Fixing and Updating The GT40 Fuel System

[h2oski]

I'm sure there is a write up like this, some where, but all I could find are bits and pieces of this or that related to this subject. Like me, I bet there are many GT40 owners out there and as I learned, just putting in fuel stabilizer or changing the fuel filters, isn't enough. I'm working through fixing and updating the GT40 fuel system in my 1999 Air Nautique.

My story begins back on Thanksgiving eve when I took the boat out. I was cruising along, 2500 RPM, calm water. About a mile or 2 from the ramp, all of the sudden, the engine stalled. Surprised, I checked all the obvious things which resulted in a tow back to the ramp. I got the boat home, removed the engine cowling, rear seat bottom and back, and the removable floor panel, tank tie down hardware, etc, now what?

I'm starting at the fuel tank. The fuel supply and fuel return fittings on the top of the tank are aluminum elbows. The tank side is 3/8" male NPT and the hose side is 1/4 female NPT. Connected to the elbows are two straight brass fittings 1/4" male NPT to 3/8" hose barb. The hose barb fitting on the fuel supply elbow is different though, it's called an Anti Siphon Valve (ASV).

Being familiar with check valves per-say, it seemed to me that the ASV could be stuck (years of Ethanol, gummed up, bugs, etc.) on the seat keeping fuel from getting to the low pressure fuel pump (LPFP). I tried to remove the ASV and it is not budging, not without breaking something, so I broke it. Who remembers what dissimilar metals do. Well the ASV is brass threaded to an aluminum elbow on the tank, not good 20+ years later when you want to take them apart.

So now I need to replace the ASV *and* the elbow. One thing at a time, need to find a new ASV. Turns out there *is* an aluminum and brass ASV available. After my fun removing my old ASV, I figured I'd opt for the aluminum one, but..... research revealed the ASV is calibrated, what!

I found...

- Aluminum ASV is for 200 HP or less
- Brass ASV is used for 200 HP or more

My original ASV is brass and the GT40 produces >200HP, so that makes sense, I guess (even though dissimilar metals is not optimal). What does the metal have to do with it? I could see one having more spring tension than the other. Maybe the brass version has a stiffer spring for high suction pumps vs the aluminum, I don't know.

If found something on CCF that resembles the +/- 200HP calibration phenomenon, but not based on type of metal...

- 15 (WC) calibrated ASV equals .541 PSI or 27.99 mm HG of mercury
- 30 (WC) calibrated ASV equals 1.083 PSI or 55.99 mm HG of mercury

(I think WC means water column. Imagine a vertical tube, one 15 inches and the other 30 inches)

Industry standard regarding use:

- 201 HP Engine and Higher = 15 calibrated ASV
- 200 HP Engine and Less = 30 calibrated ASV

Then I found this on another site selling the ASV...nothing about type of metal or HP...

Features 15 calibration yields 15 in. vertical height protection.
Features 30 calibration yields 30 in. vertical height protection.

This makes more sense to me. The ASV is calibrated for the height difference between the ASV and the potential for a leak (rubber fuel line in the bilge splits open). So if the fuel line dips below the ASV, like it does in my boat, gravity is pulling on the fuel, trying to overcome the spring tension in the ASV. And the spring tension must be light enough for the LPFP to open the ASV and strong enough so the fuel in the fuel line can't open the ASV, tricky balance.

When I search the various vendors for a new ASV, I can find brass and aluminum. Attwood and Moeller do have 15 or 30 (usually etched on the fitting). SKIDIM and WL (who I expect to know this stuff) both sell the aluminum and brass fitting, but no mention of 15 or 30, let alone which one we need to use. Maybe it doesn't matter at all, just use what ever. I do know the USCG requires the ASV and I'm sure my insurance company won't be happy if I remove it. A final concern is if I choose wrong, one that needs more vacuum to unseat the ball in the ASV, what will that do to my LPFP? Can it cause fuel starvation issues?

So, the question is, what do I need to buy?

-Scott

Next up.... the fuel pickup tube.

[core-rider]

Sounds like you already have your answer... Stick with the brass ASV (30" wc) and while you're there replace the fittings with brass hardware as well.

Have you determined with that is the cause of your engine stalling issue? Should be simple enough to run the engine without the ASV quickly to decide if you need further investigation.

[Peter6000]

I'm sure there is a write up like this, some where, but all I could find are bits and pieces of this or that related to this subject. Like me, I bet there are many GT40 owners out there and as I learned, just putting in fuel stabilizer or changing the fuel filters, isn't enough. I'm working through fixing and updating the GT40 fuel system in my 1999 Air Nautique.

