Riddle me this... IAC question...

[Eric VerValin]

Why would a car run up to 3000 RPM, then stall out.... and when unplugging the IAC... the car idles rough, but stays running...... I found that second part odd. Maybe because the car is still in "open loop" mode?

Not my car, just trying to help diagnose an issue.... I was thinking vac leak, or the IAC crapped... but I don't know...


Oh yea, no codes... well... once instance of a 542, but I'm assuming thats from when the car dies...

 

[ShadetreeSHOguy]

Crank Cancer

 

[Eric VerValin]

 

[rubydist]

I'm going to suggest that the iac is very sticky, but still moves. Because it is sticky, it responds too slowly to the command to open, so that it gets opened too far - causing the revs to go way up. Then, the pcm overcorrects on the down side and closes it completely which causes the engine to die because the rpms drop so rapidly. If its unplugged, the amount of air let past the throttle body is enough to barely keep it running.

 

[Eric VerValin]

And thats pretty much what I was figuring too... can't really think of anything else it could be. I just remember cleaning out mine and how light that spring is in there.

Another thing tho... I've been informed that the intake gaskets in question have some paint missing...... so I'm wondering if its still just a simple intake leak.

 

[boat]

I beleive Eric was also asking this to help me out. B/c it is my SHO that is doing this. I was at a place w/no internet, so, I couldn't really post or search as I was working.

The intake to head gaskets, don't have much of any paint missing. And they sit nice and flat. All mating surfaces of the intake mount squarely to the heads. All intake couplers are tight.

I also had an IAC from an ATX intake, tried that and that was the one that went up to 3k rpm and died. The one from my MTX intake does the same, but up to 2500 rpm and died.

Thanks, rubydist for that explination.

That makes a lot more sense. Especially after checking everything I touched, I did not find anything out of place, after replacing the thermostat housing b/c of my over-tightening the thermostat neck nut, to replace the thermostat, busted the top stud off . Which all these shenaigans came after fixing a lower heater core hose coming off the coolant distrubtion block on the end of the rear head. Luckily Eric had one of these housings. Thanks, Eric

EDIT:
One thing to add, prior to this every other time on start-up, the rpms would jump up to about 2k-2500, and then idle back down and smooth out. Would it be safe to say replacing the IAC would fix this problem?

 

[Bob Gervais]

Vacuum leak. Especially since two IAC's were tried, and still had the same symptoms.

 

[boat]

Ok, no problem. But where do I start and/or how do I find it? I really didn't see anything where I was working at that was a vacuum line. I checked to make sure everything was plugged in, that I unplugged or touched. I very easily I could have overlooked something for as long as I was working on it, trying to find the problem. Seems like it would have to be a big vacuum leak, if it is dying as fast as it is. Wouldn't it?

Vacuum leak. Especially since two IAC's were tried, and still had the same symptoms.

 

[Bob Gervais]

First step would be a vacuum gauge, and checking how much vacuum you're producing (or not producing).

http://www.fordf150.net/howto/diagnoseengine.php

Here's a pretty decent article on using a vacuum gauge to diagnose engine problems.

 

[Eric VerValin]

From what Dan told me yesterday I'd lose those old gaskets. It dosen't take much if anything to cause a vac leak. Maybe double check the other ends of the hoses coming off the intake too.

I'm thinking it almost has to be... because I kept buggin you about something not being tightm, and you swear everything is.... so your pretty much stuck with bad gaskets, or a cracked hose somewhere you can't see. Follow the vac hoses and make sure the far end is tight too.

 

[Bob Gervais]

Maybe there's a crank bolt stuck in the IAC?

 

[boat]

nice :laugh_ti:

I deserved that. lol

Maybe there's a crank bolt stuck in the IAC?

 

[Eric VerValin]

So that little bit of paint missing is enough huh....

 

[bubba]

rgr, Just got done putting it back together, the car now, starts and the idle is still alittle high.... but it now idles at about 500... doesn't die just idles rough... engine shaking....

 

[Bob Gervais]

So that little bit of paint missing is enough huh....

Nah, that's probably not it. First step though, is to get a vac gauge on that and see if it really is a vac leak.

 

[boat]

sounds like it could be. Bubba said it still "chugs" stays running on its own with the IAC plugged in, dies when unplugged, from what I understand from our talks yesterday, thats what it should do. He said he didn't disconnect the positive on the battery, to reset the computer. And I did swap in a different plug wire than what I had on there, so maybe a bad plug wire or 2.

Maybe a combo of those things... Going to check here soon...

So that little bit of paint missing is enough huh....

 

[Eric VerValin]

Nah, that's probably not it. First step though, is to get a vac gauge on that and see if it really is a vac leak.

I was talking about missing paint on the intake gaskets... how do u figure that can't create a vacuum leak? lol Considering he didn't change anything it kind of has to be the culprit ya know? Considering the car ran and drove fine before the uhhh... thermostat swap...

LoL.. that last sentence sounds funny to me...


Edit: Swap those cables back over... try to diagnose one thing at a time. Reuse everything except those crap gaskets. It should work with what you have.

 

[Bob Gervais]

Well, the thread started off asking about a faulty IAC. Then it moved to a vac leak, but not determined to be one because a vac gauge wasn't used to diagnose.

Gaskets and such came into question later on.

Having used stock metal gaskets many times over and over, with paint chips, without paint chips, and sometimes cleaned up, repainted and used over again, I can safely say that the gaskets probably aren't the issue. At least not a big enough issue to cause the problems he's having.

 

[Eric VerValin]

Well... since he has no internet where he's at now, I'll help ya out a little... his car before all this started... liked to idle high when started.. but it would come back down and "act right" after that.

He pulled the intake when he replaced the thermostat, put it back together and this is when everything started.

It started as an IAC question just to figure out why it still ran when unplugged.

 

[rubydist]

Vacuum leak. Especially since two IAC's were tried, and still had the same symptoms.

I've never had a vacuum leak that resulted in the symptoms they described, but I have had sticky iac valves that caused symptoms very much like that. And, just because a couple of iac valves were tried, that doesn't mean they were good....

But, it is possible that there is enough of a vacuum leak that it revs up on startup (when the injectors put in lots of fuel to get it started) and then leans out so much that it dies. Under that scenario, unplugging the iac would reduce the amount of air going in enough to allow it to keep running.

So, it seems that a very careful check for vacuum leaks like Bob has suggested would be a very good idea at this point.