MAG MPI 5.7 low-RPM hesitation

[2100V]

Hey guys,

I've had a hesitation in my engine for a while - thought it was just in need of a tune-up but after new properly gapped plugs, wires, cap/rotor, cleaning the throttle body and sensors/switches, and new fuel/water separator filter, it hasn't changed a bit. 820 hours on it.

When I first crank it, if I try to rev the engine it hesitates very badly a couple of seconds before increasing speed. If I let it drop straight back to idle, it dies. It won't die if I slowly increase/decrease speed, such as when idling the boat around in the water. It's the worst when it's cold, but even warm I can feel the engine shaking a little when I am putting around at low speed.

It runs fine, however, under load and will pull all day long at cruising speed with ballast loaded.

I cleaned the MAP, TPS, and IAC. I accidentally broke the IAC man handling it so it has a brand new one and obviously that's not the issue. New fuel pump a couple of summers ago. Otherwise it's pretty much factory parts. Oil is clean (no water). I also checked compression (dry): ~120 psi on all except the first 2 cylinders on the port side, which were ~140.

Thanks in advance.

[NICKYPOO]

As a simple place to start, have you cleaned the throttle body/flame arrestor/IAC valve?

[2100V]

Please read through my post - I've already answered those and a lot more questions (except flame arrestor, and yes I cleaned it).

[boatwakes]

It sounds like injector issues. Simple test but check the fuel pressure when the engine is off and watch if the gauge drops after the fuel pump finishes its prime. We've seen injectors stick open (usually smells rich in the exhaust) but more likely is a stuck closed or intermittently opening injector. You can test them with the engine running at idle and disconnect and reconnect each injector. Carefully listen for a change in rpm or if you have a digital timing gun connect that and watch the rpm from there. You'll see about a 60-70 rpm dip when a working cylinder is disconnected and you'll hear a change in the engine when it happens. If there's no change in rpm or sound of the engine with a particular cylinder is disconnected, you found a bad one. Test them all but if you find a problem, with those hours and year (pre-ethanol fuel system design) I would suggest replacing them all, its pretty easy. Keep us posted.