My story begins back on Thanksgiving eve when I took the boat out. I was cruising along, 2500 RPM, calm water. About a mile or 2 from the ramp, all of the sudden, the engine stalled. Surprised, I checked all the obvious things which resulted in a tow back to the ramp. I got the boat home, removed the engine cowling, rear seat bottom and back, and the removable floor panel, tank tie down hardware, etc, now what?

I'm starting at the fuel tank. The fuel supply and fuel return fittings on the top of the tank are aluminum elbows. The tank side is 3/8" male NPT and the hose side is 1/4 female NPT. Connected to the elbows are two straight brass fittings 1/4" male NPT to 3/8" hose barb. The hose barb fitting on the fuel supply elbow is different though, it's called an Anti Siphon Valve (ASV).

Being familiar with check valves per-say, it seemed to me that the ASV could be stuck (years of Ethanol, gummed up, bugs, etc.) on the seat keeping fuel from getting to the low pressure fuel pump (LPFP). I tried to remove the ASV and it is not budging, not without breaking something, so I broke it. Who remembers what dissimilar metals do. Well the ASV is brass threaded to an aluminum elbow on the tank, not good 20+ years later when you want to take them apart.

So now I need to replace the ASV *and* the elbow. One thing at a time, need to find a new ASV. Turns out there *is* an aluminum and brass ASV available. After my fun removing my old ASV, I figured I'd opt for the aluminum one, but..... research revealed the ASV is calibrated, what!

I found...

- Aluminum ASV is for 200 HP or less
- Brass ASV is used for 200 HP or more

My original ASV is brass and the GT40 produces >200HP, so that makes sense, I guess (even though dissimilar metals is not optimal). What does the metal have to do with it? I could see one having more spring tension than the other. Maybe the brass version has a stiffer spring for high suction pumps vs the aluminum, I don't know.

If found something on CCF that resembles the +/- 200HP calibration phenomenon, but not based on type of metal...

- 15 (WC) calibrated ASV equals .541 PSI or 27.99 mm HG of mercury
- 30 (WC) calibrated ASV equals 1.083 PSI or 55.99 mm HG of mercury

(I think WC means water column. Imagine a vertical tube, one 15 inches and the other 30 inches)

Industry standard regarding use:

- 201 HP Engine and Higher = 15 calibrated ASV
- 200 HP Engine and Less = 30 calibrated ASV

Then I found this on another site selling the ASV...nothing about type of metal or HP...

Features 15 calibration yields 15 in. vertical height protection.
Features 30 calibration yields 30 in. vertical height protection.

This makes more sense to me. The ASV is calibrated for the height difference between the ASV and the potential for a leak (rubber fuel line in the bilge splits open). So if the fuel line dips below the ASV, like it does in my boat, gravity is pulling on the fuel, trying to overcome the spring tension in the ASV. And the spring tension must be light enough for the LPFP to open the ASV and strong enough so the fuel in the fuel line can't open the ASV, tricky balance.

When I search the various vendors for a new ASV, I can find brass and aluminum. Attwood and Moeller do have 15 or 30 (usually etched on the fitting). SKIDIM and WL (who I expect to know this stuff) both sell the aluminum and brass fitting, but no mention of 15 or 30, let alone which one we need to use. Maybe it doesn't matter at all, just use what ever. I do know the USCG requires the ASV and I'm sure my insurance company won't be happy if I remove it. A final concern is if I choose wrong, one that needs more vacuum to unseat the ball in the ASV, what will that do to my LPFP? Can it cause fuel starvation issues?

So, the question is, what do I need to buy?

-Scott

Next up.... the fuel pickup tube.

Put a fuel pressure gauge on the rail, if the valve starves the engine fuel pressure would be the best indicator.
Have you checked the ProBoss service manual? I dont remember seeing it in there but it should be.


Sent from my SM-G892A using Tapatalk

[h2oski]

Thanks for the replies.....
core-rider The 30 is very popular on vendor sites, the 15 is difficult to find for some reason. Some ASV's I found don't mention 15 or 30 with no details about it. The whole brass is >200HP and aluminum is for <200HP is misleading, because Moeller, Attwood, and Seachoice both offer a 15 (must call factory) and 30 brass and aluminum ASV. Regarding the brass vs aluminum fittings, the fuel tank boss are aluminum so the lessor of two evils is keep the aluminum elbows. Ideally replace the brass with aluminum fittings (use a thread sealant with PTFE for lubricant/anti-galling) if you can find them. If you just like brass fittings better, there are some compounds to resist galvanic corrosion. In hindsight, I should have tested the factory ASV before trying to remove it. However, if I didn't break the ASV, I never would have discovered how brittle my 20+ year old fuel pickup tube is. I have determined the cause of the engine stalling, I emptied the FCC and almost no fuel came out, so something between the low pressure fuel pump (LPFP) and the tank. I'm pretty sure the LPFP is unable to draw fuel from the tank. In later years, CC/PCM added a pre-filter between the fuel tank and LPFP.... I wonder why

