Fuel ‘88 GT Runs Rich Cold — Fixes Itself When I Unplug Sensors (ECT, MAF, O2, etc.)

[Sufarry]

Hey guys,

I’m chasing a weird issue on my 1988 Mustang GT 5.0 and hoping someone here has run into something similar.

Here’s the setup and background:

The car sat for 17 years, so I replaced a ton of stuff just to get it reliable again.

New parts: IAC, MAF, TPS, throttle body, ECT, air temp sensor, injectors, spark plugs, all ignition parts except the distributor itself, fuel pump, fuel filter, fuel pressure regulator, water pump, radiator, thermostat.

ECM has had the capacitors replaced.

EGR system deleted (block-off plate, all vacuum lines capped). The EGR connector is just hanging, not plugged into anything.

Compression is solid — 150–160 psi across all cylinders.

All vacuum leaks have been ruled out.


Everything has been replaced with OEM-spec or equivalent parts — not random aftermarket stuff.


---

The problem:

When the engine is cold, it runs super rich.
It grey-smokes like crazy, smells like raw fuel, misfires, and just runs awful.

BUT — if I unplug a critical sensor like the ECT, MAF, or O2 sensor, it instantly cleans up and runs great.
It’ll idle smooth, sound strong, and maybe run a touch rich (expected since it’s running on default fuel tables).

Once it warms up, it still doesn’t run perfect — just better — but that initial cold-run condition is horrible.


---

Codes:

Only getting smog-related codes (expected since that’s deleted) and Code 34, which I assume is for the EGR system being unplugged.


---

Engine info:

1988 Mustang GT 5.0

GT40P heads

Ford Motorsport headers

E303 cam (or equivalent specs)

Explorer upper/lower intake

BBK 70 mm throttle body

Mac Prochamber exhaust setup

---

My question(s):

Could the unplugged EGR connector be messing with how the ECM runs in open loop, or tricking it into dumping fuel?

Is there a known wiring or sensor ground issue that would cause it to run rich until a major input is disconnected?

Anything I might’ve missed that could cause this weird “runs great when unplugged” situation?


Any insight or tests I can try would be greatly appreciated. I’ve gone through everything mechanical I can think of, so I’m sta
rting to suspect something electrical or ECM-related.

Thanks in advance!

 

[Noobz347]

Those components that you listed all share a common ground including, the fuel injector harness.

I would start by cleaning and verifying those grounds.

I suspect that when you're pulling the plugs on these sensors that one or more of those components or the fuel injectors themselves, are finding a path to ground and making it 'better'.

Miss-wiring can case these kinds of issues too. Check that everything is installed/connected in the correct places.

 

[Mustang5L5]

When you start unplugging critical sensors, the ECU defaults to it's internal programming without sensor inputs, which is why the car runs "better". It's ignoring the bad readings and running usually off minimal inputs. Usually when this happens, one of the sensors is reading erroneously.

Since you've had the ECU repaired, going to assume a pin 46 trace burn is not the issue.

Have you dumped codes? There is no fuctional check engine light in 1988 (although can be added) so you need to dump codes with the engine off and engine running and see what it spits out in terms of a potential bad sensor reading. Replacing the parts doesn't necessarily fix the issue...especially if it's a broken/corroded wire that was the culprit.

EDIT: just saw it's an '88 CA car. That would have a functional CEL.

 

[AeroCoupe]

There should be an orange wire (its #19 in the picture below) behind the passenger side head that needs to be grounded. I believe this can be attached to either a bell housing bolt or one on the back of the cylinder head. I have mine on a bell housing bolt.

There is also another ground here that will cause all kinds of issues if it is not grounding well.

 

[General karthief]

88with MAF? Cali car or converted?

 

[nickyb]

Was your ecu tested after the caps were replaced?

 

[Sufarry]

California car!

88with MAF? Cali car or converted?

 

[Sufarry]

Was your ecu tested after the caps were replaced?

No, but I had a professional PC repair shop do it and it looks great. I can't imagine the repair being faulty.

 

[Sufarry]

There should be an orange wire (its #19 in the picture below) behind the passenger side head that needs to be grounded. I believe this can be attached to either a bell housing bolt or one on the back of the cylinder head. I have mine on a bell housing bolt.

There is also another ground here that will cause all kinds of issues if it is not grounding well.

Thank you. I will absolutely verify these ground when I'm around the shop tomorrow!

 

[Sufarry]

When you start unplugging critical sensors, the ECU defaults to it's internal programming without sensor inputs, which is why the car runs "better". It's ignoring the bad readings and running usually off minimal inputs. Usually when this happens, one of the sensors is reading erroneously.

Since you've had the ECU repaired, going to assume a pin 46 trace burn is not the issue.

Have you dumped codes? There is no fuctional check engine light in 1988 (although can be added) so you need to dump codes with the engine off and engine running and see what it spits out in terms of a potential bad sensor reading. Replacing the parts doesn't necessarily fix the issue...especially if it's a broken/corroded wire that was the culprit.

The only KOEO codes are related to smog stuff. The KOER codes are EVP below minimum and Thermactor (removed/bypassed). My EGR has been removed and blocked off.

 

[AeroCoupe]

No, but I had a professional PC repair shop do it and it looks great. I can't imagine the repair being faulty.

Were they able to test it or just replace them? If they didn’t test it I would not be surprised if it doesn’t have other issues.

 

[Monkeybutt2000]

You say you replaced the ECT,did you ohm check it? I replaced mine when I rebuilt my car,and replaced all sensors "just because". Car ran burn your eyes rich, and that "new" sensor was bad out of the box. Put the old one back in been fine ever since.

 

[Mustang5L5]

No, but I had a professional PC repair shop do it and it looks great. I can't imagine the repair being faulty.

Did you ever test the pin 46 trace to see if that was burned out?


With ECU unplugged, you would test continuity from pin 40/60 to 46. It should ring out for all combinations.


Also, which sensors do you unplug and it runs better? Does any one sensor make it run better than others?