Low Idle Problem

[Crazy Jake]

Hello, I've been reading and searching this forum for a while, but I can not find anything that seems to fit my exact problem.

After idling for about 5 minutes, or just about 20 seconds when it is cold, my 99 Explorer Sport shakes bad, and the RPMs drop. However, after being in the low RPMs for a while, the truck will rev itself, then drop back to the low RPMs again and begin to shake.

Could this be the spark plugs? The oxygen sensors?

Any help is appreciated! Thank you!

 

[koda2000]

does your engine have an IAC (idle air control valve)? if so it sounds like it may need to be cleaned or replaced. try searching the forum for "IAC" threads. While you're at it, clean your MAF sensor.

 

[Crazy Jake]

I'm pretty sure I do, the car is 100% stock to my knowledge, with the exception of an aftermarket alternator.

However, after reading, it says that a faulty IACV usually causes a loud hum or whine, and I don't seem to be experiencing that. Is this true?

 

[koda2000]

there's an electical component and a valve component to the IAC. i don't know that a dirty valve (and they can become very dirty) would make a noise. mine's (a 2001 4.0 SOHC) and the IAC is located under the plastic engine cover, about the middle of the intake manifold, has 2 wires connected to it and is held down by 2 small bolts. once you have it off, look at the bottom of it. there's a needle valve which controls the amount of air introduced at idle according to a signal from the PCM. If the needle is not clean, you can clean it with some parts cleaner and an old toothbrush. if you're also going to clean your MAF sensor, you need MAF cleaner. don't use parts cleaner on the MAF.

 

[gonenuke]

You might also want to change the IAC gasket its rubber and crushes over the years.

 

[LoadedLimited]

CrazyJake,

I am having the EXACT same problem as you, except this problem started for me right after changing my spark plugs and wires. My plugs were so worn out when I changed them that they were almost double the recommended gap! The car starts up then drops to about 500 rpms as it passes through 1000 rpm's it idles very roughly and the whole engine shakes. it gets to 500 RPM's and settles down. Every time I go through 1000 rpms the engine shakes badly. I am wondering if lots of miles with bad plugs has over-taxed my ignition system. Everything was running fine before I replaced the plugs and wires and uncovered/created a host of new idle and shaking problems.

 

[steadyhand]

You can clean the IAC. If that doesn't work, I would try replacing the ECT sensor before any others. If the ECM does not know what the engine temp is due to the ECT being bad it could cause the vehicle to run rough when cold or even die completely.

The ECM uses the coolant temp sensor to determine how much fuel/spark the engine needs. As the engine warms up, the ECM gets the signal from the ECT and gives the cylinders less fuel and your RPM's will normally lower to a more normal idle speed.

I have had this problem in the past and replacing the ECT took care of my idle problem when cold.

 

[LoadedLimited]

Fixed

Unbelievable. Cleaning the IAC cured my violent shaking problem at low/very low idle speeds. I thought for sure I had a spark plug or wiring issue, but turns out my IAC was caked with carbon and was not sliding freely.

It was super simple to take off and clean. Two bolts and that's it. Scrubbed it with some Seafoam spray, which took off most of the carbon.

To remove the last bit of carbon I tried using some electrical parts cleaner in an ultrasonic bath. That didn't seem to work very well. The carbon didnt just dissolve off like I was hoping it would. Maybe Seafoam in an ultrasonic would have been better. Anyhow, it is clean enough and now working great. I blew the IAC out with some compressed air then lightly lubed with some StrikeHold.

I would recommend someone give this quick cleaning a try if they have issues with a rough idle.

Thanks, for all the input.

 

[mp223]

I'm also having a similar problem. Engine idles right at 500 rpm, but is very steady, not like a vacuum leak at all. No CEL, I've already cleaned MAF & throttle body and replaced the IAC, what are the most likely issues? This is a 4.0l SOHC with 150k on the clock.

 

[steadyhand]

I'm also having a similar problem. Engine idles right at 500 rpm, but is very steady, not like a vacuum leak at all. No CEL, I've already cleaned MAF & throttle body and replaced the IAC, what are the most likely issues? This is a 4.0l SOHC with 150k on the clock.

If the truck is idling smooth then what is the problem? My 99 Mountaineer 5.0 idles around 500 rpm when it is warmed up. There is nothing wrong with it.

My 91 GT 5.0 idles at 600 rpm. Both engines are running smoothly.

 

[mp223]

If the truck is idling smooth then what is the problem? My 99 Mountaineer 5.0 idles around 500 rpm when it is warmed up. There is nothing wrong with it.

My 91 GT 5.0 idles at 600 rpm. Both engines are running smoothly.

Because the alternator is not spinning fast enough at idle. When running a lot of accessories (heater, defroster, fog lights, etc.) my battery gauge will drown to the red L. The idle should be 600-700 rpm by default.

 

[Erik727]

I would think that's another issue with your alternator..I also have a 99 EB with SOHC, and with A/C and other stuff on mine idles good,but shows 500 on the tach..These tachs are not completely accurate..With my scanner hooked up ,reading live data,it reads 650 RPM and my alt is putting out 14.0 volts...I would get your alt and battery checked.

 

[mp223]

Nah I think its a vacuum leak or gasket or something. if I tap the throttle then let off the engine it drops to ~250 rpm and starts stumbling really bad, and after a few seconds it catches itself and rpms go back to a steady 500. For example, 500 idle > tap throttle up to 900 rpm > foot off accelerator and rpms drop to 250 ish > rough idle for a few secs then returns to steady 500 rpm.