed tightened securely into the block. Scrape any and all paint from the stud’s mounting surface where the wires will attach. Must be clean, shiny and free of any oil, grease, or paint.
Inspect the wire terminals. Check to see that none of the terminals are crimped over wire insulation instead of bare wire. Be sure the crimps are tight. It wouldn’t hurt to re-crimp them just as a matter of course. Sand and polish the wire terminals until clean and shiny on both sides. Reinstall all the wires to the stud and tighten the nut down securely.
While you’re in that general area, locate the battery negative cable which is fastened to the engine block just forward of the dipstick stud. Remove the bolt, scrape the block to bare metal, clean and polish the cable terminal, and reattach securely.
Another area where the grounding system on Renix era Jeeps was lacking is the engine to chassis ground. There is a braided cable from the back of the cylinder head that also attaches to the driver’s side of the firewall. This cable is undersized for it’s intended use and subject to corrosion and poor connections at each end.
First off, remove the cable end from the firewall using a 15mm wrench or socket. Scrape the paint off down to bare metal and clean the wire terminal. Reattach securely.
Remove the other end of the cable from the rear of the head using a 3’4" socket. Clean all the oil, paint and crud from the stud. Clean the wire terminal of the cable and reattach securely.
A suggestion regarding the braided cable:
I prefer to add a #4 Gauge cable from the firewall to a bolt on the rear of the intake manifold, either to a heat shield bolt or fuel rail bolt. A cable about 18" long with a 3/8" lug on each end works great and you can get one at any parts store already made up. Napa has them as part number 781116.
A further improvement to the grounding system can be made using a #4 cable, about 10" long with 3/8" terminals at each end. Attach one end of this cable to the negative battery bolt and the other end under the closest 10mm headed bolt on the radiator support just forward of the battery. Napa part number 781115.
If you want to upgrade your grounds and battery cables in general, contact Jon at www.kelleyswip.com. He makes an incredible cable upgrade for a very reasonable price.
Revised 11-28-2011
Thread Starter
Newbie
Joined: Dec 2011
Posts: 14
Likes: 0
From: Salt Lake City, UT
Year: 1990
Model: Cherokee
Engine: 4.0L
great, i will try this, thank you. i will try and clean some of the electrical connections too, because i recently degreased the engine and sprayed everything down with the hose real good too, so im sure i got water and maybe other stuff in the connections.
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