what causes 4.0's run rich?

[fireguy12117]

what might be usual suspects as to why a 4.0 EFI engine is going to run rich?

im referring to a rebuilt motor, so even improper engine setup could be involved (say for instance the machine shop putting the wrong heads on changing cylinder volumes enough for detonation).

i am going to verify ECT, MAF, and O2 sensors as they are the ones i know would affect the fuel tables. is there anything else i should look into?

after a week of driving this thing (its not a DD) i noticed black deposits creeping up the tail pipe, and a nice pungent smell from the exhaust. i am not loosing coolant, so i assume there isn't a coolant leak causing smelly exhaust.

the only thing different from stock is an acell coil pack and headers. Also no CEL's so if a sensor is out, its only out enough to throw off the fuel tables, not throw a code.

 

[Big Jim M]

Fireguy... Not enough information!... What makes you think RICH? When running rich there would be small flakes of soot coming out the back.. The color of the tail pipe usually tells us little. How long are the trips that ride is making? Looong trips tell us more than a series of short trips.
Does it otherwise than the tail pipe run good? The head differences possible would make possible compression differences which might result in clatter... but not result in richer mixture...
Big Jim

 

[Ranger44]

Leaky Injectors
bad ECT
Overheating
bad MAF
Bad HO2S's
bad FPR
Improper Ignition

Just to name a few.......

But Big Jim is right, we need more INFO.

I'd take care of this if it is running rich. Catalytic converters don't like gobs of fuel going into them. Then you'll have a clogged exhaust system.

 

[Kona]

look up plug color checking, then get a new set of cheap copper plugs for you ranger and follow instructions for getting a color reading on them. It is pretty much definitive for how your engine is running.

 

[mwood1985]

what year engine are we talking about?

 

[AllanD]

reading plugs can be like reading tea leaves, many see things that simply aren't there while others miss things that are there.

Any idiot can read a tailpipe.

But the silliest things can cause issues,

Like an exhaust leak often leaks BOTH ways and the HEGO can detect that....

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[naford]

Because of emissions all new motors run a little rich. What is gas mileage like? Has it changed any?

 

[Big Jim M]

Wow!

Because of emissions all new motors run a little rich.

Whoda thunk it?
Big JIm

 

[AllanD]

Rich running, provided it is't rich enough to overheat the cat it easier to deal with than running lean.

because HC is easily consumed,
CO is somewhat harder to get rid of, but if running lean
the engine will make more NOx and that is the very devil to get rid of...

but generally speaking the default that the computer runs to if there is ANY
issue is to run the engine rich.

Lean isn't also "dirty" Lean also melts parts...

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[fireguy12117]

wow, uh...the engine has the correct heads, the year is in the signature (a 93) i was just using that as an example for how far fetched we could go and it looks like we could go on for ever...dunno what the fuel mileage on a new motor is going to be- this is all based around a rebuilt 4.0.

i have good compression, newer plugs, proper base time, runs smooth through entire rpm range, coolant system checks good, wiring harness recently rebuilt and verified.

the trip length for the time i payed attention was 5 miles city, 30 miles highway twice daily. i was basically going by pungent smelly exhaust and after i eliminated coolant leaks, i assumed i had rich running conditions, i mean, it smells rich, like i am the jack ass your sitting behind at a light thinking somethings up with that motor.

what i was seeking about the sensors are usual suspects for an otherwise well running motor. for example: can the ECT be throwing bad data at the PCM but not throw a code. could having the wrong MAF sensor, say from a 5.0 V-8 give the wrong data. what are the major players as far as sensors go for establishing that the PCM is getting the right data.

if all that is getting into the motor is air and fuel, and all cylinders are firing with good compression, why in the heck am i getting a rich condition with no indication of a fault in the electrical system of the motor.

so far ive established i need to go back and look for exhaust leaks...

i guess i am looking for better understanding of what makes the engine go vroom. the MAF provides a measure of air, the ECT provides a measure of engine temp, O2's close the loop for fuel mixture- something like that right? so if i know whats supposed to do what, when, and give what readings, i can work through it, ect...