Help Me Diagnose Sudden Low Idle on Zetec Focus

Drove to store, everything normal. Came out of store, idle is suddenly low enough that the engine will die if I don't keep my foot on the gas to keep the revs around 1k. At higher revs, while driving, everything behaves normally. No CEL.

I'm tempted to just reset the idle screw on the throttle body to bump up the idle, that that seems like a band-aid.

Further background that probably doesn't have any bearing: 145k mi., desperately needs a timing belt service and passenger side motor mount, plugs and wires replaced 5k mi. ago, fuel pump maybe 4k mi. ago.

Any thoughts?

alex wrote: I'm tempted to just reset the idle screw on the throttle body to bump up the idle, that that seems like a band-aid.

Yep, it's a band-aid but it works till you find the problem. It may end up giving you an idle-overspeed error if you go to far.

1. Clean Mass Air Flow (MAF) sensor. Use MAF cleaner or Electrical Contact cleaner and be very careful. Easy to break sensor wire! A dirty one will not give a CEL untill it is really dirty. Also check wires where they enter the plug on the MAF. They are known to break.

2. Throttle Position Sensor (TPS) You will need a scanner to check that it is functioning correctly. Just a rheostat and can develop bad spots. Also check the wires on this. Usually no CEL untill it is really bad.

3. Idle Air Control (IAC) valve Most likely culprit but a real PITA to get to(on intake manifold between manifold and block) . You will have to get under the car to even see it. They do get dirty and need cleaning/replacement. Usually they do give a CEL.

Thanks, Hal.

Am I looking to clean the MAF sensor itself or, its contacts?

I'm assuming I can do a basic resistance check of the TPS with a multimeter. Do you happen to have the specs at idle? (I don't have a manual, he said ashamedly.)

And the IAC... Well, I'll just hope it's not that for now.

Oh, and I'm assuming an idle over-speed error throws a CEL code. Izzat right?

Most likely the IAC. On the ZX2 they are not very hard to get too. You may have to move a few things out of the way. The ZX2 have frequent vacuum leaks, PCV elbows etc. don't know about Foci.

check the simple stuff first. Look for a vac hose that popped off.

You want to clean the sensor wire. When you look into the MAF housing tube you will see a plastic part with a hole in it sticking into the airstream. The sensor is a very fine wire stretched across that hole. It is heated and over time will build deposits. You want to clean the wire.

On the Zetec when you let off the gas the throttle plate closes but there are some bypass holes in the TB. They feed air to the IAC and the IAC controls air flow at idle. If you have the throttle plate set so it doesn't close properly the IAC is not able to pull the idle speed down and you get a CEL for idle over-speed.

I don't have any specs for the TPS. But you can do a resistance check and make sure it changes smoothly as you open and close the throttle. Don't even need to have the car running.

And as iceracer and ignorant have stated, check all hoses to make sure they are connected and don't have any cracks. There is one PCV hose toward the back of the engine that has a tendency to get soft and collapse under vacuum.

Cleaning the IAC on a Zetec Conturd solved the problem for me. Also on a Ranger, but thats a different engine.