Vacuum Leak?????

Dangerfield23 · Aug 16, 2005

I made a previous post about a high idle problem after installing a new intake, and some of the feedback I got suggested that it could be a vacuum leak.

After acceleration it idles between 1700 and 2000 rpm. I can unplug the IAC and it drops back to normal. I plug it back in and it jumps up to 2000. I've tried a restriction plate on the IAC, but that just created different problems so I took it off. I replaced the IAC assembly and it is clean and not stuck, installed new throttle body, installed new computer, and I have a bypass plate on the IAC but nothing has helped.

Is this characteristic of a vacuum leak? I figured if it were a vacuum leak it would idle high no matter what, but I can unplug the IAC and it idles normal. I can't figure out why changing the intake would make the computer keep the IAC valve open.

ram360 · Aug 16, 2005

if it were a vacuum it would idle high all the time. I have a similar prob on my car. It eventually drops but take like 30 seconds for it to finally come back down. Still trying to fix that myself. I tried cleaning my IAC and adjusting the TPS on my car. Seems better but still not right.

vristang · Aug 16, 2005

I can't really help as I am working on a similar problem.

I start the car and it idles exactly where I set the idle to. After driving for any length of time the idle will not drop below 1000rpm. I have adjusted the tps but I am unsure what WOT should register as at the tps, don't know if it even matters?

My next effort will be to adjust the timing again, after retiming and readjusting the idle, I hope everything is fine.
I should also mention that the problem was much worse when I had a vac leak common to the crankcase. You would think the idle would always be high, but after starting the car would idle fine, just not after any driving.

custom89stang · Aug 16, 2005

YES IT MATTERS, my mechanic said it should have a certain volts at idle .99 or .09 or w/e i dont even know...uve said before and at WOT it should be like some other number ...4.66 or 6.44 or something like that, im just saying, YES IT MATTERS

Dangerfield23 · Aug 16, 2005

vristang said:
I can't really help as I am working on a similar problem.

I start the car and it idles exactly where I set the idle to. After driving for any length of time the idle will not drop below 1000rpm. I have adjusted the tps but I am unsure what WOT should register as at the tps, don't know if it even matters?

My next effort will be to adjust the timing again, after retiming and readjusting the idle, I hope everything is fine.
I should also mention that the problem was much worse when I had a vac leak common to the crankcase. You would think the idle would always be high, but after starting the car would idle fine, just not after any driving.

Idealy the green wire on the TPS should be .98 volts at closed throttle, but actually Ford says anywhere from .7 to 1 volt is fine. At wide open throttle it should be close to 5 volts (same as the reference voltage) but it usually ends up being around 4.

But none of this has made any difference on the idle with mine. My problem is that the computer is holding open the IAC valve for some reason.

vristang · Aug 16, 2005

Dangerfield23 said:
Idealy the green wire on the TPS should be .98 volts at closed throttle, but actually Ford says anywhere from .7 to 1 volt is fine. At wide open throttle it should be close to 5 volts (same as the reference voltage) but it usually ends up being around 4.

But none of this has made any difference on the idle with mine. My problem is that the computer is holding open the IAC valve for some reason.

Thanks for answering that for me. I will head out and check that soon.

As far as your problem, I would look at systems that induce a load at idle, requiring an increase in engine speed at idle. What comes to mind is the A/C, and other electrical loads. For example, if there is a problem with the a/c and the computer thinks it is on when it isn't, then the idle would increase. Just a thought, hope it makes sense.

Dangerfield23 · Aug 16, 2005

vristang said:
Thanks for answering that for me. I will head out and check that soon.

As far as your problem, I would look at systems that induce a load at idle, requiring an increase in engine speed at idle. What comes to mind is the A/C, and other electrical loads. For example, if there is a problem with the a/c and the computer thinks it is on when it isn't, then the idle would increase. Just a thought, hope it makes sense.

Yes, I thought about that, but I don't have anything hooked up to compensate for the A/C or power steering. This engine is in a '84 F150, and I didn't hook any of that up when I did the conversion.

It was doing perfect until I put the Explorer intake on it. It runs very strong, but hasn't idled right since. After I drive it, it likes to idle at 2000 rpm.

jrichker · Aug 16, 2005

A defective TPS will cause idle problems, and setting or resetting the TPS will not fix them. Turn the engine off and the restart it. If the idles speed drops down, the TPS is at fault and needs to be replaced. If the idle does not drop, the TPS is no longer suspect.

Replace the TPS only after you have determined there are no vacuum leaks.

Dangerfield23 · Aug 16, 2005

I got a new TPS with the new throttle body, so that's probably not it.

If I have a vacuum leak, would it somehow make the computer keep the IAC open? Because I can unplug it and it idles fine.

vristang · Aug 16, 2005

jrichker said:
A defective TPS will cause idle problems, and setting or resetting the TPS will not fix them. Turn the engine off and the restart it. If the idles speed drops down, the TPS is at fault and needs to be replaced. If the idle does not drop, the TPS is no longer suspect.

Replace the TPS only after you have determined there are no vacuum leaks.

I purchased a new one a while back and never put it on, since the problem was something else. I will try that as soon as I get a chance and post the results.

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