I figured I’d come here and ask as a last-ditch effort to see what’s going on before I do a carb conversion on my 1988 F350 351W with T18 manual transmission farm dump truck. I purchased this truck a few weeks ago with this problem.

Here’s what it does: After it gets warm, out of nowhere it starts bucking, surging, skipping and occasionally backfires on deceleration. The two backfires I have experienced were the wicked loud ones coming from the exhaust that could probably be heard 2 miles away. While driving, I need to keep the throttle at least 50% to keep it running and it has no power and loses speed. I usually need to downshift and floor it to get the speed and momentum back. The last time I think I noticed some black smoke coming from the exhaust. If feels like an engine running with the choke fully engaged and timing way off. Then out of nowhere – it clears right up. There is no warning or gradual anticipation going in or out of this problem. It’s like a switch – it runs like crap all of a sudden then just snaps out of it after a few min.

What I have done: I replaced the ignition module before driving the truck home when I purchased it with a motorcraft part since the PO owner said “it might stall when it gets warmed up and sometimes it will start back up and sometimes you need to wait.” Since this did not correct anything, I then replaced the pick-up coil, TPS, MAP, ignition coil, and soldered new capacitor in ECM. I then disconnected the vacuum to the TAD/TAB and took it for a test drive and still did not change anything. Fuel pressure is about 33psi and no raw fuel from the regulator and pressure jumps to 42 when I disconnect the vacuum hose. Engine stumbles and RPM’s drop when I apply the hand pump to the EGR. The IAC valve looks new and the engine rpm’s drop and stumble when I unplug the connector to it. Timing is set at 10 degrees with the jumper removed. I even tested voltage and checked ohms on the O2 sensor.

I just ordered a new ECT sensor thinking maybe it’s intermittently bad throwing the engine into “freezing cold” mode when it wants to. But I've never had a temp sensor cause havoc but who knows.

I did pull codes but I do not have them with me at the moment. The codes did not seem too relevant since the engine wasn't up to normal operating temp. I will try to pull new ones this afternoon. The truck is not really drivable especially if I were to get stuck at a light or around any traffic or need to stop. This will idle all day long and drive around the property with no problems. Get it on the road and she acts up. I've never had anything behave like this without an obvious solution. And being intermittent is making it really difficult.

Sorry for the long write-up. I am trying to be thorough. I am really considering a carb conversion since this truck probably won’t ever see 1000 miles a year. It’s not a daily and is used around the property and an occasion trip to get a yard or two of gravel.

 

The coolant temp sensor and intake air sensor modify the fuel and timing maps.

The early efi trucks with the ignition module mounted on the distributor are prone to heat related problems with the ignition module.

The PIP sensor inside the distributor can also have heat related failures.

 

Also, never a bad idea to pull the computer and inspect it for damage.

 

https://lmr.com/item/FF-423706B/must...-86-93-423706b

This gem right here stopped my issues completely after fighting for 3 months to figure it out. You would think a new one would stop it, not for me.

 

I replaced the ignition module with a motorcraft and replaced the PIP with no change. I soldered a new capacitor in the ECM - which looks like the ECM may have been replaced at some point in it's life. I suppose I could relocate the ignition module. I did purchase a heat sink for it I just have not used it yet since nothing had changed. I know the quality of parts are not very good for older vehicles which is why I am considering the carb conversion if the coolant and intake air sensors don't change things.

 

I know exactly where you are mentally. I spent over 3 months with a tuner and we couldn’t figure it out. Finally, I said this was the last thing I was trying as it didn’t make sense. It did though even with a new motorcraft Tfi. I ask you try it and then pull the trigger if not!

 

It you’re building your own wire harness, make short you shield the pip wire to and from the distro to Tfi with a drain wire. Keeps RFI away.