High Revs

[1ylwjeep]

2000 Jeep Cherokee, 4WD, Automatic 4.0

Immediately upon starting with the transmission in P, the engine revs to 5,000 RPMs, so I immediately turn it off.

If immediately upon starting with the transmission in P, the engine revs to 5,000 RPMs , but if I move the transmission to D, it comes down to 1,700 RPMs and stays there. Still too high for D and not moving, but at least the car does not sound like it’s going to blow up.

There are no codes displayed and I cannot drive it on the road for sake of it going out of control.

Any thoughts as to the problem?

I took throttle body off and cleaned it well. No change.
I replaced throttle position sensor and idle air control motor. No change.

Could it be the vehicle speed sensor?

[john505]

If you cleaned and replaced the IAC and TPS, then check for a major vacuum leak and make sure your Throttle plate is closing all the way.

[1ylwjeep]

I'm really baffled now.

It all started when I replaced a freeze out plug. I took intake and exhaust manifolds off and put back on, as well as hooked up all sensors correctly. I've triple checked. Then I replaced TPS and IAC. So after no change in the high revs, I decided to eliminate all sensors as a possible problem by unplugging them all- the coolant temperature sensor, IAT sensor, MAP sensor, both O2 sensors from the down pipes, TPS, and IAC motor. Blocked off the brake booster tube. Then I took the air filter connection off the throttle body and capped the Throttle body with cap from a paint spray can. Fits perfect. Then I blockd the CCV module and fresh air inlet tube from the valve cover to the air filter and valve over to the intake manifold.

Even after this, it still cranks over fine and runs in D at 1900 RPMs with the brake and emergency brake on, and at 4,000 RPMs in Park and Neutral. It has a mind of its own.

I agree it should chock itself out, except if there is a vacuum leak, but at this stage, it would have to be a big vacuum leak, like what was suggested.

All bolts on intake and exhaust manifolds are tight and still no codes. In Drive, engine runs smooth but obviously higher than normal as I would characterize normal as 500-700 RPMs.

Any thoughts on where the engine is getting air from (like a broken head gasket) or could it be something else (like a CPU that is defaulting to 5,000 RPMs in Park and Neutral)?

[1ylwjeep]

Thanks for the suggestions.

I ripped it apart again and put back together. Runs like a charm! Big time operator error.

[HamiltonLJ]

Glad to hear you got it straightened out!