OMC 3.8l dying at high rpm and weird cranking issue.

[Serf27]

Hey all.
I’ve asked for help here with great success from most of you, but I can’t catch a break on this 4yr project.

3.8l OMC V6 2barrel carb.
Old fuel completely drained
New main fuel line from tank to pump. New fuel water separator and new fuel filter.

First, the cranking issue:
The old fuel had some water mixed into it and so did the fuel water separator.
I cleared all that out and the engine cranks (most of the time) in short bursts.
Like it cranks over half a spin and then half a spin and then cranks normally until it starts. I thought a valve/piston bent due to water in the fuel, but cyl compression yielded 150 psi across all 6.
It sounds like there is some resistance and it can’t fully crank. With the spark plugs removed during compression testing, it was spinning over fine.
What would cause this? I will get a video of it doing it, it does it 75% of the time.
Almost like there is too much compression…?
I will pull the accessory belts off too because I’ve had that issue on some cars.
The battery is brand new.
And I used 87 octane fuel.
I hope the explanation of the issue makes sense.


Dying issue:
It can probably idle until it runs out of fuel and can coast at 2K rpm.
Last week I was getting up to 4k rpm and it would die after about 30 seconds. It always started back up.

Today I was at about 3500 rpm for 30 minutes and it was fine. Later in the day, it would start to sputter and die and wouldn’t want to start, not until I cranked it over a few cycles. (Cranked 1, 2 and 3 separate times)
Sucked fuel out of the main fuel line, took about 2 seconds to get fuel out and the fuel water separator had fuel in it too. I opened the fuel fill cap and then it started and I just coasted back to the dock at 1500 rpm.

But because the engine doesn’t want to turn over as freely, it takes a toll on the battery I believe because it needs extra power to turn the engine.

Could the cranking issue be a bad ground?

And the dying issue?
I’ve read to look for leaks in the fuel hose, clogged tank vent, fuel pressure after the pump.
I have not touched the carb, that is the next item to do, but if there is something I’ve missed, please let me know.

I do know the fuel fill cap is attached to the tank via a 1.5” or so hose, and the clamps on that hose are not tight, but would that just act as a vent as well? Since the main fuel line is clamped tight to the tank and the pump?

Thanks.

 

[Serf27]

I’ve just looked at the specifications in the manual and it is saying the 3.8l compression pressure is 125 psi.
Either the tester I used was defective, excess fuel is getting into the cylinders during cranking or something mechanical is off.

This cranking issue is random.
Sometimes it cranks and fires right up. Other times it stuffers.

The issue sounds similar to the issue in this video at 2:07 minutes, but mine turns over a bit faster, but the same deal:
Half crank-half crank or 1 crank-1or 2 cranks- starts up.

 

[kenny nunez]

Try using a 6 gallon outboard tank connected to the fuel filter, this way you will narrow down where the supply problem is.
Is the carburetor clean on the inside?

 

[Serf27]

Thanks again.
While the “new main line” from tank to pump hasn’t been ran on the water, it is still 2-3.

I will try running it on an external tank or just replace the main line entirely.

I pulled the top half of the carb off and it looked clean, the float needle valve was in good condition.
I’ve ordered a carburetor rebuild kit and electric fuel pump to install.

The fuel tank is just the following right?
Fuel tank
Fuel filler cap/hose: does this need to be air tight?
Fuel vent: basically a hole in the tank that has a filter on it to prevent debris from getting into the tank.
Main fuel line going to the pump: this is the line that needs to be air tight yes..?

Unsure of the pick up tube condition inside the tank.

 

[kenny nunez]

There is a very fine filter in the suction tube in the tank. The tube unscrews from the fitting where the fuel line is connected. After removing the tube use a small rod to push the filter out reassemble the fitting.

 

[Serf27]

I will remove the tank and check out the pick up tube.

Flow from main line seems to be unrestricted though because I pumped old fuel out a few weeks ago and it flowed out easily.

 

[Serf27]

Here is my tank.
I blew air into the tank through the breather vent on top and fuel came out of the tank.
So is the breather good? Since it has air flow?

I took the fuel fitting off, the one that sends fuel to the pump.
There is no long rod that comes out. The pick up tube looks welded in.
The fitting has some orange rubber piece in it. I took it out, but now I think it was suppose to be there…
Is this the fine filter you mentioned?
It has a spring inside it that acts as a valve. There is no air flow unless the spring is pushed up.

Should I replace this fitting with a clear 90° elbow fitting or remove this spring valve and keep this fitting?

 

[jimmbo]

That spring thing is called an Anti-Syphon valve, required by USCG Regulation. It is supposed to prevent the tank from draining it self, but the Lower Pressure provided by the Fuel Pump is supposed to unseat it.
There is no removable tube inside the fitting on the tank? Not a great design if you can't get to the Pickup in the Tank( suspect the tank is Homemade Affair), That Orange thing, is it a washer, or a glob of RTV? Looking a the 2nd Pic, I would guess RTV, never a good choice around Gasoline. The Orange stuff could certainly starve the engine of fuel. As for the line being clear a few weeks back. Well every line, Fuel or Sewer, is clear, until it isn't.
Can you get a camera to look down the Fitting on the tank? I wonder if there is a tube in there that can be pulled up.
Edit: you say you are pulling the Tank out, That makes it easier to look at things

You asked if the Fill Cap has to be Air Tight, no, as the tank is vented anyway, but it should seal well enough not to let water in, or gas out

About the Slow Cranking. A weak Starter, bad Battery, bad/corroded Connections, corroded/undersized Cables, even the Solenoid can contribute to cranking Problems

 

[Serf27]

I sucked on the valve from where the line to the tank goes. I couldn’t get any air flow until I pushed on the spring. But the pump may create more vacuum and open the valve? Or should the valve open with light vacuum?
A closer look at the valve, shows the orange stuff is in the spring too. Probably slipped past the spring and is stuck there.

