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SlownesS
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Discussion Starter #1
Hi there. 91 stealth twin turbo.

Just changed the IAC and I am positive it works.

Sometimes when I push the clutch pedal in the rpms will drop to nearly 0, then climb back up a little over 1k then idle lowly after that.

Does this sound like a BISS problem?

Thanks in advance
 

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My first guess would have been the IAC servo. Did you do a continuity check between all 6 pins mentioned on Jeff's ISC servo page? Stealth 316 - ISC Servo Motro

I recently replaced the original IAC servo in my VR4 due some idle issues similar to what you have described. When I pulled the IAC servo to replace it, I tested it and it had no continuity between two of the pins. My car has been idling perfectly now since I replaced the servo (I was sure to check the continuity between the pins again on the replacement servo before I installed it).

One way to check if the IAC servo is working is to turn on the A/C when your car is idling. The RPMs should increase by a couple hundred. Previously, when I would turn on my A/C my idle stayed exactly at 600 RPM and my lights would dim due to the big increase in current draw by the compressor and fans. Sometimes the RPMs would drop to nearly 0 as you described and the engine would almost die. Not only was the idle speed not changing when it should have been but 600 RPMs is a little low for these cars from my understanding. Now she idles somewhere around 800RPM.

One other thing you could check to help diagnose the problem is the voltage across the battery at idle to see what the alternator is charging at. Also, how many miles are on the car?
 

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SlownesS
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Discussion Starter #3
The iac reads perfect within the ranges.

It looks as if my biss screw is all the way in. Ill have to bust out the data logger and try to set the idle. Thanks for the help!
 

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Wheee! 2 wheels is fun!
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Sometimes when I push the clutch pedal in the rpms will drop to nearly 0, then climb back up a little over 1k then idle lowly after that.

Does this sound like a BISS problem?
Nope. Its still an IAC problem. If the IAC itself is OK, then the passage in the TB is probably dirty/blocked and not allowing the IAC to function.

The only time you should ever have to mess with the BISS screw is if someone else messed with it and you have to put it back correctly.

-SP
 

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Clean your IAC, check hoses near the throttle body for vacuum leaks.

Ahh You have a Twin Turbo.

What kind of BOV/Blow off valves are you running? Are you running it to open atmosphere or closed loop(recirculating)?

Running a Open atmosphere bov can cause the car to stall when pushing in the clutch. The only way to fix this is to cut or make a hold in the clutch vacuum line, so that it wont stall anymore.

I say this because, I was running an open atmosphere BOV on the stock MAF sensor, when I fixed the vacuum leak in my clutch vacuum assist line, my car would stall when I push in the clutch. The vacuum leak also made my car idle at 1500 but after fixing the vacuum leak, it would idle at 800-900 rpm. Sometime when pushing in the clutch it will drop to 100-200 and then pick up to 850rpm. Sometimes it would die.
 

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SlownesS
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Discussion Starter #6
Clean your IAC, check hoses near the throttle body for vacuum leaks.

Ahh You have a Twin Turbo.

What kind of BOV/Blow off valves are you running? Are you running it to open atmosphere or closed loop(recirculating)?

Running a Open atmosphere bov can cause the car to stall when pushing in the clutch. The only way to fix this is to cut or make a hold in the clutch vacuum line, so that it wont stall anymore.

I say this because, I was running an open atmosphere BOV on the stock MAF sensor, when I fixed the vacuum leak in my clutch vacuum assist line, my car would stall when I push in the clutch. The vacuum leak also made my car idle at 1500 but after fixing the vacuum leak, it would idle at 800-900 rpm. Sometime when pushing in the clutch it will drop to 100-200 and then pick up to 850rpm. Sometimes it would die.
I literally have every single symptom that you mentioned, and I have a Turbo XS RFL blow off valve. Honestly it's too loud for my liking. Idles at 1500, but wants do die when pushing the clutch in after reving it. So now I have to check for vacuum leaks...hmm the previous owner did a bunch of hose deletions.

So what exactly do I have to do. Should I show a pic of how it's set up?
 

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I literally have every single symptom that you mentioned, and I have a Turbo XS RFL blow off valve. Honestly it's too loud for my liking. Idles at 1500, but wants do die when pushing the clutch in after reving it. So now I have to check for vacuum leaks...hmm the previous owner did a bunch of hose deletions.

So what exactly do I have to do. Should I show a pic of how it's set up?
Funny that you mentioned that...I also have the TurboXs RFL bov, I never had a stalling problem until I fixed the vacuum leak that I was talking about.

Now the car stalled when pushing in the clutch.

You could make a cut or slice in the vacuum hose and see if that helps.

In the picture below is the Clutch Vacuum hard line, if you follow the metal hard line going up towards the firewall, you will see about a 4-6 inch rubber fuel hose, mine was cracked and making a hissing sound(vacuum leak), once I replaced that line, BAM car would stall when pushing in the clutch. This is due to the TurboXs RFL being a Open Atmosphere BOV...

Only way around it is to make a cut in that vacuum line, or go to recirculation bov.

The vacuum line only made my car idle at 1500 rpm maybe 1400 that was the only side effect. Other then that the car would run great with the TurboXs RFL.
 

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Fix a problem by intentionally inducing a vacuum leak? That doesn't sound like fixing a problem to me. Maybe masking one problem with another.

You're missing your dashpot. What that does is keep the TB butterfly from slamming shut abruptly and allows for a more gradual transition to the idle circuit (via the IAC). I'd look replacing the dashpot or maybe the entire TB. Your other post indicates you're running a NA TB, right?
 

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Fix a problem by intentionally inducing a vacuum leak? That doesn't sound like fixing a problem to me. Maybe masking one problem with another.

You're missing your dashpot. What that does is keep the TB butterfly from slamming shut abruptly and allows for a more gradual transition to the idle circuit (via the IAC). I'd look replacing the dashpot or maybe the entire TB. Your other post indicates you're running a NA TB, right?
I have a dashpot and mine stalls. The reasons some people are able to run open atmosphere on the stock maf is because they have a vacuum leak some where they don't know about.
 

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