Still running rich - need some ideas of what to check next

[john42]

Engine: 306 (1990 5.0) 0.030 over
Heads: Trick Flow
Cam: Trick Flow Stage 1
Intake: Trick Flow
Computer: Stock Ford
24# injectors

Runs rich... Blows black smoke under hard acceleration.

What I've done so far and have not made any difference:
New O2 sensors
New Rotor cap
New Spark plug wires (it had cheapes.. now it has Ford Racing)
New TPS (the old one read 1.70 volts no matter where the throttle was)
Adjusted the fuel regulator. (It was low at 37lbs. It is now 42lbs)

It idles and runs fairly well. It's just fouling spark plugs and making black smoke at high rpm. Also, noticed I was burning more fuel then my buddies on the 2000 mile round trip to London Ohio...

[jab351w]

Non-exhaustive (no pun intended) list:

- Bad coolant temp sensor
- Thermostat stuck open or too cool (i.e. 160 degree)
- Bad air charge temp sensor (not sure if this one would make it run that rich, but worth checking)
- Ruptured fuel pressure regulator diaphragm leaking fuel into intake
- MAF not calibrated to injector size (assuming not covered by a tune)
- Upstream exhaust leak making O2 read lean

Are any codes set? What's the coolant temp when warmed up?

[john42]

Non-exhaustive (no pun intended) list:

- Bad coolant temp sensor
- Thermostat stuck open or too cool (i.e. 160 degree)
- Bad air charge temp sensor (not sure if this one would make it run that rich, but worth checking)
- Ruptured fuel pressure regulator diaphragm leaking fuel into intake
- MAF not calibrated to injector size (assuming not covered by a tune)
- Upstream exhaust leak making O2 read lean

Are any codes set? What's the coolant temp when warmed up?

I've also replaced the Thermostat. It's a 160 degree. The 190 was making things too hot. With the 160 things run 185-210.
Air charge temp sensor? where is this?
fuel pressure regulator - hmmm. it "looks" good. how would I check that?
MAF is a Summit Racing 75mm Bullet - which I believe is correct
I've check and check and re-checked for exhaust leaks. Found none.

Codes: this is not something I've yet mastered with this 1990's era Ford computer.

[SBC BUILDER]

If you can get another computer and plug that in and see if anything changes. Does this computer have a way to get inside and check for codes? Some places you can rent a code reader.

[jab351w]

I've also replaced the Thermostat. It's a 160 degree. The 190 was making things too hot. With the 160 things run 185-210.

Are you positive it's really running that hot? With a 160 T-Stat and a functional cooling system it should be running in the 160-180 range. If it is, the ECU may be staying in cold enrichment mode. If you haven't already, aim an infrared temp gun at the coolant crossover in the lower intake with the engine running at temp. It should match the dash gauge reading within a few degrees.


Air charge temp sensor? where is this?

It's in the lower intake manifold in one of the intake runners. Should have a 2-wire connector. Not to be confused with the coolant temp sensor for the ECU which also has two wires and would normally be in the heater pipe running from the coolant port in the lower intake behind the thermostat. If that pipe doesn't exist on your car (no heater), it may be directly in the coolant port in the lower intake coolant crossover instead. A bad coolant temp sensor could easily cause a rich condition if the ECU thinks the engine/coolant is cold.


fuel pressure regulator - hmmm. it "looks" good. how would I check that?

Pull the vacuum line connected to the regulator. Does it have fuel in it? If you apply a vacuum to the regulator does it hold? If you have an aftermarket regulator that isn't vacuum referenced then this obviously doesn't apply.


MAF is a Summit Racing 75mm Bullet - which I believe is correct

If it's calibrated for 19 lb injectors and the ECU is running the stock tune it would run rich with 24 lb injectors.


I've check and check and re-checked for exhaust leaks. Found none.

Codes: this is not something I've yet mastered with this 1990's era Ford computer.

It's easy with the EEC-IV, just need a jumper wire, paperclip, etc. This assumes you can locate the EEC test connector and your car has a check engine light. If not you can temporarily wire a test light. These should get you started:

https://www.fordtruckzone.com/threads/how-to-retrieve-trouble-codes-on-your-eec-iv-system-without-using-a-code-scanner.659319/
https://www.foxstang.com/reading-mustang-trouble-codes-eec/
https://www.ford-trucks.com/forums/112295-how-to-pull-codes-from-an-eec-iv.html

The second link has a listing of both 2- and 3-digit codes. IMS Ford switched from the 2 to 3-digit codes in the early 90s.

One other possibility: pull the lid off the ECU and take a close look at the electrolytic caps. Any of them bulged or leaking? That's a common failure in these older ECUs and can be fixed with new caps assuming no PCB damage from the leaked electrolyte.

[MB750]

I'd be curious to know what your backpressure is with that custom muffler, although it's kinda hard to measure. You'd need to physically connect a heat-resistant hose to the exhaust before the muffler and use a manometer to measure water column.

It may flow great at low RPM's, but as revs and speed increases if the muffler can't flow the air you'll run rich. I'd still think the ECM would compensate though...