rejeep Posted January 13, 2012 Share Posted January 13, 2012 Runs great, drives great... Once I let off the gas it idles low and rough... Any ideas... Link to comment Share on other sites More sharing options...
txjeeptx Posted January 13, 2012 Share Posted January 13, 2012 Likely vacuum leak. Check intake to block bolt tightness, then look for bad hoses. There's one that forms a tight 90degree behind the throttle body on Renix TBI 2.5s. I replace it with a longer hose(like 6 inches compared to 2.5), and run it up in a loop, so the hose doesn't clamp closed in the tight bend that would be required to replace the original with a similar length of hose, and it lasts longer. The original is a molded rubber elbow, and its always gone bad on every one I've ever seen. Link to comment Share on other sites More sharing options...
Bhfear Posted January 13, 2012 Share Posted January 13, 2012 Replace/clean Idle air control valve, IAC is the cylinder looking sensor in the throttle body, I would check there first then Vacuum :thumbsup: Link to comment Share on other sites More sharing options...
rejeep Posted January 13, 2012 Author Share Posted January 13, 2012 Just replaced my exhaust manifold back.. And I was fairly good about the fittment.. I was thinking IAC as well... Think I might grab one tomorrow and throw it in.. Link to comment Share on other sites More sharing options...
cruiser54 Posted January 13, 2012 Share Posted January 13, 2012 All those short hoses around the throttle body tend to deteriorate causing vacuum leaks and rough idle. Quite common. Link to comment Share on other sites More sharing options...
Lowrange2 Posted January 13, 2012 Share Posted January 13, 2012 My 2.5s idle fluctuates up and down. I've yet to really look into as I just bought the truck on Tuesday. I'll be around getting ideas. Link to comment Share on other sites More sharing options...
txjeeptx Posted January 13, 2012 Share Posted January 13, 2012 Ok, so you're pretty sure the manifold is sealed to the head. My 4banger TBI experience comes from owning my '89 YJ for 15 years, but it all applies to the XJ/MJ 2.5 TBI system. The 4banger TBI has a Idle Speed Actuator, which externally alters the throttle blade position for idle, not an Idle Air Control valve like the 4.oL. The ISA motor serves as an output device and as a sensor for closed throttle position, and they wear out and can get corrosion inside on the closed throttle switch. Its the 4-wire Wrather-Pack connected device in front of the TB. Could try checking its connection or even replacing it. Be gentle with the idle adjustment bolt(which touches an arm that moves the throttle shaft) - you can damage the stepper motor by trying to adjust it if the bolt is bound or corroded. Also have had o-rings on the fuel injector in the TB fail(forgot to mention this one, it was a problem a long time ago on my 2.5L TBI), causing rough/no idle condition. Be careful not to drop anything down the TB when pulling the injector, and get replacement o-rings before checking them - they usually discintigrate due to old age and Ethanol in our fuel. The injector is somewhat expensive, or at least it was back when I had issues around it. If the hose to the MAP sensor on the firewall is old or leaks, the engine won't idle right, either. Finally, if the EGR valve is stuck open, you would have trouble trying to idle. You should be able to move the diaphragm with your fingers reaching around and under the "UFO" shaped diaphragm. I have had a sticky EGR valve before. I could hear it squeak as the diaphragm moved while driving, a weird sort of chirping sound. It is controlled by a vacuum solenoid valve with a hose leading to it, one leading away from it, and a small plastic filter that allows it to purge vacuum to atmosphere when necessary. That tiny filter fitting can get clogged. Also have had the solenoid quit working, so I replaced it. All things EGR related for mine were long ago. Gah, one other thing - make sure the brake booster isn't rusted underneath. A pinhole of rust can make for a large vacuum leak. You'd know if it was leaking, though, because all you'd have is manual brakes - little to no stopping power. If it is rusted/no good, consider the WJ or '95-up XJ dual-diaphragm booster/master cylinder upgrade/swap - its incredible how much it increases braking performance, and is the first upgrade I'd do to any older XJ/MJ I'll ever own. Link to comment Share on other sites More sharing options...
carnuck Posted January 13, 2012 Share Posted January 13, 2012 I had to clean the throttlebody on mine Link to comment Share on other sites More sharing options...
airspeed Posted January 13, 2012 Share Posted January 13, 2012 My 88 2.5l had a slow rough idle and would stall at times, it turned out to be the EGR valve... Link to comment Share on other sites More sharing options...
rejeep Posted January 13, 2012 Author Share Posted January 13, 2012 thanks.. will be looking into this next week... going to try and troubleshoot it a bit more before dumping parts at it.. Link to comment Share on other sites More sharing options...
rejeep Posted May 31, 2016 Author Share Posted May 31, 2016 just want to follow up on a 4 year old post.. both forum and Google search brings this post up so I figured it would be good to bring it full circle my MJ is a hauler / utility truck so I don't log many hours in it any more.. its actually driven more by friends wanting to borrow a pickup than anything else.. pulled it into the garage this past weekend for some tune up work.. All I can say is the night before I did the service I decided to check all my manifold bolts.. they weren't loose, but they weren't tight.. so for the record.. the tightening of my manifold bolts fixed the erratic idle.. 30 min of "yoga wrenching" and my "issue" I have been putting off was miraculously solved.. moral of the story.. don't UNDEERESTIMATE THE POWER OF A VACCUM LEAK!!! Link to comment Share on other sites More sharing options...
cruiser54 Posted May 31, 2016 Share Posted May 31, 2016 just want to follow up on a 4 year old post.. both forum and Google search brings this post up so I figured it would be good to bring it full circle my MJ is a hauler / utility truck so I don't log many hours in it any more.. its actually driven more by friends wanting to borrow a pickup than anything else.. pulled it into the garage this past weekend for some tune up work.. All I can say is the night before I did the service I decided to check all my manifold bolts.. they weren't loose, but they weren't tight.. so for the record.. the tightening of my manifold bolts fixed the erratic idle.. 30 min of "yoga wrenching" and my "issue" I have been putting off was miraculously solved.. moral of the story.. don't UNDEERESTIMATE THE POWER OF A VACCUM LEAK!!! Excellent!! Thanks for posting this up. Link to comment Share on other sites More sharing options...
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