Diesel smell in the aft cabin...down the rabbit hole

Jan 12, 2016
268
Hunter 410 Ladysmith, BC
We were getting just the faintest possible diesel smell in our aft master cabin, but only after refueling and for a day or two afterwards. This post is for the owner that has the same issue. As often with boats when you look closely at things you soon see issues in other areas. If you have a late 90's Hunter hopefully this post can be helpful for you. None of this work requires a paid mechanic. Simple tools, time and patience are all that is required.

1) Fuel vent hose. The vent hose installed was actually too long. It should run up vertically from the tank and then loop downward to the hull clam-shell vent. On ours it actually had a second dip, not a loop but a definite sag. This would keep some fuel trapped in the line at the lower sag as it couldn't flow back to the tank.

The solution for this was very simple. We cut about 5" off the vent hose from the tank side and re-attached. No more sags, and most importantly no more trapped diesel.

2) Leaks off any tank top fittings? I have a large inspection plate, as well as fittings for the fuel fill, tank vent, Espar heater fuel supply , engine fuel supply, engine return fuel, and the fuel sender for the tank gauge by the chart table. In addition there are brackets that are screwed into the top of the tank that secure it against the bulkhead. I needed a safe way to check if there were any leaks from any place a hole was made on the top of the tank.

First I tightened the hex nuts on the inspection plate. Most were fine but two were surprisingly loose (barely finger tight). A quick re-torque with a socket wrench was all that was required. Next I needed a way to gently pressurize the tank, so I could spray the source spots with soapy water to see if anything was blowing bubbles. For this task I took a separate one foot section of fuel hose that was the same size as the fuel vent hose, slid it onto the vent fitting, and then attached the other end to my dingy foot pump to add just a small amount of positive pressure. It didn't take much air at all to see the tank slightly bulge like a balloon.

Thankfully all looked good, no bubbles anywhere.. Thinking we'd solved it with torquing up the inspection plate nuts, I re-attached the tanks vent hose and we super cleaned the area around the diesel tank with bilge soap in a spray bottle, a garden hose, and a wet dry vac.

Feeling smug that we've got this solved, (there was zero diesel smell, mind you the tank was down 50 litres from full), we decided to fill up the tank. As soon as we're done re-fueling I check the top of the tank. DAMN IT. There is a trace amount of fuel on top of the tank. ( 1-2 teaspoons max), However at last we find the source..... The fuel sender. :banghead:

Our boat was fitted with a Wema Fuel Sender from I'm guessing the either the Hunter factory or the tank fabricator. I find the appropriate cork gasket for the sender on amazon for under 10 bucks and order it. A week later it shows up and its time to get back to work. I watched this YouTube Instructional Video for installing Wema Sender, then went to the boat.


3) Fuel Sender Surprises....

There is not enough spare cable to pull the old sender metal arm completely out of the tank and rest it on top of the tank. This sucks as I can not mask the hole shut while cleaning off the old sealant. Either I would need to cut the cable and rewire it later or, be very careful and have my wife hold the sender up as far as the slack allows and clean the area. We chose the latter as it was less work. The diesel had wet this area so many times that most of the old sealant pealed away with very little effort. A rag and a tiny bit of acetone cleaned the rest of it to like new condition. I guess 22 years of bathing in diesel is a good way to soften any RTV sealant.:) Thankfully nothing fell into the tank while cleaning.

The biggest surprise, which may be of interest to anyone who has a Hunter of this era with a Wema Fuel Sender...THERE WAS NO CORK GASKET INSTALLED! Instead they just gooped lots of RTV Sealant under the cap. This isn't ideal and I have a hunch that anyone who has had a diesel smell in the vicinity of the tank may want to check this out for yourself. The gasket is about as thick as two quarters and won't compress the same way as RTV sealant does. When the cover gets screwed back down it will seat far better against the cork gasket than it will against pure RTV sealant alone. Maybe my boat was a one off on this, maybe not. If you can smell diesel in your boat, and the engine room is 100% leak free, this is worth exploring. Just make sure you have a replacement gasket in hand before removing the sender.

We removed all the old sealant carefully keeping all the waste from dropping in the open hole the sender passes through, then laid a small bead of RTV sealant on both sides of the new cork gasket, and on all the screws and reassembled everything. Because we couldn't easily remove the sender arm we chose to cut a small slit in the gasket to allow it over the sender arm. This slit area received a tiny bit extra RTV sealant on both sides. Other than it was a little awkward getting the first two screws into their threads, it all reassembled easily.

