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 (b-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!
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 (b-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!