5.0 Ford MPI surging at idle and at 3500 rpm

[mpsyamaha]

When cold, the engine starts and runs good at first. After warmed up the idle surges very slightly, just enough to see the tach move a little and you can hear it. Feels smooth getting on plane, but after cruising at 3500 or so for a few minutes it will surge more violently. If I accelerate hard from idle to 4500+ rpm it doesnt do it, runs smooth at high speed. But if i slow down to around 3500 it will surge hard again and you can not increase rpm past that.

I have replace the cap/rotor, spark plugs, plug wires, fuel filter. This made no change to how it runs. Fuel looks good, no water. Temp, oil pressure, and voltage are good. Where should i be looking next? Any help appreciated!

 

[ESGWheel]

Let’s see if we can get the conversation started. My short answer is this needs to be scanned while running with the type that records. With that there are folks on the forum that can interpret those records and provide guidance. These are expensive but can also rent them (link).

Barring that, my suggestion is to check fuel pressure, pull the MAP sensor and clean it, check the TPS resistance is smooth throughout its range as well as look for any loose or corroded connections associated with the sensors and computer.

 

[mpsyamaha]

Let’s see if we can get the conversation started. My short answer is this needs to be scanned while running with the type that records. With that there are folks on the forum that can interpret those records and provide guidance. These are expensive but can also rent them (link).

Barring that, my suggestion is to check fuel pressure, pull the MAP sensor and clean it, check the TPS resistance is smooth throughout its range as well as look for any loose or corroded connections associated with the sensors and computer.

Thanks for the response!
I do not have the correct style connector to pull diagnostic data from this engine, unfortunately. Its different than any other VP I have encountered. I did do a thorough check of connections/plugs, and battery terminals/main power. I do have good fuel pressure and spark. I will check the MAP sensor, thanks.

I did a bunch of internet sleuthing and found others that had similar issues had bad EFI control module. It was hard to track down, as it is NLA, but found that Ford still offers the same part under a different part number. Its not expensive, so will give that a shot. This part is mounted right next the the ignition coil, for reference. I will update after test running the boat again.

 

[cyclops222]

If that fails. Do a search for air / vacuum leaks using a safe spray on liquid.

 

[alldodge]

Does fuel pressure stay steady between 31-39 psi when surging?

Manual list several items but most revolve around fuel pressure, throttle body and vacuum leaks

 

[mpsyamaha]

Does fuel pressure stay steady between 31-39 psi when surging?

Manual list several items but most revolve around fuel pressure, throttle body and vacuum leaks

Yes fuel pressure is good, even when surging. I will check for vacuum leaks. Ill get back into the boat in the next couple days, once the new part arrives.