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The pressure rising and falling is caused by the thermostats opening and closing. This happens on all engines but it is not nearly as noticeable on engines that use a PRV.

Outboard thermostats aren't 140 degrees because of running wide open, they are that temp to limit corrosion and mineral separation in the cooling system.

In my opinion the vast majority of SHO...

PSM207 you are 100% correct. Lack of warm up, cool down and maintenance is what causes some SHO engines to fail early. There are lots of 4.2 offshore outboards out there with 3000 and more hours on...

What choochoo stated above is 100% correct. The worst thing you can do with an SHO outboard (or any outboard for that matter) is treat it like your favorite pro uses their boat on YouTube. Take the...

I have seen several engines with the Nizpro tune that exhibit the exact condition mentioned here. It seems that their baseline starting map causes the engine to run overly rich for a few seconds when...

Has the engine been flashed with a performance tune?

A bad IAC valve is a possibility.

When was the impeller and housing last changed?

15400-PFB-014

Make sure the manual relief valve is tight if you haven't checked it yet.

This noise is almost always caused by a dry driveshaft bushing.... I'd question the mechanic saying it is caused by the timing belt tensioner.

While a 20 hour service isn't required on your engine, it is highly recommended by many/most technicians (including myself) as an investment into the long term life of your engine. Engine Oil and...

I’d recommend sending in an oil sample for analysis if you are concerned.

I haven't seen any differences in power output.

I haven't seen any differences in power through the years.

The noise is normal. It is caused by the variable valve timing cam phasers being dry. It doesn't sound great but it is harmless, almost every yamaha 4.2 liter I see does it.

Could be many things..... the start of a gearcase failure, very loose valve, internal engine issue etc.

How many hours on the motor? When was the engine oil and gear lube last changed?

This is what I posted on a thread a while back regarding this issue:

I wanted to post an update here regarding the use of aftermarket oil filters on Yamaha 4.2L outboards.


I purchased a few...

I'd start by dropping the gearcase to see if that frees it up. A gear failure is possible.

I'm not sure on that one.... but i'm curious to hear the answer. That would make for a pretty cool combo on a light boat.

Make sure all service is up to date and that the thermostats are working correctly. Keep in mind these engines share identical powerheads with the 4.2 offshore outboards.... I see the offshore...

As yam911 showed, the factory says 5500 max. That being said, many people run them at 5800-6000 without issue.

Yes some of the early 4.2’s had problems, and Yamaha took care of the vast majority of them with the powerhead replacement program as you stated. The 4.2’s in the offshore market tend to last longer...

Not all that hard. I recommend purchasing a factory service manual. The bolts that hold the flywheel on are one time use.

Anyone that can’t get 100 hours out of an SHO either has a setup problem...

Just for curiosity sake I went through the parts diagrams. It appears to me that the heads and cams are identical in the 200, 225 and 300 ProXS series engines, but I could be missing something. The...