Formula67

I have a 1993 Mercruiser 502 EFI Magnum with HP 500 heads and EMI exhaust.
I have a problem with very lean mixture, plugs are very clean, no water in oil and it runs just up to 4500 rpmīs and then misfires badly and then continue to rev up to 5400 rpm on a TEMPEST 25" with BRAVO ONE.
In neutral it revs good up to 4500 rpm and cuts out rpm.
I have a FORMULA F3 THREE LS 21ī6" 1982.
This is a great boat !
I have changed plugs AC-MR43T, Ignition leads, Ignition coil, module in distributor, distributor cap, rotor, fuel filter e.t.c.
PLEASE HELP !

H2Xmark

like Sutphen said check fuel pressure with a gauge on a long hose hook it up to the test port on the fuel rail, watch the gauge will someone is running the boat and see if the pressure drops when the engine cuts out, this is what my port side engine is doing right at about 4500 to 4600 rpm's , I am changing the VST filter in the tank and checking the needle and seat for problems, hope this helps a bit

Formula67

I will check the fuel pressure.
Can make it higher if itīs too low ?
PLEASE HELP !

H2Xmark

if its low then you might have a problem, like clogged fuel filter, blown fuel pressure regulator, are a needle&seat problem are filter problem in the VST tank,
[tank at back of intake manifold] you could also have a bad mec. fuel pump that supplies your elect. fuel pump in the VST tank, but if you checked it at least you would know if you have a fuel problem are not, it could be an ignition are elect. problem

Dave1972

I had a similar problem but my engine would actually stall. It ended up being the small screen/filter on the bottom of the high pressure pump in the VST tank. The screen was so severly blocked that there wasn't enough fuel flow. I also took the injectors out and there is a small screen/filter in front of each injector that I also cleaned. Problem solved. Only problem with the 502EFI (ok, not the ONLY problem) is that the fuel pressure is not adjustable and you may have to purchase an aftermarket regulator (AZ speed&marine?). Just to make sure....did you have the ECM remapped for the better flowing heads? That could be causing some of your problem. You will probably want to use the Merc scan tool to check for any codes as well. The ECM does shut down the ignition if there is a serious problem to limit RPMs but I think typically that is 2000RPM.

Let us know what you find!

Formula67

Hi !

I bought the engine as is and I donīt believe it has a recalibrated ECM.
When I bought it, it had cracks in the valve seats because of the overheat/lean mixture.
I fixed the heads and I have excellent compression on all 8 = 150 PSI.
I will purchase an adjustable fuel regulator.
Is it possible to put it in between the return hose and the VST inlet return thread ?
I will have the ECM recalibrated and I am going to purchase Diacomīs PC program for my Lap top for scanning.
Iīve checked everything else...
Thanxs a million for the help...