Unsteady or surging idle

[sched44]

I'm new to the world of Ford Explorers. I'm real impressed with the overall engineering as well as the ability of the vehicle to do what it is designed for. I'm having a hard time locating factory manuals at this time so I'm a bit low on good technical info.

You folks on this forum seem knowledgeable about a lot of the common problems these trucks exibit.

I have a 95 explorer with the OHV 4 litre/automatic. I bought it a few weeks ago and had to replace the thermostadt. The gauge read low and it was functional. The thermostadt was bad and now the engine temperature reads correctly.

The engine, when fully warm surges or hunts at an idle. When sitting at a light in drive, the engine will surge between 750 and 1000 rpm, up and back down. It does this about 2 times every ten seconds.

The engine idle surged before the thermostadt replacement.

Any ideas on what would cause this unsteady idle?

Thanks in advance for any help or suggestions.

 

[sortsol]

I had the exact same problem when I first bought my Explorer. It seems like I had to change the throttle positioning sensor (cheap and easy) and then reset the computer. It hasn't done it since. I would also try cleaning the MAS.

 

[01EB5.0]

also try turning off the climate control system...the compresser will surge if it is running

 

[TPLYNCH]

See if it does it with the climate control on Off or Panel. If it does, try cleaning the Idle Air Control (IAC) It's a cylindrical part on the side of the throttle body. Search for IAC for lots of threads descibing problems, symptoms and cleaning techniques. If it doesn't do it, then it is just the IAC adjusting idle to account for the compressor turning on/off (which will happen in every position except panel and off on a '95)

 

[sched44]

Doesn't do it with climate control off. Thanks all for the posts and help. I'M going to check out the IAC.

Thanks again.

 

[01EB5.0]

OK, well let us know how it goes. i was pretty sure that the surging was from the A/C compressor. I mean if the suges arn;t very big it might even be normal...that compressor puts a load on the engine and would cuase the RPM to change. I don;t think there is an engine out there that the RPM won;t change on when the A/C kicks on...

 

[sched44]

Stood in front of the engine and watched to see if the air conditioning compressor was coming on or not. It wasn't. RPM change (surge) is from an idle, around 750 and surges up to about 1000 RPM. I said in the original post that it happens about 2 times in 10 seconds but I stand corrected. As soon as you turn the climate control one click to the right the engine surges up and down about once every 2 seconds. As soon as you turn the knob to off it stops.

 

[01EB5.0]

well, thats wierd..if the compressor isn;t coming on, but yet when the climate control is off it doesn;t surge...that just doesn;t make sense. I thnk it's still the climate control in conjunction with a bad IAC valve.

 

[sched44]

Is the climate control run by vacuum? If so, turning it on could cause a vacuum leak to the intake maniflod. Does anyone know how this works?

 

[TPLYNCH]

If it is clicking every 2 seconds with the climate control on, it means you have insufficient freon pressure. Get your A/C system checked for leaks and recharged. The compressor won't kick in if the pressure is too low, but I still believe the IAC will kick in based on the signal that is supposed to start the compressor. So the Idle kicks up, but it is more noticable because the compressor never kicks in creating the drag the IAC was compensating for....... catch my drift?

 

[01EB5.0]

Thats a great point TPLYNCH.
my explorer cycles on and off all the time and it is eveident in the idle. The compressor actually goes on and off, but i'm low on freon by about 2 pounds. my A/C still works but it doesn;t get real cold. I don;t care though I'm not paying $200 for them to evac. and recharge the system. I think the cycling is because of ht elow freon level. on my mom's 94 van the compressor goes on for longer then justa few seconds before it shuts off.

 

[sched44]

One click to the right is temperature controlled air to both the panel and floor and will not cycle the air conditioning compressor. I stood in front of the engine with it running and the compressor was not trying to cycle.

Can anyone answer the question as to whether or not a vacuum leak (causing a vacuum leak at the engine) in the control for the various doors that direct air under the dashboard could cause this problem?

When I got to work today I tried something. With the function selector control knob to "off" it did not surge. One click to the left of "off" or "outside air to panel" it did not surge. One click to the right of "off" or "panel and floor" and it will surge but while it is surging, the compressor is not trying to engage. While I'm doing this the outside temperature is around 40 degrees fahrenheit or less. At thse temps the compressor shouldn't run.

Remember that I'm a newbie to these vehicles. Thanks again for any help or information.

 

[01EB5.0]

If you get air in all of the locations I think your vaccuum system is fine. turning to the floor/panel will cause the compressor to go..every position except panel and off will run the compressor. i am not sure what your problem would be unless your A/C is low on freon.

 

[sched44]

Clutch cycling pressure switch was shot. Engine was idling up in anticipation of compressor starting, which didn't. Replaced switch ($ 21.00 with tax, 25% off list) All is fine now. Thanks for all the posts.

 

[Robb]

Congrats sched44 on fixing your problem. This switch is new to me, so please explain. Did it receive a signal that compressor was needed, when compressor really wasn't? Did it receive signal when compressor should have, but compressor didn't work?

Wouldn't it have made more sense (for Ford) to create a switch that sensed compressor on, then idle up engine? Isn't there an idle speed sensor which would take care of it anyway?

Which came first, chic.............J/k

I'm all questioned out, I'm just curious about this
Robb

 

[sched44]

Sorry took so long to reply, been much busy.

The switch is called a "clutch cycling pressure switch." On my 95 Xplorer it lives attached to the top of the liquid receiver on the right side of the engine compartment. Voltage flows through this switch on its way to the air conditioning compressor clutch. I troubleshooted it with a 1992 F-150 factory manual. It was similiar and Fords troubleshooting charts are second to none. The bad switch had 43 ohms of resistance at it. I jumped out the plug to this switch and the compressor ran fine. It can't be left this way because as the compressor runs and the refrigeration system pressure rises, it requires this switch to shut it down. The switch keeps a properly charged air conditioning system between 25 and 45 psi on/off or there-abouts on the Low pressure side of the system.

I believe the surge was the PCM or whatever ford calls it, idling up the engine rpm's in anticipation of the compressor starting. When it didn't sence the load coming on , it idled back down and then tried again, thus surging.

The first part of this reply is relative fact. The last part is relative speculation. I'm happy to have replied.

 

[Robb]

Thanks schedd44,
That was a heck of an answer. This sounds like possibly the cure to other's erratic (surging) problems.
Robb