I have a 1985 Formula 302 with twin stock 1998 502mpi mags.

I am having trouble with the starboard engine after about 20min of running hard or cruise it begins to surge bouncing between 2000 and 3000 rpms. punching the throttle does nothing however pulling back to 2500rpm or under it stops surging and will run just fine. Running the throttles back up above 2500 will result in the surging. If shut down and wait for the engine to cool it takes another 20min for the issue to present itself again.

Normal engine temp
good oil pressure
92 octane fuel fresh and treated
fuel filters replaced in fall
tune up in fall plugs wires etc.

Fuel issue?

IDEAS?

Does your engine have that IAC sensor/valve? From what I've read this could be the culprit-- I'm not an expert by any stretch!

3pointstar

Would that present itself after some normal operating time?

I doubt its the IAC as that has predominant affects during the idle regime. It could possibly be the TPS sensor gets whacky when warm but that is a stretch. Swap sensors between motors and see the the problem goes to the other motor.

The MAP sensor could be another possible culprit, but again that is likely a stretch.

Fuel issue? Just thinking out loud.

Does it have VST or cool fuel? There's a screen in the VST system that gets dirty, will back wash itself, appear to be running good then do it all over again. +1 thinking out loud.

I would get a fuel pressure gauge hooked up to the rail and see whats its doing, is it a mefi 3 or what ?

Fuel Pressure is good and has been tested.

Biggus suggested it could be the pickup coil in the distributor.

what Im saying is put a gauge on it and watch it as it starts surging, is that what you did, it has the smell of vapor lock to me, u turn it off, cools down, runs fine, till it heats back up, or did I miss something

Sure sounds like fuel supply issue to me too.

Biggus maybe correct, but the near 100+ EST modules I've seen go bad, typically will not restart right after turned off after a run.

I'd also check the fuel psi right when it is doing this.

Fuel deliver problem of some sort or possible TPS sensor

The hotter the fuel pump can get the worst they get. Run a temp gun on the electric fuel pump and record temps.

Anyone scan this engine ?

any details how you tested fuel pressure ? and what was the readings @ idle - 1500 rpms - 2000 rpms - 2500 rpms and 3000 rpms ?