EFI 351w running way rich

[kunar]

Buddy of mine has a 94 f150, its got an EFI 351 thats running crazy rich. It runs rough and occasionally bucks under load. It also starts hard sometimes, and sometimes wont even start at all. He's checked the FPR, no sign of a leak there. Replaced plugs and wires, cap and rotor, helped with the starting issue and misfire, but not entirely gone. Plugs turn black pretty quickly. It's throwing a code for the o2 sensor, always rich, but i dont think thats a bad sensor, i think thats a symptom of another issue. Also doesnt seem to be quite getting to temp. it does get warm, but not hot. I know it could be stuck in warm up mode, running rich from that. The thermostat has been replaced, temp gauge goes to about 1/4 at the most. He's been getting about 7mpg, mind you this truck is a regular cab and its 2wd so its not a big huge monster. looking for any ideas or suggestions, anything to check for cheap/free is best. we're all on a budget!

 

[RonD]

What was the temp of the t-stat?

Best operating temp for gas engine is 200-230degF, this gives best MPG and best lubrication, all oil is rated/tested at 212degF
General rule of thumb with EFI computer controlled systems is if the t-stat is on the upper rad hose(out to rad) you should use a 190-195degF t-stat.
If t-stat is on lower rad hose(in from rad) then a 180degF t-stat is used(15deg drop from top of rad to bottom, so 180 at bottom hose = 195 at top hose).

Any t-stat used needs to have a hole in its plate, often called a "jiggle valve", this allows air to escape when refilling the system, this hole should be in the 12:00 position.
You can always drill an 1/8" hole in the plate if it doesn't have a jiggle valve.

'94 f-150 5.8l(351) was still using the Speed Density system, so instead of a MAF(mass air flow) sensor, it uses a MAP(manifold absolute pressure) sensor, for calculating fuel/air mix.

The MAP sensor will be on the passenger side firewall up at the top, it will have a 3 wire connector and a vacuum line that runs to the intake manifold.
Remove and check that vacuum line closely.
Remove and check connector, make sure contacts are clean and dry.
A MAP issue will usually cause a CEL because it is an important sensor, so I doubt MAP is bad but worth testing.

Google: Ford MAP sensor test

The MAP sensor is the main device that tells the computer how much air is in the intake, the computer bases ALL fuel/air mix ratios on that one reading.
O2 sensors are not used when engine is cold, and even when warmed up the O2 sensor data is for fine tuning fuel/air, so they are probably not the cause of the rich mix you describe.

The FPR was test with fuel pressure gauge?
should be 35 to 40psi, over that would put too much fuel in the intake.
Computer doesn't monitor fuel pressure, it assume 38psi and opens an injector for XX time to get X amount of fuel, if pressure is higher X.5 amount of fuel would come out, making a rich mix

A leaking injector could also be ID with pressure test.
A quick test for fuel leak into intake is to turn key to on
Press gas pedal to the floor and hold it there
Crank engine
Engine should NOT fire or start, if it does then fuel is leaking in.

EFI computers have a routine to clear a flooded engine.
With key on, throttle at WOT(wide open), the computer will shut off injectors when cranking, so flooded engine can be dried out.


The 2 wire ECT(engine coolant temp) sensor can cause engine to stay rich if it never warms up or never reports it is warmed up, BUT...... computer has a timer that starts when engine does, if ECT sensor data is not changing or doesn't get to "warmed up" status within 10 minutes, the computer will turn on CEL.

 

[kunar]

thanks for the info Ron. i already told him about the flood-clear mode for startup. he says he does have to use it sometimes to get it to start. ill pass this info along. thanks again.

 

[RonD]

If it fires in "flood-clear mode" then he has leaky injector(s), engine should just crank with no on again off again firing.

I know FPR has been checked but pull off it's vacuum line and check it again for fuel or fuel smell, that's one sneaky way fuel can get into the intake and cause a rich mix