Freshly rebuilt 1999 502 MAG MPI

Had some water revision issues after rebuild due to leaking risers that got sorted. Mentioned only because there was water discovered in the oil.


On Bank 1 (1/3/5/7) you can hear it missing at idle. Slight popping/hiccup out of the thru hull exhaust. Not audible with thru hub engaged.

Off idle, even slightly, appears to clean up. WOT runs 4800-5000rpm as it always has. 68MPH.Touches 71 in ideal conditions. This may be slightly slower before rebuild by about 1-2mph but also had to replace a drive during the process so maybe there.. not too worried about this part ATM.


Cap/rotor/wires/plugs all new, but I did swap rotors and cap just to check. Timing is set correctly.


Fuel Pressure on my DIY gauge set up..

30 PSI Key ON

Drops 2 psi over about 5 mins with Key off.

~32PSI at idle - tough to tell as gauge bounces/vibrates very fast

Unplug vacuum line to fuel regulator and PSI jumps to ~40PSI

Off idle as motor revs fuel pressure drops significantly. Appears to be in the 20's, tough to really tell with gauge bouncing. Maybe we come back to this

Vacuum is 13 in HG at idle, jumps to around 18 in HG off idle.


I've had a borescope into the cylinders and not seen anything that caught my attention. Had a hard tim trying to see the valves clearly though. Trying this again shorlty.

Compression test revealed 140PSI across the board last time I checked with exception of #3 was 120PSI. Not great, especially #3 but my gauge has always read a bit low. This was pretty early after rebuild, maybe rings not seated yet. Going to redo this also.


I know the suggestion is probably take it back to the guy who did the rebuild.. there's a long story there that I hope to avoid if I can figure this out independently.

Thanks all for your help.

do a leak down test,,sounds like a leaky exhaust valve.

Compression test would show that though right? BTW I just started doing a compression test and that #3 cylinder is up to 130PSI now, #1 is 135PSI, haven't got to 5&7 yet..

Edit - finished 5 & 7 with good compression results. Found that #5 plug gap was zero. Must have dropped the plug by accident last time.. but no improvement to the chugging out that exhaust.

no,leak down will tell you where the leaking air is/where the popping is

so mefi1 vst or mefi3? fuel pressure should NOT drop to 20 anything with either one running fwiw

Ya that's odd for sure.. pressure gauge needle was vibrating so badly it was difficult to tell how low, but it definitely dropped. Boat runs great though right up through 4800 rpm.
Cool Fuel

couple thoughts..
The fuel pressure gauge was one of those small dial ones, with a homemade shraeder adapter. Really no reason it shouldn't read pressure, but the needle shake was so bad while running it was a guessing game. So is that just a crappy gauge set up or sign of another issue? So it dawned on me that since the rebuild I had them replace my fuel pump as my original one was getting noisy..
It's a knock off pump off amazon..

So do I have a crappy gauge set up or maybe a crappy fuel pump. As said the boat turns good RPM, no hesitation, no bog. Just has a weird sound out of the bank 1 exhaust..

I had similar issues way back when I had the MPI Cool Fuel setup on my 502s. Ended up being a bad hose going from the tank to the water separator input - the idle would be blubbery but it would seem to run somewhat okay on plane. In the end, that engine ended up cooking a couple of pistons (still ran okay but was f'd - found it when I was removing the heads to go with aluminum). EFI fuel pressure is vital so if it's showing you something that isn't right, I'd investigate that until you're sure it's not a problem.

Fuel pressure for Cool Fuel MPI's should be 42psi

Follow S30’s advice. :thumbs

“Off idle as motor revs fuel pressure drops significantly. Appears to be in the 20's, tough to really tell with gauge bouncing. Maybe we come back to this”

Is this with drive in neutral , or in the water under load ?

Autozone, Oreilly’s etc, rent free tools including fuel psi testers.

What was the reason for the engine rebuild ?