So the IAC is not trying to slow down the idle because you have seen the pintle is retracted ,you can see it .So turn ignition on engine OFF,look into the hole and see if the IAC is now closed all the way ,it should be until you start it,if it is closed all the way ,that shows it is in fact being commanded
to that position ,ok that brings me back to the temp sensor .You can replace the temp sensor instead of checking it ,doing any test with it .They are not
real expensive .It is right in front on the engine ,pointing forward .You did the TBI gasket ,eliminates that anyway ,and your much more familiar with that
whole setup now which is a plus .overlanders has a point also that the IAC
is just not closing all the way,but the test I suggested should tell if that is the case.
I already posted this once, but I don't see it anywhere, so I will post it again.
Thinking ahead, I already checked the Coolant Sensor. It checks out good. It is receiving 5.2 volts from the Computer and it measures 980 ohms resistance when the engine is warm to the touch. My Haynes Manual says that is about normal for that temperature. I will recheck the resistance again when the engine is hot, to see if the resistance is somewhere around 200 ohms, that is suppose to be normal.
Back to the IAC. I tried posting this problem on the GM Truck Forums and just got a response from a GM Tech. Here is what he said in response to a comment about the IAC, THE COMMENT "The gasket between the TBI & manifold is probably the bad one but the IAC can be bad too and it should be closed if you shut off the running engine and remove the TBI and look at the seat. It should be closed. If not its bad. THE GM TECH's RESPONSE, " IAC counts go down, not up, with a vacuum leak. An IAC count of 0 is an excellent indicator of a major vacuum leak. It reads 0 because the computer is attempting to slow the engine down by shutting off IAC air (0 counts = 100% closed).
Also the IAC will not be closed (assuming it's working normally) if you shut the engine off, take it apart and look. The computer retracts (opens) the IAC on shut down in preparation for the next start.
To know for sure, unplug the IAC while the vehicle is idling, then shut it off. The computer can't do anything with it disconnected and you can see what state it's in at idle... which may or may not tell you anything."
I'm going to unplug the IAC while the engine is running and then turn the engine off. Then look in the bean hole to see if the pentle is retracted. This will confirm, for certain, if the IAC is OPEN while the engine is idleing at 1000 RPMs. If it is, does that mean it's bad, for certain? And BTW, I've got the PCV vacuum port plugged during all this...if it were connected to the PCV valve, the idle speed would be even higher! Bill
I believe I have confirmed that ECM believes the idle speed is just fine and is not trying any heroic measures with the IAC to lower it down." Here's why: This morning, I was going to do the test mentioned by the GM Tech for the IAC position "engine off", is it closed or open? So, with engine on and at 1000 RPMs idle, and using a mirror and light, I looked down the air port, (bean hole) that is just above the IAC on top of the TB, to see if I could tell what position the IAC pentle was in. I could just barely see the tip of the pintle, indicating a partially open position. I still had the PCV vacuum port plugged with a golf tee, (the results of other tests), so I removed the tee and the idle speed increased considerably and then settled back at about 1200 RPMs. I re-checked the IAC pentle position and it was fully closed, purtruding noticably into the air port. The computer was trying to get the idle back down to 1000 RPMs by commanding the IAC to close. That suggests that the IAC is following the commands of the computer and is able to move back and forth, including the fully closed position. Something else is telling the computer that the idle speed is OK at 1000 RPMs, when it is NOT!! Question still is, what is doing that? Any more ideas? I've already checked the Coolant Sensor and it checks OK. Bill
Bill , what you found out from the truck site on the IAC is correct ,thats what I was getting at ,key on engine off the pintle is extended the IAC commanded closed .So the coolant sensor sounds like its working ok . the final thought I
then have is the TPS throttle position sensor being out of range .A digital volt meter can tell you .key on ,you will have a 5 volt reference at one wire ,ground on one wire ,and the last wire will be the voltage the PCM sees.
should be less than 1 volt ,like .83 or .90 ,if it is 1.5 volts at closed position
it is bad .it is on the throttle shaft on the throttle body.Easy to check without replacing it .Now the other thought that come to mind at this moment is the ignition timing could be wrong .Could be advanced.It should be checked and set with the single wire (tan with the black tracer stripe)disconnected .The connector is usually under or next to that black cover on the firewall to the left pass side ,page 1-32 fig 43.4b shows a picture of the
connector ,unplug it and then set timing to correct setting.Your IAC sounds like it is all working right from your description at this time.
