I have a pair of 1997 Merc 502 MPI engine with 500 efi cams and stainless marine exhaust. The MEFI 1 PCMs were refalshed for this configuration. Long story short, I ended up running the engines at 36 psi fuel pressure and disconnected the knock sensors and did bad things to the engines. The damage is identical on both engines: The spark plug melted on the # 3 cylinder with no piston damage and the piston melted on the # 6 cylinder with no spark plug damage. All four of these are the same cylinder location in the head castings. Is there a cooling problem the the head in around the second cylinder from the left end of the head when looking from the exhaust port side? Is there a flow problem in the intake manifold / head which could cause these cylinders to be leaner than the others? Could there be a mismatch between the 500 efi cams and the 415 MPI intakes? Could the PCM program be causing a unique fuel flow issue on these cylinders? Could COMETIC head gaskets cause a cooling flow issue? Are a melted piston and a melted center electrode on a spark plug caused by the same condition or is one detonation and one pre ignition? Thanks for any insight you can provide.

Could the PCM program be causing a unique fuel flow issue on these cylinders?
No, the mefi is batch fire and fires 1/2 the motor every rev, those cylinders simply were the leaneest out of all of them and melted down first. Who did your reflash? The only safe way to start switching cams and changing fuel pressure (although yours was higher than stock) is to ideally dyno motor while watching temps/02 sensor and at a minimum check motor with 02 sensor in the boat and re-tune fuel ables or at least throw extra fuel at it with higher rail pressure (a band aid), Smitty

The PCMs were flashed by Tyler Crockett. I thought the factory fuel pressure was @ 40 psi with no vacuum to the regulator? If the system batch fires are cyliners 3 and 6 at the beginning or end of the firing sequence of each batch? Still trying to understand why the exact same failure mode on the same cylinders in two different engines. Also, using an O2 sensor would be reading a mixture of all exhaust from one bank, would it catch one unique cylinder?

Thanks.

Not sure how the fuel pressure with no vacuum is on a stock merc but when I was first tuning my 502efi prochrged engine we were setting base pressure around 32 psi so I would think 38 would have been in your favor.
As far as I remember the batch fire it fires 4 cylinders 1,2,5,6 then another revolution later 3,4,7,8, it just ends up spraying fuel crudely into ports without alot of importance to piston position/timing, the fact that you melted the same hole in both motors tells me that those holes must typically run lean ( someone who sees alot of damaged motors like Eddie Young might chime in if 3/6burn up first). As far as a 02 sensor not seeing a individual cylinder, only seeing that bank, as long as you don't have a severly lean cylinder from a strange problem you are going to tune to a safe afr , not to edge of burn down anyways, individual egts on dyno would tell you more but alot of tunes have been done in boats using 02 readings and it usually works, Smitty

I might have the batch fire order wrong but thats how I remember the injector plugs going acrossed the intak but here is a previous thread:
http://www.offshoreonly.com/forums/g...efi4b-ecu.html

Batch "A"1467, Batch "B"2358

Thanks for posting that, I stand corrected. Back to original question- guys complaint is 3 and 6 got hot and melted down, they might run lean as far as these motors go but its NOT because of injector firing sequence or injector timing, Smitty

Why where the knock sensors disconnected ?

Just want to add MEFI 1 ECM's not the greatest to begin with and have a very limited range as far as reprograming. MEFI 1,2 & 3 are NLA even thru Mercruiser. If need one of those vintage ECM's its MEFI 4 that takes its place now a days. Also 1997 ECM's should not have been MEFI 1's to begin with as that should have been MEFI 2 or 3 by then. Your engine serial # would dictate what ECM should have been installed when this motor was in stock configuration. Good Luck

Long story, the engines were not making the power I expected based on boat speed. So I connected my Rinda software to the boat and recoreded what what happening. I discovered a lot of spark retard in both engines. Since they are stock compression and the fuel pressure was set to 36 psi as recommended with the current tune in the PCMs I looked for other conditions which would set off the knock sensors. I read on this site that the drive shafts used with Stellings Extension boxes could set off the knock sensors so I decided to disconnect the sensors. I made two passes with these disconnected and there was no spark retard recorded but that was enough time to damage the engines.