Tohatsu MD90B TLDI, Running poorly and loud, Fuel pressure lower than air pressure

Figured I would start a new thread since my other issue was resolved. I am still working on getting this MD90B running, previous owner reported it ran well when he pulled it, ~8 months ago.

When I got it installed on my boat I could get it to run but it was running very poorly and would die when we rev'd it past ~3K, mechanic diagnosed it as a low air problem, only had ~40psi on the air rail and motor seemed to run well on shop air.

I recently replaced the air compressor hoping it would fix my problem, however the problem remains. Still running poorly. I rigged up a psi gauge to test the air pump directly and it seems to check out ok

heres a vid of me testing the pump, you can see it jumps up to 80-100psi after a few cranks - https://youtu.be/pSJWHQdMJoM


The engine has no codes, starts right up and idles, but it is running very loud and rough

I checked the pressures at rails and I am getting 65psi fuel, and 75psi air at idle

Everything I am reading says that it should be the other way around? Fuel should be 10psi higher than air? Any ideas why my pressures are so low?

Is it possible to test the fuel regulator/air regulator to check them out? I am also noticing a bit of fuel on the air compressor/air rail side not sure what is going on there.

I am in mexico and parts are damn hard to get down here so hopefully I can get it figured out this go round!

thanks for the help guys!

Welp, figured out how to remove the air and fuel regulators today, they come out with a little prying from a flat head to defeat the o-ring seals

fuel regulator looked brand new, no tears or debris, seals all in good shape


the air regulator on the other hand... not so hot! O-rings shredded, hole in the diaphragm, and clogged full of dirt!





i pulled it out and cleaned up all the dirt that was built up behind it in the rail, there was also a lot of yellow foam and fuel smell there

i assume this explains why I was getting fuel in my air rail Schrader valve and fuel smell all the way into my air compressor

went ahead and dissembled the entire rail to clean it out w gasoline, air injectors look fine, I’ve pulled one fuel injector so far and it’s black on the top but looks ok overall

I am going to see if I can find someone here in town that has an injector cleaning machine, though I doubt they will have the proper equip to clean up these style of injectors

ordering a new air regulator now from the US, going to go ahead and order a spare fuel regulator and a low pressure fuel pump rebuild kit as well, just in case...