Peter6000 I did rent a Autozone fuel pressure gauge as the first thing to check and it didn't register any pressure. I was stubborn and dismissed the zero reading because the pumps ran and sounded fine. I later discovered the FCC had almost no fuel in it, which explains the zero reading. I have the ProBoss service manual, but as you said, I didn't find any documentation about the anti-siphon valve (ASV). I tried to contact PCM, but I only found a contact us (no phone number) on their website.

-Scott

[core-rider]

h2oski You've mentioned the fuel pickup tube twice now, but haven't detailed what is wrong with it. Is it broken, cracked, plugged??? You don't have fuel in the FCC... obvious issue which should be easy to determine the reason behind as there aren't many parts between the fuel tank and FCC. Starting at the source and working your way towards the FCC to identify any leaks, blockage, etc.

[h2oski]

core-rider I'm trying to create sort of a step by step thread. Not so much about helping me fix why my engine won't start, but share the issues and concerns that I found along the way to fixing it. Maybe alert others to what is aging, effected by Ethanol, improved on, changed, etc. I could replace the LPFP and call it day. But as you say, there aren't that many parts, so I'm going to replace them all. The cost to update those few parts vs a boat full of people and engine that won't restart is worth the cost/effort to me.

[core-rider]

Okay makes more sense that way.


Sent from my iPhone using Tapatalk

[h2oski]

I decided to go with a brass CAL 15 ASV. The research points to a common calibration characteristic, that CAL 15 is for >200HP applications, be it brass or aluminum. The information about brass for >200HP and aluminum for <200HP might be true, but I need to see it etched on the fitting or something on the packaging to indicate the calibration. For curiosity more than anything I plan to test each ASV with a length of tubing. What's suppose to happen is with the tube in a vertical position, you fill the tube with gasoline (not water, it's heavier than gasoline) and the spring tension inside the ASV should hold the fuel in the tubing.

As I said, when I tried to remove the factory ASV fitting from the fuel supply elbow, I broke it off at the threads. I don't know how I could remove the threads from the elbow without damaging it, I need to replace the fuel pickup assembly. I unscrewed the elbow from the boss on the fuel tank and gently pulled up on the elbow. I was shocked at how the tube looked. I wish I had a picture of it. I expected it to be yellowish, and it was, but it was also dark green, brown, with black all mixed together. Of course, I blew through the elbow and end of the tube and it was clear. Since I can't reuse the fuel pickup assembly, I wanted to see how brittle the plastic tube is. I held the tube between my index finger and thumb, slight squeeze and it cracked in 15+ pieces. The other thing I noticed, there isn't any sort of screen or mesh at the bottom of the fuel pickup tube. I half expected something to protect the ASV or anything between it and the low pressure fuel pump (LPFP). Perhaps something got lodged or was passed through effecting the ASV or worse, damage the LPFP.

I checked our vendor websites and only WL shows a fuel pickup option, I didn't see an option on SKIDIM. The fuel pickup assembly is two parts, the fuel pickup elbow and a fuel pickup tube. The issue I found is neither part on the WL website mentions any details, like elbow thread sizes or how long the fuel pickup tube is. If you decide to order the two parts from WL, then you have to figure out how to attach the fuel pickup tube to the elbow. Remember, where the tube attaches to the elbow will be exposed to an air space at the top of the tank, so it must be air tight. I decided to find a complete fuel pickup assembly.

I went to the Moeller website and found a replacement fuel pickup assembly that will fit the boss on the fuel tank. I took the model number and searched around and found a number of options. A warning, be careful of what you're looking at, the fuel pickup tube should be nylon or polyethylene. There are a number of "Made in China" products that claim to be fuel resistant but don't mention what materials they used. I found a company that sells an authentic Moeller fuel pickup tube in 12" and 24" lengths. Unfortunately, my 1999 Air Nautique fuel tank needs approximately 16", figures. I ordered the 24" assembly knowing I need to trim it to length.

