Help with '92 F-150

LX Dave

Active Member
Jul 2, 2017
I know there some really smart people on here so I wanted to pick your brains on what's going on with my truck.

Details first....

1992 F-150 with the 300 I-6 and E4OD transmission. 4WD, 3.55 gears and 31x10.5-15 tires.

When I got the truck, it was in pieces. Apparently somebody was going to fix it up, but gave up instead.

Engine fully rebuilt, transmission rebuilt, rear axle, front axle, you know the game here. Nothing was done before I got the truck.

What's going on...

Upon first start, I thought everything was good. When I went for a test drive if felt extremely under-powered. I'm guessing about half power, takes about a block to gain 20 mph from rolling at 25 mph.

With the transmission in park, if the throttle is opened just a little (around 1200-1500 rpms), the engine is missing really bad. Shakes everything when it's doing that.
If I unplug the MAP sensor, it smooths out and runs great. I'm guessing this puts it in Limp Mode.
I have tried 4 different MAP sensors with repeatable results. Two MAPs were verified good on another truck, and one was new.

I also picked up another PCM, and it's the exact same thing with that one. I have not sent my old one in for a core yet.

The engine harness was tested for continuity, and tested the inline resistors and diodes. Everything is good.
I have tested the wiring going to the PCM also.

Another thing on the harness, I can NOT pull codes from the connector in the engine bay. I have to jump the connector and watch the check engine light in the dash.
It came back good with KOEO. With the KEOR test, I got a "Insufficient RPM change during ER dynamic response test", and "Spout circuit open". Tested the wiring for the spout connector,and everything seems good.

Fuel pressure is at 50 psi with the vacuum line on, 60 psi with it off.

Base timing is set to 10 degrees with spout removed.

All of the sensors (including the pick up in the distributor) are Motorcraft.

It seems like the timing is moving like it should ( timing light on while operating the throttle).

TPS has a smooth sweep on the meter with no jumps anywhere throughout the range.

At this point, I'm at a loss. I now have the engine bay half torn apart checking everything and nothing has helped.
Going to just lay the harness back in place a re-test everything making sure the connectors are making good contact.

What would cause it to run good with the MAP unplugged and/or what is related to the MAP making it do that? Or is it just going in to Limp Mode what's making it run good?

Thanks for any help or ideas. I'm willing to try anything at this point as I really need this truck on the road.


ETA: Does this really only take 14 lb injectors?
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