Bad mileage

[93'Stranger]

Well i had the exact same problem he had, and the same effects, and the cause was the cat...and i did tests on it and it never showed up, until i took off my exhaust and the cat was plugged, and it really does reduce back pressure because the more back pressure the more hp, i know its happened to me

 

[Wicked_Sludge]

there are LOTS of possible causes for these EXACT symptoms, the most likely being excess fuel in the cylinders due to a bad FPR, stuck injector, bad MAF/MAP, bad O2's, clogged air filter, PCM failure, physically damaged exhaust valve...shall i go on?

what kind of test did you perform? there are several ways to test the cat on the vehicle aside from physical inspection.

and as for this statement:

and it really does reduce back pressure because the more back pressure the more hp, i know its happened to me

i think you need to brush up on your fluid dynamics. a restriction in flow (such as a plugged converter) will cause pressure to build up BEHIND the obstruction. would you please explain to my how stuffing a potatoe in the tailpipe reduces backpressure?

even if it did, more backpressure does not mean more horsepower on any 4 cycle engine in the world. ideally, you would completely eliminate all backpressure in order to increase your engines VE, but doing so is not possible, so we have to settle with as little backpressure as possible.

 

[DailyDriver]

PCM failure???

I didn't think the truck would run if this happened? Could you shed a little light on this please?

I'm still fighting this problem myself (bad mileage) & I don't have any codes showing either.

 

[Wicked_Sludge]

just like any other complicated peice of electronics, the PCM can fail in sections. a specific circuit or component can take a dump without preventing the vehicle from starting and running.

 

[valleyvinyl84]

would you please explain to my how stuffing a potatoe in the tailpipe reduces backpressure?

Go Wicked!

 

[DailyDriver]

just like any other complicated peice of electronics, the PCM can fail in sections. a specific circuit or component can take a dump without preventing the vehicle from starting and running.

Well I'll be...........ok. Didn't know that. Any way to specifically diagnose this? When the mileage problems started a few months back it was a gradual thing. It progressively got worse. However, I noticed during this time period if I disconnected the negative battery cable & cleared the "memory" the normal mileage returned for a tank or 2 & then started to decline again. I attributed this to crappy gas & the computer trying to compensate for such. A buddy mentioned it may have been the computer but, well, as you can see I'm ignorant lol.

This isn't working anymore & the mileage sucks all the time.

 

[Derek The Great]

My bro had a 93 crown vic, it was smoking black for the longest time b/c he needed o2 sensors. Then it just started to run real bad, wouldnt accellerate, stumbled and took forever to hit 25. My guess was a plugged cat, as running too rich all of the time will plug thos things up pretty quick. So we had a shop cut thos out (it had 4...) drop 02 sensors in it and replace with straight pipes. It ran like a gazelle after that

 

[Wicked_Sludge]

theres really no way that i know of to diagnose a bad PCM...its really the failure of all other attempts to fix a problem that usually leads to replacing the PCM.

 

[bluebombersfan]

wicked sludge
how much does a new pcm go for? sounds crazy expensive
also if you disconected sensors and engine light came on would this prove that the pcm is working to some degree?

 

[Wicked_Sludge]

depends on where you get it. the junkyard is your best bet since PCM failures arent common. check with a local junkyard for prices. new from the dealer your probably looking at a couple hundred bux.

 

[Bryan22]

I got a PCM at a junkyard for $50. But i wouldn't reccomend buying one until you've tried other things first. Tests are free if you have the tools and confidence to do them. Why spend money when you don't have to?

 

[bluebombersfan]

But I was still wondering if you disconnected different sensors and the CEL went on would that count as a test to indicate if computer was still somewhat functionall

DGEORGE You may have solved my poor mileage problem
I did what you said, just took a look at my crank pulley and its all over the place, hope this will solve my mileage problem

 

[Wicked_Sludge]

if the engine starts and runs, thats a sign that the PCM is going SOMETHING.