Fox '87 GT can't get to stay running

spilly · Mar 29, 2026

Hi everyone. First post on the forum.

A little back story on the car. I recently inherited this '87 GT from my uncle. I remember this car very fondly from my childhood and am very happy to be the one to bring it back to life. Through my teenage years I remember the car was a constant work in progress.

The car has been parked for a very long time. Last time it was insured was 2008. It has been parked since then. I took possession of it a few weeks ago and have been going through it attempting to get it running. Rodents had gotten into the car and made a bit of a mess of things, the odd wire chewed sort of thing. I've been able to get it to start and idle (very poorly) but that has been about it. I'm not able to touch the throttle without stalling the engine instantly. When it does eventually idle half decent it's running very rich. Still can't touch the throttle tho.

Here's some details about the car;

'87 GT 5.0
Powerdyne supercharger
Cobra intake
Fully rollerized
Don't know what heads are on it
Bbk equal length headers
Msd boost timing controller
Built AOD w/ high stall torque converter
B&M hammer shifter
5 lug conversion

New gas tank, new walbro 255 lph pump, new fuel filter, new sending unit. New spark plug wires, spark plugs and new fluids, new TPS sensor.

Smoke testing the vacuum system didn't reveal anything either.

Now the stuff that's got me confused...

The car has an ecu from a 93 cobra (according to my research)

The engine harness is from '87

There is a MAF sensor on the intake with wires going up to the firewall and seem to have never been connected to the ecu?

Map sensor on the firewall has vacuum line removed and the line coming from the upper intake has been plugged. The car actually idled worse when I reconnected the vacuum line to the map sensor.

There are all kinds of vacuum lines pulled off and plugged near where the engine harness goes through the firewall. Presumably some emissions related stuff?

The engine bay isn't super tidy and seems like things have been eliminated but left behind. There are several plugs on the engine harness that are not connected to anything and I'm not sure if that has been intentional or not. Not sure if there is a tune on the ECU to tune out the maf or not. It's been a little difficult going through things because I don't know where things were left when my uncle stopped working on the car and unfortunately he's longer around to ask.

Anyone who can help point me in the right direction would be greatly appreciated.

Thank you!

86HO5.0 · Mar 30, 2026

That looks like a stock HO intake with a Cobra badge.

Pull codes.

The vacuum lines nears the engine harness are likely for the TAB/TAD connectors and EGR/EVP. They're not needed, it would be a good idea to delete and clean up all of the unused/unnecessary vacuum lines.

If you don't have a wiring schematic, get one. That will help identify what the unused connectors are for and importance of said connector or lack thereof.

The 93 Cobra ECU is natively MAF. The MAP sensor on MAF cars doesn't go to manifold vacuum, vent to atmosphere. I'd recommend wiring up the MAF.

It also looks like there's a 'piggyback' or intermediate connector on the ECU?

Since rodents have partied in the engine bay it would be a good idea to pull the upper intake to inspect wiring.

gkomo · Mar 30, 2026

Maybe he started a MAF conversion but never finished the process?

nicholase · Mar 30, 2026

I'm not up on the MAF conversions, but wouldn't a Cobra ECU in auto need some additional wiring changes? Maybe look into that and make sure that part was completed. I'm sure others will know more regarding that.

LILCBRA · Mar 30, 2026

Welcome to the site!

So in your 5th pic, that black intermediate connector is a mass air conversion piggyback for an SD ECU. The ECU is apparently from a run of the mill 89-93 5.0 manual car, not a Cobra. At least this is what a couple Google seraches tell me. So what I [think] may be part of the problem is that intermediate connector. Mass air and SD harnesses are pinned differently. So, aside from cleaning up the mess that the rodents made, you may want to repin the harness to add the mass air unit and eliminate the piggyback part. The next thing to check is what injectors is it running? If the ECU hasn't been altered and it's running something other than orange top 19 pound injectors, it will undoubtedly run rich. I think what I'd try to do is to confirm to the best of my ability that the engine itself runs as it should, then I'd approach it from the supercharged standpoint. At least that way you'd know you have a sound engine [and] you'd be able to find out what you'd need to install to get it to run well with the blower. Just my 2 cents, if it's worth that....

