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Nhra747
02-26-2017, 08:19 PM
My 1991 supercoupe has been dying at idle in gear whenever the car has warmed up
Idle fine in gear while it is cold, no check engine lights or codes when doing engine running test runs great
Completely stock with new plugs, fuel filter, air filter, o2 sensors, and iac valve
Vac gauge reads 20 at idle, 12psi boost at 5000rpm
Any ideas?

TbirdSCFan
02-26-2017, 10:29 PM
My 1991 supercoupe has been dying at idle in gear whenever the car has warmed up
Idle fine in gear while it is cold, no check engine lights or codes when doing engine running test runs great
Completely stock with new plugs, fuel filter, air filter, o2 sensors, and iac valve
Vac gauge reads 20 at idle, 12psi boost at 5000rpm
Any ideas? Check your engine mounts. And no.. Im NOT joking. :cool: If they are collapsed and the limiter arm(s) is resting on the lower part of the bracket, they need to be replaced. Another way to tell is if your oil pan is resting on the subframe.. they're gone.

Nhra747
02-27-2017, 10:42 PM
Inspected the motor mounts they look fairly new and the oil pan is away from the kmember

Creighton
02-27-2017, 11:55 PM
Chasing same issue. VAC reading in D ? What is RPM at idle? Bad new O2 sensors are out there. Let us know what you find.
Thanks!
Creighton

TBirdJKC
02-28-2017, 12:00 AM
Is it an auto? Your torque converter may be trying to lockup when it warms up.

Nhra747
02-28-2017, 12:18 AM
At idle in d is 15ish
Approx 650rpm in d when it doesn't die, 900 in park

Did the same thing when the o2 were bad so I don't think that's the issue, plus they eliminated the codes I had for running lean

The car is auto, I don't think the converter is locking at idle but how would I be able to check that?

sam jones
02-28-2017, 04:58 AM
Good morning


Check fuel pressure. When was the last time the fuel filter was changed?

TbirdSCFan
02-28-2017, 05:52 AM
At idle in d is 15ish
Approx 650rpm in d when it doesn't die, 900 in park

Did the same thing when the o2 were bad so I don't think that's the issue, plus they eliminated the codes I had for running lean

The car is auto, I don't think the converter is locking at idle but how would I be able to check that?

900 is a tad high. Have you checked for vacuum leaks?

Something else you can try is to disconnect the battery for a couple of minutes and then restart and let it idle for 5-10 minutes to relearn idle settings. Then see how it behaves.

Nhra747
02-28-2017, 10:19 AM
I have checked everywhere you can think of for vac leaks, sprayed ether around and notice no change in idle
I'm going to check fuel psi tonight

aroot1
02-28-2017, 03:58 PM
I have checked everywhere you can think of for vac leaks, sprayed ether around and notice no change in idle
I'm going to check fuel psi tonight

The old either/carb cleaner trick really doesn't work on these rigs. That was fine for old carb/vac distributor stuff, but the dis system will compensate with near instant timing and other changes, unless the leak is massive. Best to do a smoke test. I have been thinking of buying one of the $50-80 "paint can"/baby oil home made units from fee-bay, worth it 4 me to NOT have to spend the time running down and assembling the mish-mash of parts.

Adam

Nhra747
03-01-2017, 09:53 PM
Fuel pressure is a steady 37ish at idle, goes up as gas/boost is applied

sam jones
03-02-2017, 12:34 AM
Good evening

Did you check fuel pressure with the vacuum line disconnected from the fuel pressure regulator with the engine running? Was the engine warmed/operating temperature when you recorded fuel pressure? What was the engine key on engine off fuel pressure? Was it steady or was it low and dropped off suddenly?

With the fuel pressure gauge tester installed and the engine running try putting it in gear. If the engine shut off, what is the fuel pressure?

Have you inspected the fuel pump relay? Are the fuel pump relay pins corroded? Are the fuel pump relay connector wiring burned, damaged and or un-serviceable?

Creighton
03-02-2017, 03:57 AM
Great thread going here. Test for TC lockup at idle?
Thanks
Creighton

SuperCoupeDave
03-02-2017, 12:31 PM
My 89 is doing a similar thing, but its not idling well in park either. Idle is somewhat erratic. RPM gauge moves around a lot. If I rev it in park when the engine speed comes down it wants to die but revives its self.
My problem could be that the car is only driven maybe a few hundred miles a year. Its a showroom condition low mile SC, So I'm trying to keep it pristine. Probably need to run a good fuel system cleaner through it.

KMT
03-02-2017, 01:48 PM
When doing a cold start, does the idle sit high (1100~1500 rpm?) for a few moments and then step down?

How old is the TPS?

What brand O2 sensors?

What is the warm idle rpm with the IAC disconnected? It should struggle, but not die outright. Don't adjust the throttle plate stop to make it happy...that's the IAC's job.

Inspect the interior of the throttle body around the plate for carbon build-up. Remove the throttle cable and check throttle plate movement off idle to confirm it isn't hanging up, which could indicate a build up that could be limiting airflow which could affect the system's normal ability to maintain idle.

SuperCoupeDave
03-02-2017, 03:12 PM
When doing a cold start, does the idle sit high (1100~1500 rpm?) for a few moments and then step down?

How old is the TPS?

What brand O2 sensors?

What is the warm idle rpm with the IAC disconnected?

Mine when I'm doing a cold start it will do a quick high idle, quick being 1-2 seconds then drop to normal idle speed. Doesn't matter how cold it is in the garage. All my sensors are factory as far as I know. Bought the car 3 years ago with 46k miles. Only thing done to motor was water pump.

KMT
03-02-2017, 03:44 PM
Mine when I'm doing a cold start it will do a quick high idle ~ All my sensors are factory as far as I know.

Sorry, I was trying to debug Nhra747's idle issue.

Ken

BLOWN38
03-02-2017, 04:27 PM
Unplug the IAC valve after it has warmed up and see if it will idle. If not you need to open the throttle blade a little or some of the TB's have a bypass screw on them covered by a rubber plug. You can open that if its there. Open either so that the car idles about 600 rpm with the IAC unplugged.

figulaz
03-02-2017, 06:18 PM
If it idled fine before my money is still a vac leak. Are you seeing 20hg at you boost vac guage or a hand held vac guage...world of difference. Someone mentioned a smoke test above...if you're ok with smoking i've used a cigar to find leaks with some fair results..not a perfect solution but has helped with some tricky leaks. Any time you work on these cars (or any 20+ yr old survivor) there is some kind of potential to generate a new vac/boost leak. Not a bad idea to replace lines if you haven't. Good luck.

TBirdJKC
03-02-2017, 09:04 PM
I still think its your torque converter. Have you tried putting the rear up on jack stands (wheels off the ground of course) and shift into gear? With nothing stopping the wheels it shouldn't die.

Creighton
03-02-2017, 11:48 PM
>I still think its your torque converter. Have you tried putting the rear up on jack stands (wheels off the ground of course) and shift into gear? With nothing stopping the wheels it shouldn't die.

Thank you sir. That was the test I was looking for.
Creighton