The tank does look home made but looks to fit perfect.
The orange glob did look and feel like rtv. Looking at the fuel level sending unit, I see orange rtv all around it. Guessing previous owner used too much and it fell into the tank. Here’s photos of the valve with orange glob in it and the fuel level sender.

I think the tank had a bracket holding it down and I put new flooring over that bracket. I need to lift the carpet up and remove the floor in that section to remove the tank…. Will try tomorrow.
The vent hose looks dry and cracked. I see no issue as it’s supposed to vent, but will be good to put a new hose there.

I also stuck a pick into the pick up fitting on the tank to see if I could lift any pick up tube out and nothing.

The cranking issue seems to have stopped in the last couple of days I’ve been working on it. It cranks normally. Cranks 1-2 times and starts.
Would there be any reason for it to crank slowly on the water? I’ve read of fuel vapor lock..?

 

[jimmbo]

Back Pressure, as all the Exhaust outlets are underwater and full.
Vapour Lock is where Fuel Boils in the Fuel Line between the Tank and the Fuel Pump. All liquids boil/flash at lower temps as the Atmospheric Pressure is lowered. Lower pressure in the Fuel Line is what causes the Atmospheric Pressure in the Tank to push the Fuel to the Fuel Pump. Some People will say the Pump is sucking the Fuel out of the tank, but any Physics Teacher will say what I said. Fuel between the Pump and Carb will not vapour lock, but can Percolate. That is where the Fuel is under Pressure has a Higher Boiling/Flash Point, but when the the inlet valve of the Carb opens it can suddenly Boil/Flash and flood the carb. A easy way to visualize it, is to shake a bottle/can of Pop or Beer. Inside the bottle/can, there is a balance between the Liquid and Gas. After shaking it, open it quickly...

 

[Serf27]

So it’s not vapor lock then.
The fuel line is wrapped around the rear of the engine and engine temps are 180°F.
Should I be running with the bilge air vent fan turned on?

Possibly back pressure, but wouldn’t all boats have this issue?
How could I correct it? Running dual batteries?

 

[jimmbo]

All engines that discharge the Exhaust underwater face this, In outboard an Idle Relief is in the design to let exhaust gases out above the Waterline at Idle and low Speed. I/Os tend not to have them above the Water Line.
180 is too high for a Raw Water cooled Engine. I wonder what Thermostat is in the Engine, should be around 140

You are legally required to run the Bilge Blower 5 minutes prior to Starting the Engine, and keeping it running any time the Boat's speed is below 10mph, stopped/anchored.
I fail to see how Dual Batteries would reduce Back Pressure?

 

[Serf27]

Its about 170°-180°.
The factory thermostat is 160°.

Thanks for the 5 minute rule. I only ran it for about a minute with engine cover open.
Will keep it on while in the no wake zones.

I was thinking that dual batteries = more power to crank the engine faster and over power the back pressure.
So is the stuttering engine cranking normal?
Did you get a chance to view the video at the 2:07 minute that I listed above? That’s how it sounds but a little bit faster.

Any idea (except a faulty tester) why my cyl compression test read 150 psi and the manual says 125psi?

 

[Serf27]

I removed the tank and the pick up tube cannot come out. I was able to look inside the tank a bit with my camera.
The pick up tube does not have a a screen on the bottom. I stuck a 1/4” wire down the tube and it was free of any other blockage.

I can’t get the last piece of rtv out of the anti siphon valve, so I’ll probably need a new one.

As for no screen on the pick up tube.
I am thinking of configuring this new set up.

Clear 90° elbow fitting goes onto the tank pick up tube.
Hose from the elbow runs to an inline filter
Right after the filter, I install a new anti siphon valve
Then I run the main line from the valve to the pump.


The filter before the siphon valve would catch any debris(though the tank looks clean).

Thoughts?

Here’s a picture of the tank/pick up tube.

 

[jimmbo]

At least I can't see any water, at least in this shot. Make sure the filter has a Lot of Media. as that will reduce the Pressure Drop across it. With todays Crappy Gasolines being refined with FI in mind, and the Fuel Pump in the Tank, some have a high Flash Temp, and can Vapour Lock more quickly than fuels of Decades ago
The Anti Syphon Valve is required to be at the Tank,

 

[Serf27]

What do you mean a lot of media?
I ordered this one. Does it look good?
FRAM G3 In-Line Fuel Filter https://a.co/d/cyTXe6i

That is fresh fuel in the tank.
No strainer in sight. So adding the fuel filter.

Is the engine starting issue just something normal that I have to deal with then?

 

[jimmbo]

I was referring to Water Separating Filters that look like, and are about same size as the Oil FIlters used on the the old Ford and Chevy V8s

 

[Serf27]

Ahh, yes I have a new oem one of those already installed.

 

[Scott Danforth]

pull the starter and have it tested

Pull the drive and see if it turns over easier. if it does, you have a drive issue to look into

check your tank vent. or the next time it dies, pull the fuel cap. if you hear hissing, your tank vent is clogged.

 

[Serf27]

I hooked up a clear hose to the pump and ran it to a 1gallon fuel tank.
Here’s how the fuel flow looks. Is it normal to have air pockets in the line?
If it’s not normal, is the pump bad or how do I get rid of the air pockets?

It idled for about 10 min and then 5 min at 2K rpm and the flow looked the same.

Thanks.