For the two past years that I have owned this truck (1991, 2.3 l, 5 spd.) I have had periods when the engine ran at high idle speeds. It didn't make any difference if the engine was hot or cold. I could start out on a trip with the engine warmed up and normal idle speed and then drive 40 mile at expressway speeds and when I pulled off, idle speed would be 1000 - 1200 rpm. Other times it would go to a normal idle fairly soon after a cold start. I never knew what it would do. I don't know if it was in open or closed loop. Gas mileage was 18 - 20 mpg. During those two years 4 different mechanics tried to solve the problem. There were no trouble codes. They could not find anything wrong or come up with any explanation for all of this.

Last week, I had the truck in the shop to replace the speedometer cable (the spedo needle was flopping all over the place). Since then, the engine starts at expected high idle speed and fairly quickly comes down to the normal (650 rpm) warm engine idle speed. Its been doing this consistently.

The spedo cable couldn't have caused the idle problem. So what happened? Did the mechanic (an old Ford guy) do something with the electrical connection for the VSS (speed sensor at the tail of the transmission)? I finally talked to him today. "Oh," he says, "I found a small vacuum leak and replace a section of tubing." "We didn't even charge you for it."

The point? When you have a peculiar problem, keep after it. There is a solution.

 

Bump. Any thoughts or reactions?

 

Where was the vacuum leak? My daughters 93 Tempo has been doing the very same thing for the past 5 months. In desperation, I finally had to cut the power wire to the IAC and run it through a switch. When the idle comes up, she switches the IAC off. (I had to turn up the manual idle screw so that it would run at stop signs) She turns the switch on for cold starts. I have done everything that I could think of to correct this problem. I installed a new IAC, (I did this twice), new heat sensor, cleaned the MAF, cleaned the throttle body, replaced the vacuum purge valve on the firewall. I checked for vacuum leaks by spraying the vacuum hoses with carb cleaner. I found no leaks but that doesn't mean that I didn't miss one.

 

I'll see the mechanic who fixed it on Monday. I'm curious to ask him and will pass on the info.

 

I saw the mechanic yesterday. When I asked him which vacuum line he repaired, he looked all over the engine and said he couldn't remember. Nuts! In the meantime, I have come to realize the high idle problem is still with me, only it is not so severe. On a restart, the engine idles normally and the minimum idle speed doesn't fluctuate like it used to do.

 

My daughters car does that too. Sometimes the idle is what I would consider a normal high idle, like the old carbureted engines used to have. Other times it roars, like the gas is half way to the floor. I was told that a bad T-stat can cause that kind of a problem. I need to install a block heater for her so I'll change the T-stat at the same time while I have the antifreeze drained. In the mean time, her IAC switch is working just fine. After the car warms up enough to run on its own, she shuts the IAC off.

 

try replacing the PCV valve. it solved my otherwise unsolvable high idle problem/annoyance.

 

The PCV valve was replaced about a year and a half ago. The idle problem existed before the change and continued afterwards. But anyway, I pulled the PCV valve out yesterday to show the guy in the auto parts store what it looked like (that was after he gave me one with two nipples on it while my Motorcaft one had only one nipple). I went ahead and bough the new valve and a new piece of tubing that goes from the valve to the throttle body. The new parts will be installed tomorrow (I hope). To day I had an errend to run this afternoon and drove the truck. It had idled down to a normal idle speed (600 rpm) by the time I reached the end of the driveway so, 97rangerxlt, you may have come up with the solution. Thanks alot.

 

Mine does this also but does settle back down to normal idle after being stopped for 5 seconds or so. No trouble codes.

 

I have to say I'm a little bit surprised... vacuum leak is about the very first thing I would think of with a symptom of high idle; intermittent or otherwise.

Glad to hear it's fixed for ya.

 

Changed the PCV valve. Checked hose from valve to throttle body. Problem still occurs. Have not been able to find any source of vacuum leak. Whatever the problem is, it seems to be temperature related. Once the engine is warmed up, a subsequent restart results in normal idle speed behavior. I'm stumped.

 

did u disconnect the battery after you changed the valve to reset the computer?

also i would look at the coolant temperature sensor. something is making the computer think the engine is still cold so it idles high all the time.

 

Thanks, 97ranger XLT, I forgot to reset the computer. I'll give it a try. The engine coolant temperature (ECT) was one of the first things I changed over 2 years ago. That didn't make any difference. The ECU has been reset several times after that as other things were done.

 

just to make sure, the engine warms up within a few minutes of driving after a cold start and the temperature gauge shows normal once the engine is warm... correct? and the heat works well?
and no codes being thrown or cel on..?

the only other thing i replaced with my high idle was the TPS, but it didn't do anything. my sole problem was the bad pcv valve creating a vacuum leak.
also my high idle was all the time not just cold.

thinking out loud here....

 

Yes. 97ranger xlt, the engine does all of those things. The idle speed is initially about 1200 rpm. If I let the engine warm up, fairly quickly the idle speed drops to 1000 rpm, as though it is times. If I let it continue to warm up (not driving the truck) the idle speed will slowly drop to about 600 rpm. However, if I start driving the truck, the idle speed stays indefinitely at 1000 rpm. Temp on the gauge comes up, the heater puts out plenty of heat, no codes or cel light coming on. If I shut the engine down and then later restart it, it will idle down to 600 rpm fairly quickly.

Puzzling isn't it.