502 mpi gen v rebuild - advice needed
 

I made the decision to have my motor pulled and gone through. After talking to some engine builders in the area I'm pretty sure I decided where I'm taking my engine to be rebuilt but have a couple questions.

The shop told me they would do the following:

Bore block 0.030 over
hone cylinders
deck block
deck head & complete valve job - he said he would regrind valves or replace as necessary
SRP pistons - I believe he said dome pistons but wondering if they should be flat top?
Comp Cam - he said this cam will be very similar to stock but have 0.020 more lift - Will this cause reversion with stock exhaust and Corsa silent choice? Do I need to reprogram ECM?
ALL ARP bolts - Heads, rods, mains.
Felpro gaskets

I have the Gen V block - so it is a flat tappet valve train.

Hoping someone can chime in on the piston selection regarding dome or flat top and the cam especially.

All in all - just looking for basically a stock rebuild to maintain drivability and durability.

How much HP do you think this would make? Would this increase my compression ratio where I won't be able to run 87 octane? I did research when I bought my boat years ago and remember reading the 502 runs best on 87 octane, any higher the computer would retard the timing.

Apologize for all the loaded questions here just looking for some good advice! Hoping someone can answer ASAP as the engine is already out of the boat and about to go to the machine shop!

Stock 502 Pistons have a small dome, just keep the stock cam,

Regarding the cam, assuming it has the same duration AND ramp profile, LSA and all valve event timing the extra lift will likely not cause the motor to revert. I would install wide band AFR to see how much ECM tuning you would need. It will run on the stock tune but would likely benefit from some table modification.

Regarding the compression ratio and 87 octane let me address the timing pull you are referring too. There were some early build 454/502 EFI motors that did a knock sensor test during start. Basically the timing was advanced to get knock verifying the sensor was good. If the knock sensor test was flagged as a failure during start it would default to a reduced timing mode (limp mode). Running higher than 87 octane fuel would in some instances not allow knock to be sensed during the start up test giving a bogus sensor failure and resultant limp mode. There was a Merc Bulletin addressing this issue many years ago. Again it was on early 454/502 builds. You would need your engine serial number (BUP, I saved you from having to state this;) to verify if this issue affected your motor.

Beyond the start up test of the sensor any time knock is sensed the ECM will pull timing. How much it pulls, how long it pulls it, and how quickly it feeds it back in are all parameters set in the ECM. All of the happens regardless of fuel used.

Best of luck.

raw water or closed cooling? fresh or salt? if salt and raw, replace the heads. they rot at #2 and #7 exhaust valve guides.

raw water cooling - fresh water entire life.

The 502 can pull more then 500 HP, the guys that know how to do it are here but don't know why they are not chiming in. You have the VST and a MEFI-1 (always a good reason to tell everyone what you have) so there is no code 44 for the knock sensor, it just retards timing IF a knock is sensed. To build power you need to increase compression, and a flat tappet cam is not going to do it, you need a roller cam. You can keep the compression a bit lower and still build power, I'm running 89 octane

BTW Smitty has a great build on a 502

I never heard of mefi advancing the timing on start up to test the knock sensor, new to me. The stock 502 tune is terrible in the regards that max timing is only about 26 degrees and afrs are pig rich, i recently built a 502 w lighly ported iron heads, flat top pistons , 500efi cam and highly modified intake w bored out throttle body. It made over 560 hp and about 590 ftlbs on dyno, runs best on 89 octane, boat is 10 mph faster than it was w stock 502. They need more cam than the piece of garbage merc puts in them and a little tune, a guy could easily make 500 hp with just the normal.mods to intake, a cam and a little tuning, fwiw, Smitty

[QUOTE=articfriends;4486051]I never heard of mefi advancing the timing on start up to test the knock sensor, new to me. /QUOTE]

Its Merc Service Bulletin 94-12....see text below.


