Fast Idle while driving

On my way home from work last night (3 mile trip) I noticed an idle issue that repeated again this morning. The CP is starting normally with a fast idle for about 30 seconds, then it kicks down to a "normal idle". After driving the car for about a mile or so, the idle jumps back up. With the car in drive, the car continues to pull ahead when you take your foot off the gas. When stopped at a red light, putting the car in park or neutral, the engine is racing quite fast. Tapping the accelerator in park and neutral did not settle it back down. After turning the car off and restarting, the idle will drop back to normal, but then will increase again after driving for a couple of minutes. It's a good thing I got those front brakes fixed last weekend...

 

My guess would be either throttle linkage (cable or sticky) or vaccum. Where it started all of the sudden I would lean toward a vaccum issue, check over your hoses carefully.

 

Check the PCV valve too.

 

sounds like a cracked vacuum hose contracting and expanding with temp change.

 

whatever it is...my 89 does the same thing

whe you start it cold...it revs up a bit then goes down for a split second then goes to warp speed!!!!!!!!!
then it fluxs while driving too !

....(i hate technology)..........

 

First, check for vacuum leaks, then check the throttle position sensor. Of course, run the diagnostics on the computer as well, and see what codes come up. If you have not changed them in the last 60,000 miles or so, slap in new O2 sensors. You have 2, one on each exhaust manifold. Make sure you buy the right ones from Ford, and you will need the Calibration Code for the car. There were about 7 different calibration codes for the 302, depending on where the car was sold originally, and a few other variables I've never been able to pin down.

 

I'm trying to remember back that far. Didn't they have an IAC that used to coke up and make them stick.

 

Ok, so I was doing some reading over on the Grand Marquis forum. Up under the breather is an idle control solenoid that runs strictly off the vacuum. Apparently they can get sticky and cause the rpms to race suddenly. Planning to remove/clean and see what happens.

 

so...Mercwood....try this

as Im drivin home the other nite...IM thinkin..."geez my feet are cookin with the heater on"....Im sitting at a light....I turn off the heat to the floor...and guess what????
the RPM's drop !!!!!!!!!!!
I turn it back to floor....the RPMS come back up !!......WTH????

so im sittin there screwing around with the 'heater controls'...amazed that the RPMS go up and down with the different settings !!!

It is truly a vacuum leak somehwhere thats makin mine go wacky...now i gotta find the leak

 

quite often on Fords they have vacuum dash pots in the car built into the dash that control blend doors. Quite often these vacuum hoses get over looked. Depending on year model etc. if you just pull the glove box and look around you can find several. If you have a vacuum pump (hand operated will do fine) go out under the hood and apply vacuum to hoses going through the fire wall and either have a buddy or run back and forth and switch it do different things. If one of the dash pots never moves the rod controlling the blend doors fallow that vacuum line back to the AC controls if no problem is found fallow it back out to the engine bay. On cars that are about 20 years old these vacuum hoses are more like powder than rubber and all it takes is that one last set of speed bumps.

Also- A Ford with IAC problems no way!!! I would defiantly have a look at IAC counts and diagnose the valve itself if vacuum leaks don't seem to be the culprit. Fords are known for IAC valves only to be overshadowed by DPFE sensors. Also back then it was common to have like 5,000,000 thermal ported vacuum switches for all kinds of different things. If a vacuum line on the switched side of one of those was leaking the effects would be intermittent.

~Couch

 

Ok...you guys have got me thinking now about the heater. I remember this past winter that the heater only worked on the "vent" setting. If I set it on the "floor" setting, it would not work at all. I will have to check that out.

I pulled off the idle control solenoid (IAC?) and cleaned with carb cleaner yesterday. That did not correct the problem. I took the car on an errand and the engine began racing again. When I got in the parking lot, I disconnected the vacuum hose which connects to the idle control and there was really no suction when I put my finger over the end of the hose. As soon as I shut off and restarted the car, the suction returned. I reconnected the vacuum line and it ran fine until I got back home.

 

So there is a big enough vacuum leak that the car can idle with no IAC valve circuit. I'm thinking chasing vacuum hoses is the right track to fix this problem good luck.

~Couch

 

I had a vacuum hose that came loose from off of the back of the AC/heater box.
Had to pull apart the entire dash and HVAC system to get to it.
But it gave me a chance to clean and refurbish everything.

 

Well good did it fix the problem?

~Couch

 

Mines exactly he same MW !!!!!

and tonite when i got home ....i noticed the car was revving like mad !!!!

and I swear I can hear a 'vac' leak when i hiot the brakes...so im checking the booster tomorrow....as well as the Vacuum release on the Ebrake


the real problem with mine is that it has 300,000 plus on it !!~.and when it revs that fast ....the motor sounds like a bag of marbles:banghead3: