So, last Thursday my idle starts climbing up to 1,500 RPM in my 2.8. First thing I do is check for vacuum leaks, find none (carb spray method). So I pull the snorkel and stick my finger over the IAC hole in the bottom of the throttle body. Bam, strong vacuum and I can bring the idle down to a stall by blocking the hole entirely. So, the problem's with the IAC... I pull it and spray out the hole, then clean the pintle. Put it back in and the idle is higher.

I pull the IAC back out and leave it plugged in, IIRC the IAC is supposed to extend to full closed when turning the key to RUN before cranking. The pintle doesn't move at all, not even a twitch. Next I go grab two known good IACs from my parts cars, cars that drove at one time with a normal idle. Each IAC yielded similar results, except with an even higher idle. I tried turning the pintle out on one of them so that at least I could get the idle down somewhere reasonable and it wouldn't turn. Forced it and it snapped, turned out there's a plastic key in the nose of the shaft hole that indexes a slot in the side of the shaft below the pintle. Oops, scratch one IAC. Hmmmm...

Next I figured the driver is bad in the ECM, so I swap in a brand new ECM, no change.

So, out comes the Fluke 88 and I check for coil resistance from the ECM connector, and both A and B coils in the IAC show a hair over 63 ohms and more importantly, this verifies wire continuity to the IAC. I plugged the ECM back in and backprobed the IAC wires, getting 10V dc on both the blue pair and the green pair when I turn the key to RUN but don't crank. I know that stepper motors don't use straight DC, but the presence of voltage indicates that the ECM is trying to do something.

Now I'm stumped.

Wiring's good.

It's extremely unlikely that all three IACs would fail the same way at the same time.

ECM's known good (brand new).

I don't know what to check next, and could use some ideas...

JazzMan

[This message has been edited by JazzMan (edited 02-05-2008).]

There is another check you could try. I have read this but not tried it.
You jump the aldl AB pins. Then when you turn the key on the IAC is suppose to keep driving out.
Your suppose to be able to check the four wires on the IAC plug and they are suppose to each be pulsing.

The reason you read 10v on all the wires is because the outputs all set high (10v) and to drive the motor the other wire of the pair pulls to ground.

So with AB jumped you should see each line pulsing to ground.

Or so I've read....

Ok, I'll check that out. Wish I had a scope, this would be oh so much easier. One of the things complicating this is that my Helms manuals are boxed up somewhere and I can't find them.

Another problem is that I have to bleed my clutch every couple of miles and have been having to use RPM matching to get it into 1st when coming to lights and stop signs. If I miss the gear then I have to shut the engine off and restart in 1st. Oh, and the 1st gear synchro is blown so it only stays in first maybe 1/10 of the time nowadays. With it in gear and the engine running at a light it drags so hard at 2k RPM it's like an automatic, and after half a minute I start to smell clutch smoke.

I'm trying real hard not to hate my Formula right now...

JazzMan

Have you reset the ecm to see if that miht help also.

Sorry your having clutch problems... I'm sure you know all the advice available on that and arms aren't long enough to help...

Physically removing the ECMs reset them, hehehe...

Oh, forgot to mention that despite hours and hours of idling 1,000 RPM above spec my ECM has never illuminated the SES light (though a code may be set, I haven't checked that yet) despite the fact that it is supposed to. Neither ECM.

The clutch problem is that the sh(*&$@(%&ty Lucas Girling design that GM bought sucks. The slave seal isn't spring loaded so the slightest vacuum (such as thermal fluid contraction or parking downhill) sucks air into the slave.

I'm thinking about using an ABS pump to build a continuously-bleeding system now.

JazzMan

Do you have a scan tool or WinALDL?
You might have a coolant temp sensor that's lying to the ECM, and telling it that your engine is cold.

quote Originally posted by JazzMan: ...The clutch problem is that the sh(*&$@(%&ty Lucas Girling design that GM bought sucks. The slave seal isn't spring loaded so the slightest vacuum (such as thermal fluid contraction or parking downhill) sucks air into the slave. I'm thinking about using an ABS pump to build a continuously-bleeding system now. JazzMan

Any chance that you can park "nose up"?

[This message has been edited by Raydar (edited 02-05-2008).]

Did you backprobe for TPS volts? You know if your TPS ate it and was feeding higher than about .60 volts at idle, the idle malfunction indicator wouldn't work AND the IAC would be left to pretty much full bypass.

GM TPSs "buy the farm" a lot more often than do IACs....

