78F350

I recently bought a 1987 F150 with the 4.9/300 6 and EFI. It runs fine in Neutral and in Park and also runs well on the road until it reaches normal operating temperature. Once it has warmed up it will nearly stall out any time I accelerate or put much of a load on it.

I have replaced:
Spark plugs
Distributor cap
plug wires
Throttle position sensor
IAC
cleaned and reinsulated the wiring to the coil
replaced the EGR valve, which was jammed

The Air (smog) pump has been removed
The gear position indicator is a little off and I usually have to put it in Neutral to start it.
Previous owner says he replaced fuel pumps and filters recently.

Where should I look next?

byrd48

Check the timing

Somery

I had an ignition coil do that. Until one day it wouldn't start. But I used a backup that made it run. But mines the 5.0

farmer12

Check the coil..or just replace it. They're not that expensive. Or the TFI module on the distributor. I had something similar happen on my 5.8. Ended up being the TFI.

78F350

Thanks for the info. I'm going to get a new distributor and coil. I was surprised how inexpensive they are.
I pulled the distributor to get a look at the internals and TFI. There was corrosion inside and my 1 week-old distributor cap already had some white crust on the contacts.

farmer12

Make sure you go with brass contacts on your cap and rotor.

byrd48

I have a new distributor on mine and new tps and I have the same problem. It just started a few weeks ago and only happens in 4th or 5th gear when it's in lower rpms under a load (I first noticed it going up a long incline in the mountains, as it started slowing down in 5th gear, it started shaking, almost like the clutch was slipping). When I would downshift to 4th, it was fine again until it started slowing and it did the same thing again.
When I bought the truck a year ago, it had a bad tps and did the same thing, but much worse. I have the 4.9L. Let me know if the tfi solves your problem

78F350

Update...
I replace the Distributor and Coil today and set the timing. The problem is still there. It does run better and smoother than it had, but after it warms up it still cuts out when it it under a load.
I did not replace the TFI module yet. I'll probably swap that out tomorrow.

farmer12

The new distributor didn't come with one?

78F350

They didn't have the one with the module in stock, so I took what they had and ordered the module separate. The module will be in at the store in the morning.
Considering its 3:30 AM and I'm working... I'll probably wait 'till Monday to do it. I'll post again when I see what happens.