Please help with HPDI issues

Stilletto

Cadet
Joined
Jul 14, 2025
Messages
12
Hi Everyone. I am in desperate need for ideas on why my 2002 HDPI 200 z motor is sick.

I think it is important to give a quick backstory on what I have done so far.
When I got the boat it was a year before I could actually put it in the water.
I started preempted maintenance last year. I found a completely dead injector , replaced and replaced all injector basket filters..
replaced spark plugs, cleaned o2 sensor. replaced VST filter. ran seafoam through.
I noticed the little white collar at the top of the vst pump was missing, replaced that and o ring.
I wonder if someone left it out on purpose....more on that later. The gasket that goes between the top and bottom of the VST was also leaking, replaced that.
Replace the tank vent and fuel fill hose, and replace the sending unit because it was letting in water into the tank. Changed out the fuel water separator.
Cpmpression was 105/110 each cylinder cold.


The motor has always ran a touch rich, and never has a smooth idle or ideal sound at any rpm.
When we first took it out, it ran fairly good at any rpm under about 2500. It has a strong 2 stroke smell, but it ran. Gets on plane fast, and it will fall off after about 30 seconds, and then stalls out. You can restart immediately and repeat the process.

So after that day I took apart the VST and realized that the aftermarket VST filters sit offset, and cause the fuel pump to tilt a bit when closing the vst up. Lesson learned, replaced with OEM.
Replaced BOTH low pressure pumps (OEM) because one was leaking from the back. Changed the filter in the under cowl water separator, switched out the anti siphon valve on the tank for a straight through barb, at least for a while to test. Changed the fuel hose to the tank.
Inspected the spark plugs, looked OK except for the bottom two that had carbon build up.

Hooked up to YDS and verified no codes, low pressure was at 4.8psi and high pressure was bouncing from 675 to 720. Ran the rpms up to 2800 and everything seemed stable until about 30 seconds when fuel pressure starting dropping on both the low side and high side and it shut off. I couldn't tell which pressure dropped first.

I changed the mystery filter in the HP pump. put everything back together. Engine wouldn't stay running, and was backfiring and popping. Looked at the spark plugs and they were all black and dry, like charcoal. Cleaned them, engine ran "ok" until I tried the test again, back to black sooty plugs.

The only thing I haven't done is verify the pressure coming from the VST because I don't have an adapter, but I HAVE tried two different electric pumps. I also couldn't get the fuel pressure regulator off the top of the VST to inspect without the screws rounding out.

So I am running out of funds and patience, and I really want to get this thing running decently. It only has 409 hours on it, and I think I am logically tracing the issues.

Why would a motor blacken the plugs in 2 or 3 min of running? too much fuel or oil? I adjusted the oil rod. Seems to me too much oil wouldn't cause the cutting out issue, unless it was ridiculous amount of course.
Just before the engine cuts out at 2800rpm it momentarily increases rpm on it's own, probably the ecu's last resort before cutting off from a lean condition? I think they call that flaring?

I would bite the bullet and purchase anything if I was SURE it was the issue, but chasing parts is breaking the bank.

I suspected the HP pump is failing, but wouldn't that show lower pressure at all times?
If the HP pump was running out of fuel from the VST, why would I have to clean the black off my plugs so quickly? Unless, the ECU is dumping oil to correct???

Any help here is GREATLY appreciated.
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
43,421
Welcome
And to start, I have never worked on a Yam or Suz
Why would a motor blacken the plugs in 2 or 3 min of running?
Running rich will blacken the plugs quick, and your posting shows it is

Just before the engine cuts out at 2800rpm it momentarily increases rpm on it's own, probably the ecu's last resort before cutting off from a lean condition?
No, most likely the fuel supply is running out so it starts to run with correct amount of fuel before it runs out of fuel

Hooked up to YDS and verified no codes, low pressure was at 4.8psi and high pressure was bouncing from 675 to 720. Ran the rpms up to 2800 and everything seemed stable until about 30 seconds when fuel pressure starting dropping on both the low side and high side and it shut off. I couldn't tell which pressure dropped first.
Sounds like the power is being removed from both pumps

You need to measure the high pressure fuel pump side. I'm just guessing but do think you have a regulator issue but need to test to verify
 

Stilletto

Cadet
Joined
Jul 14, 2025
Messages
12
No, most likely the fuel supply is running out so it starts to run with correct amount of fuel before it runs out of fuel
Thanks alldodge. I'm trying to get my head around what you are saying. So, the motor is getting too much gas, until the flow slows down and then it is getting the right amount for that throttle setting so it revs up a little, mixture good. But the flow is decreasing, so then it dies out. Is that right?
 

boscoe99

Commander
Joined
Aug 22, 2013
Messages
2,132
Why no mention of the medium pressure pump?

Low pressure pumps feed the medium pressure pump. Medium pressure pump feeds the high pressure pump.

