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loping RPM in neutral

nooberiffic

Well-Known Member
Joined
August 19, 2009
Messages
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City, State
Utah
Year, Model & Trim Level
93 XLT
Have a problem with loping RPM's when in neutral. Its not throwing any codes, I checked for intake leaks, cleaned the MAS, the IAC, TB, adjusted the TPS correctly (I believe, .96 volts at closed closed throttle), and reset the computer.
It only happens under a certain set of conditions.

Engine is not under load. Vehicle is moving (if I am at a complete stop or just crawling to a stop the issue goes away), and the transmission (5 speed) is in neutral. Only does this when the engine is at operating temp. If it is cold and put it in neutral and coast it kicks down once, then goes back up to cold idle rpm.

Any ideas guys?
Thanks

edit: whoops, this is happening on my '90 4.0 Ranger, not my 93 Ex.
 



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that might be a bad coolant temp sensor.
 






I would start with changing the temputure sensor, sense it only does it when it warms up maybe there connected somehow?
 






good idea. I assume you mean the coolant temp sensor?
Another thing, if I even just push the clutch in 1/4 inch, it stops. Also, The engine is new, with less than 5,000 on it.
 






yes, test the coolant temp sensor, not the one for the gauge, the one for the ECU.

When you say you push the clutch in 1/4 of the way it stops? You mean you can't push the clutch in further than 1/4 of the way?
 






no, I noticed that when I pushed in on the clutch (all the way, to put it in gear) the surging stopped. So, to see how far the clutch had to be pressed for it to stop I played around with it, and it only takes the clutch being depressed about 1/4 of an inch for it to stop.

Also, its not so much of a surge (up) in RPM, as a RPM drop. I know where the CTS is for the gauge, where is the one for the ECU?

Thanks guys
 






no, I noticed that when I pushed in on the clutch (all the way, to put it in gear) the surging stopped. So, to see how far the clutch had to be pressed for it to stop I played around with it, and it only takes the clutch being depressed about 1/4 of an inch for it to stop.

Also, its not so much of a surge (up) in RPM, as a RPM drop. I know where the CTS is for the gauge, where is the one for the ECU?

Thanks guys

The one for the gauge is called a sending unit, and has a single, small wire going to it. The wire "plugs" into a threaded 'stud" sticking up from the sending unit. The one for the ECU is called a sensor, and it has 2 wires, and "normal" underhood "locking" type plug. It is located in the intake manifold, just above the thermostat housing, on the drivers side. It's basically right across from the sending unit. A 1" combination wrench should be the right tool to get it out. You'll want to remove the air intake tube first though (from the air cleaner to the throttle body). Also, remember that there is coolant under that sensor. If the system is full, you'll have a pretty good size spill. I would drain a little bit out, through the radiator drain plug first. Also remember to use a little teflon tape on the threads.

All that said, I wouldn't just replace it, without testing. If you have access to a scanner (not just a code reader, you need live sensor data), you can do it yourself. Let it sit overnight ("cold soak" is the term). Before you start it up, hook up the scanner, and look at the readings for both the coolant temp sensor (ECT), as well as the inlet air temp sensor (IAT)(looks like the coolant sensor, but it is screwed into the upper intake manifold, on the driver's side) If either one of these reads much different then ambient air temp, then replace it. Also, start up the truck, and monitor both of these sensor readings as the truck warms up. Keep a close eye on them, and make sure that they increase smoothly and steadily as the truck warms up.
Also, I still wouldn't rule out the IAC, just because you cleaned it. Fords are notorious for IAC valves causing idle issues. Next time it is in the middle of one of these idle drop issues, unplug the IAC w/ the engine running. If the idle changes and smooths out, then there is your problem.

You could also have a bad MAFS (MAS), despite your cleaning. Just because a sensor isn't throwing a code, doesn't mean it's working right. Especially on OBD1, unless a sensor gets WAY out of range, the computer will just see that the sensor is still sending a signal, and assume that the data is correct; when it fact, it may be very wrong. For example, let's say your ECT sensor reads 195deg when you first start it up and the engine temp is really only 50deg. Well, as far as the computer is concerned, 195deg IS a valid reading that occurs during normal engine operations. The reading is within the normal range it would expect to see from the sensor, so it doesn't set a code. It accepts the 195deg as the engine temp, and acts accordingly. For one, it would command less fuel, when in fact the engine is still cold, and needs cold start fuel enrichment to run properly. Now, on the other hand, if the sensor was sending a reading of 295deg, then the computer would likely set a code, because it "knows" this is outside the normal range.
If you don't have access to a scanner, I think this is a case for taking it to a mechanic w/ the proper equipment to diagnose it.
 






