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1G 91 Talon TSi stalls at idle

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MyTalonTSi1991

Proven Member
48
28
Jul 12, 2019
Bloomington, Illinois
I have a full stock configuration 91 TSI FWD Automatic.
Car has been running absolutely perfect and it randomly in the last month or so has started sputtering at idle and stalling.

Most of the time it drives fine with no issues but sometimes it gets very unhappy and refuses to idle. Yesterday it stalled and had a hard time starting, had to give it part throttle to get going and the tach was kind of jumping around which it never did and it took me a good probably 5-10 minutes of this to get it to stay running and idle on its own. Got it running and proceeded to drive it back to my garage and it drove just fine. It did this a couple weeks ago, originally suspected bad gas so when the tank neared empty I put in a bottle of Lucas and fresh 93 ethanol free fuel. At that time since I had some spare parts on hand, I replaced the plugs, wires and Idle motor, idle is about 750 warm when in drive, if I turn on the AC it dips to about 600. I am kind of at a loss here.

The only "mod" on the car is a Walbro 255 fuel pump since when the original one went I had a hard time sourcing an OE spec one. Could it be my ECU? Coil? Bad plug wires? This one has me really scratching my head since this car has performed flawlessly up until this.
 
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Update.......new ECU in, idle speed properly adjusted, new CAS set up properly. Car still bogs down. My mechanic thought it was the cheap universal cat converter on it.....he took that off, checked for vacuum and boost leaks, has good fuel pressure......at a complete loss here. Thinking its going to be time to just cut my losses and sell the car.
 
Update.......new ECU in, idle speed properly adjusted, new CAS set up properly. Car still bogs down. My mechanic thought it was the cheap universal cat converter on it.....he took that off, checked for vacuum and boost leaks, has good fuel pressure......at a complete loss here. Thinking its going to be time to just cut my losses and sell the car.
When it exhibits the symptoms, does it seem like it's only running on 2 cylinders? Do you have a data logger to get any insight that way? What about the check engine light, any codes from that?

Don't get down on it. You'll figure it out and then all your hard work will pay off!
 
So this issue has been solved......TMO Logger showed no issues, all sensors reading proper levels. Shop found the fuel pump didnt appear to be working properly. To their amazement and mine, whoever owned the car before me had the wrong fuel pump installed with zip ties. A proper OE spec Denso fuel pump was installed along with a new filter....problem solved!
 
So this issue has been solved......TMO Logger showed no issues, all sensors reading proper levels. Shop found the fuel pump didnt appear to be working properly. To their amazement and mine, whoever owned the car before me had the wrong fuel pump installed with zip ties. A proper OE spec Denso fuel pump was installed along with a new filter....problem solved!
Yep, these problems always get fixed. It just takes time.
 
Have you checked your closed throttle switch? There's a switch on the throttle valve that tells the ECU "his foot's off the gas -- you're in charge of the RPM" and if that switch fails you will get stumbling/stalling, hard starting, on manual trans. you'll get difficult shifting. A failed/failing/sluggish ISC valve will give much the same symptoms but the switch tests okay.

There are three ways for air to get through the throttle valve assembly: One is the fixed idle air adjustment -- that's a screw that's adjusted following a procedure in the manual and then left alone -- it sets a basic minimum air flow through an internal passage around the TV.

Then there's the throttle valve. It controls speed under control of the gas pedal or cruise control.

Finally there's the Idle Speed Control (ISC). This is a small motor driven valve in the bottom of the throttle valve assembly. It does four main jobs:

1. Sets the correct idle RPM -- 750 in most of our cars I think unless something's going on.

2. Takes care of fast idle according to coolant temperature etc. under command of the ECU.

3. Compensates for sudden extra loads on the engine -- You turn the steering wheel suddenly when the car's sitting so the power steering puts on a load. This gives a signal to the ECU to say "give more air now'' when the event happens. (You've seen that wire that goes to the top of the PS pump, right? That's the switch that does it.) The A/C also signals the ECU but most cars do an 'idle up' -- increase idle RPM a bit for better air conditioning.

If the ISC is working properly then when sitting at idle in neutral the PS pump event should be almost imperceptible on your tach -- just a very slight drop and a bit more blip up when the load goes off. Substantial RPM changes but it does recover suggest a sluggish ISC. A/C clutch engaging you should get a smooth, prompt increase of RPM with no sag first.

Also loads that don't signal like turning on your headlights (increased load from alternator) will be compensated when the RPM drops.

ISCs are killed by four things: Use of carb cleaning solvents in the throttle without plugging the hole to the ISC. The solvents wash out the lube and may damage the plastic valve. Dirt -- run without a filter or with a rodent hole in it, you'll have ISC problems eventually. Age -- the plastic will eventually wear enough to jam on its own dust. Misadjustment of other things like the fixed idle air passage can cause the ISC to repeatedly slam closed and break the valve pintle which will give either sluggish operation or a complete jam.

4. Serves as a 'dashpot' to slow down the reduction of RPM when you take your foot off the throttle while cruising. If the throttle closes suddenly, the engine may die if the clutch is disengaged -- say coming to a stop sign. When the ISC is working right the closed throttle switch signal says "open the ISC quite a ways and then gradually close it until the correct idle RPM is reached."

Those four functions make a failed ISC or throttle switch the source of mucho driveability problems.

The test of the closed throttle switch is easy -- just put an ohmmeter on the right pins on the plug and note whether they're shorted when the throttle is closed and open when the throttle starts to open. If the switch is okay but you have the symptoms then suspect the ISC.

These two parts are both much more common failures than ECUs and a lot cheaper to fix. I've got four early '90's Mitsubishi passenger cars; all three of the high-mileage ones have had ISC failures and the one with 250k miles has had two of those and one switch failure.
 
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