'97 Mountaineer V8 won't stay running

[jjue]

Well, I have a problem with the Mountaineer keeping the engine running. So far, I have changed the fuel filter, and I know that I am getting 40 psi at the test port on the fuel rail. I cleaned the IAC, and I changed the air filter just in case. I plugged in the backup computer just in case it was the Apten chip, which I switch back to stock when all of this started. I also know that this problem didn't start until I filled my tank with 20 gallons of fuel this morning.

The vehicle fires off immediately, but has a hard time staying running, if I can keep it running. I know that the few times that it will actually idle, opening the throttle body might let the engine rev to 2000 rpm, if it doesn't kill the engine first. In those cases, the engine will actually run better without pressing the gas.

So, am I looking at bad gas, a bad mass air sensor, a leak, or what?

 

[Jason_25]

Sounds like a bad IAC to me.

 

[jjue]

I thought about it, and that's one reason why I cleaned it, but it doesn't make sense for that to be the problem yet since I can add air by pressing the throttle (I've replaced it once already), and it wants to die rather than rev higher most of the time. The last time that it was the IAC, I had to drive with two feet most of the day until I could pick up a new one from the dealership.

 

[dogfriend]

If its the IAC, cracking the throttle open a little bit will simulate a working IAC and you should get a reasonable idle. It sounds like cracking the throttle doesn't help in your case, so I don't think it is the IAC.

It does sound like something is throwing off the fuel/air ratio, though - so the MAF sensor, IAT sensor, etc would be likely suspects. You would expect to get a CEL for one of the sensors.

If you aren't getting a CEL, then look at the things that won't set a light - vac leaks, bad fuel pump (there are tests for both pressure and flow), leaking injector(s ) or clogged injector(s). My book says that a clogged injector will cause one cyl to go lean, the PCM sends more fuel to the others to compensate which makes the others run rich, causing rough idle, stumbling etc.

Maybe bad gas = clogged injector(s)?

 

[jjue]

Well, I'm hoping that it's bad gas. I'm about to run out to get several gas cans so that I can empty the tank and put some fresh gas in from a reliable gas station (even though it will cost me about $0.10 more per gallon), but if it doesn't work then, I will have even more gas for the boat, and the lawn mower, weed eater, etc. I do find it strange that all of this started after I filled up at a name brand gas station between the Ford dealership and the Chevy/Hummer dealership.

Update: I accidently left the Mass Air Meter unplugged while messing around with a few things this morning, and it now runs fine, although the air/fuel mixture is definitely rich. This is the first time that I have every seen a Mass Air EFI vehicle run fine without the Mass Air plugged in. So, what does this mean? Bad Mass Air Meter?

 

[dogfriend]

I'm not sure that it necessarily means you have a bad MAF sensor. Ford designs the system to go into 'limp home' mode if one or more sensors are malfunctioning. On the other hand, a bad MAF will definately mess up the fuel/air ratio.

You should be able to use a digital voltmeter to test for the correct voltage from the MAF.

 

[jjue]

Well, if you can believe this, I apparently didn't clean my mass air meter well enough, and this was causing my lean out condition. I decided to clean it again before I tested the output voltage with my multimeter, and when I crank up and ran smoothly, the corresponding voltage assured me that a proper meter reading is being performed. Well, if nothing else, I now know other ways to limp home without having to pay for towing.