have 502 mag mpi with bizzare injection problem. have scan tool and run diagnostics and am well versed in this type of injection but would really like to have a one on one dialog w/ someone that has done a LOT of these to maybe keep me from re inventing the wheel.

if that person is out there, please e mail me at [email protected]

I have learned ALOT about 502 mag mpi and could possibly answer your question but the person who is a expert at it is Tyler Crockett-give him a call 810-324-2720,he is a great gut and will be able to help you,Smitty

Could you give us an idea of what the symptoms are? Posting the problem and eventual solution may help some else with the same problem some day.

Dave

true enough.... its long but here is the whole story

have 1996 502 mag mpi. they were takeouts w/ 125 hours on them. came absolutely complete and factory. after boat rebuilt to some significant degree, i installed the motors, fired them up and they ran absolutely perfectly. as documented elsewhere in this forum, went thru the trimming out process and learning curve ( am vastly experienced in other mechanical arena but no boat stuff) and had settled into nice top gun that would go 65 mph at 4900 rpm all day w/ good temps and pressures. it did this for 300 miles.

at 300 miles filled it w/ fuel ( also documented here ) and did analysis of performance/economy and all was wonderfull. on that same day AFTER fueling went to lunch...30 mile round trip more or less... as fast as the boat would go both ways just fine.

parked the boat... and weather turned lousy... so for about 25 days the boat stayed parked.

last sunday while the weather was STILL pretty bad, i decide to go out anyway... boat started and ran perfectly the 2 miles to get out to sea and i zapped it and all was wonderful except for the 4 ft seas... which meant that when i turned the corner at 15 miles the seas were going to be 6' .... so i said the hell with it and turned around and started home...

about 3 minutes later the port motor just went flat. temps and pressures good but would only idle ... any advance in throttle just gave a big nothing... starnord motor ran fine but not great...

i said to myself " hmmm throttle position sensor just fell off" so shut down port motor and went home on one just fine.

rain all day the next day... tuesday i go and start the motor and it sounds completely flat... tps still in place... i rattle all the connectors and screw around a bit looking for the easy fix but find nothing and the starboard motor doesn't sound particularly crisp and the weather is still lousy so i pack up and go home and order up a mercruiser scan tool. figure gonna need it anyway and will save time...

it arrives the next day...

so.... i check fuel pressure ok

i check all the sensor outputs and get some anomolies on the port motor but not so bad...

there is no water in the fuel that i can detect

i swap the throttle bodies around ... that gives me a good TPS and iac mtor and it makes a little diff but not much... i unplug the sensors one at a time to get them to default... no change...

its sitting idling flat and i rattle the coil wires... the lt inputs at the plug and WHAM the thing is clean on 8 and everything is great... i smile shove off get 1 mile and same thing blah flat ... and starboard motor STILL not great...

so... now i am absolutely convinced that somehow i now have 300 gallons of bad fuel.... no way it can be anything else... i cant see any water, filters are clean etc... but all was perefect bef fillup and now not... end of day 1

day 2 today.... 7 am i am at the dock w/ 2 5 gal cans of fresh fuel. i lash them into the engine compartment and hard plumb them to the fuel system completely isolating the boat tanks. i flush the fuel rails, filters and surge tanks...

i hit the switches and WHAM i am on a clean sharp 16 cylinders.... i shove off ( weather NICE today) get outside, nail the sticks and i am at 5100 rpm so fast i can't believe it... boat running better than ever and i see 67 mph 2 mph better than prev best....

WELL i say... that settles that... so i stop, throw out an anchor and switch the fuel system back to original... double blind test.... port motor goes flat immediatly ...starboard is ratty...

sounds simple ... except when i flush eveything and put it all back to the 5 gal containers, the starboard motor gets fine but the port stays flat. scanner says all the sensors are low voltage... which means cold which means wildly over rich which all adds up.... but now i have conflicting and coincidental problems which i don't believe....

go back on one motor which runs ok...

now... drain and barrel roughly 300 gals of fuel... cant SEE anything wrong but who knows at this point.... put in 50 gals fresh per side ( from dif source) and get rid of jerry cans before i blow myself up and put fuel sytem back to original,

starboard motor sounds MUCH better... port motor still flat... so i start to head back from fuel dock and after a bit there is this surge and suddenly everything is wonderful....

i head out to sea... 5100/67 mph instantly.... i smile... run a bit and turn and head back... and wham... port motor goes to idle with sticks at wot. i stop... it stalls. it takes me 10 mins to start it and when it does it blows unburned fuel out the exhaut for a full minute and is flat.... thing has gone full rich and washed the plugs instantly scanner says ALL the sensors are low voltage. starboard motor is perfect...

