stoutg

My dad and I just bought a 1996 Cherokee. We test drove it in the city and on the highway. The previous owner said there were no problems that he was aware of.
Driving on the highway from my house to my dad's, he used the cruise. While on cruise and relatively flat ground it would rev up from time to time and it sped up. After turning off the cruise with the button on the steering wheel and not touching the gas, it kept itself at about 65mph. My dad slowed down to about 20 to go through a round about then let off the brake, without touching the gas it sped up to 55. When he got to his house he pushed the brake and turned into his gravel driveway. The brakes stopped the front tires, the RPMs reved, and the back tires turned and skid the Cherokee until it ran into a dirt pile and my dad turned the engine off. After waiting a bit, we turned it back on and it immediately reved up high and we turned it off. We disconnected and reconnected the battery and then turned it on and everything was fine.
I connected a cheap Bluetooth scan tool and manually rotated the throttle. The TPS showed going smoothly from 5% to 75%. We went for a drive with the scan tool connected and the RPMS stayed around 1750 to 1800. The TPS would fluctuate slightly and seemed to match the pedal inputs. We drove for a while and had no problems. We turned on the cruise and it appeared to do what it was supposed to. When we turned off the cruise the Cherokee would rev and the TPS stayed constant, no fluctuation, at 24%. When we hit the brake it would rev to around 2200 rpm and when we let off the brake it would speed up. We slowed down and put it in neutral to stop and the rpms went immediately to 3500 before we shut it off.
It appears that the TPS is bad and so the computer thinks the throttle isn't moving so it sends the signal to increase the throttle causing the engine to rev, just a guess. But why does this only happen after using the cruise.
Any help will be appreciated as we are scratching our heads. We are planning on looking at the throttle body tomorrow.

freegdr

Sounds like vacuum leak or brake switch issue for cruise it on pedal next to brake light switch.

stoutg

I hooked up a vacuum gauge to the intake manifold after the engine was warmed up and we got a consistent 22 in Hg. What I've read says normal vacuum is 15 to 20 so our engine vacuum is high and a vacuum leak would drop it down to 5 to 7.

I am only seeing one switch on the break pedal and I am assuming it is the brake light switch. I will take another look.

Kung Fuhrer

There should be a fairly useless switch on the cruise handle meant to speed up while in cruise (but there's no way to keep this adjustment so why not just use the gas pedal? Thanks Obama!). Maybe you've got some crossing there? This adjust your throttle up slowly so if your engine speed is JUMPING I'd doubt it's that, really just brainstorming here with you.

TPS itself isn't too hard to pull and investigate, I'd start there.

stoutg

The speed steadily increases to a point then stops acelerating without us touching the gas. We drove it this morning and it sped up to about 30 on its own, if we sped up then let off the gas it would slow down. We never used cruise this time.

I have checked the TPS through an OBDII scanner and it appears to be working correctly. With the engine reving at about 3,000 RPMS the TPS showed the throttle was closed (22%) and looking at the throttle I could see it was closed. And pushing the gas caused the engine to Rev higher, the TPS to show the throttle opening, and I could see the throttle opening.
I haven't used a multimeter to check it though, I don't know if that would give me any more information.

Thanks for helping brainstorm, the more brains, the better.

Kung Fuhrer

At what speed does it stop accelerating? Cruise only goes up to 65-70 before it'll stop... at least that's as high as I've ever been able to set mine.

stoutg

It seems to vary each time. The first time it happened itspead up to 65, today it only went up to about 30 before it stopped acceleration.

Turbo X_J

stoutg

I saw that video, unfortunately I don't smoke. You can also find vacuum leaks with soapy water, propane, and carb cleaner. Some of those come with a fire risk.

I've measured the vacuum on the engine and there doesn't appear to be a leak, I have 22 in Hg. If there was a vacuum leak I would have seen 5 to 7 in Hg.

Turbo X_J

Vacuum leak won't result in a lowered reading on a gauge. Try it with your gauge hooked up, pull off a vacuum line the gauge won't drop unless the leak is massive enough to stall the engine.

freegdr

X2 vacuum leak will raise idle with out changing tps reading. Ecm thinks a vacuum leak is throttle opening thus adds fuel. Try plugging vacuum supply line to cruise control if issue stops problem been narrowed down at least.

freegdr

Info here...http://www.morris4x4center.com/knowledge-base/index.htm

jpmarine6786

I have used mine at 75-80 on I-70 through Kansas.

stoutg

You are correct, I had the vacuum gauge hooked up and the Jeep running and pulled a vacuum hose off and didn't see any change.

We tried blowing smoke into the engine and we didn't see any smoke come out anywhere that we saw.

stoutg

The vacuum hose on the intake manifold nearest the firewall goes to the cruise control, climate control, and 4 wheel drive according to the Chilton manual. The reving only happens after using the cruise control so I am making an assumption that a vacuum leak would be somewhere in the cruise control part of the vacuum system. Going on that assumption it would seem that I could symulate a vacuum leak there by disconnecting the vacuum hose from the intake manifold that connects to it. I expected the engine to rev when I did that just like it does when using the cruise but it didn't. There was no change in the engine.

Is my assumption or logic incorrect? Is the something else I should try?

We did the smoke test and didn't see any smoke. Cleaned the throttle body and replaced the IAC and still have the same problem.