ragtop409

Do you guys know if the black motors are the same way (the nock sencer test) and the bad distributer ground? Freinds regular 502 dose these same things. Charlie

John B

Now this is getting very interesting. Especially the knock sensor situation. This could cause the black transom problems.....Thanks for the tips. JB

bobl

The floating tach syndrom is a sure sign of a bad ground. Check all of the grounds from the EFI and main harness. They must have a good ground to the block.

Dennis Moore

Mercruiser Service Bulletin 94-12 states that the engine will go through a self test on the knock sensor for only certain models.

This feature was later dropped from the computer on newer models.

There were only a small amount of engines produced with the self test. These engines were;

5.7 liter EFI serial numbers OF415136 and below
454 and 502 EFI Magnum serial numbers OF417515 and below

Using high octane fuel in any other Mercruiser engine will not effect engine operation (it will just be a waste of money).

Sincerely
Dennis Moore
Mercruiser Master Tech

FAMILY AND PERFORMANCE BOATING MAGAZINE

ragtop409

Thanks Charlie

John B

Fuel pressure is 41 -42 lbs all the way to top end. No detonation sensed with Mercury tool. Timing, a different story. Initial set at 8, went to 26 at 2100 rpm and stayed there until 5000. Went up to 35 at 5300. This was similar on both motors! Anybody know what Mercury is thinking? This may be what I feel at 5000, a big change in timing.????

Anybody have any ideas? or verification.

John B

How come there is a black spot on my envelope, left side of the post?

BryanTuvell

OK guys, I am having the SAME problem Dennis described at 4000-4500 RPMs, as soon as I plane out I go to a retard mode and limp home. I also had an idle start problem which a new IAC fixed.
I am using a Cert Merc mechanic. No codes on motor (2001 502 OM serial #, 150 hours). Plugs are rich after retard mode, black transom too, new wires, plugs and tested with a entire new distributor, issue is still there.
Comp check 150 and #2 cyl was a little lower.

I had changed the fuel/water separator at winterizing, plan to change it again today, then we will check the fuel PSI per page one of this thread today.

Frustrated that the boat ran fine before winter, strong 5100rpm, and at a new season I can't get going.
Bryan

BryanTuvell

Went and picked up a fuel/separator while I wait on the mechanic to come by this afternoon, at the biggest repair shop in town (Lynnhaven Marine).
A master Mech there said there who has done maitenance for me said there are 3 or 4 things that forces the Gaurdian/limp home mode, it is in the MEFI3 Manual.

Anyone have a minute to look that up?
Bryan

I saw a very nice 2000 38TG w500's in for service, sweet!

Adivanman

A question and a couple of comments...

Question:

Does your 500EFI really spin 5300. Not to split hairs but the rev limiter should be 5250...you may have some tach float.

Comments:

The erratic tach is, absolutely, a ground issue, most likely solved by running a short jumper wire off one of the cap hold-down's to a good engine ground.

The most common reason for fouling plugs when running 93 (especially in Nothern climates) is bad gas due to storage or seasonal variations (detergents and antifreeze, etc.)

The timing on the 500EFI's is inherently soft by Mercury design. Good for longevity but questionable for all out performance.

Before you attribute your RPM loss to the engine, be sure to consider the drive or prop. Increased load on the engine could be due to problems downstream.

Just my two cents...