Fuel Tank Venting Check

Engine is Volvo Penta 350 (570 MS4), inboard, Prestolite BID distrib, qject carb. Boat is a 94 Chris Crowne 340.

I want check a fuel tank for a gradual vacuum build up. I have a plastic fill cap and thought I could fit it with a hose barb and hose to a vac gauge. If the vent works properly should the tank vacuum stay at sea level? Slightly lower?

I read the Adults Only thread on Fuel Testing but before I do that I want to eliminate a fuel vent restriction.

This 90 gal tank has a Racor (maybe LG50 ) inline fuel/air sep. It might have second separator up where the vent goes thru the hull, possibly Atwood, though I haven't gone to the trouble to get it since it's a real pain to reach.

The engine runs OK for 20-40 minutes at cruise, 3800 RPM, then slowly develops a miss that will gradually kill the engine. It will restart after a short shutdown, run fine at idle, even at 1000-1500 rpm, but won't do full RPM until it's been shutdown for overnight or so. Last year I opened the gas cap while it was doing it's missing routine and don't remember it helping but I want to be certain before I go looking elsewhere.

This boat has been a project over the last 2 years. It's had plugs (look fine), plug wires, cap, rotor, BID module, coil, anti-siphon, fuel pump, and a carb rebuild. I've check the anti-siphon, fuel/water sep, and carb filter, multiple times and all look fine.

Re: Fuel Tank Venting Check

I wrote the fuel test and it will show a tank vacuum build up. If you T in the gauge between the pump and the tank (or in between the fuel filter and tank). If you start to see a restriction build up, open the gas cap and see if it goes away. If it does, its a tank venting issue. If it doesn't, then it's not the tank.

On a side note, 20-40 minutes at 3800 rpms is really to long to be a tank venting issue. If it was building up a vacuum then you'd notice in a few minutes... not half an hour.

I'd be looking at something electrical, possibly the coil. Sounds like something is heating up and dying. After it shuts down check immediately for spark. After it shuts down OHM out your coil.

Re: Fuel Tank Venting Check

Thanks a bunch. I'll check spark and ohms when it happens. I replaced the coil last year but I know you can get bad ones off the shelf.

Looking ahead, assuming an electrical problem, forward the coil on Purple wire are a breaker, a fuse, and the key switch. Might any of these (breaker) gradually breakdown (up resistance) and drop coil voltage enough for the spark to weaken? Would a lead directly from the battery+ to coil+ test for this?

Re: Fuel Tank Venting Check

rdc54yty said:

Thanks a bunch. I'll check spark and ohms when it happens. I replaced the coil last year but I know you can get bad ones off the shelf.

Looking ahead, assuming an electrical problem, forward the coil on Purple wire are a breaker, a fuse, and the key switch. Might any of these (breaker) gradually breakdown (up resistance) and drop coil voltage enough for the spark to weaken? Would a lead directly from the battery+ to coil+ test for this?

Click to expand...

doubtful the wire or breakers are increasing in resistance, likely the coil itself is getting too much vibration.

Another thought might be some form of vaporlock? is your motor staying cool or might you have a restriction in your cooling system, which is slowly raising the temp over time.

Re: Fuel Tank Venting Check

Coil vibration damage? No idea, how do you tell? It ohms ok.

Overheat... Temps seems right, 150-170F at the gauges. The gauges were compared to an IR gun shot at the manifold goose neck when I bought the boat so I think they are still fairly close. The worst reading was within 10 degrees. I can put hands on both manifolds comfortably after a full throttle run with no problem.

Vapor lock... I've wondered. Steel lines from the fuel filter to the pump and pump to carb. It's an FWC engine so there's a vertical heat exchanger mounted up front next to the water pump (making it a real pain to work by the way). The pump to carb line runs up between the engine block, heat exchanger, and manifold. I suppose it could be too hot.

The other engine never acted this way but then the problem engine is on the starboard side so I guess the fuel line could be getting extra heat from the port engine.

Re: Fuel Tank Venting Check

I removed the fuel tank pickup on the problem engine. It was probably about 50% blocked with brown/black crap from the tank. I blew off the little screen filter with some carb cleaner and it looked shiny bright new, just like the gen set pickup that I had to remove to get to the engine pickup out - who the heck designs this stuff???

Unfortunately in my zeal to clean the pickup screen I forgot to collect the dirt to see if it was rust or sludge. Hope it's sludge...

Ran like a champ the rest of the summer.

K.I.S.S