93 Merc 2.5 EFI detonation

[DLVos]

My 200 hp merc has been completely rebuilt 7 times in the past 4 1/2 years by Mercury trained mechanics. The piston domes erode and break between 0 and no more than 30 hours of operation. I have had the switch boxes replaced as a set, all new reeds OEM., all injectors replaced, water pump replaced several times, oil pump replaced, entire lower unit replaced. Rebuilt powerhead ordered and installed by dealership (lasted 3 hours). Merc Rep called in twice with no results.<br />Damaged pistons sent to Fon du Lac for determination of cause. No results.<br /><br /> coils read...pri-Gnd Sec-Gnd Pri-sec<br /> #1 0 .2 .2<br /> #2 .025 .25 .275<br /> #3 0 .2 .2<br /> #4 0 .4 .4<br /> #5 0 .2 .2<br /> #6 0 .2 .2<br /> New switch boxes read 15K on bias ckt.<br /> Compression is 125 + or - 5%<br /> <br /> Mercury mechanics have been unable to diagnose the cause of what I consider to be detonation. I have ordered Seloc manual to self diagnose. Observation shows yellow wires from stator to rectifiers overheated and melted insulators at plug connectors. I suspect overcharging with damage to stator.<br /><br /> Can anyone help with test procedure and normal values for stator, trigger and rectifiers?<br /><br /> Any opinions will be appreciated.

 

[Kenny Bush]

Re: 93 Merc 2.5 EFI detonation

Whew...... What a doozy.. I have a similar problem with a '92 200hp that eats pistons like donuts.... Through my year of searching, I have found no solution, other than to back off the Max timing a little and hope for the best. A tech at Fon du lac told me that Mercury had a detonation problem that is hard to diag. I too bought everything electrical and oil related..... I do not think I can specifically help but I am with you watching........waiting...and maybe someone will chime in with the magic cure.....

 

[rodbolt]

Re: 93 Merc 2.5 EFI detonation

hello well you know what parts are good anyway.<br />the cause of the piston failure must be determined or it will happen again and again. could it be a restricted water passge causing an overheat? try a much colder set of plugs. maybe resize the combustion chambers to lower the compression. there is a simple formula for figgueing mechanical compression ratio by using swept volume . if you make the chamber larger then the compression reduces.<br />have you done a fuel system vacum check> and have you done a complete fuel system pressure test? I hope merc is still paying unless the hull is used in competition.<br /> post more info on hull size and type along with usage<br /> good luck and keep posting

 

[DLVos]

Re: 93 Merc 2.5 EFI detonation

Thanks for the quick reply guys. Hey Bushcat, seems we may be of use to each other. Let's compare notes and maybe figure this thing out.<br />I know the stator and trigger can test normal and still be bad, and that during these years the stator winding gage wire was too small. <br /><br /> While waiting for my manual, I am investigating tests for detecting corona discharge and slot discharge in the alternator windings....could be???<br /><br /> If you would elaborate on the testing and components you have replaced on your motor, I will compare and see if we can find the culprit.<br /><br /> I am devising tests using a 10mhz dual trace scope to check out the dynamic circuits. Perhaps I will find what Mercury can't.<br /> <br /> And....No Mercury is not paying. It's quite expensive buht I never give up at any cost.<br /><br /> dave

 

[DLVos]

Re: 93 Merc 2.5 EFI detonation

Hi Rodbolt,<br /> All cylinders at this point have been bored. If not mistaken, one is .40 two .30 and three .20. Hasn't seemed to make any difference.<br />Fuel vacuum and pressure tests were supposed to have been done by dealership. I am going to start from scratch. Will advise. <br /> <br /> Hull is 20ft. Astro FDX Bass Boat sporting a 25 stainless three blade prop. Thought about overpropping but it dosen't figure into the problem since the erosion moves around to different cylinders and only affects one or two cyls. at a time.<br /><br /> Found what looked like brown silicone tank sealer in the filter. Checkin it out as we speak.<br />When I know you will.<br /><br /> dave

 

[rodbolt]

Re: 93 Merc 2.5 EFI detonation

hello<br /> it still sounds more fuel than ignition related. if it were ignition its easy to check. run it at the ramp with a yamaha 200 test wheel while watching the spark. use a protractor or a dail indicator and place a TDC mark on the flywheel for each cylinder. I use a ferret instruments tach/timing light with an advace button . that way you can watch timing in relation to crankshaft at high speeds.if your engine still has the speed stabilizer you may wish to eliminate it. it has a tendancy to add 5-8 degrees advance above 5000 rpm. I read an article about a place in california that repairs tests and remanufactures the injectors. I still think its running a bit on the lean side due to a short injector pulse time or a high pressure regulator malfunction or a restriction in the rail. while your testing the spark for scatter keep a high pressure gauge on the rail. also do an oil system consumption test. eroded pistons at the piston edges indicate excessive heat. there is only so many ways to generate the heat. thousands of the EFI 2.5's run every day with no piston problems. so dont get to caught up with stators and such. its the same set up that has been used since the early 90's. that is where my IDC dyno and a seawall comes in hany. I once ran an 88 spl on it for 1 1/2 hours to find an intermittant ign problem but a test wheel can be your friend. on a bass boat I am going to assume its on a jack plate . make sure the water pressure stays above 15 psi above 2000 rpm. but now your down to you will either need a test wheel or a dyno so you can check for proper ign timing at all 6 cylinders.<br /> good luck and keep posting

 

[mellowyellow]

Re: 93 Merc 2.5 EFI detonation

just a couple of footnotes to add to the expert<br />advice you have already been given... different<br />octane fuels can cause det. problems in some OB's.<br />a good build up of carbon on piston crowns can<br />help dissapate heat in some cases where a new/<br />shiny clean crown won't...<br />this info. is in addition to, not in place of<br />previous advice BTW.<br />good luck,<br />M.Y.

 

[Kenny Bush]

Re: 93 Merc 2.5 EFI detonation

Man DVLos.... You sound like "SuperTech"..... You go..... I think you may be on to something with the spark corona. The tech at merc I talked to said it was major double firing. Stay on target wildman.... You may just write the next technical process for the Merc manual... I agree with the post earlier with water pickup pressure. Make sure that baby is cooling through the upper RPM's. My pistons(1,3,4) all were eating away at the edges of the top around the exhaust port region(Indicating Heat). I replaced everything electrical(trigger plate, stator, ECU's, coils, wires, plugs, rectifier), fuel(carbs gone through, fuel diaghrams, hoses, bulb, oil system checks), cooling(impellar, restrictions in head, tstats, pressure check). Chasing ghosts gets a little expensive and frustrating....... I backed the Max timing off about 8 degrees and so far it has lasted about 60 hours, but I have not hammered down on it either. Anyway, stay on it.

 

[DLVos]

Re: 93 Merc 2.5 EFI detonation

man y'all get on the stick when help is needed.<br /> I intend to examine ALL the suggestions you all have offered. I do have an electronics degree so I naturally lean to that arena. I WILL find this problem and make us all happy.<br /> I am building the necessary test equipment to check induced harmonic voltages into the trigger circuit.....heh heh heh. Sounds good huh?<br /> Will stay on top and advise of progress. <br /><br /> Thanks guys.<br /><br /> Dave