My engine stumbles and just never seems to open up when getting on the throttle. It feels like it's choking or restricted somehow, and has no power. I've checked the normal things, air filter, injectors, coils, etc....
My original engine started knocking at 78,000 miles, shortly after i bought the car. I installed a used 31,000 mile 2002 4.6 engine, and it ran IDENTICAL (aside from the knock of course), still sputtering and bogging. I determined it to be something that stayed on the car between the engine swap. I have already changed the fuel pump. $220 down the drain, didn't make a difference. I've been running a k&n filter since I bought the car, and have cleaned the MAF multiple times. I have also changed the fuel filter several times. The only things I can still think of being the problem, are the MAF, a cam/crank, etc... sensor, although I'm not throwing a code, a plugged/restricted exhaust, or a computer issue. I have a UPR X-Pipe, and Hooker Max Flow mufflers, and already had an exhaust shop test the flow at the tailpipes and before/after the cats, and everything looked good. The exhaust also always has an airy sound to it (like theres a leak) when its under any kind of load, but I quadruple checked, and I have no leaks.
I have a laptop diagnostic system. Does anyone know what the MAF should be reading at a given speed/rpm?
Have you checked all the vacuum hoses over to make sure they're connected and not cracked/split? The MAF will definitely make the car run like total crap if it's not operating properly.
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Sometimes I think that government fits that old-fashioned definition of a baby: An alimentary canal with an appetite at one end and no sense of responsibility at the other.
- Ronald Reagan
My MAF Sensor was bad and my car was running like total crap!
I cleaned it with alcohol and a Q-tip several times (the only recommended method I have been told by a A-1 certified mechanic). I cleaned it with 0-2 safe throttle body cleaner (not reccomended as I was told by the same mechanic, and that I ruined it using throttle body cleaner) I read a post on here that told how to see if your MAF Sensor is bad. It worked for me. Unplug the MAF Sensor and start the car. Plug the sensor back in while the car is running. If the car dies, as mine did, or runs worse, it's the sensor. I bought a new sensor from a Ford Dealer and my Mustang has never ran better. Hope this helps.
93 GT Mustang
paperchasn365,
I have checked the voltages/resistances of everything listed in my Haynes manual, and everything was within spec.
BlueStreak03,
My new engine is a complete 31k 2002 4.6. All hoses look pretty good but i'll double check and maybe take a spray bottle around the engine to see if there are any bubbles.
93mustang,
I'll double check, but I think by coincidence I tried that exact same thing in the past and the car died. I'll also get wierd spikes and times where the computer loses data for a few seconds while under WOT on my laptop diagnostics. It all may stem from the MAF. Did yours ever throw a code while going bad? I have never seen a CEL, except for when I had the DPFE hoses backwards right after the engine install.
is there, was there, any chip ever installed in the car? sounds like maybe the computer could be having some problems. try reseting the computer(and leave the battery disconnected for atleast a 1/2 hour). and is it a stick or auto?
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2000 GT VERT-- Far from stock, far from where I want it...
The check engine light did come on a couple of times when it started running really weird , like jumping up and down in rpms, like I was pushing on and off the gas. I'd stop, wait a minute, and restart the car and the light would go off.
Have you checked the coolant temps sensor, manifold absolute pressure sensor and O2 sensors by any chance?
Is there any rhyme or reason to all the spikes and stalls and rough idling or does it all appear to be completely random?
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Sometimes I think that government fits that old-fashioned definition of a baby: An alimentary canal with an appetite at one end and no sense of responsibility at the other.
- Ronald Reagan
If you still didn't figure it out, it might be a good idea to see a dyno shop and do a couple of dyno runns.
They can easily detect a "lean condition" they can also re-map your PCM. Under normal load at average rpm's, our engines run way to lean. This causes pinging, stumbling and hesitation. Numerical high A/F ratio is only done for emission and gas mileage reasons.
If you know the A/F ratio in comparison to RPM and engine load, then you know where to start searching.
paperchasn365,
It's a 5spd. I've had the battery disconnected several times while messing with my stereo. Hasn't made a difference.
BlueStreak03,
I am getting readings from those sensors on my laptop diags. I'm not sure what they should be reading but I am getting data from them. I don't believe I have a MAP sensor, not that I could find, anyway. Everything seems to be random. All of the o2's are peaking and dropping in voltage like they're supposed to be. I replaced the fuel rail pressure sensor, thinking I should have been getting a PSI reading from it, and I still aren't.
engineer76,
It's funny you mention that. I was already thinking of putting it on a dyno. Theres a shop around here (Lamotta Performance), that gives you 2 runs with A/F for like $79. Not sure what remapping the PCM would cost though.
I was ready to have the dealer install the 2002 computer (I'm still running on the 1999), until they told me I'd need another key, and it would cost $125 for that, plus their $99 PATS reprogramming fee.
....
I have just tried a new MAF sensor, and it didn't make a difference. Good thing I work at a local auto parts store, they should let me return it.
Sometimes I think that government fits that old-fashioned definition of a baby: An alimentary canal with an appetite at one end and no sense of responsibility at the other.
- Ronald Reagan
Would I hurt anything unplugging the rear o2's? It seems thats where my problems started. When i first bought the car it ran great. A week later, the check engine light came on. I took the car to the shop that does all the work for the dealer I bought the car from. They diagnosed the o2 sims that had been installed were bad (I have an aftermarket UPR X-pipe), and gave me a pair of o2 sensors from an 03 cobra. The light never came on again, but the car slowly started running worse from then on.
A mustang specialized dyno shop, usually does the dyno runs and custom tune for around $500. They might even offer a special on a certain day. Just ask around.
I'd say, if you can't find out what the problem is, just give it a shot.
I would take it to a local dealership and try to have them reflash the computer. If the previous owner had a tune in the computer it wouldnt go away by just disconnecting the battery. He may have used a tuner which rewrote the programming and the only way to fix it would be to have the computer reflashed.
Is there any way to tell if the MAF is bad? 
