PLEASE HELP Crazy mistery issue

[alexedit]

Hey Guys -
I have a 1998 Suzuki DF70 outboard motor that is giving me issues. Two outboard mechanics have not been able to figure out what is going on.

It will not go past 3000rpm, it backfires (just a bit not much), and it has very little power. When I crimp the gas hose that goes from the bottom of the fuel injector rail back to the VST, the motor works great, but if I unclamp it, it goes back to behaving all messed up.

Only has about 400 hours. 180psi compression on all four cylinders. This is everything that I have done on it:

New VST and the high pressure fuel pump (Vapor Separator Tank)
New Spark plugs, Oil, impeller and anodes (part of regular maintenance)
New Fuel Pressure Regulator (located at the bottom of the rail - see video)
New mechanical fuel pump
New Fuel Filters

A boat mechanic connected it to the computer and didn't get any codes.

Always use REC90 gas

I've ran out of options. I don't know what else it could possibly be! Please help

watch video please.

 

[99yam40]

so why has no one tested fuel pressures

 

[alexedit]

so why has no one tested fuel pressures

If I find that the fuel pressure is now, what should I do?

 

[alexedit]

Cause that's the thing - so the fuel pressure is low, but what could be causing it?

 

[99yam40]

seems you do not know what the fuel pressures are,
or you do not want to post the info

 

[alexedit]

seems you do not know what the fuel pressures are,
or you do not want to post the info

I don't know what the fuel pressure is, but let's assume it's low. What should be the next step?
What are the issues that cause low fuel pressure?

 

[Scott Danforth]

I don't know what the fuel pressure is, but let's assume it's low.

Let's assume you actually measure fuel pressure and then diagnose from there

 

[StarTed]

It sounds to me that your fuel pressure in the hose you crimp for testing is too high or the flow is too great. Is that a bleed back hose? Sounds like you're having fuel pressure issues and need to check them

 

[alexedit]

Thanks StarTed-
Yes it's a bleed back into the vst tank.

And yes, I understand that it's a fuel pressure thing. today I will put a gauge on the rail to see what pressure I am getting, but that the question guys. Finding out what the problem is does not give me direction. I am trying to work one step ahead knowing that there will be pressure issues.

The question is: If I have low or high pressure, ok what then? where do I turn?

thank you guys!

 

[alexedit]

Let's assume you actually measure fuel pressure and then diagnose from there

ok, pressure is low... What do I do now?

this is driving me and two other mechanics crazy!

 

[mjf55]

How low, what is the actual pressure you or your mechanic measured? That might help the experts here.

 

[Scott Danforth]

Low is subjective like mfj55 posted what are the actual numbers.

Also, if the other two "mechanics" are factory trained, they would have done a fuel pressure test immediately, then did a fuel pump output test, and a regulator test.

 

[StarTed]

What about fuel injector leaking back into the feedback system? Just a guess from your initial post. It sounds to me like something leaking back through the hose.

You need psi readings posted here and where they're taken for appropriate help.

 

[mjf55]

You need psi readings posted here and where they're taken for appropriate help.

That is correct, StarTed, in PSI.

Also, can you answers Scott's question regarding a fuel pump output test, and a regulator test, whether it was perform and the results. That will help get folks helping you.

 

[alexedit]

How low, what is the actual pressure you or your mechanic measured? That might help the experts here.

Hey guys!
Thank you so much for all the help here. Here are the results and what we did.

We measured it at 34psi. I don't know what it should be, but that's the current reading. When I clamp the return line completely, we get 58+ psi.

I got a clamp and gradually clamped the return line until the motor behaved normal, this turned out to be 41psi.

At 41psi the motor behaves like a dream.

So for now we have a work around, BUT....what could this issue be?

 

[99yam40]

what all did the new VST include?
Did you test pressures at idle or when problems showed up?
If new VST includes new filter and pump, did you have the injectors cleaned or replaced?

I have no idea what pressure it should be running, but sounds like it is too low.
Find a service manual for your motor and it will have specs for everything

 

[alexedit]

what all did the new VST include?
Did you test pressures at idle or when problems showed up?
If new VST includes new filter and pump, did you have the injectors cleaned or replaced?

I have no idea what pressure it should be running, but sounds like it is too low.
Find a service manual for your motor and it will have specs for everything

The vst came with the pump as well -

I do know now that the issue is because it has low pressure. It's at 32 and it should be at 40 (manual)

But I clamped the line and the boat is now working - this video is dedicated to all those who helped

https://youtu.be/gKoPP0Mbqf4

I still don't know why the pressure is low

 

[clanton]

Connect gauge to high pressure side, turn key on, pump run time 3 seconds spec is 34 psi and hold for 5 minutes after key is turned off. This checks OK, run engine in water under load should hold 34 psi. This for a 2000 motor.

 

[99yam40]

Could be the OPs gauge is not calibrated/accurate and his 32 is the 34 it is suppose to be at.
pressure regulator should hold at the designed pressure at all times.

If the injector screens/filters are being restricted there would be the need for more pressure to get enough fuel into the cylinders for them to burn properly.


But pressures need to be read when the problem shows up too, with a calibrated gauge and make sure it is in spec.

His motor is not fixed yet if this is true

edit I just saw he(OP) said 34 on one post and 32 on another one
So my bet is on the injectors if the motor is designed to run at 34 psi
injectors can be serviced cleaned and flow tested if sent to the right shop. beats buying new ones

 

[jeff1rey]

I have a similar problem with a 115 hp and boat is at the shop with mechanics scratching their heads. What has happened with your problem currently??