86HO5.0 · Mar 30, 2026

Never knew a MAF piggyback existed. Learn something new every day.

LILCBRA · Mar 30, 2026

86HO5.0 said:
Never knew a MAF piggyback existed. Learn something new every day.

I didn't know either, had to Google the part number because it looked strange.

spilly · Mar 30, 2026

LILCBRA said:
Welcome to the site!

So in your 5th pic, that black intermediate connector is a mass air conversion piggyback for an SD ECU. The ECU is apparently from a run of the mill 89-93 5.0 manual car, not a Cobra. At least this is what a couple Google seraches tell me. So what I [think] may be part of the problem is that intermediate connector. Mass air and SD harnesses are pinned differently. So, aside from cleaning up the mess that the rodents made, you may want to repin the harness to add the mass air unit and eliminate the piggyback part. The next thing to check is what injectors is it running? If the ECU hasn't been altered and it's running something other than orange top 19 pound injectors, it will undoubtedly run rich. I think what I'd try to do is to confirm to the best of my ability that the engine itself runs as it should, then I'd approach it from the supercharged standpoint. At least that way you'd know you have a sound engine [and] you'd be able to find out what you'd need to install to get it to run well with the blower. Just my 2 cents, if it's worth that....

Thank you, this information sent me down a Google path that helped a fair bit. I have been using groc to upload pictures and ask generic questions and that's where I got the "cobra ECU" from. Groc also confirmed it was an automatic ECU? But after digging deeper, the intermediary adapter should have a pigtail coming off it for the maf sensor. I thought that a rat chewed the wire initially but I was never able to find the other end in the harness. Turns out I'm looking in the wrong spot all together though. The pigtail must have fallen through the firewall and that's why I haven't located it yet. The car supposedly drove with this set up in the past.

As far as injectors, it is the red topped (30lb from what I found online) I'm not sure if the eec has been chipped or tuned for that either.

Would it make sense to track down a 89-93 MAF harness and put that in the car?does it work like that? I honestly considered just going standalone like a holley terminator x system if I couldn't get this mess sorted. I'm really not up to speed with wiring and engine related issues as I've always been a 4x4 guy. I know my way around suspension but this stuff is pretty new to me.

I appreciate everyone's comments! Hopefully it's not to much to get things up and running again!

OX1 · Mar 30, 2026

Does it have an FMU?

This is the "mass air" wiring diag

And typical SD to MAF wiring.

An if you need it, "salt/pepper" connector pinouts.

And if you want to try to test sensors, this is typical "idle" values........

General karthief · Mar 30, 2026

Let an 'old guy ' give you a 'what I would do', take all the blower stuff off, get the engine stuff as close to stock as possible, get the engine running good then work on adding blower stuff, you can keep the ecu and wire in the MAF but that adds a bit of complexity to diagnostics,
You could go back to speed density but that requires a different ECU.
Another thing is send that computer to a place like The ECUExchange to update/repair, it's nearly like 35 yrs old (math sucks) and capacitors and power supply stuff go bad, see any 30 year old home computers still in use?
Got a good set of jack stands?

spilly · Mar 30, 2026

Well I hooked up the MAF to the intermediary plug and the car fired up and stumbled for a moment then started to actually run half decent. I was able to touch the throttle without killing the car! It idled pretty well for about two minutes then started to stumble again, occasionally jumping back up in rpm for a moment. I shut it off and now crank no start. Pressed the Schrader valve on the fuel rail and a tiny spurt of fuel then nothing. I don't currently have a gauge to test pressure but I'd guess it'd say zero.

I recently replaced the tank, frame filter, and pump with walbro 255lph in tank pump. New hose from pump to hanger installed too. After I installed it I pulled the line from the filter when priming and had a ton of fuel then. I also pulled the vacuum line off of the regulator just now and no fuel or smell. I can hear the pump prime when I turn the key, just builds a tiny bit of pressure then nothing more.

