hillisml

My 91 convertible likes to stall at idle when the A/C's on. I have no problems first thing in the morning, but after the car is hot it tends to stall unless I goose the gas a bit. The hotter the weather gets, the worse the problem. I'm in Puerto Rico so this is a constant problem due to the heat. If I turn the A/C off, when I get to a light the RPM will drop to about 400 and then back up to 750 for a normal idle.

A quick search points to the TPS as a good suspect. I shorted the 1-pin green connector and watch the code on the "Check Engine" light. I get a code 18, which is Throttle Sensor, Narrow Range. Great, I figure, this is exactly what I expected. Following the shop manual, I measured throttle open and closed voltages at the TPS for both narrow and full range. All tested good. I connected a TPS test light to the 3-pin green connector and one light was on and one was off, which says the TPS is correctly adjusted.

So, the ECU says the TPS is bad or out of adjustment, but everything about the TPS tests good. Bad ECU? Bad something else? Any ideas?

presetlimits

the TPS can 'test' to be good, but still be faulty

downwinddave

My idle is low to start with. I have the same problem on my 91 vert. I am going to swop TPS and ECU next weekend. (Am off to mainland)

Let us know what you find out.

Dave in Honolulu

Rotorific

iTrader: (1)

You could also test your BAC valve being that is key for idling with the ac

downwinddave

Thanks will check it also!

chris_stampe

If the bac valve is good it might be the transistor in the ecu that controls the bac. Your problem fits the description of this perfectly.

chris_stampe

O yeah, clean the **** outta your bac w/ wd40 and test to see if it clicks w/ 12v on one terminal and ground on the other. If it still doesnt work, search for that transistor fix. Only like $2 from radio shack and doesnt take too much work.

hillisml

I'll check my BAC over the weekend. Can anyone tell me more about the transistor fix in the ECU? I'm very comfortable with a soldering iron and working on the ECU doesn't give me the heebeejeebees. Does the ECU general suffer from the cold solder joint problems that other 2nd gen electrical components have?

hugues

Look at this for idle diagnosis

It has the ecu fix as well, if this happens to be your problem

http://www.teamfc3s.org/info/articles/idle.html

Have a good one,
Hugues -

hillisml

I did lots for troubleshooting this weekend and still have an idle problem. I followed the idle FAQ. I verified my TPS is set correctly, and as mentioned before, it measures right even though the ECU gives a code 18 saying the TPS is bad. I moved on to the BAC. I disconnected the BAC connector with the engine at idle, as per the workshop book, but the there was no change in idle speed. I have the correct voltage at the connector so the ECU should be good. I next removed the BAC and measured its resistance. Also good. I applied 12V to the BAC and it clicks, so it looks like the BAC is good.

So, all this info seems to conflict. The BAC seems to be good but disconnecting the BAC with the engine running should result in a drop in idle speed, and that's not happening. What else should I look at?

Again, the car runs fine in the morning when the weather is cool. In the afternoon when the weather gets ungodly hot the car tries like mad to stall at idle with the A/C running, and sometimes even with the A/C off.

I was getting a code 18 (low-range TPS) last week. I cleared it as part of my weekend work and now I'm getting a code 17 (O2 sensor).