bp71k5
3/4 ton status
Ive developed a high idle after the truck warms up (1200rpm) on my tbi setup. Should I suspect a vaccuum leak or bad IAC?
CK5
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TBI - high idle = vacuum leak?
- Thread starter bp71k5
- Start date
You can "force" the IAC motor closed and then see what the RPM's are.
First you need to...
(1) Jumper the A and B terminals of the ALDL connector
(2) Key on, engine off (DO NOT start engine while ALDL is jumpered as you can blow the ECM)
(3) Wait 30 seconds then disconnect IAC wire connector (at TBI IAC motor)
(4) Remove Jumper wire from ALDL connector
(5) Start engine and observe engine RPM
First you need to...
(1) Jumper the A and B terminals of the ALDL connector
(2) Key on, engine off (DO NOT start engine while ALDL is jumpered as you can blow the ECM)
(3) Wait 30 seconds then disconnect IAC wire connector (at TBI IAC motor)
(4) Remove Jumper wire from ALDL connector
(5) Start engine and observe engine RPM
bp71k5
3/4 ton status
I did that procedure a while back and the idle was in spec at the time. I'll try it again and check the idle speed again. I just don't want to fool with it if a vacuum leak is a more likely cause. The throttlebody base gaskets appear to be in good shape but I should probably replace them just to be sure.
In your case there are only a couple vacuum lines so i wouls start by plugging ALL of them (brake booster included) then see if the RPM's drop. If they do i would start by hooking the brake booster hose back up first then check again. If the brake booster diaphragm goes bad it will create a vacuum leak that will take you forever to find UNLESS you've had prior experience with a bad booster (go ahead, ask me how i know). 
In your case there are only a couple vacuum lines so i wouls start by plugging ALL of them (brake booster included) then see if the RPM's drop. If they do i would start by hooking the brake booster hose back up first then check again. If the brake booster diaphragm goes bad it will create a vacuum leak that will take you forever to find UNLESS you've had prior experience with a bad booster (go ahead, ask me how i know).
Could you tell me how you know? I'm still having problems with my motor and I suspect a vac leak.
Thank you,
Ed
I'm assuming you mean how i know that a booster is bad? If you have a bad booster (diaphragm leaking) that is a vacuum leak and disconnecting the vacuum hose and plugging it should make the engine run better. Sometimes you can also hear a "hiss" from the booster which is the vacuum leak.
I have never had a booster actually go bad, but I have had the donut gasket crack and fail (don't know what the name really is, but the gasket that goes between the master and booster shell)
Vacuum leaks are hard to find especially when the idle is high because it's blowing air around.
Do what you can to lower idle like driving the IAC in and disconnect. Turn off fans if you can. Open air like outside is best for this test because it's dangerous!!!!!
With engine idling use starting fluid and small bursts to anywhere you could have a vacuum leak and watch for the idle to raise.
I have heard people say to use all sorts of things but starting fluid works. I have sprayed water, wd40, carb and choke, brake clean etc on a known vacuum leak and could not see, hear a difference even on a RPM gauge. But it will show with starting fluid.
Do what you can to lower idle like driving the IAC in and disconnect. Turn off fans if you can. Open air like outside is best for this test because it's dangerous!!!!!
With engine idling use starting fluid and small bursts to anywhere you could have a vacuum leak and watch for the idle to raise.
I have heard people say to use all sorts of things but starting fluid works. I have sprayed water, wd40, carb and choke, brake clean etc on a known vacuum leak and could not see, hear a difference even on a RPM gauge. But it will show with starting fluid.
bp71k5
3/4 ton status
I haven't replaced anything yet, I can't find any obvious leaks. The problem is that the high idle is really intermittent. Maybe I can try waiting until it happens and then start plugging hoses to narrow it down.
No starter fluid for me. Let's just say I've had bad luck with that method in the past.
No starter fluid for me. Let's just say I've had bad luck with that method in the past.
Yeah starting fluid can be exciting!
Next best thing I found is a propane soldering torch, unlit, around all possible vacuum leak spots.
I assume you have checked the linkage back to the TBI unit so it is returning all the way closed. I have seen a worn TBI shaft stick a little open when close to closed/idle.
If you restart the engine when this happens does the idle return to normal?
Next best thing I found is a propane soldering torch, unlit, around all possible vacuum leak spots.
I assume you have checked the linkage back to the TBI unit so it is returning all the way closed. I have seen a worn TBI shaft stick a little open when close to closed/idle.
If you restart the engine when this happens does the idle return to normal?
bp71k5
3/4 ton status
it's been bored and a bushings installed, but the shaft still may be worn. I did some testing yesterday and wasn't able to get the idle to drop by pushing on the linkage.
Nope. Oncr the engine is warmed up and the idle flips higher, the idle stays above 1100rpm until it's been off for a good while.
Also checked all my accessories and there's no difference with those plugged so I think a leak is probably somewhere else. One odd thing is my pcv valve. I can feel that thing chattering while the engine is running. I can feel and hear it opening and closing really fast. I didn't think it was supposed to close unless there was too much crankcase pressure.
Hmmmmm... I wonder if that PCV valve is bad? I have never noticed a lot of noise from them but some... can you put a vacuum gauge on the engine and see if the vacuum is steady at idle? Preferably a tee in the vacuum line to the MAP sensor.
I will look at my PCV valve later and see if it acts the same as yours. It is a controlled vacuum leak to evacuate engine of fumes but I never noticed if it was as eratic as you describe.
I will look at my PCV valve later and see if it acts the same as yours. It is a controlled vacuum leak to evacuate engine of fumes but I never noticed if it was as eratic as you describe.
bp71k5
3/4 ton status
It's my second one, but I suppose it could be bad. It's only a year or so old. I have an old one I can try replacing it with to at least see if it's consistant.
Easier yet is to just remove and plug that vacuum port for testing purposes.
bp71k5
3/4 ton status
I did pull the valve out and plug the end with my thumb and it didn't make a difference.
But I just got back from the parts store with a new valve and the motor started idling normal again with the existing valve. I got out to check it and the valve wasn't chattering anymore. So I compared the old to the new and the old valve does seem to have some oil in it.
I'll drive it around with the new valve for a bit and see if that's a coincidence or whether that was really the problem.
also a vacuum reading if possible. Not just the reading but if it is steady or eratic.
bp71k5
3/4 ton status
I'll pickup a new guage tomorrow. If the vacuum is fluxuating a lot, what does that mean? My old brake bleeder vacuum guage tool show the vacuum did fluxuate high and low really fast, but I figured the guage just wasn't accurate.
If it's fluctuating as you described it's driving the MAP sensor NUTS! Not good! Indacates a leaky intake valve. Unless you have a large cam, that's another bag of worms on a Spedd/Density system.
bp71k5
3/4 ton status
Hmm, I'll have to make sure of the vacuum diagnosis before I pull off the valve covers that took me two years to get to finally stop leaking...
Cam is a TBI friendly version from a reputable builder.
bp71k5
3/4 ton status
Ok, vacuum is fairly steady floating slowly between 17 and 18 inHg. The truck is now running very nicely with a 750 rpm idle. I find it strange that it stops the problem 5 minutes before I change the pcv valve. I'll keep the vacuum guage handy and take a reading when it goes high again. Thanks for all the help everyone.
