IAC isn’t controlling idle properly

[redfury]

So, my V8 Astro isn’t idling out of the gate anymore. I have to turn the key on first, then put my seat belt on in order for it to idle when I start it up. If I just turn the key and start the van, it starts and immediately stalls, or idles around 450 rpm and then dies.

Sometimes the idle will run upwards of 1200 rpm and eventually settle down. Sometimes the idle will start off low and want to stall, but eventually adjusts to where it is supposed to be. My long term fuel trims are at 19.5 on each side equally, which makes me suspect that the IAC isn’t closing all the way. the IAC is BRAND NEW, and acts the same way as the previous IAC valve I removed.

When looking at the van with an OTC Genysis 5.0 scanner, it shows my IAC counts at 180 at 650 rpm and was showing that as a percentage of 15xx% on the scan tool. I took a picture of the scan tool screen, but I don’t have the phone handy at the moment.

I could operate the IAC through the scan tool, but it was slow to respond, though that might be the scan tool itself, I’m not entirely sure as it’s my shops tool, so I’m not used to using it daily.

I hadn’t had problems with idle so much other than a blurb on occasion where the idle would drop, it seemed to react to the A/C clutch and dropping into gear just fine before, and does it sometimes.

Considering the nature of a stepper motor, there isn’t much I know to do to check the wiring or signal coming from the computer.

I thought perhaps the problem might be with the TPS, but I’m not convinced that the problem lies there.

Honestly, I need some direction as to where to look first.

Topic

[David]

I’d make sure you dont have a vacuum leak somewhere. I know my Buick was like that when I changed out the upper intake manifold gasket and didnt tighten down the back bolts properly.

[Kenneth]

How’s the fuel pressure look?

[James O’Hara]

If you have a vacuum controlled one you have a vacuum leak. If it has wires running to it then my guess is either you got a bad new IAC (rare but, happens) or you have a short or resistance in your harness. I am guessing because you hooked to it and got 15xx% that it is electrical and that you probably have a issue with the wires going too and from the IAC. First check the IAC to make sure it ohms out. Then I would follow the wires with you hand and feel/look for abrasions, cuts, and/or melted wires. If you have none of that you should check inside the connectors and look for corrosion or loose pins. Basically it should be shiny and/or covered with dielectric grease.

[Erin]

[quote=”Sovek” post=121604]I’d make sure you dont have a vacuum leak somewhere. I know my Buick was like that when I changed out the upper intake manifold gasket and didnt tighten down the back bolts properly.[/quote]

Very much what the problem sounds like. I had this intake vacuum leak twice –
On a 94 grand prix – when cold, it would rev up, stall, rev, stall, etc until it reached operating temp.
On my 99 cougar – at first it ran a little rough at times but got to the point where it ran rough ALL the time.

So for the OP – does engine temperature change anything? Intake leaks can cause some weird stuff and in your case, I would almost bet that if you cannot find any other leaks – start looking at the intake upper and lower gaskets. Both of your IAC are probably fine. If there is a vacuum leak, it is going to be working overtime trying to find a stable idle, a mere symptom instead of cause. Of course having to replace intake gaskets is the worst case in fixing a vacuum leak so exhaust all other options first.

[redfury]

A lower intake gasket leak is certainly a possibility, but strange IAC behavior has been there for a while. The original had to be plugged in, key on, then unplugged.. Rinse and repeat a few times, otherwise I would have a super high idle that wouldn’t self correct. That was the first one. The joy pull and the new ones worked using the key on, key off approach. My freeze frame for the po171 comes on at idle or at 1000+ rpm. The fuel pump is new, the ignition minus plugs is new.

[redfury]

This is what has me scratching my head about this. I started thinking while posting in another forum or two today that I wonder if the EVAP hose ( hard line with a quick connect ) that runs next to the distributor and coil might have something to do with it. I did have to work by it recently when checking out the coil, and there’s not much space/wiggle room with the wire loom that is running through that area of the intake. I checked the distributor for play, but it just rides up the teeth of the gears and my timing is fine, and I checked my CMP as well, just to make sure. it’s still sitting happily at 2 degrees BTDC, right where it’s supposed to be at.

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[redfury]

[quote=”Summer_Night” post=121644][quote=”Sovek” post=121604]I’d make sure you dont have a vacuum leak somewhere. I know my Buick was like that when I changed out the upper intake manifold gasket and didnt tighten down the back bolts properly.[/quote]

Very much what the problem sounds like. I had this intake vacuum leak twice –
On a 94 grand prix – when cold, it would rev up, stall, rev, stall, etc until it reached operating temp.
On my 99 cougar – at first it ran a little rough at times but got to the point where it ran rough ALL the time.

So for the OP – does engine temperature change anything? Intake leaks can cause some weird stuff and in your case, I would almost bet that if you cannot find any other leaks – start looking at the intake upper and lower gaskets. Both of your IAC are probably fine. If there is a vacuum leak, it is going to be working overtime trying to find a stable idle, a mere symptom instead of cause. Of course having to replace intake gaskets is the worst case in fixing a vacuum leak so exhaust all other options first.[/quote]

It’s doubly bad since it’s an intake gasket and it’s in an Astro. The fact that it’s a V8 makes no matter, it would be the same problem if it had the V6. I guess the only thing I can add is that when I take the oil cap off, the fuel trims definitely change for the worse, and putting Propane in through the oil fill hose stabilized the readings. I have video of it on my computer somewhere. I think the fact of the matter is, I have a lower intake gasket leak, which can’t be confirmed externally, I can only assume that it’s sucking out the crank case. I’ve done a water test, propane test, carb cleaner test all around the upper and lower intake and the base of the throttle body.

I don’t want to do an intake, but I’m getting to the point that I’m out of other viable reasons/options for the high LTFT numbers. I loose an infinitesimal amount of coolant. I haven’t had any problems with the EVAP system, other than one code way back, which wasn’t a small or large leak code….never to return.

[Author]

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