1998 Explorer 4L OHV High RPM Surge

[tinkering]

I thought I had it all solved but my intermittent High RPM surging has reared it's ugly head again.
My related threads are here:
http://www.explorerforum.com/forums/...d.php?t=411696
http://www.explorerforum.com/forums/....php?p=3434910

OK. After getting gas, we started the engine up again, it surged up to 3000 RPM and stayed there. We shut it off and the same thing happened on the second start. The third start it was OK.
We didn't touch the gas pedal at all during all this.

On the way home we could feel some intermittent surging while on the highway.

For safety sake it is parked at home until this gets settled.

There is no engine light on.
Might there be codes found with a scan even though there is no engine light on?

I need to get aquainted again with all the sensors and solenoids etc.. Is there an idle speed control solenoid that could be malfunctioning; could the main computer be insane?

Any ideers?

[shorod]

There could be pending diagnostic codes that are not "regular" enough to trigger the Check Engine Light. You should check for any pending codes as well as active codes.

Your 1998 will have an Idle Air Control (IAC) servo mounted at the throttle body. It could cause these issues, but generally if the idle is fluctuating, there's a good chance the IAC is working and partially enabling the fluctuation rather than allowing the engine to stall. It's more likely that you have a vacuum leak somewhere. Be sure to check all the obvious places plus the not so obvious places like the PCV elbow, the bellows/folds of the intake air tubing, the clamps for the intake air tubing, and the intake manifold gaskets.

-Rod

[tinkering]

Thanks for getting back Rod.
I will wait for a good weather day to do a scan and check all the vacuum lines etc..
I always try to post my results too

[tinkering]

Originally Posted by shorod

There could be pending diagnostic codes that are not "regular" enough to trigger the Check Engine Light. You should check for any pending codes as well as active codes.

Your 1998 will have an Idle Air Control (IAC) servo mounted at the throttle body. It could cause these issues, but generally if the idle is fluctuating, there's a good chance the IAC is working and partially enabling the fluctuation rather than allowing the engine to stall. It's more likely that you have a vacuum leak somewhere. Be sure to check all the obvious places plus the not so obvious places like the PCV elbow, the bellows/folds of the intake air tubing, the clamps for the intake air tubing, and the intake manifold gaskets.

-Rod

I ran the code scan. There were No Codes found.

The IAC is new last fall. We tried 2 or 3 of them from the wreckers; they all made that same squawking noise. A couple days before this recent RPM problem happened the new IAC was squawking a bit while driving on the ragged mountain road we use. That's the first time it has made any noise since installing it last fall.

I checked all the vacuum lines I could find (maybe there are a few hiding).

Do you mean the big black pipe from the air filter? I have a K&N after market filter setup on there. Everything appears tight.

I didn't check the PCV valve rubber; I will do that.

I just replaced the upper and lower intake manifold gaskets last winter (anti freeze leak). I suppose that doesn't guarantee there isn't a small leak there. Could I use a vacuum gauge somehow to check for manifold leaks?

I did find that the nut that holds the main battery wire on the back of the alternator hadn't been run on after I did the manifold gaskets. Do you think that connector flopping and sparking on it's terminal might have caused some funny stuff with the computer; that it was sending a bogus signal for the idle speed, or something like that?

I drove the vehicle for four hours and of course it has not displayed that RPM problem again

[shorod]

Originally Posted by tinkering

Do you mean the big black pipe from the air filter? I have a K&N after market filter setup on there. Everything appears tight.

[snip]

I did find that the nut that holds the main battery wire on the back of the alternator hadn't been run on after I did the manifold gaskets. Do you think that connector flopping and sparking on it's terminal might have caused some funny stuff with the computer; that it was sending a bogus signal for the idle speed, or something like that?

I drove the vehicle for four hours and of course it has not displayed that RPM problem again

A couple of interesting tidbits of additional in formation there. Yes, I was referring to the big tube from the stock air filter housing to the throttle body. The fact that you have a K&N makes me wonder if by chance you cleaned and re-oiled it recently. It is REALLY easy to over-oil reusable filters, and when that happens (and sometimes even with the OEM oiling), the oil coats the MAF sensor and can cause driveability concerns ranging from just reduced fuel economy to rough idle/stumbling and lack of power. I'd suggest you remove the MAF sensor and carefully clean it. If it's not too bad the CRC MAF Sensor Cleaner aerosol will work. If it's baked on you may need to get very gentle with a solvent-soaked cotton swab.

As for the loose cable on the alternator, that's certainly a possibility as well.

-Rod

[tinkering]

Originally Posted by shorod

A couple of interesting tidbits of additional in formation there. Yes, I was referring to the big tube from the stock air filter housing to the throttle body. The fact that you have a K&N makes me wonder if by chance you cleaned and re-oiled it recently. It is REALLY easy to over-oil reusable filters, and when that happens (and sometimes even with the OEM oiling), the oil coats the MAF sensor and can cause driveability concerns ranging from just reduced fuel economy to rough idle/stumbling and lack of power. I'd suggest you remove the MAF sensor and carefully clean it. If it's not too bad the CRC MAF Sensor Cleaner aerosol will work. If it's baked on you may need to get very gentle with a solvent-soaked cotton swab.

As for the loose cable on the alternator, that's certainly a possibility as well.

-Rod

Yes, thanks. This past winter I did clean and oil the K+N element.
I suppose the MAF could cause an idle speed surge too? Regardless, it should be cleaned. I imagine that MAF has never been looked at.
Does carb cleaner or brakeclean work?

I am glad you think the loose cable is a possibility, seeing that is all that I actually physically found while looking for a vacuum leak.

Thanks

[shorod]

Carb cleaner might be a bit aggressive for cleaning the MAF, but brake cleaner likely would be fine. I remember when it used to be called "Brake and Electronic Parts Cleaner."

-Rod