Printed in U.S.A. - 1 - 94-12 994
service bulletin
TO: SERVICE MANAGER TECHNICIANS No. 94-12
PARTS MANAGER
A. EFI GASOLINE RECOMMENDATION CHANGES NUMBER 1 and 2
Models
All 350, 454 and 502 cid EFI Stern Drive or Ski Inboard Engines
Change Number 1
MCM 5.7L Alpha or Bravo EFI Stern Drive Engines, Serial Number 0F415136 and Below.
MCM/MIE 454 & 502 Magnum EFI Stern Drive or Ski Engines, Serial Number 0F417515 and Below.
MIE 350 Magnum EFI Ski Engines, Serial Number 0F350545 and Below.
USA and Canada: 87 (R+M)/2 Unleaded Regular Gasoline without Alcohol.
DO NOT USE Unleaded Premium or Leaded Regular Fuels.
Outside USA and Canada: 90 RON Minimum Octane Major Brand of Unleaded Regular Gasoline without
Alcohol.
DO NOT USE Premium Fuel.
Reason For Change Number 1
Use only unleaded regular gasoline in these engines. DO NOT USE premium fuel because it can and does
cause certain problems such as retarding spark advance, using more fuel and setting Code 43.
Every time the engine is started, the ECM goes thru a self spark knock sensor system test when the engine
reaches the specified test RPM for that particular engine. The ECM adds spark until it “hears” a spark knock.
This test tells the ECM that the system is operating properly. When premium gasoline is used, the ECM may
not get the engine to “knock” when it goes thru the self test. The ECM now “thinks” the knock sensor system
is not operating. The ECM retards spark several degrees and adds more fuel. It will also set Code 43 because
it “thinks” the knock sensor system is not working. This code will stay active during this running period.
If the fuel is switched back to unleaded regular, the ECM will do the self test the next time the engine is started
and goes above its specified RPM. The engine will now run normally, however, the Code 43 will be stored in
the ECM’s memory. Code 43 will stay in the ECM’s memory until the engine is started/stopped 100 times or
until the technician clears the code.
Revised September, 1994
94-12 994 - 2 -
Change Number 2
MCM 5.7L Alpha or Bravo EFI Stern Drive Engines, Serial Number 0F415137 and Above.
MCM/MIE 454 & 502 Magnum EFI Stern Drive or Ski Engines, Serial Number 0F417516 and Above.
MIE 350 Magnum EFI Ski Engines, Serial Number 0F350546 and Above.
USA and Canada: 87 (R+M)/2 Unleaded Regular or Premium Gasoline Without Alcohol.
DO NOT USE Leaded Regular Fuel.
Outside USA and Canada: 90 RON Minimum Octane Major Brand of Unleaded Regular or Premium
Gasoline Without Alcohol.
Reason For Change Number 2
The use of unleaded regular or premium gasoline in these engines is acceptable. The newest ECM used on
these engines will not do the “self spark sensor system test” each time the engine is started, like the older
ECM’s.
These newer ECM’s will still retard spark and add more fuel if it detects a spark knock during its normal operation
just as the older ECM’s did.
All service replacement ECM’s will have this latest change in them also. The “Check-Sum” code number
placed on the latest ECM’s that can use either regular or premium gasoline are as follows:
350 cid EFI engines: 983E
454 cid EFI engines: 93AB
502 cid EFI engines: 939D
72801
a
a.- “Check-Sum” Code Number Location.

Smitty the MEFI 3 and 4 do check to make sure the knock sensor is working, main reason I know is it kept showing up on my motor. It checks when the motor is first throttled up for a split second and does not check again until a restart. If it doesn't get a knock it sets a code 44 and doesn't allow the timing to fully advance. Mine was a bad connection on the connector, so I bought a pig tail and replaced and a MEFI 3.

Don't know if the 555 and higher does or not

[QUOTE=Trash;4486065]So sounds like a early mefi1 thing? Like i said, new to me, we learn something new everday,thanks!!!

Also interesting, ran mefi4 on my blower motor and various other projects plus ran and tuned quite a few mefi3s now, must be in flags and schalars table, i do turn most the code crap off anything modified, will have to open up a bin file when i get back to shop and see what i can find, Smitty

[QUOTE=articfriends;4486161]Yes by the bulletin date and more specifically the serial numbers mentioned within the bulletin. Merc altered the test later to preclude this as being an issue.

[QUOTE=Trash;4486193]Ever heard of it on a mefi 3 or four?, I know on passenger car when you first start them if the KNOCK sensor is shorted out or has an open circuit with no resistance whatsoever it could throw a code but I've never heard of a mefi 3 or four testing actual knock upon start up based on octane and then deciding to go to a limp default mode?

I was a bit in error for MEFI 3 and 4, it shows up for 2, 3 and 4. Was not part of MEFI 1

http://forums.iboats.com/filedata/fe...5484&type=full

So I just left the engine shop and this motor has already been rebuilt once before. Also a roller cam was added at the first rebuild. Bearings are not horrible and Pistons have light scuffing on the skirts. Probably only needs a honing and new Pistons/rings and head job. Probably only needed to replace head gaskets. I should have pulled the heads before sending it off but oh well I'll have a fresh engine again. If I knew more about having the ECM programmed and wanted to buy some good exhaust I would try to wake it up a little bit but just staying stock.

[QUOTE=articfriends;4486264]I have not. This is the only bulletin I've seen regarding this issue. I ran into it way back in 1994 when these motors first came out.

[QUOTE=Trash;4486527]I called bob at mefiburn and he told me that indeed there IS this possibility on some, not all, mefi3's and 4s, he said. The essiest way to ensure it never happens Is to un-check the flag for code44, knock sensor will still work IF it sees knock but wont do no unexpected things do to a start up table.

[QUOTE=articfriends;4488323]Thanks for passing that info on!

I looked at several tunes I have, looks like most the stuff Tyler crockett has done has #44 unchecked, a stock hp 500 does have it checked but there are other things in the tune that will affect wether it "test" knock sensor and what it does IF it doesnt see knock on intital startup, some you can see with std mefi burn, some you need mefi pro to see,seems strange that merc had a bulletin saying it was a concern on early mefi1s but not a concern later, Bob wasnt specific as to what had it and didnt have it ie maybe stock ramjet 502 does but not mercruiser fwiw, Smitty

IF I dyno my stock hp500 efis this winter I will check timing at a certain map/rpm then disconnect the knock sensor and see if total timing is reduced somewhere and post my results, will be after January probably though, Smitty