rssshen vette

OK guys, My last change/upgrade was the installation of a F.A.S.T. EFI 2.0 (self learning) ECM system. My cam's overlap was an issue so I went back to the Factory ECM. See my Signature for engine detail.

I have a high idle problem. As the car warms up to operating temp the idle increases up to 1400 rpm. I replaced the vacuum lines. Still had high idle. I disconnected all vacuum lines from the intake ports and capped the ports with rubber plugs. With the car running, I sprayed carburetor cleaner around the base of the intake, throttle body, and accordion tube. I also replaced the throttle body and IAC housing gasket. Results... No leaks detected.
Next I brought a new IAC valve. I took the TB off and cleaned the TB blades and IAC housing. I installed the new IAC put everything back together. I followed the IAC calibration procedure according to the FSM. The idle is back to normal. I got good crisp throttle response. everything is good! I let the car run at idle for 4-5 minutes and shut it off. Five or six minutes later I start the car to go for a test drive and the idle is back at 1400 rpm. So I shut the engine off to do a visual test of the IAC. I took IAC off the TB and kept IAC valve in the IAC housing. I turned the ignition key to the on position and waited 30 seconds. I check the IAC pintel and it didn't move. There was about a 1/4 opening from the pintel to the seat and no vibration from the IAC motor. I check the voltage going to the IAC and there was 12 volts to two of the four wires. So I then grounded the AB terminal, turned the key to the on position, and check the pintel. The pintel fully extended and seated in the housing and I could here the IAC motor vibrating. So I then unplug the IAC wires, turned the key off, disconnect the AB ground, reconnected the IAC plug wire, and turned the key on. The pintel retracted back to the original positon 1/4 inch from the seat!
WTF! I'm scratching my head on this one.
Has anyone had this issue? The only way to get the idle down to 700 rpm is to unscrew the idle screw until the TB blades are completely closed. Any help suggestion would be greatly appreciated! TIA!

aklim

I think that is your first mistake. You do NOT have a stock ECM program nor do you have a stock system, I think.

Going off the way I prefer to do it might work. I go off IAC counts. On a stock system, I'd get everything running till it is hot, check timing to make sure and go from there. You may need to see what your base idle speed is supposed to be from the computer program.

After that I would keep the engine running with everything but the motor off so there is no demand for a higher idle like the AC coming on. After which, I see what the IAC counts are. Target is about 30, give or take. Assuming your command idle is set right for the engine, I would see what the counts are. So if it is 50, it means the ECM thinks it needs more air to do the job. That means you turn the screw in which opens up the blades. If it is say 5 counts, it means you have too much air. In which case, you turn the screw out. Each adjustment you make, rev the motor and let is settle down. It should be about 30 counts, give or take a few.

IDK if that works for you but might as well try. So if you cannot achieve it because it is pegged at 0, I would suspect an air leak. You can put your hand over the TB and see if it still runs. If it does, there is a massive leak somewhere.

Kevova

A lot different than a 85 TPI. What are TPS MAP IAC and MAF at idle and min air rate? Auto or manual ?

C409

..... Keep in mind that the IAC has a "parked" position that it goes to when you power up the ignition to give the engine more air to start ... after the engine starts , the IAC will then move to begin to control the idle speed after other conditions are met ... as in ... Hold time , decay time , and ramp down time ... all these are in the ECU , some are user adjustable some are not but their purpose is to ease the idle to its set speed as gently as it can to avoid abrupt changes in RPM and stalling or bucking ...
..... Aklim's suggested IAC setting is very easy to do and will work on just about any car with an IAC ... just remember to check/reset the TPS when you get it set where you want it ...
..... Did you disconnect the vacuum line to the brake booster ? ... try putting a carb style throttle return spring on the throttle linkage and see what happens ... and check your software to see if it gives you an IAC position readout .....

rssshen vette

My car is an 85 year model, but I have a 1227165 ECM from a 87. The chip in the ECM is a mail order tune from PCMFORLESS. My idle with this tune was working fine before switching to the F.A.S.T. ECM. So there should not have been an idle issue after switching back to the 165 ECM. You're correct I have a vacuum leak it's at the IAC. The IAC works in diagnostic mode, but it is not responding to the commands of ecm in normal run mode to regulate the idle. I'm considering buying another new IAC valve. It's possible the one I have is defective.

The TPS is .54 There is no map for the setup. My data log shows 160 counts for the IAC with the transmission in park the MAF at 23.00 700r4 transmission.

aklim

Good point. I forgot since my year and 90, IIRC have been made with NON-ADJUSTABLE TPS.

Or maybe disconnect one hose at a time and see if the IAC changes?

aklim

IDK. At 160 counts, it means that the ECM is commanding it to be wide open. IOW, it can't get enough air.

rssshen vette

Yeah I disconnected the vacuum line to the brake booster. There was no change in idle. I'll try Aklim's suggestion tomorrow.

rssshen vette

I did that when I replaced the vacuum lines. I replaced them one at a time. I'll go out tomorrow and try again.

rssshen vette

Is it possible the O2 sensor or the temp sensor be the cause of the high idle? The idle is fine under 120* after that as the engine temp climbs the idles creeps up as well. by the time the temp reaches 160 the idle is at 1400 rpm.

Kevova

If IAC works in diagnostic mode it's not defective. MAF is too high IAC is commanded too high. What it min air rate? If car was running right why go to fast efi?

rssshen vette

I had plans to build another engine to replace the 383 and use the F.A.S.T. ECM to run that engine and eliminate mail order tuning. I don't what the min air rate is. I don't see that table in Tuner Pro RT.

aklim

I really doubt it but you can always hit it with an infrared thermometer and see if the ECM agrees with the sensor. Something is commanding the idle. Can you scan for the command idle to see what the ECM wants? Either Command Idle or Desired Idle or something along those lines?

aklim

Not sure. I'd just get a dyno tune instead of fooling around with "guess a tune" stuff. Is there no dyno facility there?

Maybe he is referring to the Minimum Air adjustment as outlined by the FSM which is totally useless in this case.

Kevova

Min air rate is not ecm controlled. With timing is fixed. IAC fully extended. It is the base idle and it's important just the factory spec doesn't apply With what you have, I would say 550+/- 50. From there you need to reset tps. You may have to make it adjustable. It would be worthwhile to convert to a Speed Density using a MAP sensor not MAF.
The high maf reading indicates large volume of air being drawn into throttle body ecm would add fuel. A low number and high idle would indicate vacuum leak or air leak between sensor and throttle body.. Air would bypass sensor entering engine.

rssshen vette

There are several dyno shop in and around the Nashville area, but none of them will tune a OBD1 car. At least that's what they tell me when I call to inquire about tuning my car.

rssshen vette

Thanks guys for your replies! I'll get back on it tomorrow. I'll keep you post of my progress.

C409

..... The ECM is commanding the IAC to 160 counts for some reason ... double check the wiring to the IAC ... wiring color codes to correct pins and continuity back to the ECM ... also IAC pin locations at the ECM since you went with the one from an '87 ... the IAC should be around 30 and the MAF around 10 @ normal idle speed ... if there was a vacuum leak , the IAC would close to zero trying to reduce the idle speed ..... Check the desired idle speed in the software ... make sure there isn't an extra zero in there ... Where is your TPS at idle ? .....

C409

..... The FAST should use a MAP sensor ... no direct connection to a MAF .....

aklim

He may need to track down the wire from the solenoid to the ECM to seesee it git cut somewhsomesome Ask me how I know.

If that works out I would wiwond what the command idle is.