Throttle could be stuck a little farther open too ('course that would also cause the TPS to read high, and turn off idle malf indicator)


Kurt

I put this isn another high idle thread about the IAC checking, you did some of it already.... it is from the 86 fsm:

quote couple of interesting checks from the fsm: for code35, but the info is relevant and also a good test: from step 3: If idle is too high, stop engine. Ignition on. Ground diagnostic terminal. Wait a few seconds for IAC to seat ,then disconnect IAC. Start engine. If idle speed is above 800rpm +- 50, locate and correct vacuum leak. to check IAC windings: pins a/b and c/d are the pairs, should be more than 20 ohms for each pair, they don;t give a spec for the highest, but an open is bad. to check the ecm drive lines, disconnect iac connector, at harness end, use test light, ignition on, diagnostic pins shorted, check each terminal to ground, they should all flash. ( this is because it is trying to seat the connector, it will just keep going) if all are flashing, that is good. You can verify the mechanics with a helper, but be careful you don;t launch the pintle on the IAC: remove the iac, screw the pintle all the way in. with it connected to harness, hold the unit with finger pushing against the pintle to hold it in to the IAC, have helper turn ignition on, and short the diagnostic pins (a nd b on the aldl connector). you should feel the iac driving the pintle out. without your finger on it, it will end up somewhere you will never find it, so be careful!!! Actually that last test will check everything easily, may want to start there....

For the clutch, it's been mentioned if you remove the slave's rubber boot and fill it with grease, you can stop the air suckback. cheap and easy workaround at least....

quote Originally posted by Raydar: Do you have a scan tool or WinALDL? You might have a coolant temp sensor that's lying to the ECM, and telling it that your engine is cold. Any chance that you can park "nose up"?

Yes, but then fuel starts running out of the top of the tank where the hose connects. Not having a place to work on my Fiero is really crimping my style.

I've got WinALDL but the laptop it originally ran on died. My current laptop should run it because it's a Win98 machine but I haven't tried it yet. That's my weekend project. I pulled codes last night and no joy, there's no stored code other than 12. I grounded the ALDL diagnostic connector and started it, the idle hovered around 1,100-1,200 until it went into closed loop (as indicated by the steady slow on/off cycling of the SES light) and immediately climbed to 2,000 RPM, even with the connector grounded. So, I'm thinking that the ECM is actually still in control of the IAC. I'm going to disconnect the coolant temp sensor tonight, that should rule out a sensor saying the temp is too cold.

quote Originally posted by tjm4fun: For the clutch, it's been mentioned if you remove the slave's rubber boot and fill it with grease, you can stop the air suckback. cheap and easy workaround at least....

I tried that, there's a whole 4oz tube of silicone grease in that boot now, it's squishy. No effect.

JazzMan

[This message has been edited by JazzMan (edited 02-06-2008).]

Winaldl works great with Win98, I use one of my laptops that way all the time.

If you disconnect the CTS that is the same as telling the engine it's very cold so would also keep the idle high. Lower resistance = hotter.... I suppose the ecm is set to go to an error state with the cts unplugged but don't know what that might be...

If the IAC is cleaned or has been replaced you need to drive the car over 35mph for the ECM to reset it.
You will have high idle until the ECM resets it.

Look at the second answer down:
http://chevrolet.justanswer...er-1995-chevy-camaro

And from Auto Zone's Help Pages:
Reset the IAC valve pintle:

There are 2 possible methods to resetting the IAC valve. First, depress the accelerator pedal slightly, then start and run the engine for 5 seconds. Shut the engine OFF for 10 seconds, then restart it and check for proper idle operation. If this does not work, start and run the engine until it reaches normal operating temperature. Drive the vehicle and the IAC valve will reset once vehicle speed is above 35 mph.

[This message has been edited by Oreif (edited 02-06-2008).]

quote Originally posted by Oreif: If the IAC is cleaned or has been replaced you need to drive the car over 35mph for the ECM to reset it. You will have high idle until the ECM resets it. Look at the second answer down: http://chevrolet.justanswer...er-1995-chevy-camaro And from Auto Zone's Help Pages: Reset the IAC valve pintle: There are 2 possible methods to resetting the IAC valve. First, depress the accelerator pedal slightly, then start and run the engine for 5 seconds. Shut the engine OFF for 10 seconds, then restart it and check for proper idle operation. If this does not work, start and run the engine until it reaches normal operating temperature. Drive the vehicle and the IAC valve will reset once vehicle speed is above 35 mph.

I've been driving the car at speeds over 70mph for hours. In the past when I've worked with the IAC I've observed the learning process, and it's never been extended and never to 2,000 RPM which is what it's been doing this last week.

