1988 cherokee auto 4.0 RPM's wont drop.

[Jewel]

Hello,

I have a 1988 Cherokee Laredo automatic 4x4 6cyl.

Lately, I have been having acceleration problems. It began with the surging problems that so many other Jeeps have complained about . . . as I have replaced (brand new) TPS and the Idle control sensor, cleaned throttle body, check linkage etc.

At this point more often than not the idle will be at normal when I start my Jeep . . . it will run a short while in this mode. Then the idle will rev up as the engine warms up, which requires me to turn the motor off and restart or put it in 3rd and drive.

As long as I do not let my Jeep go to 1200 RPM's it will run well. If however, I go to/over 1200 RPM's it will get stuck and this starts a problem with my Jeep . . . because it will not kick down (RPM's) when necessary, such as stopping (stuck at 1200 RPM's) now it is accelerating on its own (or untill I turn off the engine).

What I have observed lately is my accelartion (pedal) when it is accelerated (increase fuel) my RPM's go down and run normally to the engine speed. On the other hand, if I do not accelerate the RPM's go very high 2000 RPM's and will not kick down untill I accelerate to just above 1200 RPM's and it feels/sounds like the stress level of my Jeep is normal again (speed is between 45/50 miles per hour). Nevertheless, as I slow down such as to . . . lower my speed the RPM's remain stuck (1200 RPM's) and the gas and the brake become an issue as they are now working against one another (not good on snow as I slide untill I stop).

I thought it might be a worn Throttle body . . . or does my Jeep have an acceleration pedal sensor that needs replaced. If I could figure out why my RPM's get stuck at 1200 RPM's and not drop down I think I could figure out my problem as it is I have snow now and it is a little scary to drive in the mountains where I live.

Thanks, Jewel

 

[MudderChuck]

I've had all the same idle problems you have described, multiple times. Various causes and fixes.
You can plug off every vacuum line to the intake with a short piece of tubing with a screw in the end (except the brake booster) and drive it. The motor doesn't need any of them to run. If nothing changes, you have eliminated, possible rubber vacuum line leaks as the cause.
The vacuum line under the battery going to the vacuum resivoir behind the passengers side front bumper, often rots and leaks. The bolt tabs for the vacuum canister break and it beats itself to death.
The vacuum line from the MAP sensor to the TB needs to be connected and in good shape. I usually remove it very carefully (it gets brittle with age) hold my finger on one end, suck, then plug the end with my tongue, to see if it holds vacuum. You have to be careful which hole on the TB you plug it into, one is a blind hole and has no vacuum.
Listen to your brake vacuum booster for a hiss and/or a change in RPM when you hit the brakes.
The intake/exhaust manifold bolts come loose (be real careful of the end studs, I usually leave them alone), vacuum leaks can change with how hot the motor is. The gasket for the TB can shrink and the TB bolts will be loose and may cause a vacuum leak.
Use an ohm meter and test the ground wire at your TPS. Check the ohms between the ground wire at the TPS connector and chassis ground for excessive resistance. Clean the connetor really well, scrub and use a quality contact or electrical cleaning spray and a small brush or whatever. Shake the injector harness up by the firewall some during the ground resistance check (there is a splice in the harness under the tape on the firewall that can cuase almost exactly what you describe, if the crimp connector is loose).

Not set and fast rules, but a 1200 RPM idle often means a small vacuum leak, either a vacuum line or the intake. A 2000 RPM idle is a large vacuum leak or the IAC is sticking open. A 3000 RPM idle usually means the TPS is water soaked or your brake booster line is disconnected and sucking vacuum.
A hunting idle (with no bucking or serious motor misses at around 2000 RPM's) is often a TPS ground issue. Remember the TPS is actually two TPS's (or two halves) one half for the transmission and the other half for the motor (the square TPS connector is for the transmission). A hunting idle sometimes an O2 sensor problem, but an O2 sensor problem often will cause a bucking or miss somewhere around 2000 RPM while driving (moderate acceleration) and an O2 senor problem will often default to lean and won't often cause high idle, but can cause the idle to hunt (up and down) some.
The IAC can get sticky even if it is new. Sometimes low voltage will cause it to function poorly. If your battery is iffy and/or the charging system isn't up to snuff, it can cause the IAC to be sluggish. Along the same lines, any connection between the battery and/or the ignition switch and the ECU, from the ECU to the IAC, that is sufficiently corroded will act just like a resistor and lower the available voltage available for good function.
I scrub out most all of my sensor connectors periodically, with one of those little tooth brushes for cleaning between your teeth and a top quality contact and electronic spray.
My 88 idles really good, it took me awhile to chase down all the gremlins.
Whatever you do don't use WD-40 or spray oil in a sensor connector, your shooting yourself in the foot if you do.

 

[Jewel]

Hello,

Thank you for your reply . . . after I wrote you I decided to work on my Jeep and took it for a test drive only to realize I did not fix the problem after stepping on the brake with acceleration I ended up on an embankment a tilt away from a deep ditch (mountain roads). Of course there was a few inches of fresh snow on the road. I managed to get out of the embankment without falling into the ditch myself and drove home with no damage.

I recieved your reply and started down the suggestions you mentioned. I found three issues I did not see with out your aid. It became dark before I could check others. As it stands I drove up and down my steep driveway without the idle getting stuck it seemed to bounce back to normal. So, thank you for your quick response . . . and I will continue checking my Jeep through your suggestions.

Peace, Jewel

 

[carnuck]

If it happens again, take apart all the connections (except the injectors) and spray electronic cleaner on them. Especially the C101 connector (above the brake booster) Bad connection corrosion causes the computer to receive the wrong signals and rev like crazy or stall out. (it's weird)