See what the timing is ,it will show advanced significantly if the wire is
connected (tan black tracer).So that is the next step ,should have been the first ,but I didn't think about it right away . you said you purchased the truck with the fast idle maybe the last tuneup timing wasn't set right .
Well Scott, this will be my last post on this problem for a while (I'm sure all the readers are delighted). Did we fix the Fast Idle problem.....NO....not yet!! I just finished checking the Throttle Positon Sensor (TPS) and it checked out PERFECT!! 5 volts from the computer, .78 volts at idle position, gradually increasing to 4.5 volts at full throttle. PERFECT!! I don't have a Timing Light, so I cannot check the engine timing right now, but my Brother-in-Law has one. The best that money can buy, he says!! But, it will be a while before I can borrow it and check the timing. Thinking about it, that just could be the problem!!
I don't know about you, but I am sick of this......I have removed that air filter cover at least 663 times (smile) in the past several days! I have to do this work in my detached garage where the temperture is about 100 degrees here in Alabama, and the humidity about the same number. I do have a fan that blows hot, humid air. I usually am sopping wet when I finish checking these things.
I have learned a lot about Detroit built products, and about my 94 GMC Suburban Tow Vehicle in particular. My vehicle maintanence experiences are mostly associated with Volvos. And they are different from Detroit products. I very much appreciate your advice and the other comments by readers of this thread. Cheers for now, Bill
P. S. - Scott, what would you charge, in your shop, to do a timing check on a 94 GMC Suburban 2500 series? Maybe I will just take it to a local shop and get that done.
Ok Scott, and all the rest of guys, here we go again. I borrowed my Brother-in Law's timing light and decided, TODAY I would check the timing on the 94 GMC Suburban Tow Vehicle, to see if THAT might be the reason for the fast idle. Remember, I said NOTHING ever goes easy for me! So, this task started off with the same traits. The timing marks were so dirty I couldn't even see them... and they were a bear to clean off, mostly because I couldn't get at them. Onced they were clean, I hooked up the timing light and I was ready to go. Then, I remembered that I have to bypass the Electronic Spark Timing system (EST) before I check or set the timing. The Haynes book says the plug that disables it is on the firewall under the relay cover on the passenger side, ID by a tan and black wire. Took that cover off and looked everywhere and no plug!! Read the book again, it said on later models it's located near the distributor, except on 95 models, it's under the dash, below the glove box!. Looked all around the distributor...no plug. I thought, OK, this is typical, the chances of me finding that plug under the dash is NIL. WRONG! I looked.... and there it was in plain sight!! Hooray!! Then I checked the timing and guess what?!! It was 4 degrees before TDC, right on the button.....exactly what it's suppose to be! That's good...... and that's bad.
So, anybody have any more ideas? I guess the manifold gasket could be leaking, I haven't checked that yet, but that is an expensive fix....and if that is what it is, then I will just live with the fast idle. Bill
Bill, next time you get the urge to drive somewhere, bring your Suburban down here. I can hook it up to our life-support machines, and maybe we can figure out what is wrong with it. If you comne down this weekend, you can even help me put a new A/C unit on a friend's SOB MoHo...
__________________
Terry "I'd rather wake up in the middle of nowhere, than any city" Steve McQueen
AIR#2611
Bill, next time you get the urge to drive somewhere, bring your Suburban down here. I can hook it up to our life-support machines, and maybe we can figure out what is wrong with it. If you comne down this weekend, you can even help me put a new A/C unit on a friend's SOB MoHo...
Thanks for the offer, Terry, I'll keep that in mind. I surely hope you have an Air Conditioned work place. South Florida is a HOT, HUMID place this time of year!! Bill
Bummer I have the same problem and was looking forward to you having a fix. My problem came with the RV. Logged over 5,000 miles with the fast idle.
The part I hate most is putting it in gear, I know that has to be hard on the transmission plus stopping with a fast idle is hard on the brakes.