Since my clutch won't release it is common for the engine to drag down at stop lights because I leave the tranny in first as I come to a stop (otherwise I have to shut off the engine to get it into gear). Back when the idle was working correctly I'd sit there holding the car from moving with the brakes and watch the ECM bring the idle up to normal, then when I started off from the light the idle between shifts would be higher as a result. Then, when I came to a stop in neutral the ECM detected the car not moving and started bringing the idle back down to normal. The ECM would not start adjusting the idle until movement ceased, I'd verified this by coasting along at highway speeds in neutral and observing the idle. If I blipped the throttle hard in neutral while coasting the ECM would then adjust the idle while the car was coasting in neutral. I'd also played with it (back when it worked properly) by holding the throttle slightly open to raise the RPMs a few hundred while the car wasn't moving and was in neutral, then watched the ECM bring it back down to normal. When I let off the throttle the RPM would dip very low and then catch back up as the ECM bumped it back up to normal. Pretty consistently, as in never acted any differently than that.

Yes, I like experimenting, it's a good way to learn. What's happening now is that for some reason two different ECMs and three different IACs with measured good wiring in between are leaving the idle at a hard 2,000 RPM.

I've done some research, and often times it's been stated that it's possible to "turn" the IAC pintle in or out to adjust it (not for the purpose of setting idle, but for the purpose of making sure it doesn't bottom out while installing it). I tried that and destroyed a perfectly good IAC. All three IACs that I have, the original one (now destroyed) and the two I took off my parts cars, have a slot in the shaft and a crimped in place plastic nose piece with a tab that rests in that slot to prevent the pintle from turning. This stands to reason because the helical shaft of the pintle goes through a motor with a fairly steep helix angle and the shaft would likely turn rather than extend when that motor rotates inside the IAC housing. I went ahead and dissected the IAC I destroyed to verify that this was the way it was designed to work. So, I don't understand how people can "turn" the pintle without breaking that plastic nose piece and causing the pintle to rotate with the motor, therefor eliminating its ability to extend/retract in any kind of consistent manner.

Damn, wish I could find my Helms manual.

JazzMan

[This message has been edited by JazzMan (edited 02-06-2008).]

I've had *all kinds* of weird running problems that were solved by replacing the coolant temp sensor.

Darn, I wish you where closer, I'd like to help with your clutch problem.

I've not had problems fixing them with my vast experience of one car...!

Is your master the gen1 style with the reservoir on the end.... humm what does that sound like....

your IAC sounds strange, as Iknow for a fact when I cleaned up my tb on the rebuild I screwed the pintle on the iac all the way in per something I read somewhere, but maybe I just pushed it in, I really can;t recall.
I would seriously try that test light test with the key on, a/b at the aldl shorted and check each wire at the iac connector to ground with a test light. that test is right from the Helms, so it is definately valid and safe... it should flash on each wire, as that motor is a stepper and each wire can be driven to 12v or to ground.
if that shows good, then maybe the iac is just a symptom and not the cause.
just out of curiosity how does it idle with the a/b pins shorted? that is preset to a 1k idle I believe...

Jazz, three things:

1) I have an '88 Helms manual
2) I have a garage you can work in (hafta clean it up first)
3) I'll sell you my Formula



-fh

First off Im feeling lazy and didnt read everyones replays so this amy have been said. Spray a litle trany fluid aroung your injectors, Then press and wiggle them a little. Let it sit overnight and see if that helps, if does your O rings are dried up. The trany fluid with make them swell up some. You can also listen with a lenght of hose near each injector.

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[IMG]



Trueleo.com/fiero.htm
RSpiderII@aol.com

As far as retracting the IAC, there are at least two different types depending on which year/engine you have. The IAC on my 84 Duke had a spring with a tang that kept the pintle from rotating. You could pull the spring back and screw the pintle in or out. Releasing the spring "locked" it from rotating. The one on my 88 GT does not have the spring and it can be retracted (sometimes) by firmly pushing on the pintle. I don't think it can be extended by pulling on it, though.

------------------
Rod Schneider, Woodstock, Ga.
"You can't have too many toys!"
1988 Fiero GT
1966 Porsche 911
Van's RV-6 airplane-under construction

quote Originally posted by Francis T: First off Im feeling lazy and didnt read everyones replays so this amy have been said. Spray a litle trany fluid aroung your injectors, Then press and wiggle them a little. Let it sit overnight and see if that helps, if does your O rings are dried up. The trany fluid with make them swell up some. You can also listen with a lenght of hose near each injector.

I used carb spray for the same test initially, if there was a leak it'd change the idle, but there wasn't a leak. I've ruled out the throttle cable by the simple expedient of being able to stall the motor by blocking the IAC hole in the bottom of the throttle body with my fingertip. If there was enough airflow, either from a vacuum leak or a stuck throttle, to keep the engine running I wouldn't be able to stall the motor this way.

I'm going to check end-to-end continuity of the wiring next, the last check combined the coil resistance and continuity checks.

A recap:

Turning the key to RUN without starting the motor does not cause the IAC to extend. I think I remember that it's supposed to, but I may be misremembering.

