More people end up in "the house" so to speak with Jack Nickleson (remember the movie???) than become good mechanics!! Unfortunately, I am now having to go back to work full time & 100 miles away from the boat!! Only on weekends to fight this thing & I still want to play some (the wife insists on it!).

Pantera- My timing IS set to 28* on both engines, just one of them indicates on the scan tool that the spark advance is low at 19-21* when the crank is showing 28*, Then a 3000 rpm, both engines are now at 40*+ !! Az Spd has my ECM's & I have been trying to reach Shoffner all week with no luck..... I gotta get a life!!

Blown Formula, when you say that " the timing is set to 28 "degrees and "the base timing is holding 28 degrees" what do you actually mean? Please explain how you set the timing and where the 28 degrees came from. Do you first set the timing in service mode to some 8 degrees and then read 28 degrees at the crank at 2000 rpm being out of service mode?

The base (service mode) timing is set to 8*. The 28* is @2000rpm in normal mode read at the crank with a adjustable timing light. Then I check "spark advance" display in the scan tool & see one engine @ 28* @2000 rpm & one showing 19-21* same rpm. Check the crank & it is still at 28*???? Since the tool is reading the ECM, is the ECM doing something screwy??? Both read 40.1* @3000 rpm spark advance on the scan tool. There is no distributor gear slop as both gears are new. Seems like way to much advance at 40* to me.... still waiting for my call back from Az Speed.... I have a Poker Run next week & I need to solve this!!

Pantera- did you ever solve your problem??? What was it?

The scan tool and ECM have no idea where your base timing is. They can only reference the timing provided by the ECM, so moving the distributor should not change the value displayed but it sure as heck will change the actual timing as shown on the balancer. I don't trust adjustable timing lights, you really need to degree the balancer from 0 up to 40 or so. In a pinch you can put a reference mark at 2.5" from 0, this represents 36 degrees on an 8" balancer (actually it should be 2.513 but you cant measure that close) of course 1.25 will be 18 degrees. This is a good reality check.

As far as manuals, I promise the factory manual is better. Any Merc dealer can get them for you, we do normally keep them in stock.

I know this might sound crazy but two brain burners that I finally figured out.I had some prochargers on a 312 and had the exact same problem. Put a vacum guage on both motors and compare, one of my motors had a vacum leak, and it caused the ecm to search all over for a known idle quality,it was moving the timing air motor and it never performed as good as the other,and never left a code until I found the leak a upper intake gasket was pushed out ,after I fixed that ,they both ran perfect.Also for your guy's info I ran into a friends boat that was blowing huge amounts of oil out the breathers dip stick tube ect we thought it was detonating, it turned out the intake was supercharging the inside of the block by way of the egr valve and valve covers

MBAM/ Marc- I will get some degree tape Monday.... and check that. I agree the ECM doesn't know where it starts at but with base values the same (same service mode base timing) then they should both advance to close to the same place according to the scan tool. One advances to 28* as it should- the other goes to 19-20.1* per the scan tool.... so does that mean the ECM is screwing up at 2000RPM? The ECM also does not read vacuum so that should not effect that but may other things (run up/down, wild, etc...).... just need ONE firm answer!!

At least this problem is not a 1* taper in a valve guide problem like Pantera24 was.... what a nightmare!! Glad he finally found it, but it had to break before they located the trouble.....

thanks guys.... and Iwill put a vacuum gauge on it too.... although my Boost/vacuum gauges do agree with each other.

The engine does read vacuum/boost through the MAP sensor. can be checked with the scantool if it delivers reasonable and consistent values between the engines.
Another idea; crossfire or some screw up with the sparkplug wires?

I understand the MEFI 1 does not read boost or vacuum but the MEFI #3 does...... wiring was checked.....
onward and upward!!

On the standard MPIs the ECM reads manifold pressure also with the MEFI1 SW. If the ECM does not read manifold pressure it is probably more sensitive to leaks. I think you really need Arizona speed & marine to help out in the trouble shooting.

Blown Formula. I can now read your attached chart with data. A few comments;
TPS; all values look normal
MAP; all values look normal. Do I understand you correctly that the MAP input is not used by the ECM in this set-up?
IAC POS, etc; Obviously different between the engines. Is it trying to compensate for something?
Base timing @crank; 28 already at idle?! not normal on a standard MPI engine. usually fluctuating betwenn -10 and 10. However, Az S&M may have done something. Stabilizing around 28 @ 2000 rpm is OK. Did you ever check timing at crank at@ 3000?
Spark adv per scantool; 18-20 on bad engine at idle. That is high.I dont understand how it can be different from what you measure at the crank!?
I read in the manual that if the ECM fails , the ignition module (in the distributor) will take control of the timing and advance it to 27 degrees. Is that what is happening????
I think you need Az S&M to comment on all this.