wwbobb

Hello all

I have a 1995 502efi completely stock other than Gil offshore exhaust. According to my Rinda Mercruiser scan tool, the advance is showing 40 degrees at 3500 rpm. I have base timing set at 8 btdc. I also tried 0 btdc to see if it would make any difference. It did not. Still 40.
Anybody ever see this problem? I'm stumped.

Thanks
Warren

28Eliminator

The timing is programmed into the ECM, what does the timing do at other rpms like 4000, 4500? It has a knock sensor that will pull timing if it senses knock, but 40 does seem to be a little on the high side. Does it indicate any knock happening?

wwbobb

Didn't want to run it that high on the hose. Figured it was all in at 3500. No knock sensor activity according to the scanner. I'm leaning toward the ECM but checksum comes back with no errors.

28Eliminator

That is not how a computer controls timing. On the hose there is no load, the vacuum is high, no air flow, no commanded torque and the such. You have to check it when it is under load in the water. I am not surprised you see 40 on the trailer, as it's not even a valid test. Connect your Rinda and run the boat and you will see much different numbers probably closer to 32 deg of timing.

articfriends

iTrader: (1)

were you driving the boat when you saw this 40 degrees at 4000 rpm's? The hp 500 uses quite a bit of timing in the timing tables BUT its really based on the base timing being at the factory setting, if you retard the base timing by 2 degrees you effectively reduce that number your seeing to a actual 38 and on other hand IF you bump up the base timing by 2 degrees over stock that 40 your seeing is actually 42. Now lets look at something else, if you WERE free revving it and not driving it you were in a 50-70 map table, if you were actually driving this would be closer to around a 90 map table, What this means the best way I can explain it is 0-30 inches of vacuum is equal to 100 to 0 map, each 3.3 or so inches of vacuum is 10 map, when free revving your motor you could have 15 inches of vacuum which would equate to about 50 map, your boat would never be a 50 map under a 4000 rpm load , more like 90 map. I have reviewed a hp 500 clone tune before, at the lower map settings I remember seeing it having timing as high as 38 degrees in the upper rpm bands, maybe it was 40 BUT either way at 80 map it was more like 36 degrees and as it got closer to 100 map it went down closer to 34 give or take a little. To make a long story short the timing free revving with no load is different than loaded so unless someone's been screwing with your tune its a NON issue, Smitty

articfriends

iTrader: (1)

Your dead on, I was typing and didn't see you post or I wouldn't have bothered but yes, you are entirely correct, Smitty

wwbobb

Thanks for the quick replies. Makes sense regarding no load conditions. This stuff ain't in the manual. I'll throw it in the river in a day or two and see what happens and report back.

28Eliminator

Thanks Smitty, I liked your detailed explanation. I haven't programmed any boat stuff, but i have cars and most people don't understand how EFI all works.

wwbobb

Update

I finally took her out this week and full advance was stable at 34 degrees at WOT. Thank you 28Eliminator and Smitty for the education!

Warren

Trash

Just to add another data point to what Smitty said, the Merc timing tables can be an interesting animal.

My bone stock tune has 40.1 degrees of timing in the 40 MAP column from max rpm down to around 1800 or so. There are only two conditions you can see this kind of ignition advance;

One is on the trailer, no load as the MAP will be relatively low / vacuum relatively high.

The other is on a snap decel (running say 4000 rpm or more and snapping the throttle to idle, closing the T/B blades causing high vacuum). It is my guess that they throw all the advance during decel to keep the engine lit off as it transitions to idle during this condition.

Smitty stated that free revving you'd see 50-70 kPa on the MAP, but I've also seen it in the 38-40 range as well. Snap decels will pull it down further, but it's very transient.