Putting the ECM into Field Service Mode (jumper diagnostic to ground in the ALDL with the engine running) reduced idle to 1,200 so for a couple of minutes, but it went back to 2,000 as soon as it went into closed loop.

I have not put the ECM into diagnostic mode using the 10k resistor yet, I'll be doing that this weekend when I get the laptop and inverter set up to run WinALDL.

I did some research on the IAC as used on the 2.8 and there's precious little technical info on it. Megasquirt's site has the most usable info on it with a good description of our type of IAC (it's bipolar!), so each pair (blue or green) wires goes to a separate coil, of which there are two. That means that when the ECM is attempting to move the IAC each pair of wires will be sequencing on and off and reversing polarity. I can build a simple series of switches in an H-bridge configuration to manually extend and retract the motor, so if all else fails I can disconnect the IAC and manually adjust the idle to a usable range until I have the time and resources to fix the original problem.


Fierohobby, drop me a PM with your number, I think I'll be giving you a call.

JazzMan

the iac will only extend if the a/b are shorted with key on, according the fsm.

as for the iac wiring, the ab are the opposite ends of the same coil, and is driven pulse mode with polarity reversal to move the motor. Least that's how almost every other stepper motor in the work is run.

grabbed this out of the 86 fsm which I have the electricla split out in a pdf:


should help ya do up a circuit...

[This message has been edited by tjm4fun (edited 02-06-2008).]

Well, made progress of sorts. Last night I repeated the test that I did the night before, shorting A-B in the ALDL connector then turning the key to RUN without cranking. I let it sit for a minute then shut off the key. I then unplugged the IAC and started the motor, bam, 2k RPM, no change. Hmmm... Shut off the motor but left the key in RUN and went back to plug the IAC in before thinking about what to do next. As soon as I plugged in the IAC it started making a rattling kind of noise, so I let it sit there for a while then unplugged it again before starting the motor. Success! The car wouldn't hold an idle, would die as soon as I let off throttle.

Now, most people would consider this a serious problem but given what I've been through with the idle this got me halfway home.

What I did next was to manually adjust the throttle stop open a few turns and plug the IAC back in. Leaving the A-B jumper in I started the motor then rushed to the back of the car and grabbed the IAC connector. As the idle started to climb up past 1,200 RPM I unplugged it, then adjusted the manual stop back closed to bring the idle to a nice stable 1,000. This is a temporary fix, of course, but it makes the car oh so much easier to shift with the bad clutch.

No success go unpunished in my life, however, so literally at that moment of getting the idle to 1,000 RPM I started smelling coolant. The guage showed the thermostat temperature (it's accurate) so I looked under the car. Coolant was running out of the front so I took a look up front and it appears that the driver's side tank seal or tank itself has failed. Sigh. Well, I don't have a spare radiator that I can get to any time soon so I just filled up all the 1-gallon jugs I could find and put them in the trunk, and will drive this way until I can deal with the leak.

JazzMan

Good thing you live in D/FW and can run just water without freezing...

Lady luck is not smiling on you this week.

Man, I feel your pain! I hate when things just start to snowball like that. that was the reason mine came off the road and sat for 3 years, it was just one thing on top of another. letting it sit wasn;t the best thing,k but allowed me to save up the funds to rebuild it mechanically from front to back. But I;m sure you know that drill.
I have seen epoxyies for sealing plastic tank cracks, have no experience with em, but sometimes they might slow the leak.
hard to find, but there are also lower pressure radiator caps that will work, might also slow/.stop the leak.

If you find the crack and it's not right where the flange meets the metal I have had good luck fixing them with JB Weld and some fiberglass cloth. I clean and lightly sand the area, put a layer of JB Weld down, press in the cloth and cover with more JB weld.

I fixed an old Honda raditator on a car my son was driving while he was rebuilding his firebird.
He sold it to a girl two years ago and it's still running aound town.
Also fixed the plastic gas tank on my tractor mower this way as well as a poly sprayer tank and they're still holding.

Just an idea...

For the radiator a friend of mine has promised to loan me his for a few weeks (his car's down for a swap) today, so I should be able to get it installed tonight. Tomorrow I'll run the original one (and it is original for the car with 242,000 miles on it) over to Al's Radiator Dungeon and get it rebuilt.

I pulled the thermostat this morning and dumped just over a gallon in, downside of course is the idle is now too low due to the lower idling temperature. No problem, a screwdriver can fix that.

One observation, with a manual idle control there's no more hunting; the idle is rock steady. I long ago traced my hunting issue down to the VE varying sharply between 900 and 1000 RPM. The ECM would bump the idle, the increased VE would amplify the idle speed increase, the ECM would bump it down, the decrease in VE would drop it even more, into a hysteresis loop around 3-4 seconds in length. You could see it pretty clearly on the wideband/linear O2 sensor readings.

JazzMan