94 Explorer Running High

[Deleted member 279606]

Help Needed. My explorer has starting running high for the 3rd time in about a year. When I say running high, I mean the rpms are offset 1-2k high, both in park and drive. It takes extra brake pressure to come to a stop and when I let off the brake, it self accelerates. It's as if it wants to run itself at @35mph, depending on the incline of the road. When I put it in park, after driving long enough for it to warm up, it revs way up to ~3krpm until I turn it off. The CEL is not on.

The first time this happened it was accompanied by a whining sound when turning. I found the power steering fluid was low, and when I refilled it, it fixed both problems. It was as if the engine was trying to pull nonexistant fluid constantly, creating an extra load on the engine. Problem solved, or so I thought...

About 9months went by and it happened again. All fluid levels were good. I checked and cleaned the IAC and throttle valves (it was remarkably clean to start), I checked the TPS and it functioned nominally. There was nothing wrong with the throttle cable. Eventually I cleaned the MAF, and after a few drive cycles it ran normal again.

That was about 2months ago, and now it is happening yet again. I don't hear any vacuum line hissing sounds, but it could be that I just cant hear it over the roar of the engine.

Not sure if it's related, but I'll also add that it takes several turns for the engine to start up, more when cold, even after I replaced the battery.

Thoughts? What else can I try? Thanks!

 

[Gadget X]

I would definitely start by capping vacuum lines one at a time to see if any of them cause the issue. Check the one that goes to the fuel pressure regulator on the passenger side of the engine, near the front of the fuel rail. It's a little hard to see, but I had an issue with similar problems a while back, and I traced it to there. Good luck with it!

 

[the_don]

also, pull the intake hose and make sure the throttle plate is closing all the way.

 

[Deleted member 279606]

Wow, thanks for the fast replies.

the_don - I already checked the throttle plate and it does close.

MayJan - what do I look for when capping vacuum lines?

 

[natenkiki2004]

also, pull the intake hose and make sure the throttle plate is closing all the way.

x2 on this. Better yet, have someone in the driver's seat floor the gas then let off completely while checking the throttle plate. It could just be sticky from a cable or the return spring.

 

[Deleted member 279606]

natenkiki2004 - When I was inspecting the throttle plate, I manipulated the throttle cable by hand and the plate was very responsive.

 

[natenkiki2004]

Well when you think about this situation for a bit, it has to be something really severe. These computers are pretty dumb and can only adjust fuel a limited amount. If you're really at 1,500 or 2,000 RPM at a stop light, something's really wrong. Like a big hole in your intake or the MAF is telling the engine that a ton of air is always flowing in. Leaking injectors or a small vacuum leak wouldn't do this. I had several vacuum leaks on my Ex when I got it and aside from hissing it ran fine. There's something seriously wrong with the major intake components like the TPS or MAF or the throttle/IAC being stuck open.

If the throttle blade and IAC are fine, test the TPS. Not just for smooth graduating resistance but also for the reference voltage and the output voltage. If that's fine, check & clean the MAF. I'm not sure what electrical tests you can do but there's gotta be something written up somewhere.

 

[Deleted member 279606]

A big hole in the intake... like this one?

I'm not sure how I missed this before, but I took it off to re-inspect the throttle plate and noticed it this time. Clearly it needs replaced, but do you think it will be the fix I need?

 

[Deleted member 279606]

So apparently I can't post a picture, but suffice to say, the underside of the flexible ends of the air intake tube were very deteriorated with major holes.

Given that this is downstream of the MAF, I would imagine that this allows more air to intake than the MAF is calculating, causing the engine to run lean. But would that in-turn cause it to rev high like it is?

 

[natenkiki2004]

Eventually that extra air not flowing through the MAF will be ignited, burned and sent out the tail pipe where the O2 sensor sits. It will see all the extra unburned air and tell the computer to dump more fuel in to compensate.

This is bad but it doesn't identify the high idle, your throttle plate, TPS and IAC should still regulate the engine speed.

If I were you, I'd wrap the intake tube in tape to cover up the holes, disconnect the battery for a while, reconnect it and take it for a drive, see what happens.

 

[Deleted member 279606]

So I taped up the duct as best I could, far from a perfect seal, but still better, and disconnected the battery for a while. When driving it this morning, the offset was less extreme, but still certainly running high.

I've ordered a new IAC and some replacement ducting. I'll let you know if that does the trick.

 

[the_don]

I know its kind of tough to get to but i would pull the vacuum line to the fuel pressure regulator and smell the line for fuel. these things are known for 15+ year plastics breaking down and developing vacuum leaks, add in a ruptured FPR diaphragm and you have a runaway idle.

 

[natenkiki2004]

I know its kind of tough to get to but i would pull the vacuum line to the fuel pressure regulator and smell the line for fuel. these things are known for 15+ year plastics breaking down and developing vacuum leaks, add in a ruptured FPR diaphragm and you have a runaway idle.

Actually, not really. Too high fuel pressure means that there should be more fuel sprayed by the injector which would cause fouling due to running rich. The computer would never know either and continue doing so. MPG would go downhill and plugs would foul quickly, it wouldn't cause the engine to rev up. Remember, the O2 sensor only detects unburned oxygen, not unburned fuel.

 

[the_don]

im thinking if the fuel is leaking past the diaphragm in to the vacuum system, it would only happen when the vacuum is at its highest, or at idle. if one of the vacuum lines(or the intake pipe) had a leak it would be when vacuum is at it highest. IF this is happening the ecm would "think" the tps is out of alignment, and being the great learners that is the EEC-IV this could have happened slowly. with just a vacuum leak it would not want to rev at 2000 rpm without extra fuel, and the only places the fuel system comes close to the combustion chambers are the injectors, the FPR, and the evap system.

 

[Deleted member 279606]

FIXED!

Replacing the IAC valve did the trick. I was pleasantly surprised it was just the IAC, from what I've found online, bad IACs were responsible for stalling or rough idle, not running high constantly like I experienced, but it worked nonetheless. All for ~$35 and 10min of "labor".

Thanks everyone for your responses, hopefully this is the end of the line for this problem...