Testing the IAC

[smcphc]

I've been scouring the tech forum trying to cure a low idle problem that started over the weekend. I have a '93 80 with 138k and normally on cold start it idles around 1400 or so and then drops down to 650 after a few minutes. A couple weeks ago on cold start it idled immediately around 800 or so and when in gear and at a stop dropped to 500 and was on the edge of stalling. I poked around the IAC, checked the connections and it cleared up.

On Saturday, it happened again and now it's permanent. This morning I replaced the PCV valve, grommet, and hoses, took the IAC off (what fun that was) and cleaned it up a bit. No joy.

Before I plunk a few hundred dollars down for a new IAC, I tried to test it. I tested the resistance of the IAC and it was in spec, but when I got to the valve operation test I'm a bit confused. I lifted this image from another thread...

From what I understand, I apply voltage to B1 and B2, but for grounding in sequence, do I keep the ground on each pin as I move to the next one, or do I lift the ground off and move to the next pin? I tried it in sequence lifting off the ground and moving to the next one and I just got a bit of clicking when I got to S3 and S4. I don't have enough small jumpers to do them all at once...

As for the IAC, do these things rarely fail? I disassembled it and it looked clean and fairly simple. Anything else in the system I should be looking at?

FWIW, headgasket was done about 200 miles ago.

Thanks - Scott

 

[bpenn1980]

You might want to try cleaning the whole throttle body. It can be removed from the car, cleaned and reinstalled in a few hours. After I cleaned mine (and cleaned the IAC) my high start up idle and low warm idle completely disappeared.
.02

 

[smcphc]

I might try that. I'd hate to replace the IAC and find out it wasn't the problem.

Scott

 

[Junk]

What condition are the wires in leading to it? When I had an idle issue, and was being told to replace the IAC, there were some scorched wires that needed some attention. Dunno if that will help, but I'd check'em.

 

[smcphc]

What condition are the wires in leading to it? When I had an idle issue, and was being told to replace the IAC, there were some scorched wires that needed some attention. Dunno if that will help, but I'd check'em.

Wires look good. Just spent a half hour looking for one of the IAC screws that dropped when I was taking the IAC off, I swear those things are cursed. When I took it off for the first time, both screw were frozen and neither would budge with the phillips head. I loosened the upper with a vise grip, but I couldn't get a good bite with the needle nose vise grips on the lower screw, finally tried a small chisel and worked it loose enough to get it off with the phillips head screwdriver. I'll be replacing them with either allen or hex heads. Dropped the upper IAC screw, didn't make a sound, no clinking. No sign of it anywhere, finally crawled up underneath and felt along the motor mount and and there it was. I hate having missing fasteners, never know where they might end up and I stress until I find them.

Anyway, at least it starts and runs, I'll button it up for now and might do the throttle body cleaning tomorrow unless some other suggestions come my way.

Thanks - Scott

 

[airlaird]

Please let us know

Let us know..the IAC with my discount would have cost me about $340.00. Not cheap. Now, a Toyo mechanic told me if the idle go up when you turn on the A/C...then the IAC is fine. I don't know if that is true but that is what he claimed. He is an "on call" ASE certified Toyota mechanic. I too still have a low idle...I've done it all and no change. Still idles at 580 rpm.
airlaird

 

[smcphc]

Let us know..the IAC with my discount would have cost me about $340.00. Not cheap. Now, a Toyo mechanic told me if the idle go up when you turn on the A/C...then the IAC is fine. I don't know if that is true but that is what he claimed. He is an "on call" ASE certified Toyota mechanic. I too still have a low idle...I've done it all and no change. Still idles at 580 rpm.
airlaird

Yeah, that was about the price I was looking at. A bit of an expensive gamble if it's not the culprit. I'm pretty sure the idle goes up if I turn on the a/c, I'll double check tomorrow. I think I'll try cleaning the throttle body before trying a new IAC. I'll keep the thread updated as I go along.

Scott

 

[smcphc]

Unfortunately I didn't do this according to the scientific method so I'm not sure what caused the cure, but it's back to normal. A nice package arrived from Slee yesterday afternoon (driving light harness) and I was anxious to get it installed so I went ahead and did it which required disconnecting the battery. I went for a short drive last night with the usual stumbling while the ECU reprogrammed itself.

This morning, I started it up, didn't get the high idle but cold start idle was at 800 and settled in at 600, much better than the 450-500 I was getting before. I tried the cdan method of reprogramming, and after it was warm I shut it down and restarted it 10 times. It sat for 5 hours, I went out to start it and idle went to 1600 and then settled in at 650 after a minute or two.

So, I'll cross my fingers and see what happens. I do know that it was not idling correctly after I had cleaned the IAC and replaced the PCV valve. I had done that and checked before disconnecting power to the ECU. The only thing I can think of is that the ECU may have needed to compensate for the new PCV valve and the clean IAC, but I don't know if that makes sense.

In any case, thanks for all the suggestions. As with some of the other idle problems, this one doesn't seem to have a definitive answer.

Scott

 

[Tapage]

Did you clean your entire throtle body .?

 

[smcphc]

Did you clean your entire throtle body .?

No, I replaced the PCV valve and grommet with new OEM and disassembled and cleaned the IAC. I was going to clean the throttle body today but I noticed the improvement this morning so I thought I'd give the ECU reprogram a chance.

Scott