Working on a 502 Mag EFI with an old Whipple side mount. Just had Whipple rebuilt over the winter. Re-installed exactly as it was removed. Did not remove any other parts except charger and upper plenum. As soon as the motor fires it will not idle below 3000 rpm. Checked every possible item. Twin engine boat so we swapped all sensors ECU etc. from good motor. Smoke tested for vacuum leaks. Found none. When running even with throttles closed it is still pulling in a bunch of air. Took off blower belt and still sits at 3K. Actually pulls enough air to spin blower. Only time it will not run is when I tape off all of the air cleaner. If I leave a row of tape open open, it goes to 3K
. And the IAC air has also been blocked off.

Check all sensors with Rinda scan tool and they all show good. I am at a loss at this point. Anybody got any ideas ???

Thanks !!!

Smitty

So the lower plenum and dist. stayed in place, timing didn't change ? did the scannerr see the throttle positioning sensor,

Only upper plenum along with blower and throttle body were removed. Using Rinda scanner, all sensors on both motors are operating within normal ranges. Also swapped one sensor at a time from other motor including ECU. Even pulled blower belt off, it it still went to 3 K . It was pulling air right thru blower and making it spin. TPS shows correct voltage at .49 and show position all the way to 100%. Only thing we did not check was TPS position at the 3K mark. Did not want to leave a cold motor revving at 3K sometimes bouncing towards 4K. Totally crazy stuff.

really sounds like a vac leak.maybe it pinched one of the o ring seals that seal the upper to the lower.

Thought it was a vacuum leak at first. Double checked everything and then used a smoke machine and no leaks. Have had whole assy off 3 times.

Man I would have thought a vacuum leak as well. I know how all smoke machines work but we use propane to find vacuum leaks anyways passing that along for a cheap route for anyone. Maybe give that try as externally trying to find a leak, Are you sure you do not have a crack in the intake plenum. IMO would have to be the TPS or IAC or last thing the coolant temp sensor..

I take the block off IAC was for testing ? The other motor exactly same set up on everything & sensors?. I would also check voltage, grounds and all connections to sensors.

I would also compare your diacom scan thru out from the good motor compared to your lunch eater engine. See if all the specs and ranges are the same or are they off somewhere.

i have no experiance with a whipple side mount,is it innercooled?if the throttle plates are closed and iac is closed where is it getting the air that is allowing it to rev to 3000 rpm?

I agree thats why I asked about the blocked off IAC., I guess the throttle plates then are not closed 100 % all the way to have a set idle speed when this engine is running or does the throttle plate have a air hole in it or is this what you met blocked air ?

IMO then a totally 100 % closed throttle plate(s) should stall this motor with a non working IAC / blocked air. the throttle plate has to have a hole it or are not closed 100%.

Also Mercruiser throttles plates were set with set screws to allow just enough air past regardless then have the computer monitor what the IAC pintle should be set at (moving in or out) to maintain proper idle speed. Engine Idle speed is a function of total air flow into the engine based on IAC pintle position.

How about bad or missing O- ring on the IAC or a bad seal of the IAC itself ? Never had one missing so throwing a dart here.

How is the pcv valve plumbed? You won't see the smoke if it's dumping into the valve covers
What year is the engine could the vacuum line to the fpr swapped with the fuel pump vent?