1986 GT stumbles badly off idle (Page 2/4)
balaseca APR 17, 01:45 PM

quote
Originally posted by Patrick:

I don't think you're trying hard enough.




Hahaha!

Yes, I verified with both a small screwdriver and a borescope. The wide slot in the balancer lines up with 0 deg at TDC on compression stroke for Cyl #1.

I know the IAT is an issue, but can it cause what I'm experiencing? Only happens after the car warms up. My other thought was residual gunk in my injectors. I know fuel system cleaner in a can is okay, but the car sat for 15 years, so I'm trying to pick the collective brain of the Fiero community. I normally stick to carbs. lol
Patrick APR 17, 02:03 PM

quote
Originally posted by balaseca:

Yes, I verified with both a small screwdriver and a borescope. The wide slot in the balancer lines up with 0 deg at TDC on compression stroke for Cyl #1.



Okay fine. Ya gotta know that we get all types of new members posting questions here. Some are mechanically knowledgeable, some... aren't.


quote
Originally posted by balaseca:

I know the IAT is an issue, but can it cause what I'm experiencing?



It can't be helping. When the engine is warmed up, it goes into closed loop operation... and that's when its relying on info from the sensors.

I doubt it's the cat. I've had a plugged cat, and it affected the engine mostly when the revs got up... which stands to reason as it couldn't breathe. Off idle was basically fine.

Have you checked that both banks of injectors are firing? There are two fuses, one for each bank. If only one bank is firing, the engine can almost seem fine while idling, but then falls on its face when trying to accelerate.

[This message has been edited by Patrick (edited 04-17-2025).]

balaseca APR 17, 02:17 PM

quote
Originally posted by Patrick:

It can't be helping. When the engine is warmed up, it goes into closed loop operation... and that's when its relying on info from the sensors.

I doubt it's the cat. I've had a plugged cat, and it affected the engine mostly when the revs got up... which stands to reason as it couldn't breathe. Off idle was basically fine.

Have you checked that both banks of injectors are firing? There are two fuses, one for each bank. If only one bank is firing, the engine can almost seem fine while idling, but then falls on its face when trying to accelerate.




I know that the fuses are good, but actually haven't checked to see if both banks are firing. What's the best way to check that?

The car seems to have no issues on open loop (aka right when cranked, and at WOT). It's that right off idle that really kills it. Also, there's light surging when holding a steady speed (around 25-30mph).
Patrick APR 17, 02:31 PM

quote
Originally posted by balaseca:

I know that the fuses are good, but actually haven't checked to see if both banks are firing. What's the best way to check that?



A noid light, I guess... but I've never used one myself. What I have done is used a mechanics stethoscope with the metal probe attached and listened to each injector individually to make sure it was clicking. And that was after checking the resistance of each injector to make sure they were all in spec.

I just wanted to mention that the reason for doing a fuel pressure test is two-fold. Yes, it shows the fuel pressure while the engine is running... but just as importantly, it shows how quickly the fuel pressure drops when the engine is turned off. It shouldn't drop quickly. It should maintain pressure for... oh, I dunno... ten minutes or so before slowly dropping. Quickly losing fuel pressure could possibly indicate stuck open and/or leaky injectors.

[This message has been edited by Patrick (edited 04-17-2025).]

balaseca APR 17, 03:17 PM
Makes sense. I'll have to do the mechanic's stethoscope thing. I'll also make sure I do a fuel pressure test.

I just took a drive in her and it seems like its worse when at a stop on an incline.
Vintage-Nut APR 17, 03:35 PM

quote
As to EGR, there are blockoff plates on both the intake and exhaust, no tube or valve there. The solenoid is plugged in, but one vacuum line is pluged with a small screw and the other still travels under the intake.



There are differing opinions, however I suggest verifying if the ECM chip was reprogrammed to override the EGR solenoid input.

balaseca APR 17, 05:16 PM
Well, I did the pressure test. 41 PSI with key on and engine off. After 10 mins, it had dropped 6 PSI.

New cap and rotor, still bogs badly if you stomp on the pedal (or even just get on the gas). Pretty much have to rev up the motor slowly from a stop or it will just sit and stumble all over itself till you let off the gas totally.
olejoedad APR 17, 08:08 PM
After you timed it, did you remove the jumper from the ALDL plug?
balaseca APR 18, 03:10 AM
Yes.

When I get home from work today, I'm going to take a short video clip of what it does, see if hearing/seeing it helps at all.
Patrick APR 18, 03:42 AM

quote
Originally posted by balaseca:

Well, I did the pressure test. 41 PSI with key on and engine off. After 10 mins, it had dropped 6 PSI.



IMO, that's dropping way too fast. I'm not saying that this is necessarily the reason for the symptoms you've mentioned, but I suspect you've got at least one leaky injector... or that short length of hose in the fuel tank is possibly leaking.

Have you checked the TPS? What you're looking for is dead spots, and I believe at idle it's supposed be .5v

And then there's the MAP sensor. It'll definitely cause a bog if it's faulty, but I don't know the procedure to check it.