mercruiser mefi 3 discussion

[bud37]

Came upon some interesting info reading a mercruiser service manual.....it stated in the battery requirement section, that on dual engine installations the requirement was for each engine to have a separate dedicated battery because the ecm's were susceptible to voltage spike issues.Near as I can tell, we have two batteries from the factory, with the choices being 1,2 or both.Maybe that was old info that has been updated but interesting nevertheless.......it did go on to explain a few of the issues that would arise from that and how the ecm would react to it..............

[bud37]

Question, what is better for diagnostics for mercruiser mefi 3 , the mercruiser G3 or diacom.....

[Viper]

Diacom. G3 is overkill for the MEFI. If I recall correctly, G3 doesn't go that far back, you'd need Merc CDS which is pre G3.

[bud37]

Thanks for that.....going to order the diacom..... have to get used laptop with windows but I think it will be the best route.......

[mjk1040]

Earlier on you mentioned crap in the Throttle body. May I suggest a can of carb cleaner and a tooth brush and clean the inside of the throttle body. The idle air control valve may be dirty.

[km1125]

Check any vacuum hoses too... you could have a vacuum leak that the IAC compensates for under most conditions but upon that quick throttle move it can't and the engine goes lean and stalls.

When you get the diacom, compare idle manifold vacuum on both engines.

[bud37]

Thanks guys.....first thing this spring before the boat went in I cleaned both throttle bodies, checked the plates, there are no mufflers installed in them.....removed the IAC' s cleaned them with sensor clean ( carefully)...whats with the black paint in the torx, that was the hardest part of the job....adjusted the cables to spec....cleaned the distributors inside ( rust stuck to sensors, don't ask about that....not me!)...caps and rotor assemblies are new.Gone over and thru all connectors, vac lines,PVC valves.
Both engines start and run fine.....IAC counts are a little high on stbd 60 or so ( marina tech )......there is something in the programming or something that is common to both engines, because they both exhibit this loss of reference type symptom, but they do not stall every time.

Quite honestly it could just be fuel related, but still I like the ability to check things out in depth without having to be at the mercy of others.I am looking forward to doing this.......

[km1125]

There is also a throttle plate adjustment for minimum flow... perhaps that is why the IAC counts are high on both engines?

[bud37]

So, an update.... I got the diacom, great program....so far I have 7 recordings with small changes for each.The iac counts are still high on the port side, and all the sensors appear to be working fine..all the relationships appear to be correct.....one thing I have noticed is the engine stall issue goes away when engine warms up, so I changed the iac and tps and the engine is much happier now, but the iac counts are still high, tried adjusting the tps but I have to make the mounting holes larger to get the tps voltage down.
Made some shims ( .010 to .030 ) to put under the throttle arm where it contacts the adjustment screw ( saves removing the throttle body).When I put the .010 shim there, the iac counts drop to 40 but the rpm is up ( 750 or so)....need to figure out how to get the rpm down.This ecm has the floating desired idle.
When I inspected the throttle arm where it contacts the stop, it is actually worn from being jammed against the stop over time, so the shim puts it back about where it should be......i will leave the shim out for now.Going to drive the boat to see if it learns... out tomorrow for a night or so if this rain ever stops.....so just an update, more to come if anyone is interested in this stuff......

[km1125]

I don't understand why you need the shim. Is it not possible to adjust the adjustment screw without removing the throttle body??

You should not have to adjust the TPS, but might have to reset the TPS settings in the ECM and relearn the TPS range. Not sure how to do that though on these engines.