-Scott

[h2oski]

As I said I decided on the CAL 15 ASV, but I was still on the fence with the CAL 30. After all, the CAL 30 is readily available, so why isn't the CAL 15? I know I'm over thinking this whole ASV thing, but I don't want to ruin the LPFP or have fuel starvation issues. I called Moeller (not Moeller any more) and spoke to some one in product support. The fella was helpful and he knew exactly what I was talking about. He confirmed that the calibration is indeed related to the height of the fuel tank in relation to the fuel pump. Which makes sense because the ASV is suppose to hold the fuel in the hose and not let it siphon from the fuel tank. The longer the hose (USCG says up to 20ft long), the more fuel it has to hold back. He added that the fuel pump must be able to create enough vacuum to draw the fuel from the tank though. The CAL 30 has a stronger spring than a CAL 15. He did not know how much vacuum was needed to overcome the ASV spring tension though for maximum flow through the ASV. I asked why the CAL 15 is difficult to find and he replied with the part number and added it's only available in brass 3/8" MNPT x 3/8" barb straight; they don't make a CAL 15 1/4" MNPT any more.

Next, I contacted PCM to see what they'd say. I spoke to the warrantee support person who was extremely helpful. We discussed the situation, concerns, testing, etc. In the end, we decided in order to make sure we have the correct ASV calibration, I should test the vacuum between the fuel tank and the LPFP. We should see about 3in of vacuum. He also gave me the same part number for the CAL 15 ASV and confirmed it's only available in brass 3/8" MNPT. UGH, because the new fuel pickup assembly I have has 1/4" FNPT. I can use an 1/4"x3/8" adapter, but now I have an extra joint. Fortunately, the fuel pickup assemblies are not expensive. I decided to order a new fuel pickup assembly from Tanks, Inc. that has a 3/8" FNPT for the ASV.

Now remember, I have a real good idea why my engine won't start (bad LPFP), but after breaking my ASV I decided to update my entire fuel system starting with the fuel pickup assembly. So, until I replace the LPFP, I'm not able to test the vacuum on the fuel supply line. The 3/8" MNPT CAL 15 ASV and 3/8" FNPT fuel pickup assembly are on their way along with a 3/8" fuel line coupler with 1/8" test port for my vacuum gauge.

-Scott

[h2oski]

I found a CAL 15 ASV from wholesalemarine.com. Ironically, NP has the same part on their site now, except it says 5/8" which I think is a typo for 3/8". Any way, I want to test each ASV because I've read a lot of misinformation about the ASV, calibration, >200 <=200 HP, brass and aluminum, this and that, etc. The explanation that made the most sense to me is by a fella I spoke to at PCM, where 15 and 30 are 15 and 30 inches between the height of the ASV and the lowest point where the fuel line could be compromised (ie. splits, cuts, falls off a fitting, etc), essentially any failure between the ASV and the LPFP. You also have to consider the 30 requires more suction by the LPFP than the 15. More suction means the LPFP has to work harder which impacts pump service life.

To test the 15 and 30 ASV, I need to simulate the fuel system in my boat. I won't bore you with the test setup contraption, but it includes 3/8" ID clear tubing, 48" long to represent the fuel line from the ASV. I marked off 15 and 30 inches from the ASV hose barb side down towards the floor. The other end of the ASVs (pipe thread ends) are threaded into the Moeller fuel pickup assemblies (I have two, 1/4" and 3/8" MNPT) and each of the pickup assemblies are sitting in a 5 gallon bucket representing the fuel tank. I added 3 gallons of ethanol gasoline to the 5 gallon bucket. Then using a brake line bleeder for suction, I pulled a vacuum on the end of clear tubes until fuel started to get past the ASV and stopped about 1" past the hose barb side of the ASV. The fuel stays in the line because the little ball inside the ASV keeps the fuel from returning to the 5 gallon bucket. Focusing on the CAL 15, I continued to draw fuel from the 5 gallon bucket until the fuel reached the 15" mark and removed the brake line bleeder. Ready for the fuel to run out of the tube, magically it stayed in the tube, which makes total sense. When I removed the vacuum, gravity is pulling on the fuel in the clear tubing. The variable is the spring inside the ASV holding the ball against the seat to keep the 15" of fuel from running out. Next up, the CAL 30, I pulled a vacuum until fuel reached the 30" mark on the clear tubing. Again, I removed the vacuum and the fuel stayed right at 30". Next, I wanted to see what happens when I pull more fuel past the 15" mark, so I pulled more fuel a couple of inches past 15" and removed the vacuum. Yep as expected, the 15+" of fuel in the clear tube over came the CAL 15 spring tension and started to siphon slowly.

While my testing and results are not scientific, exact-fullness, maybe my altitude plays a part or the barometric pressure that day or the full moon, 3 gallons vs 2 gallons (head pressure) the test confirmed, at least to my puny brain the 15 and 30 are based on the height of the ASV and where (hopefully never) the fuel line can be compromised.

Next up, how much vacuum does the LPFP need to draw fuel from the fuel tank? PCM says no more than 3in.