'93 Escort LX revs up without gas

[fencefixer]

Hello there!

I noticed something strange which is getting worse and worse. I use manual transmission and when I push the cludge in and take my foot off of the gas pedal to switch gears, the engine revs up an additional 1000-1500rpm and slowly goes back down again. In the beginning (like 4-5 months ago) it would just go up like 100rpm. Does not even matter what RPM/gear I am at/in at the moment. Once I turn off the engine and restart it, the problem is gone but sometimes shows up again a couple minutes later.

Some days this doesn't even happen. I got a new timing belt installed like 5 months ago and don't remember this kind of thing happen before that. What could be the problem?

[mightymoose_22]

Just curious... does it idle normally or at a higher speed than usual?

Dunno if you have a tach or not... might be hard to tell.

[fencefixer]

Actually it varies. Normally it will idle at about 1500rpm. Sometimes it stays steady at 2000rpm. But almost _everytime_ after about a minute or two, it will go down below 1000rpm causing the engine to "stutter"; it never dies though.

Funny thing happened to me today, with the problem of the engine reving up on its own, I was driving in 4th gear and took the foot off of the gas. Car stayed at a steady speed as if I had cruise control.

[fencefixer]

Hmm... did some more research and found out that I am supposed to clean/replace the "idle air control valve"... How would one clean one of those?

[szigatti]

The iac is held on by two bolts to the top of the throttle body, looks like a samll electric motor. just unplug it and remove the bolts to take it off, be careful with the seal. Clean it by spraying it with carb/break cleaner, a tooth brush may help, then spray some wd40 to lube it and reinstall. The engine may rev awkwardly for a few seconds because of the wd40/carb/brake cleaner but should go away quickly and is nothing to worry about.

I would also check the vacuum hoses for leaks.

Hope this helps

[wtousand]

It's likely a vacuum leak. The IAC valves are pretty reliable. See thread ( http://www.automotiveforums.com/vbul...d.php?t=603624 ) for more info as to why it acts as it does and how to find the source of the problem. Good luck.

[mightymoose_22]

1500 rpm at idle? seems pretty high to me...

[hakachukai]

Got get a can of Carb cleaner for like $2, and spray it all around your vacuum connections, hoses, and intake gastket. If you think your intake is leaking somewhere, spray it around the whole intake.

If when you spray it somewhere, the engine revs up a little... you've just found a vacuum leak.

Keep in mind that there may be more than one vaccum leak. So don't stop checking on the first one you find (if you find any at all).

[wtousand]

Originally Posted by fencefixer

Hello there!

I noticed something strange which is getting worse and worse. I use manual transmission and when I push the cludge in and take my foot off of the gas pedal to switch gears, the engine revs up an additional 1000-1500rpm and slowly goes back down again. In the beginning (like 4-5 months ago) it would just go up like 100rpm. Does not even matter what RPM/gear I am at/in at the moment. Once I turn off the engine and restart it, the problem is gone but sometimes shows up again a couple minutes later......

.........Actually it varies. Normally it will idle at about 1500rpm. Sometimes it stays steady at 2000rpm. But almost _everytime_ after about a minute or two, it will go down below 1000rpm causing the engine to "stutter"; it never dies though.
Funny thing happened to me today, with the problem of the engine reving up on its own, I was driving in 4th gear and took the foot off of the gas. Car stayed at a steady speed as if I had cruise control.

What makes this particular problem hard to diagnose is the fact that the computer varies the fuel mix ratio automatically at different engine conditions, even if the leak stays the same. Sometimes spraying a leak-blocking liquid "clogging material" will speed up an engine if it is idling too lean at a low idle ...such as when the computer thinks the engine is warm ...and it has been at idle for several seconds and has gone to lean idle set. Or if the sprayed liquid is flammable, it may support the same function as extra fuel, that is to bring a lean condition closer to normal range. But if the engine is cold, it runs at automatic over-rich pre-set which will often support more air such as a leak, albeit at a higher idle such as 2000 rpm. Blocking the leak under such conditions will slow the idle down by robbing extra idle air since it was performance-friendly rich to start with. To top it all off, the computer tries to learn or re-learn driver habits and goes into gradual compensation mode as you drive away.

For those that want to experiment and learn on a working car, pull the PVC vacuum hose off the throttle plenum before a cold start. The engine may not idle for a half minute or so with such a large leak (about 1/4" diameter), but shortly will idle fast (2000 rpm) when cool. But as you drive, the engine will warm up, go to medium lean, and after a few seconds after a stop, refuse to idle fast enough to prevent stall, where the computer goes to full lean extended idle condition. A smaller leak can be simulated by pulling off a smaller (1/8" dia.) vacuum hose and there is such a thing as a leak so large that the engine just won't run at all. Someday when it does the same thing because of a hidden vacuum leak, it will all seem familiar.

Some of you may remember the dashpot on the smog carbs a few years ago ('80's). These dashpots were necessary because manufacturers were changing carburators from the normal rich idle to a lean setting. But when the throttle was chopped, the lean engine would often stall. The dashpot "eased" the throttle closed and prevented the lean caused stalling. Nowadays the computer allows a rich enough idle, when the throttle is chopped, to prevent stalling but after a few seconds, it sets the fuel mix to ultra-lean. That's when our leaky Escorts (and every other leaky FI car) stall. There simply isn't enough fuel to support even a teensie extra bit of air at ultra-lean.

With the classic symtoms you describe, the most common problem is likely to be merely a leak, but it could be something else. My opinion is that it is best to eliminate the leak theory first. Good luck.