I still suspect a vacum leak. I did have a broken vacum line that caused the AC to blow on defrost all the time but that wasn't enough of a leak for the fast idle. The other reson I suspect a vacume leak is it takes very little to have the AC to go into the low vacume mode. It will change from dash air to defrost or AC compressor cuts off under a medium load.
TB gasket checks OK but I haven't changed it out I don't like throwing parts until I have a better handle on the cause and not the symptom.
Ain't this fun ????????????
Best of luck and please keep us posted.
I will talk to a couple techs I deal with alot ,as we all swap ideas and hard
to solve drivability problems .Well ,you do have a weird one there,but if garry
also has it ,it must be common to 454 engines .I serviced the exhaust manifold on a 90 or 91? same engine TBI normal idle .Thought about your problem while doing this customers repairs .Ive gave out the most probable ideas I could ,but one more thing to try .Plug that air hole above the IAC with
somthing ,a piece of foam rubber that you could just stick in the hole to the
extent of not allowing any air to the IAC at all .See if you still maintane any
idle ,it should not be able to run ,also make sure the throttle plates are indeed shut completely ,no air can get by them.I will see what I can find out for you this next week and reply back here or PM .
Bill, I just did a fast scan through all this stuff, and one thing none of us has mentioned (if you did, I apoligize) is the possibility the EGR valve or moounting gasket may be leaking. If you haven't checked it yet, take out your trusty can of WD40, and spray around the EGR valve and its base.
If that is good, take your WD40 and spray along the edges of the intake, where it bolts to the heads.
This next tip is not for the faint of heart, but it will help you if this is the problem.
Remove the PCV valve from the valve cover, and leave it plugged into the vacuum line.
Start the engine.
Remove the oil fill cap, and using carbureter cleaner, squirt it into the hole where the cap goes. If the idle speeds up, you have a leaking intake manifold gasket.
__________________
Terry "I'd rather wake up in the middle of nowhere, than any city" Steve McQueen
AIR#2611
Thanks for the offer, Terry, I'll keep that in mind. I surely hope you have an Air Conditioned work place. South Florida is a HOT, HUMID place this time of year!! Bill
Like Alabama isn't...
__________________
Terry "I'd rather wake up in the middle of nowhere, than any city" Steve McQueen
AIR#2611
Bill, I just did a fast scan through all this stuff, and one thing none of us has mentioned (if you did, I apoligize) is the possibility the EGR valve or moounting gasket may be leaking. If you haven't checked it yet, take out your trusty can of WD40, and spray around the EGR valve and its base.
If that is good, take your WD40 and spray along the edges of the intake, where it bolts to the heads.
This next tip is not for the faint of heart, but it will help you if this is the problem.
Remove the PCV valve from the valve cover, and leave it plugged into the vacuum line.
Start the engine.
Remove the oil fill cap, and using carbureter cleaner, squirt it into the hole where the cap goes. If the idle speeds up, you have a leaking intake manifold gasket.
Terry, that Carb cleaner is corrosive stuff!! Are you certain that it will do no damage to the engine? Even with a manifold gasket leak, how does the Carb cleaner get into the combustion chambers and cause the engine to speed up? Bill
Bill, I just did a fast scan through all this stuff, and one thing none of us has mentioned (if you did, I apoligize) is the possibility the EGR valve or moounting gasket may be leaking. If you haven't checked it yet, take out your trusty can of WD40, and spray around the EGR valve and its base.
If that is good, take your WD40 and spray along the edges of the intake, where it bolts to the heads.
This next tip is not for the faint of heart, but it will help you if this is the problem.
Remove the PCV valve from the valve cover, and leave it plugged into the vacuum line.
Start the engine.
Remove the oil fill cap, and using carbureter cleaner, squirt it into the hole where the cap goes. If the idle speeds up, you have a leaking intake manifold gasket.
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I just finished checking the Throttle Positon Sensor (TPS) and it checked out PERFECT!! 5 volts from the computer, .78 volts at idle position, gradually increasing to 4.5 volts at full throttle. PERFECT!! I don't have a Timing Light, so I cannot check the engine timing right now, but my Brother-in-Law has one. The best that money can buy, he says!! But, it will be a while before I can borrow it and check the timing. Thinking about it, that just could be the problem!!
Bill
Even with a manifold gasket leak, how does the Carb cleaner get into the combustion chambers and cause the engine to speed up? Bill
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