91 2.3l back-fire and rough idle when cold- after air filter and MAF replace

[Tonybranger]

Okay, when I first bought the truck the motor craft MAF was unplugged the truck ran fine, just sucked gas about (10-12mpg) when plugged in the truck would start up and die immediately even if the gas pedal was floored. After I cleaned it same result.

So I went on rock auto and purchased a after market one from Delphi , also I change the cheap rusted Specter air filter to a brand new. K&n well since the truck gets better gas mileage but when it is first startedup it wants to stall out and back fires also if I don' let it warm up it had no power- like can't even shift into second gear. A buddy suggested to unplug the neg battery cable to give the computer (brain) a chance to reset and relearn the changes! Seemed to help but the next day right back to no power and backfiring ......

Any suggestions on what I need to do next or what I did wrong? Maybe buying the cheaper Delphi MAF? Please help thanks! Tony

 

[BRUTUS_T_HOG]

Delphi is not a cheap junk brand.

When the problem happens you need to shut engine off and disconnect MAF then see if it still does it. This will isolate if it is a problem with the MAF signal.

Does the engine crank over with low compression when you are having low power? As in fast cranking with little resistance to the starter? What you describe sounds a lot like worn valve seats

 

[Tonybranger]

With the MAF unplugged the truck runs great just bad mpg, I bought the MAF to help fix the mpg issue that's it. I refered to it as cheaper in relation to the price of a motor craft one.

When I have it unplugged it starts up fine, I've never done a compression check and top speed is only about 75 mph!

When the MAF is unplugged and I start the truck cold it revs up to about 3 grand then comes right down to about 1100 rpm. And has no issues what so ever after that.

With it plugged it I get a high rev then it drops almost to where it would stall if I didn't give it gas pedal. (Similar to a vacuum leak) after about five mins or so it's okay just an occasional back fire and no power then all of a sudden it takes off, is the k&n air intake give it too much air? Or tps bad?

How can I tell if the MAF signal is good or bad? Thank you!

 

[BRUTUS_T_HOG]

Make sure you don't have any vacuum leaks or air able to bypass the MAF sensor. So you're saying it runs bad cold only?

 

[Tonybranger]

Yes , after i let it warm up and hold the gas down a lil to keep a steady idle and don't let it stall , it runs fine, just backfire on take off then seems to go fine

 

[Tonybranger]

Anybody else have suggestion!?

 

[V8RangerBoy]

I will echo most of what Brutus said. Unplugging the MAF doesn't mean that it is faulty. Delphi builds some of the best stuff in the industry, I doubt that's the problem.

Unplugging the MAF suggests that the signal that it is reporting to the PCM is incorrect. Again, that doesn't mean that it is faulty - a vacuum leak or other air leak behind the MAF will mimic this symptom. We see cars in the shop all the time that have MAF faults reported because of vacuum leaks.

Basically, the MAF on a healthy engine should be reporting about 5 grams per second of flow at idle. This is the biggest input a Ford PCM uses to set the fueling strategy. If there is a leak downstream of the MAF, the MAF may only be reporting about 2 grams (just as an example) and will cause the PCM to lean the fueling out tremendously.

Then, when you unplug the MAF, the PCM has to resort to its base table to set fueling. It knows that for a certain RPM, TP signal, etc, roughly X amount of fuel is required, and the engine will run. I have seen several Tritons with holes blown in the bottom of the intake that will run with the MAF unplugged.

Quickest way to check for vacuum leaks is with a can of carburetor cleaner. Start the engine cold and spray around the entire intake tract and see if the idle smooths out. If it does, that's the area that is likely leaking.

 

[Tonybranger]

Thanks a ton for the advice, I will try the carb cleaner truck this weekend and see if I have any leaks- I have checked vacuum lines but not the intake! I'll let you know what happens!

Also if the harness is bad or sending a wrong signal, how would I know?