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RPMs stay very high

FIND

Explorer Addict
Joined
December 27, 2009
Messages
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City, State
Vermillion, SD
Year, Model & Trim Level
99 XLT V8 AWD
Was driving home from the grocery store today, truck wanted to keep revving after I got up to speed. I accelerated to 45 (not sudden acceleration either) and when I took my foot off the pedal to coast down when someone was turning in front of me, I noticed the truck didn't drop RPMs. I pulled back against the pedal and the RPMs still didnt drop, shifted to neutral and the RPMs jumped to 3000. Shifted back to drive, turned off onto my street popped the hood to check throttle. TB was in the mostly closed position. I pushed the throttle body to be sure it was fully at the stop and the RPMs still woudnt drop. Prior to accelerating to speed, the truck had no issues with RPMs.

RPMs stay above 2700 idling in neutral or park. Disconnected the MAF, RPMs dropped seriously then started climbing slightly. reconnected, RPMs jumped right back up. Disconnected IACV, RPMs stumbled momentarily. I had noticed a hissing noise so I was looking around for a vacuum leak. After searching for a couple minutes or so, I heard a bursting noise and the hissing got much louder. I looked up and saw the steam pouring out the side of the radiator. Truck wasnt overheating. Radiator had a large crack in the side of it.

Any ideas what is causing the high idle? No vacuum leaks, IACV is only a couple months old. Fuel pressure regulator was just replaced. MAF function is good. TPS is good. Truck is not equipped with EGR. Recently cleaned and Rebuilt injectors that are all functioning properly. Truck did not seem to be running overly rich. No CELs.

Has anyone experienced super high idle in a situation where the coolant drops really low? Air in system near sending unit?
 



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check vac leaks. check TPS sensor, *make sure* throttle linkage isn't holding TB open..

it's probably something simple you've just got to find it.... find it before you put it in drive again, a 2700rpm neutral drop could destroy you're trans (seen it)

and fix that radiator before you end up with a blown head gasket/cracked head!!
 






check vac leaks. check TPS sensor, *make sure* throttle linkage isn't holding TB open..

it's probably something simple you've just got to find it.... find it before you put it in drive again, a 2700rpm neutral drop could destroy you're trans (seen it)

and fix that radiator before you end up with a blown head gasket/cracked head!!

Yes I know, already checked all that. I did say that all in my post. I am usually not stumped like this, and if it was something simple like that I wouldn't have asked. I ordered the radiator HOURS ago. The TB returns to the stop screw. I have been driving the truck regularly, therefore I can rule out the stop screw being adjusted to far out.

I have been a mechanic for quite a number of years actually....... And, I wouldnt drive a vehicle around with no coolant in it even if it is only in the 50s here. Nor would I normally neutral drop an engine. This was a special case and I was coasting at a relatively high speed.
 






hmmm, any codes? My first thought is that it takes fuel and air to make the idle race like that, 2700 rpm is really freaking high. Check codes, both key on engine off and key on engine on codes, see if anything shows up. Almost sounds like the computer itself started wigging out.
 






No vacuum leaks, IACV is only a couple months old. Fuel pressure regulator was just replaced. MAF function is good. TPS is good. Truck is not equipped with EGR. Recently cleaned and Rebuilt injectors that are all functioning properly. Truck did not seem to be running overly rich. No CELs.

:rolleyes:
 






lol sorry, I reread that twice and still missed the no cel comment. Check for proper voltage signal from the TPS, proper feeds to the IAC, proper signal from the MAF and Coolant temp sensors. If all your sensor readings look good, recheck for vacuum leaks. It could be a combination of problems also, extra air getting in from some where and extra fuel being added from some component. 2700 just sounds high even for a pretty major vacuum leak. I'd also check manifold vacuum with a vacuum gauge just to be sure that its where its supposed to be.

I had a vacuum port on the manifold tee that was uncapped and even with spraying carb cleaner all around it I never heard a vacuum change. I thought I had done a good check for vacuum leaks and proceeded to replace the IAC and coolant temp sensor to try and correct a high idle problem. after those 2 changes, I still had a high idle issue, so I rechecked for vacuum leaks and found the missing cap.
 






Truck hasnt been doing it this morning at all. I am going to assume that it was a short caused by leaking coolant or something to that nature that caused the very high idle. Truck hadnt gotten warm enough yesterday to cause me to think it is temperature related, and even sitting today I couldnt get the truck to repeat what is was doing yesterday--Even though yesterday it would do it no matter how many times I started it, shut it off, let it sit for a few minutes.
 






low coolant will cause the engine to rev up in an attempt to get coolant onto the ect sensor. If the ect sensor is dry, the engine will rev. If the coolant is this low, the temp sending unit is also dry, and the temp gauge will not work.

I bet the whole issue was due to low coolant level.
 






low coolant will cause the engine to rev up in an attempt to get coolant onto the ect sensor. If the ect sensor is dry, the engine will rev. If the coolant is this low, the temp sending unit is also dry, and the temp gauge will not work.

I bet the whole issue was due to low coolant level.

Thanks, that is what I was wondering about. Picking up radiator right now, replacing thermostat and radiator hoses while I am at it. Still need to test the ECT. Does anyone know what it should read?
 






Just a quick question I am having the same problem with a friends 92 explorer I have checked everything as you and and still after a week of looking I cannot find a problem,I did notice a hole in the throttle body flap is that hole supposed to be there and one more quick question how much does the idle air control valve move . I have checked the IAC,MAF and throttle position sensors all are working and have proper voltage . This all happened after I replaced an alternator !
 






From a book I have, the IAC doesn't necessarily "move", it "dithers" back and forth very rapidly to adjust the idle. It cycles at a frequency of 60 hertz (cycles per second). The frequency is high enough that it doesn't have time to seat in either the open or closed position. The computer alters the duty cycle of the pulse as needed.

For the ECT values, here's an abbreviated list:
Temp (F) Resistance (Ohms)
50 59K
104 16K
176 3800
212 2000
248 1180

Edit: sorry, the forum wont let me format the chart properly to put a space between Temps and Resistance values. A pan of water on the stove with a thermometer should get you to the test temps needed.
 






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