1986 GT stumbles badly off idle (Page 3/4)
balaseca APR 18, 03:57 AM

quote
Originally posted by Patrick:

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.



Firstly, Patrick, Thanks for all the timely help!

I put some Marvel Mystery Oil in the tank to see if lubing up the fuel system might help things. Any other advice on a leaky injector?

The TPS goes from about .5V to 4.7V smoothly through the whole range.

Havent checked the MAP, but have heard that they're cleanable? I think...

Another thing I've noticed is that my exhaust is WAY quieter than all the stock exhaust videos on the ole YouTube. Flow seems good when revving, but I'd expect a little more (a lot more, really) oomph from the motor than I'm getting
Vintage-Nut APR 18, 06:45 AM

quote
Havent checked the MAP



The MAP is a very good idea!

The MAP output is used to calculate vacuum advance for the ignition....

Bad MAP / Bad Ignition Timing


quote
have heard that they're cleanable?


NO, you will likely destroy it. This sensor cannot be cleaned or repaired. Only replace it.

BTW
GM discontinued the original MAP and revised the sensor years ago; so, if you have the original, get the newer sensor which is more accurate.

The GM part number for this revised map sensor is 12569240 {ACDelco part number 213-1545}
*Don't be cheap - buy a GENUINE GM MAP!

PS - Manifold Absolute Pressure Sensor TEST


------------------
Original Owner of a Silver '88 GT
Under 'Production Refurbishment' @ 136k Miles

[This message has been edited by Vintage-Nut (edited 04-18-2025).]

82-T/A [At Work] APR 18, 09:38 AM

quote
Originally posted by balaseca:

The airbox is another issue. The genius that owned it before me stuck a cone filter on the end of the stock tube that goes to the throttle body (finding a new stock filter housing isn't fun).

I've looked at the cap and rotor and they don't look worn at all.




This might be an issue too... there is an Intake Air Temperature sensor that's supposed to be installed in the air cleaner assembly. If this is not connected or... who knows where it is? Then it's not properly reading the temperature of the incoming air.

Newer cars which have a MAF (Mass Air Flow sensor) have one of these built in, right in the center of the MAF sensor... and it's a critical component for the ECM to properly regulate fuel per the temperature of the incoming air. If it's just dangling, or worse... hanging just above the exhaust Y-Pipe, it's going to be giving REALLY bad readings which will dramatically affect the fuel mapping.

EDIT: Sorry, didn't read the other responses before I responded. 100% what Patrick said, the IAT is a concern. Having it disconnected is a problem. It directly affects the amount of fuel that the engine gets, and it must rely on other things instead. As Patrick said... when the engine is not warmed up, it's running "off the tape" so to speak. Once it's warmed up, then it relies on the sensors. If you've ALSO removed the EGR valve... that too affects the fuel map, since the engine expects a certain amount of exhaust gasses being cycled back into the engine... which also lowers combustion temperatures, and affects how much or how little fuel is needed. To actually remove the EGR valve, it would need the ECM to be reprogrammed otherwise you'll be running lean at different points.

[This message has been edited by 82-T/A [At Work] (edited 04-18-2025).]

balaseca APR 18, 09:57 AM
I have plugged in a new IAT, undecided on the best place for it though (besides the airbox that I'm looking for haha).

I'd love to get an EGR setup for the car, but though I can find a valve and a tube, the part that goes from the exhaust piping to the EGR valve and allows the tube to be in place is something I just dont have any clue where to find.
Vintage-Nut APR 18, 10:42 AM
1985-86 Air Cleaner / Housing

Two PFF members sells Fiero parts

*Member Larry Hubert and owns 'Fiero Depot' in Florida

*Member mrfiero {Mike Valentine} and owns 'The Fiero Factory' and has moved the base of operations to Denver, CO.

EGR Setup

quote
the part that goes from the exhaust piping to the EGR valve and allows the tube to be in place is something I just dont have any clue where to find.


GM 10034074 / Adapter, EGR Valve


GM Parts Store in Albuquerque NM might have one at $23 + shipping.....
https://www.gmpartsstore.co...f-s-adapter-10034074
82-T/A [At Work] APR 18, 10:52 AM

quote
Originally posted by balaseca:

I have plugged in a new IAT, undecided on the best place for it though (besides the airbox that I'm looking for haha).

I'd love to get an EGR setup for the car, but though I can find a valve and a tube, the part that goes from the exhaust piping to the EGR valve and allows the tube to be in place is something I just dont have any clue where to find.



Ugh, yeah... I just looked on eBay for you... and couldn't find any of that stuff. Normally that stuff is all available... I'm surprised none of it is on eBay right now.

So you need:

1 - V6 Air Cleaner Assembly
2 - V6 EGR Block (Y-pipe to EGR valve)
3 - V6 EGR Valve ... you can get this from Rock Auto
4 - V6 EGR Tube ... you can get this from the Fiero Store: https://www.fierostore.com/.../Detail.aspx?s=67019
5 - V6 EGR Valve Hardware ... you can get this from the Fiero Store: https://www.fierostore.com/.../Detail.aspx?s=66452


For the first two items, you could try reaching out to Mike's Fiero Parts:




You could also reach out to Mr.Fiero on here, and PM him: https://www.fiero.nl/cgi-bi...bio&UserName=mrfiero (send him a private message)
balaseca APR 18, 11:12 AM
I'll have to give him a shout. I'm about to make a seperate thread about my non-functioning VSS too. Haha
balaseca MAY 10, 11:30 AM
So, I've uploaded a video to youtube showing the issue.

https://youtube.com/shorts/...za5lIU?feature=share

Pressure at the schrader valve is 41 PSI when reved. 38 PSI at idle, 41 PSI with key on and motor off. There is some leak down after the key is turned off.

Car does also sometimes stumble at WOT when driving at highway speeds (60+ mph).

I'm thinking FPR coupled with gummy injectors?

Timing is at 10 degrees BTDC (checked in the last two days)
Patrick MAY 10, 05:08 PM

quote
Originally posted by balaseca:

I'm thinking FPR coupled with gummy injectors?



What's FPR ? [EDIT] Fuel pressure regulator.

Could be the MAP sensor as suggested earlier... but really, trying to diagnose an engine issue without using WinALDL and/or a scanner is just taking shots in the dark.

[This message has been edited by Patrick (edited 05-10-2025).]

balaseca MAY 10, 06:18 PM
Patrick,

I was thinking the same thing about WIN-ALDL. I need to get me a cable.