2003 Z250TXRB HPDI slowly overheats

[89 Grady White Project]

I rebuilt a 2003 Z250TXRB motor. During testing, it has a slow progressive overheating problem when using a boot on the foot. It starts up good and idles around 875 rpm, but the water temperature slowly starts to climb and does not stop. I bought a Yamaha boot, which has a better seal but that just slowed down the over heating process. The tell tale has a good stream once it reaches around 140 degrees. I've engaged forward drive and run it up to 2500 rpm, but that slight accelerates the overheating process until I have to shut it down around 190 degrees. I've checked the water pump and it looks like someone replaced it recently because it has very little wear. I verified the thermostats opened correctly before reassembling the motor plus with the tell tale with a strong output stream, I'm suspicious I did not assemble something correctly but what is the question. The oil pump seemed to not put out much oil and I made sure to bleed out any air, so then I purchased a new one but its about the same amount of output ($215 experiment) and it continues to overheat. I purchased the Yamaha diagnostics computer adapter to monitor the ECU readings. It shows all normal fuel pressures at the 3 stages, so I'm assuming it doesn't have a fuel problem. It shows everything at normal values.

It's twin, an LZ250TXRB was rebuilt at the same time and it runs great, so after comparison between them I'm starting to run out of ideas. I'm putting in a new water pump impeller, housing and o-rings in hopes it may clear up the problem but I suspect it will not. I was reading in the service manual that if the throttle valves are not opening synchronously, they could make it run rough at idle and while opening the throttle, so since the problem child does not run as smooth as its twin I'll recheck those gaps.

I'm starting to run out of ideas, so any suggestions would be appreciated.

[boats.net]

Is it overheating when boat is in the water or just when running on a flush adapter? Have you checked the pressure control valve? Number 14 in diagram Yamaha Outboard Parts 2003 Z250TXRB CYLINDER CRANKCASE 3 Diagram

I don't mean to derail your thread but i have a 200 hpdi here in Australia with a melted piston and I'm being told they are very difficult to rebuild. I would appreciate the thoughts of someone who has rebuilt one and some idea of prices over there for powerheads etc. cheers haydn

[bigfish1229]

please start a new thread for this question however they are not bad to rebuild

[89 Grady White Project]

Sorry to take so long getting back to you guys, but I was doing some research in my manual and found that pressure control valve. I had to do some minor disassemby to access it and found I must have missed cleaning it out during the rebuild. It was loaded with all kinds of crud, so I cleaned it out and put it back together. I ran it up this morning with the thermostat open so I could see if the water jackets filled up and it eventually got there, but it took longer than I would like. I then re-installed the thermostats and it appeared to be fixed and ran at 2500rpm for 10 mininutes, but then when I took it up to 3500rpm ran for sometime but eventually started overheating. I was reading in the manual about the grommet seal and mine felt like a hard piece of plastic instead of pliable, so I'm wondering if its still leaking too much water. I'm letting it cool off for now, but going after the grommet again.

Any other ideas would be appreciated.

[89 Grady White Project]

I forgot to mention. Yes, I did buy the Yamaha hose adapter boots and even those do not make a great seal on that curved housing, but they are better than the cheap boots. When I had the foot off to check out the water pump (it was new!), I hooked that boot up and I was not impressed with the amount of water that made it into the pump impeller area so I hope in the water there's more water pushed in.