Hpdi 250-300

[formula29]

I am well aware of the problems with the big block hpdi's but what is the design problem that actually causes the number 2 piston to let go which seems to be the worst problem, I have had several of them and have actually had good luck with mine, I'm just curious, every one I have bought the first thing I do is check the compression, if its in spec I go through the entire fuel system, clean and flow injectors, service the h,p pump and replace all of the basket filters inside and I have had good luck with them, I have switched over to f250s on my current boat but just helped a friend with his hpdi 250, #2 piston came apart and #4digested the pieces, the sad part is the guy was surprised and never knew the hpdis had a bad rap, he said but its almost brand new its only got 200 hrs on it? poor guy.

[rodbolt17]

dunno
Yamaha blamed it on improper oil line bleeding.
Yamaha also updated the early z250 motors several times that included oil link rods, oil pumps an ecu and a regulator/rectifier.

every z250 I did the ECU/oil pump update on blew within 25 hours.

replaced the powerhead and no more isssues.

[formula29]

Thanks rodbolt, I'm just trying to learn something about why they blow, and why its usually # 2, I can understand if its caused by a lean condition, but that hasn't been the case on the ones I have worked on. the one I just repaired was an 02 and I don't think it has had any updates on it. it had 232 hrs on it.

[pstephens46]

Have heard most high output 2 strokes are more prone to failure. Mercs, Yams, Etecs etc. A neighbor had 3 300hp Mercs on a Donzi. Horrible, always in shop.

[boscoe99]

My belief is that the damage is due to detonation. Some folks over on a bass board I frequent believe it is the piston ring locating pin coming loose that causes a ring to rotate and snag on a port causing the damage.

When I ask how the number two ring locating pin knows to be the one that comes loose more than the others they don't have an answer.

Folks that have switched to the use of gasoline with a higher octane rating seem to have less piston failures than those that don't.

The big block HPDI's don't have knock sensors. Why? Don't know. Only Yamaha Japan knows and they aren't telling. Earlier high HP models had knock sensors. Later high HP model have knock sensors. The HPDI never got them.

[formula29]

That is my recipe for longevity, under prop it rather than over prop, I run 93 octane with stabil and use synthetic oil, I prop mine to turn 6,000 rpms and run the piss out them, my thinking is people over prop them, then since there is not a knock sensor they detonate to death, still why #2?? Just lucky I guess.

[boscoe99]

That is my recipe for longevity, under prop it rather than over prop, I run 93 octane with stabil and use synthetic oil, I prop mine to turn 6,000 rpms and run the piss out them, my thinking is people over prop them, then since there is not a knock sensor they detonate to death, still why #2?? Just lucky I guess.

Since detonation is both temperature and pressure related my wild guess would be that the temperature in the number two cylinder is just a bit hotter than other cylinders.

Even when the number two goes (similar to but different than going and taking a number two) other cylinders are sometimes found with milder detonation damage. Sometimes when a motor is found with another cylinder really damaged then an inspection of the number two oft times reveals detonation damage.

I agree with your analysis. Under prop the motor, run 91/93 and use synthetic oil. To that I would add make sure the cooling system is always in good working order and let the motor warm up in gear. And maybe don't jam down on the throttle to the WOT position in a nano-second. Like the bass boys with hot foots are known to do. They seem to suffer the greater number of big block HPDI failures than are the saltwater boys.

[99yam40]

I have read that the top if one side of block tends to run hotter than the other for some reason.
do not remember which side or if it where #2 cylinder is, but I guess it very well could be

I guess the warm up in gear is to make sure all cylinders are firing during the warm up so all are up to proper temperature

[formula29]

I forgot to add that, I always get the motor in gear as soon as possible and let it warm up while idling out, I do this just to prevent plug fouling so its running on all six, I am running the 4 strokes now and they have grown on me, but I still work on friends big block hpdis from time to time, thanks for the info guys.