Manual tranny - sudden jerk when in gear & no gas

[ParkerRocker]

1992 Explorer Sport

I don't know if this is normal, but if I didn't know better, I would say that it feels like the IAC valve is suddenly closing.

When I am in any gear, if I let off the gas, after a brief amount of time (usually about 15-30 seconds) the vehicle will jerk suddenly and start to slow. I understand that it should slow in this situation because of transmission drag, but why the sudden jerk? I'll try driving around with the IAC unplugged sometime and see if it does it. Any thoughts?

Also, if I take it out of gear and coast in neutral my RPMs hover around 2000. I cannot find any vacuum leak, voltage on TPS checks out, and I just cleaned the throttle body with an approved cleaner.

FYI I just replaced the IAC valve/sensor, O2 sensor and reset the PCM.

 

[arco777]

Doesn't sound normal to me! My 93 5-speed never acted like that.

I would definitely be looking at the IAC for that symptom. Considering that you just replaced it, and assuming it was acting that same way before replacement, I'd look for a fault in the wiring for the IAC.

Also, check that your throttle body is actually closing all the way when you let off the gas. If the throttle cable is hanging up, it might be letting your throttle butterfly hang open temporarily.

When you're out of gear at any speed, engine speed should be close to idle.

 

[ParkerRocker]

So... I just replaced the IAC Valve, the old gasket was just caked to the old valve, and I didn't have a new one. Actually I totally spaced it, I have been looking for a vacuum leak to explain the issue. You think no gasket could cause enough of a vacuum leak to cause that issue?

 

[arco777]

Vac leaks can cause issues similar to yours; If the IAC was allowing more air in than it was supposed to, the extra air could increase idle speed and lean out the mixture.