RPM's surging +/- 200 rpm's
 

I've found a lot of excellent advice on TBM in reference to this problem but I figure'd I'd throw it out here also in case someone else has experienced this maddening problem. At idle but mostly at the first throttle "click" or no-wake zone speed, the motor surges approx. 100 - 200 rpms and it's making me mental. My motor is brand new, with 27 hours on it but the intake is from my old motor so it's something to do with the intake/fuel injection setup. The advice I've received so far from LKSD, Boss232 & others is to clean my throttle body which I will do today and check the vacuum lines but, just in case it doesn't go away has anyone else experienced/fixed this ??? It really is annoying, any advice is appreciated. Oh yeah, the motor is a 2007 GM Marine Division 502 Mag MPI (415hp) base engine with the intake from my old motor. Rock

is it like a stumble? lean condition off idle

filed spark plugs possibly

believe it or not I had too much timing do that too.

It does not happen at idle or when running above 1000 RPMs. It's just a constant surge, from 400 to 800, back down to 400 then back up to around 800. The only way I can get it to stop is to either back off the throttle all the way or run over 1000-1100. It's annoying because it's right at the 'slow, no wake' speed. I'm going to check the IAC motor and throttle cable adjustment as suggested by others.

Rock

If it is EFI or MPI, it is most likely your IAC,( idle air control)valve/sensor

Try this may do it if corrosion or salt is hanging up your iac .Spray some storage seal or wd -40 in the inlet to the iac while it's running.
It's the hole above the shutter under the flame arrester. It might keep you from taking things apart.
Jim

Engine Surging - FIXED !
 

Well, I took the throttle body off cleaned the sh*t out of it, installed a new IAC motor and TP sensor. The surging is gone and the idle is a little higher than before. The surging, at it's worst, got to about +/- 1000 RPM's...now it's just strong and steady. :cool-smiley-011:

One last question, though. I noticed that when I replaced the TPS that there was a little bit of room for adjustment (approx. 1/4"). What is this adjustability for and how do I know if it's adjusted properly ? Again, it's running great now, I'm just curious.

Thanks,

Rock