Any thoughts?

spilly · Mar 30, 2026

OX1 said:
Does it have an FMU?

Yes it does

General karthief · Mar 31, 2026

You need to know fuel pressure.

AeroCoupe · Mar 31, 2026

just me but I would just repin the stock harness for a MAF and run the needed wires and ditch that adapter. One less point of failure.

Pull the ECU and check the opposite end and see if there is a chip on it.

Pull codes. 87’s do not have a check engine light (CEL) so who knows if there are codes.

Electrical Thread 'How To Pull Codes From EEC-IV In 86- 95 5.0 Mustangs'

Jan 27, 2016

@Noobz347 - The code dump procedure isn't in the sticky - you may want to add it...

Dump the codes: Codes may be present even if the Check Engine Light (CEL) isn't on.

Dumping the computer diagnostic codes on 86-95 Mustangs

Revised 26-July-2011. Added need to make sure the clutch is pressed when dumping codes.

Codes may be present even if the check engine light hasn’t come on, so be sure to check for them.

Here's the way to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for...

I read new spark plug wires and plugs but what about the cap and rotor.

Pull codes, pull codes, pull codes….

spilly · Mar 31, 2026

Ordered a fuel pressure testing kit and a code reader. I will have the fuel pressure information and any codes soon too.

I had a spare fuel pump relay so I tossed it just to see if it made a difference and it certainly did. The car did run way better and without stumbling. I ran it for about 10 min. Was able to rev it up a little bit but there is some lag between depressing the pedal and rpm increase. Still smells a little rich as well, but not nearly as bad as before.

However after I shut it off, I wasn't able to restart the car. Just cranks endlessly and smells like gas. Maybe a stuck injector?

Will report back with codes and fuel pressure information when I get it.

Thank you everyone for the responses.

gkomo · Mar 31, 2026

Which fuel pressure gauge did you get?

spilly · Mar 31, 2026

gkomo said:
Which fuel pressure gauge did you get?

It was an Amazon one. I believe it was recommended by some YouTube person.

spilly · Mar 31, 2026

Also,if I were to just replace the engine harness, rather than repinning mine, can I use any 89-93 MAF harness? I found this one locally

OX1 · Apr 1, 2026

General karthief said:
Let an 'old guy ' give you a 'what I would do', take all the blower stuff off, get the engine stuff as close to stock as possible, get the engine running good then work on adding blower stuff, you can keep the ecu and wire in the MAF but that adds a bit of complexity to diagnostics,
You could go back to speed density but that requires a different ECU.
Another thing is send that computer to a place like The ECUExchange to update/repair, it's nearly like 35 yrs old (math sucks) and capacitors and power supply stuff go bad, see any 30 year old home computers still in use?
Got a good set of jack stands?

I agree with this premise, but assuming blower has it's own belt, you don't need to take it off.
I removed blower belt when I took my LTD in for body work. I even asked for a tune that limited RPM to 3000.
Idled and drove to body shop fine with air drawing right though no spinning centrifugal blower.

Since it has FMU, my guess is blower setup was based on stock engine config, and you don't have
piggyback tuner or "tune".

OX1 · Apr 1, 2026

spilly said:
Also,if I were to just replace the engine harness, rather than repinning mine, can I use any 89-93 MAF harness? I found this one locally

I don't recall the full details, but I in 86 at least, Ford did some weird stuff with O2 sensor harness, depending upon if you had man or auto trans (the only reason I could ever figure was, is it helped on assembly line, only having to change O2 harness based on trans option). Not sure if that carried over on 87 up, maybe someone else can chime in.

Fox '87 GT can't get to stay running

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now i can hopefully expect to receive the shaft

I lubed and pushed, until it was all the way in

I wish I didn't have all of these balls in the air

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I wish I didn't have all of these balls in the air

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wonder how much it would cost to ship you a pair

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wonder how much it would cost to ship you a pair

lube between the nut and the face. I know my lubes

Electrical Thread 'How To Pull Codes From EEC-IV In 86- 95 5.0 Mustangs'

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now i can hopefully expect to receive the shaft

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