I can't seem to get it right! Replaced the air bypass valve since it finally showed a code, no change in power. Now it seems the vents want to go to crap under power. Tried to check all the vacuum lines, couldn't find a bad one. No one's been under the dash since '97. Still has the original exhaust as well as 150,000 miles. Did try to clean out some of the lines with it not running. Started it & ran great for a block... then to crap. It uses no oil & has decent cylinder pressures as well. Why is this thing such a dog? It's a '94 F-150 302/4R70W with the HD towing package. I'm stumped!

 

Is it MAP or MAF? Can you be more descriptive than "crap"? What do you mean "clean out some of the lines"? What is "decent cylinder pressures" (what are the numbers)? What happened under the dash in '97? What other mods/damage/repairs? When did it start going "to crap"?

The more info you provide, the more likely you'll get useful responses.

 

If you explained yourself a little better you might get some answers.

"Now it seems the vents..."

What does that mean? Tell us more!

Regards, Robert.

 

Cylinder pressures are from 100# to 125#, all acceptable. Running the AC & accelerating, the cold air vent changes to defrost indicating a vacuum problem. This vacuum problem would also cause the ABV to not properly operate. As for cleaning the lines I was trying to dertermine if there were vacuum lines clogged. I had a stereo installed in '97 by a professional chain, had no problems until recently. There are no codes. Until 1 year ago this truck got 19 MPG & pulled my trailer very well( 16' with a #3000 car). Now it won't even climb the smallest hill without shifting from overdrive into third. The truck has no modifications except the stereo & has been very well maintained. The fuel mileage has dropped drastically as has the power, it won't even pull itself without being at around 3000 RPM. Tried playing with the timing & got it to run better. Thinking it was the chain, & checked, acceptable as well. Fuel pressures are exactly as the manual says they should be but the exhaust pipe tip is unusually white. It seems to all point to a fuel injector problem, possibly thinking about a PCV valve problem (hard to see under the upper plenum)
"Is it MAP or MAF?" Can you be more specific? As far as I know(which isn't everything Ford didn't utilize "mass air" injection on trucks, just "speed density". So I assume it has a manifold absolute pressure (MAP) sensor?

 

The Idle Air Control (air bypass valve) isn't vacuum operated, and also isn't even active off-idle or at WOT, so it's unrelated to your problem.

The vent switching can be caused by a minor vacuum leak inside the dash, so it may also be coincidental. Or it could have just been growing slowly since the stereo was installed.

I use "MAP" as a quick way to refer to speed-density EFI versus MAF, but both were used on '94-95 trucks. Do you have a single outlet from the air filter with a MAF sensor on it, or do you have 2 tubes coming out of the filter top (MAP/SD)?

The PCV can't cause enough trouble to do what you're describing, even if its hose came completely off or it was plugged solid.

Your power problem might be coming from a worn-out transmission.

 

map calculates air flow by monitoring vacuum, maf calcualtes air flow by monitoring air flow from the filter.

The map system had problems. For instance, one guy just tried to put a high performance cam into his truck, which produced lower vacuum, and the pcm couldn't cope with it. Any vacuum leak at the top of the intake manifold will also screw up the pcm as the map vac line comes from there.

The system changed right around the time your truck was built. You will have to look and tell us. The map is (was) located in the upper left, as you stand looking in, of the engine compartment. The maf, is, I think, in the filter box. I have a map, only read about the maf.

My exhaust also has white powder, and my 5.0 works fine.

There are others on this site who know far more about problems like this than I do. Stay tuned, they will surely help.

Regards, Robert.

 

Have you checked the vacuum from the manifold tee at warm idle? My '78 351M was doing poorly even after a rebuilt carburetor and full tune-up. I yanked the carb, re-torqued all the intake manifold bolts, then sprayed sealant onto new baseplate/EGR gaskets and used an actual torque wrench to reseat the carburetor to the manifold. Ran like a new engine. Could you have an intake manifold vacuum leak? I've read a few accounts of people re-torquing their lower manifolds on EFI 302's with great results, initially complaining of sluggishness and remarkable power loss over time.

 

On second thought, you might try running some vacuum line to a gauge in your cab. Doesn't have to be the Autometer permanent-mounted style, but just a diagnostic piece for temporary use. This, I have found, is one of the most useful instruments one could have for tuning and diagnostics. I think steve83 has a link to a webpage describing vacuum gauge readings and how they indicate potential problems. But if the engine is huffing and puffing at interstate speed, and your pedal is nearly floored, I'd guess either fuel delivery problem (vague, I know) or exhaust blockage. I had my aforementioned 351M pulling 21"Hg at idle, yet my '93 351W can manage only 18"Hg and has been feeling progressively weaker over the last two years Looks like we both have some tinkering to do.

 

Vacuum gauge readings

 

Sounds like a clogged cat.

CB

 

The clogged cat would be my guess.

 

If the cat was plugged wouldn't there be very little air coming from the exhaust? I'm sure it needs an exhaust change but would that cause a vacuum problem?

 

Pulled the upper intake to change PCV valve & check more hoses & all the runners are all nasty. Any clues as to why? There's only air there. I'm thinking it's time for a change of engines.

 

By nasty do you mean black, sooty, or oily? My guess would be the exhaust but that is only a starting point. Since you have no codes, I wouldn't think it'd be HEGO sensor. And the fuel pressure checked okay, but the injectors could be malfunctioning or clogged. Again you should have HEGO/O2 sensor code. When you timed the vehicle, you did remove the SPOUT connector didn't you? Lack of power/ poor fuel mileage tells me that the fuel isn't burning properly. A description of the intake/exhaust can help. The exhaust is white? And you are not heating up or losing water? Couldn't be a head gasket but they don't always show up with the compression check. Pull the plugs and descibe them. If the truck is timed correctly, I would think the problem area could be exhaust/emissions. Check your AIR pump. It can affect mileage as thermactors can too. You can just cut or unbolt the exhaust and check it. If thats not it. Just put it back together.

 

Starting to look more like the exhaust. Intake is kinda varnishy looking & wet, smells bad. Plugs are very clean, almost too clean. Last time it was timed I set it at 10 degrees with the spout disconnected. I'm still pulling stuff off but I thing a new PCV valve, upper plenum gasket, & TB gasket & I'm going to the muffler shop.