Insufficient fuel pressure from a lesser pump can and will affect the fuel pressure in a greater pump.

For instance, if and when the low pressure pumps starts to fall off it might be reasonable to see the fuel pressure in the medium fuel pump fall off. Conversely, the fuel pressure falling off in the medium pressure fuel pump can cause the fuel pressure to fall off in the high pressure fuel pump.

Can you post a screen shot of where you are seeing 4.8 psi in YDIS?
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
43,421
Thanks alldodge. I'm trying to get my head around what you are saying. So, the motor is getting too much gas, until the flow slows down and then it is getting the right amount for that throttle setting so it revs up a little, mixture good. But the flow is decreasing, so then it dies out. Is that right?
For the most part yes
Should the high pressure be between 675 and 720 psi?
but listen to Boscoe99 he has a better understanding of your motor
 

Stilletto

Cadet
Joined
Jul 14, 2025
Messages
12
I can do one better. I will take a video of the 30 seconds in YDS tomorrow. It will show the pressures flashing and then the motor cut out. Then I'll have fun wirebrushing 6 plugs again LOL.

I mentioned I did try two different medium pressure pumps though.

What is confusing me is that it has to be too much gas to blacken the plugs but not enough gas because the pressures drop and motor dies.
 
Last edited:

Stilletto

Cadet
Joined
Jul 14, 2025
Messages
12
I can do one better. I will take a video of the 30 seconds in YDS tomorrow. It will show the pressures flashing and then the motor cut out. Then I'll have fun wirebrushing 6 plugs again LOL.

I mentioned I did try two different medium pressure pumps though.

What is confusing me is that it has to be too much gas to blacken the plugs but not enough gas because the pressures drop and motor dies.
Didn't get to work on it this morning, definitely get this info in the morning.
I am also going to replace the Schrader valve on the VST so I can get a proper reading.

Does anyone know if the motor will shut down if the fuel is TOO rich?
 

Stilletto

Cadet
Joined
Jul 14, 2025
Messages
12
I suppose that I probably need to get the injectors cleaned/flow tested. Why not, got to deplete my remaining funds on something LOL. $150 for 6 seems reasonable I suppose, at least I will know they are good.
 

Stilletto

Cadet
Joined
Jul 14, 2025
Messages
12
This is what the plugs looked like this morning after running for 2 min. Was clean right before. I also took some video of YDS I will be uploading momentarily, but I couldn't rev it high enough to see the pressures drop because the motor was running so poorly (popping and sputtering, of course with plugs like that).
20250925_112404.jpg20250925_112708.jpg
 

Stilletto

Cadet
Joined
Jul 14, 2025
Messages
12
Here is a video from YDS where I was attempting to catch the cutout and pressure drop in both LP and HP. But the motor just needed to be shut off before it died.

 

boscoe99

Commander
Joined
Aug 22, 2013
Messages
2,132
Hooked up to YDS and verified no codes, low pressure was at 4.8psi and high pressure was bouncing from 675 to 720. Ran the rpms up to 2800 and everything seemed stable until about 30 seconds when fuel pressure starting dropping on both the low side and high side and it shut off. I couldn't tell which pressure dropped first.
That is not a low pressure pump value. That is the high pressure pump value just expressed in a different unit of measure.

 

Stilletto

Cadet
Joined
Jul 14, 2025
Messages
12
Well, today I removed the VST (again) and readjusted the float to sit a wee bit lower, switched to a different VST pump, removed the fuel pressure regulator and cleaned it up (seemed a little frozen up). I cleaned the needle valve and seat and adjusted the oil pump rod. It was about 1/8 inch from the stop at idle, so i adjusted it to be right at the stop at idle, more like half a mm.

The motor did sound better but cylinder 1 was still fouled and sooty after going through a series of rpms. But only that cylinder. I poured a bottle of seafoam in a gallon of gas and ran that straight to the motor bypassing the primer bulb for 15 min at various rpms. It sounded much better. I attached a video.

I shut it off for 10 min and couldn't get it to start again, probably because the can was lower than the motor and it lost its prime. Bulb feels firm but burping at the end of the squeeze. Then it got dark. But, seems to run better now. I still haven't tested if it's gonna cut off after a min at higher rpms.

 

Stilletto

Cadet
Joined
Jul 14, 2025
Messages
12
Well, the only way to get the motor to run now is to start it and quickly put it in gear. Otherwise, it will stall out. I don't know why that just suddenly started happening.
Still running rich, and still shutting off after a short while above 2800rpm.
About to thrown in the towel I think....
Big fail for me I reckon.
 

Stilletto

Cadet
Joined
Jul 14, 2025
Messages
12
I found one of the spark plug wires had no continuity, was able to cut to fresh wire and reinstall the boot. That might be my latest starting issue, as it was cylinder 4, one of the only 4 that run at neutral. The injectors are all sent to Brucato and will test when I get them back.
 
Top