it starts to rain and i go home.

now ... ive been in this business for a long long time and im not a believer in coincidental problems.... but i believe there was a contamination problem w/ the fuel... i THINK somehow on the day i got fuel there was diesel contamination in the fuel i bought but SOMEHOW ( and i offer no rational explanation) the boat ran perfectly for that 30 mile rt so i got home fine. then the boat sat 25 days and whatever was there separated.. and screwed me up...space aliens maybe but bottom line is that fresh fuel made a big and repeatable dif.

but now i have this motor reading all the sensors at low volts... not possible and i don't believe it for a minute... what i BELIEVE is/was a bad harness ground... ... which i inspected and was perfect... and then it got dark and i went home.

tom im going to unplug the primary engine harness from the good motor and plug it into the " bad" if it STAYS bad then that suggests that it isn't a bad 12v feed to the ECU but does not address the injection harness itself... which looks perfect.

the problem now, very specifically, is that motor is wildly rich because all the sensors are telling the ecu that it is ice cold. the chances of a single sensor failure is reasonable... TWO at once is really unlikely and three is absolute zero. and combine that with , everyonce in a while when it feels like it , it runs perfect... i know i know... BAD connection somewhere.. my BOOKIE could have told me that... your job is to tell me " OH... they ALL do that... look here"

what i am looking for is someone that has worked on these specifically in a professional sense for so long that he is going to read this and say " oh there you go...another pin 24 problem in those gen 6 ecu's" or something like that...

and tommorow is another day. incidentally would be a buyer for a good serviceable late mag mpi harness and sensor array etc if anyone has one kicking around that they want to sell cheap. im far enough away from the real world out here that having a complete set of spares is something i would consider...

Okay, just a thought. You say all the sensors read low voltage. Since it is a feedback system you need to have good reference voltage to each sensor to begin with. Someone will probably correct me if I'm wrong, but that should be 5 volts from the ECU to the sensor and then whatever the sensor sends back is what you read on the scanner. So first thing you need to find is iff you have 5 volts at each sensor. Now if you don't, you need to check all the + inputs to the ECU for good voltage. If the volts in are good ( I suspect they are not) and the volts out are bad then it is an ECU issue. Since you seem to be having the problem on both engines I suspect you are losing feed voltage in the boat and it is lowering reference voltage to the sensors, causing your rich condition.

of course.

if ecu bad , then ref voltage would be wrong across the board if failed in that mode... but is intermittent. so implies harness /connection. also. NOT both motors now... only port. factory service manual covers all this and harness voltage checks at pin outs on my list of things to do for today. was simply hoping that this might have been a " common " problem that someone recognized

motors/electrics are complete stand alone. share absolutely NOTHING. except common ground and THAT only because everything has same ground... they both have seperate and ground WIRES actually multiple that return to each own battery as well as common grounding point... all fresh and been checked,

is not a systemic low voltage or high resistance but could easily be injection harness SPECIFIC low voltage or high resitance somewhere...

question is where.

Another thought is the actual ECU ground-could be an issure. Sorry I haven't experienced the exact problem to tell you where to go-hope you let us know what you find. Definitely sounds like you're on the right track.

I don't have the "magic bullet" you are looking for, but could you be more specific about your fuel pressure test(s)? Did you check the fuel pressure under load? Did you check the VST for float sticking, flooding, vapor pressure regulator failure or restricted high pressure pump intake screen?

When you say all the sensors are "low voltage meaning cold", I don't understand. There is only one sensor (ECT) that tells the ECM what the engine temp is. What temp is the ECM reporting? What are the voltages of the various sensors?

Are you aware that you engines have a "Power Reduction Mode"? Are you sure your engines are not linked by a data link harness, so when one engine goes into Power Reduction Mode, the other will too? All twin engine installations of that year were supposed to be linked.

Dave

Another thing to add, as dave said if you have a vst system on the boat and the diapraghm's are going bad they will cause the motor to run rich, I had it happen to me where the bad diapraghm was causing the motor to run rich enough that it would not come up to temp, which caused the temp sensor to tell the motor to stay in "cold" or enrich mode which makes the problem worse.

The data link only purpose is so that you can hook up a scan tool connect it to one motor and be able to switch back and forth from motor to motor with the scan tool.

Also there should be a ground cable from motor to motor, it would seem to me that with it being intermittent, you have a bad ground somewhere.

I had that problem before. Belive it or not it was the ICM on the distributor(delco-remi). Just swap them and give it a try just a 5 min swap. I wont go explaining why but its been a a problem I have encounter on few of my costumers boats. No bad codes, clean fuel{no water} everything test ok, but icm modules give that problem ,they mess up some ECM functions too, the engine just wont advance or acelerate.