Not to hi jack the thread but my 93 dose this same thing when cold. The RPM's go from low normal idle when warm to a high idle back and forth every few seconds. Once warmed up it stops and give it any gas it stops or put it in gear (auto tranny) it stops. the ECT sensor is new and all voltages check out okay this just started to happen in the last week before that ran as normal as could be. This is the only thing its doing out side of normal right now.
 






Thanks guys, ill try to check all of the above this afternoon and tomorrow. I dont know if I will be able to check while it is doing it, as it only does it (while in neutral) down to about 5-6 MPH.

Thanks guys.
 






Okay, im checking out the stuff now. The CTS (for the gauge) is new, the one for the computer I pulled of the 2.9 that used to be in the truck (they had the same part number). I dont have access to any testing equipment, and cannot afford the mechanic. The part is 7 bucks, so I may replace it. I ran into something interesting, that may, or may not have anything to do with this

sometime during the swap the oil pressure gauge stopped working. I know I have pressure, because I would have seized up on the side of the road it I didn't. I swapped out the sender, and it still wouldnt work. They also said they checked it, and it was fine. Well, while I was puttering around today, I noticed that there is a second cap type connector (like the one that goes on top of the pressure sender and the engine cts) that is on the same wire loom part as the OPS, but isnt hooked up to anything. Any ideas what it's for?
 






Okay. I cleaned my MAF again, cleared the computer, and let the engine warm up. Drove it around, still having the RPM drop. Unplugged the AIC tried it. Did not do it.
So, if I am correct, its the AIC. But the bugger is brand new. Is it possible when I took it apart to clean it, i put the motor back on upside down, and somehow reversed the polarity, so the motor wouldnt work? Or am I looking for a replacement?
 






My truck did the same thing when i had my trans rebuilt....i had an automatic tho. It eventualy quit...i still have the issue on a cold morning start. My problem is that i have a small crack in my uppen intake plenum.
 






My truck did the same thing when i had my trans rebuilt....i had an automatic tho. It eventualy quit...i still have the issue on a cold morning start. My problem is that i have a small crack in my uppen intake plenum.

That’s interesting as that is my set up and also the only time mine dose it. How did you find that small crack?
 






Nope, I dont think it was the IAC. Threw a spare one I had laying around and it did the same thing. Not sure why it wasnt doing it when I ran it disconnected though... Back to the CTS I guess!
 






That’s interesting as that is my set up and also the only time mine dose it. How did you find that small crack?

I found it when i was puttting on all new upper gaskets when i first got it. the crack was right down near the bolt holes. Someone had over tightend them an caused the crack. I tried to seal it up with jb weld but it eventualy crackd off. I have another engine i pickd up from a buddy...im going to be redoing the gaskets in the near future again and will be putting the new upper intake on. If it solves the problem i will for sure let you know.
 






Guys, I have some news. When I tried to pull KOEO codes before, it wouldnt run through the self test. Turns out that it was because of a bad neutral switch. If I keep the clutch in, it does work, and it spat out "21" code. Which is CTS out of range. So, nice call guys on that. I replaced it with a spare I had, and now I get "11" (system checks okay). BUT, it still does the RMP drop. Could it be the bad neutral switch?
 






I was having bad idle issues at cold start ups and driving alitle while sitting it would not idle ran fine otherwise. I ran out of ideas after checking everything I read about on here. One evening after realizing when I unplugged maf it seemed to run better but I still didn't think that was it. I took off the black plenum completely and upon putting it back on I turned it over and noticed 3 rather large slits where it hooks back onto maf sensor. They were underneath where I had never checked before. I taped them up and it runs like a charm again. Not sure how the tears got there but appear to be from rubbing against something.
 






Okay, after I replaced the CTS and it still did it, I unplugged the ECU for an hour, and now it doesnt do it continuously, just once. small kick down about 150 RMP, then back up and stays level.

What do you guys think?
 



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