Mmurdock

I'm working on an '89 XJ with 4.0 and 5-speed.

I've been having problems with an engine stumble as reach 2500-3500 rpm (sorry no tach so I can't be exact). Today I finally got around to adjusting the Throttle Position Sensor. It was way out of adjustment and now it's set at a perfect 17% output. This has almost completely cured the high rpm stumble. On the road test I had to really put my foot into it to cause it to stumble.

I can live with this, but at idle it still not smooth. It sets at one idle speed for about 10-seconds then speeds up about 100 rpm for about 10-seconds then goes back down about 100 rpm. This cycle just continues.

I have repaired several obviously broken vacuum lines and today I got out my Mity-Vac vacuum pump to actually test them. I've never tested vacuum lines before so I'm not sure if this is the proper tool for the job. I disconnected the small vacuum line to the bumper mounted vacuum ball and pumped up the vacuum too the ball. Once I had a little vacuum showing on the guage I stoppped pumping. The vacuum dropped quickly but not steadily. I did this with the engine off. Should it be running? Any help would be most appreciated.

freegdr

have you ever cleaned the idle air control motor? or IAC

Mmurdock

Yes, I cleaned the Idle Air Control Valve a couple weeks ago and also cleaned the Throttle Body as well.

It took quite a while to clean the TB as the carbon deposits were so thick I had to use an old tooth brush and lots of carb cleaner to scrub it clean. I did my best to get all the deposits out of the hole the IAC pintle fits in but I didn't have a brush small enough to get in there. If I can't find anything else wrong I might go get a gun barrel brush and try cleaning it again.

cruiser54

Grab a can of carb cleaner and spray around the intake manifold to head area and on the vacuum lines. Loose intake manifold bolts are common. If it were me, I would snug up the intake bolts just to be sure. Do this with teh engine cold and start in the middle and work out to each end of the manifold.

Another source of "ghost" problems on your Jeep is bad grounds, especially at the dipstick tube stud. Follow the instructions below before going in any other direction.

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 pretty 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 terminations 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 terminations. Inspect to see that none of the terminations 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.
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 termination. 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 16’ long with a 3/8" lug on each end works great and you can get one at any parts store already made up.
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.

DFlintstone

X2 on the spray. Hearing that rpm change when you hit the bad spot is like hitting pay dirt. Isn't there a T from the vac-ball line to the evap canister? Not sure there but that might explain the vac drop there....