I have a 2000 Mercury Mountaineer 4.0L. Engine cycles up and down at idle when hot. I have changed the idle bypass valve three times, which works for a while.

Question edited for clarity.

Fuses 4 and 8, but there is also a relay involved.

Fuse Box Diagram Mercury Mountaineer (1997-2001)

https://fuse-box.info > mercury > mercury-mountaineer...

Here you will find fuse box diagrams of Mercury Mountaineer 1997, 1998, 1999, 2000 and 2001, get information about the location of the fuse panels inside ...

Mercury Mountaineer (1997 - 2001) - fuse box diagram

https://www.autogenius.info > mercury-mountaineer-19...

Aug 11, 2018 - Mercury Mountaineer (1997 - 2001) - fuse box diagram. Year of production: 1997, 1998, 1999, 2000, 2001. Passenger compartment fuse panel.

Mountaineer » Fuse Diagram

https://knigaproavto.ru > Fuse Box Diagram > Mercury
Fuse Box Diagram Mercury Mountaineer; engine 4.0 L Cologne V6 and 5.0 L Windsor V8 (1996, 1997, 1998, 1999, 2000, 2001)

That's a sign of a dirty throttle body or IAC valve, depending on which engine you have.

Verify that there is no vacuum leak, and that your idle stop nut is not misadjusted.

Is the check engine light on? Mountaineers are bad about developing vacuum leaks which will lean the engine out and cause it to stumble at idle while in gear. Check for p0171 and p0174 codes.

The stall problem could be a faulty idle air control valve or dirty throttle body. The computer uses the valve to adjust base idle speed depending on the load on the engine.

dirty or bad idle air control valve i would at worst the egr valve but most likely the idle air control valve which controls the air and idle from the throttle body try taking it out and cleaning the tip as well as in the throttle body where the control valve goes in if that dosent work replace it and the check engine light does not have to be on for it to be bad.good luck to you.


10 year ford tech here.

The thermostat is beneath the inlet housing from the top radiator hose. You are better off working on a cold engine and adding a little antifreeze when you finish to replace what you will lose when you remove the parts.

There should be 2 bolts holding down the fixture and there will be a round "O" ring type gasket sealing the fixture. Observe the direction of the thermostat so that you replace it right side up instead of backwards which is possible in some cars. Then replace the gasket and lightly bolt the housing down evenly. It is a soft metal and brute force will bend or break parts. Start your car and check the water level after you have turned on the heater and the car is hot. Add antifreeze to the overflow tank.

Myself, I take a half gallon bottle of hot soapy water and wash the engine area off to get rid of the antifreeze smell and it makes it easier to detect a leak if you have one.

If you have the check engine light on, rent a scanner, read out the code(s), and pursue to repairs. Possible problems are: idle air valve, vacuum leak (hoses and loose bolts) on intake manifold, EGR valve defective or not properly controlled, PCV valve defective, sticky injectors, bad plugs/wiring, O2 sensor, ect.. Use the scan codes to replace parts--don't just start swapping parts till it works.

This car you are driving is 20 years old. Have you thought of getting a newer model? It sounds like you are having too many problems.

YOU MAY HAVE A IDLE SPEED CONTROL VALVE PROBLEM, CLEAN IT FIRST, THEN DISCONNECT THE BATTERY, FOLLOW THE PROCEDURE I HAVE ATTACHED.

Here is the most common cause of this stall and idle speed issues, it is the idle speed control air-bypass valve and throttle valve, they get full of gunk over the miles and cause idle issues (stalls) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.