atgrazi

Hello everyone,

Long time reader, first time poster here. I just got a 1986 Trans Am with the 305 TPI and 700R4 Auto tranny (I had an 84 before with carb). The previous owner wasn't very helpful in telling me what all was done to it. It has a fastchip PROM in the ECU, I don't know if this is causing issues or what. It's also had the smog pump bypassed and all the pollution controls plugged up or removed. And the AC compressor bypassed. Guess he was trying to get max power...

Anyways... Here it goes. Every time I start the car (engine cold or engine hot) it idles at 700-800RPM in park and 650-700 in gear, but after about 3-4 minutes, the idle increases to 1200 and sometimes 1500RPM. I replaced the Idle Air Control Valve, but it runs the same. After warmed up with 1200RPM idle if I put it in gear, the whole car will jerk and then it will idle down to 700-900, but as I am just letting off the brakes it idles up and the car speeds up on it's own without pushing the gas up to 25MPH. Even if I get the engine totally warmed up, if I shut it off and turn it back on immediately, it has a low idle (normal, not missing) and then it eeks it's way back on up to 1200-1500RPMs again.

I am not familiar with the fastchip, but I've been told this might be what is causing this. Overall, I never get a check engine light, even if I unplug every sensor on the car. But the check engine light works if I just turn the key forward and don't start the engine (so the light is there and not blown or disconnected). So what do you all think could be causing this? Bad ECU? Bad performance chip? Or is the fast chip supposed to do this? I know the ECU is doing something, if I unplug certain sensors it will run like crap, just no check engine light. If the fastchip is causing this, I'll order a remanufactured ECU and swap it.

My dad keeps insisting it's a vacuum leak, but would that really cause it to always idle at base speed when starting hot or cold then idle up again to 1500RPMs? Wouldn't that be more consistent if it's a leak? With this idle creeping up, it is un-driveable and might hurt the transmission when changing between park and in gear.

Any ideas? Thanks

atgrazi

I got a remanufactured ECU today, and another TP sensor and another IAC valve. All original AC Delco parts... now when I start it the car idles at 1500rpm all the time. Put it in gear and it goes down to 700-800rpm. I know the computer is working now, before if I unplug any critical sensors like TPS, nothing would happen. Now if I unplug TPS, it won't run and I get a check engine light. Before with the computer in there nothing would trip the light. Is there anything else that controls the idle speed? I see some books mention high idle relay, and some mention ingine temperature sensors that can tell the computer what to idle at... where to find these and how to test them?

paulo57509

Question: Did the reman ECU come with a mamcal or did you have to swap the FastChip into it?

atgrazi

It didn't come with chip, or anything else. I had to put the fastchip into the new ECU. It was an original AC Delco from parts geek. But things did change. Instead of idling low when cold and high when warm, it idles high all the time, except when in gear.

atgrazi

Today I got a new Borg Warner TPS sensor and adjusted it to spec, about 0.54 volts closed and over 4V when I rotate the throttle wide open. Now it idles normally, but after warming up when it idles under 700, it gets rough and wants to quit. The exhaust smells quite rich. I don't know if I have a vacuum leak now or what.

paulo57509

This kind of indicates that there's an issue when the ECM goes into closed loop. However, because things have been modified in an unknown manner, it's hard to really say where to start looking.

The first thing I would do is find a stock chip and see how the engine behaves. Eliminating the FastChip as the potential problem would save you a lot of time, guessing and frustration.

atgrazi

I'm looking for a stock chip now. Also looking for an ALDL interface and some software so I can try to see what's going on. All the guesswork is making me pull my hair out. Thanks for the response.

84Elky

Running blind not knowing what's in the BIN's Calibration (the data the BIN code uses) rarely works. Would recommend getting TunerPro and the appropriate equipment from moates.net so the content of the existing BIN is known (APU1 and HDR1). This will allow you to get the contents of the current BIN (and later program a chip yourself) to see what's in the Calibration. For example, there's a table that controls closed throttle idle RPM relative to coolant temperature.

Also, if your using AUJP, a stock MEMCAL with stock chip likely cannot be found for less that $200-$250. Can spend about that and get the equipment that will allow you to do everything yourself?

johng1

iTrader: (1)

I would dare say you have a vacuum leak. See if a local garage can use their smoke machine to find out. It should not cost much and will at least rule that out. A vacuum leak will cause the car to do what your describing. I've got a fastchip for my car and have had some weird idle problems and it was a vacuum leak every time. Most likely nothing wrong with the fastchip.

atgrazi

Had it checked for vacuum leaks, didn't find any. The EGR valve might be stuck. If I want to delete that, I also need to reprogram the computer. I don't want to put another $200-300 at it unless I'm getting an intake, carburetor and getting rid of this ancient computer. Some have said Holley carburetors with TPS sensor will allow the shifting and timing advance to still work with what it has. I don't think I can maintain the TPI at this point. Even if I need to change the distributor to a vacuum advance model, all of these things together are cheaper than a new chip and interfacing tools. Even the mass air sensor is like $700 for Delco and $200 for offbrand reman parts... so all I need to motivate me is another bad sensor or computer reprogramming and I'm going carbureted. 😂

atgrazi

Seems all my weird idling goes away when I unplug the MAF. I guess the MAF sensor is bad. 😅