The final step will be to refill the tank right to the top and check for leaks. We leave soon for a few weeks of Desolation Sound Cruising. Wish us luck that we solved this diesel smell issue once and for all.
 
Jan 12, 2016
268
Hunter 410 Ladysmith, BC
Thanks Jim,

I spoke with two good marine diesel mechanics I know locally who both recommended cork with RTV sealant on both sides I expect I should get at least 5+ years of trouble free service from this. If not, I can replace it easy enough over the winter when the cruising season is over.

Funny enough on the bottom of the page from the link you provided, the KUS lawyers added this gem leading me to feel more comfortable going with the cork option:

WARNING – Please Note:

This adapter is in no way intended for use in any type of fuel application. Gasket supplied with this adapter has not been tested nor approved for fuel applications. We will not be held liable for any misuse of our products.
 
Jan 4, 2006
6,444
Hunter 310 West Vancouver, B.C.
As @JamesG161 has said, cork is like a wick and has no business being near any fuel. I strongly suggest you remove the cork gasket before completely filling your tank or then you've got the problem of draining the tank enough to work on a new gasket. And I am assuming your tank is poly and not aluminum.

I like @JamesG161 's idea of using the SHS adapter but seeing as the hole is pretty small you can cheat a bit. I removed my WEMA sender gasket early on to upgrade the factory installation and used Permatex 2 (and only #2) as a sealant. It's the only flexible sealant I've found that sticks like glue to poly.

Because we couldn't easily remove the sender arm we chose to cut a small slit in the gasket to allow it over the sender arm. This slit area received a tiny bit extra RTV sealant on both sides.
Sorry, but you've just introduced another source of leak. Diesel (or any petroleum fuel) has a really nasty habit of getting through the tiniest of fissures and holes.


I have a large inspection plate, as well as fittings for the fuel fill,
Not a good idea at all. If the smaller fittings showed signs of leakage, it's only a matter of time until this one leaks, and it may not be a small leak when it comes. There's no need for a large inspection plate on the top of the tank as there's nothing in there to see. I would suggest permanently sealing the plate with Permatex and then installing something more sensible like this:

Fuel Tank Pump Out Fitting.JPG

This enables one to insert a tube from a Pela oil extractor and pull a fuel sample off the bottom every few years to see what the fuel looks like. So far so good. All due to Biobor JF anti-fungicide.

1596909721303.png

A few years ago I really got bored one day and decided to slide the borescope through the access hole to take a look at the tank bottom. The tank was low and I didn't immerse the scope as I'm sure it would have buggered it but good. Looked clean to me with a few tiny black dots on the bottom at 15 years of age. Also, we have pretty good sources for fuel here in the lower part of B.C.

@VanIslandGuy , I want your Desolation trip to go seamlessly as I will be traveling vicariously with you this year because I can't get a crew together. We were scheduled for the Broughtons this year and then all hell broke loose. They now think I'm going to infect them with something. Spoil sports, but yes it's safer that way.

100_2245 - Turnbull Cove.JPG

In the middle of nowhere in the Broughton Archipelago
 

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Feb 14, 2014
7,400
Hunter 430 Waveland, MS
I installed the newest Wema sender with the float and reed switches. The old float had swollen from diesel soaking, but the new one is sealed.

That Gasket is for Diesel, a bit of a pain to install but...
The sender is screwed into the Adapter and the gasket stays in place. Once done never leaked, even in 6 foot seas.

sealing the plate with Permatex
That is GOOD stuff.
I used it to seal the Square metal gasket [tough to find] on my Exhaust Riser.:biggrin:
Messy black goop, but works on high temp stuff too.
Jim...

PS: My diesel tank is Aluminum.
 
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Jan 12, 2016
268
Hunter 410 Ladysmith, BC
Further down the rabbit hole, the KUS gasket I ordered is listed as cork/nitrate which I will guess means cork treated with nitrate to make it all fuels safe. Pematex RTV sealant was used on both sides so I will test and see. As the boat has gone 22 years without a proper gasket/just sealant, and only now started weeping, I'm happy using it as is for the balance of the season. From the KUS FAQ Page:

We offer the following gaskets for the SSS/SSL units:
  • Cork/Nitrile
  • Nitrile
  • Viton
 
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