This is my first post to the f150online forums. I'm hoping you guys can help me diagnose my 1993 f-150. I'll try to be as detailed as possible, so bear with the long post please.

My 1993 f-150 5.0 4x4 with 123k miles has developed a profound loss of power over the last 6 months or so. At idle it can rev, but on the road it cannot rev above 3k rpm even at wide open throttle. Little old ladies will pass me in their station wagons. I can eventually get to highway speeds but it takes a while. Here's what I've checked or replaced:

Ignition:
1. Replaced all plugs. Plugs all looked the same and looked OK.
2. Replaced the wires. Also used a timing light to confirm signal in all 8 wires.
3. Replaced cap and rotor
4. Replaced ignition coil
5. Replaced ignition module
6. Confirmed timing is correct and that timing advances appropriately with rpm's
7. Alternator current is appropriate

Fuel:
1. Confirmed fuel pressure at idle is appropriate (I don't have a way to test it while driving as I just have a gauge that attaches to the fuel rail)
2. Tried switching from one fuel tank (fuel pump) to the other, no difference
3. Blew the fuel lines out with compressed air
4. Replaced fuel filter

Air intake
1. Confirmed engine pulls a good and steady vacuum.
2. Pulls good vacuum at constant high rpm (no exhaust restriction)
3. Replaced air filter
4. Replaced Throttle position sensor
5. Confirmed that throttle is opening all the way.

Computer
1. Ran EEC-IV self test: only one code, indicating that EGR might be stuck open (egr should be closed at wide open throttle, so I plugged my EGR by placing a dime in the line, which made no difference).
2. Tried disconnecting battery (computer reset)

I'm really running out of ideas here. And this is not a subtle problem, there is a profound loss of power such that anyone riding in my truck would ask what was wrong with it. The things I'm considering at this point...

1. I don't have a compression tester so I haven't done that. The engine does not smoke.
2. I don't know how to confirm that each fuel injector is firing. The computer would not do the cylinder balance test no matter how many times I tried (which was a lot), so I can't even confirm if this a focal problem with a certain cylinder or a systemic problem. The truck idles great, and does not seem to be missing or bucking or anything.

Thanks for induling me. Any guidance would be GREATLY appreciated.

Sincerely,
Chris

 

Disconnect the Y-pipe from the exhaust manifold and go for a ride around the block. I don't trust the vacuum gauge. My bet is a clogged converter

 

or plugged muffler,,,