1G 1g dies when coming off throttle

[tkr]

I have a 91 talon awd and just recently it starting having idling issues. When I push in the clutch when coming to a stop, the RPMs drop too low and it dies. It's like it can't catch the idle. Then I start it again and it idles fine until the next stop. I've done a lot of research and almost everything pointed to the ISC, but I checked it with a multimeter and it's getting good resistance through all the coils. I'm also running the stock BOV recirculated. I was wondering if maybe a boost or vacuum leak could cause this, but it seems like it boosts and holds just fine. Car is all stock except for intake, exhaust, 20g, and a rewired walbro 255. Any help would be much appreciated!

 

[Stoneman]

Get a can of starter fluid and while its idling lightly spray your vacum lines one by one and see if the idle surges up. This should tell you if you have a vac leak and where it is.

 

[steve]

I've done a lot of research and almost everything pointed to the ISC, but I checked it with a multimeter and it's getting good resistance through all the coils.

The ISC coil resistance is only part of the story.

The idle circuit starts with the Idle Position Switch. It needs a good ground to the throttle body to work correctly. The green wire comes from the ECU and has a 5 to 12v signal on it. When the throttle is closed the signal is pulled to ground (0v) by the IPS. I've found a lot of cars over the years loose the ground strap for the throttle body and the IPS becomes intermittent. If the IPS doesn't pull the signal to ground the ECU doesn't know to control the idle speed. The other side effect is it also won't idle surge if the IPS isn't working.

Next part is the drivers on the ECU that activate the ISC Coils. After measuring the resistance of the coils at 28-32 ohms for each of the 4 coils, the easy way to test if the ISC works is to connected it to the harness when off the throttle body and watch it while someone power cycles the ECU. When you turn the ECU on the ISC pintle should move in and out making a scratching sound rather than a clicking sound and wiggling. There is a video on youtube showing this. Make sure the o-ring is good in the bottom of the ISC bore when you reinstall the ISC so it doesn't leak.

Last the throttle body needs to be clean and the Basic Idle Speed Screw needs to be correctly set. The procedure for adjusting the BISS is on the site and on the VFAQ site. If the car doesn't idle at 750 +- 50 RPM normally the idle circuit isn't working. You must have DSMLink or some other modified ECU software to idle correctly at a different speed.

As designed by the factory when cruising the ECU steps the ISC out (opens up the bypass) based on the TPS position. When the ECU sees the throttle close it slowly steps the ISC in to act like a dashpot to keep the engine from stalling.

Checking for vacuum leaks is a great idea but stalling like this is usually because of a failure in the idle circuit described.

 

[tkr]

Yea it idles at around 1000 RPMs and I don't have any tuning software so I'm guessing the idle circuit isn't working?

 

[DSM1G90]

Tach on the dash is a decent reference for idle speed...what I did after I reset my ISC from instruction via VFAQ, and set the BISS screw. I had no tuning tools to speak of as well.

If I ever have to replace the ISC motor in my 1990, it sits different than the 1991 year, meaning a removal of the throttle body.

I know it's working when I'm rowing through the gears in town traffic - coming to the light, I dump the clutch, idle wants to fall below 500 RPM, but recovers to head up 900 before settling down to the 750 setting.

True, check all of your grounds. Connections needs to be clean and bolt tight.

 

[steve]

Yea it idles at around 1000 RPMs and I don't have any tuning software so I'm guessing the idle circuit isn't working?

Correct, now you get to figure out why. If it's idling that high I wouldn't be surprised to find that the BISS is already screwed in all the way and you'll have to sort out the whole intake side of things to get it back working right.