HOW TO: Testing your boats fuel system.

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Vice Admiral
Aug 27, 2007
Note, this was originally posted courtesy of Don S. Minor edits and pictures added by CaptJason

A lot of you folks out there seem to be tearing into carbs quite a bit. Before you choose to open up a carb, you really need to look at what's feeding the carb first, and that is the boats fuel system.

Boats seem to have a lot more problems with the fuel system compared to the auto industry. Mostly due to all the "Marine Specific" parts in the system required to make the system safe in a boat. Anti-Siphon valves, special fuel lines, water separating fuel filters just to name a few.
Below is a way of testing your boats fuel system if you suspect a problem. Note, this is for testing the BOATS fuel system which includes everything up to the engines fuel pump. With any boat, you have to separate the BOAT side, from the ENGINE side, as they are 2 completely different things. It assumes (I know, bad choice of words, but you have to start somewhere) the fuel pump and carburetor are working properly.
Most of the information below was just copy and pasted out of a Volvo manual. All I did was put it in one place and separate it from other information.

The Snap-On fuel gauge mentioned below is not a necessity; any good fuel pump pressure/vacuum gauge will work or even a single vacuum gauge. I (Don S.) have one from NAPA that works fine for the tests. I (CaptJason) have a Matco branded one, and it works fine as well. If you do not know what gauge we are talking about. It is often called a compound meter. It will measure pressure and vacuum, and be made to be used on fuel systems. If you can not find one, they are available at Sears under the "Actron" brand name here.

As with any special tool... Not everything will come in the box.You also need to make up some proper fuel hoses for YOUR boat... because EVERY boat is different. Most of the extra brass connectors and fuel lines/vinyl lines can be had in between NAPA auto part stores and home depot/lowes type stores. Since you probably won't be working on very many, get the right stuff and make your job easy and safe instead of difficult and dangerous. You will need specifically sized fittings that will fit the fuel pump, and or the boat side fuel line or tank. Don't even think about cutting that steel line and hose clamping a tee for the gauge to it. Steel line should not be cut. Besides, we are testing the BOAT SIDE fuel system, where steel line is almost never found. If you were TESTING THE ENGINE SIDE fuel system, there is often steel line (and should be) and this thread does not cover ENGINE SIDE fuel system testing.

My rig is seen here. I have the compound gauge, a brass T-block, an appropriate sized adapters and barbed nipples. I have a rig for 5/16th and 3/8th sized fuel lines. Those are the 2 most popular sizes. Very small outboards may have 1/4 inch, and a plastic T is typically fine for those situations.


A couple of problems that can be caused by a defective BOAT SIDE fuel system is.
1. Low WOT (Wide Open Throttle) rpms. (When the engine used to be fine)
2. Engine dies out when running at any rpm.
3. Engine surging at any rpms.

Step 1 - Testing for an airbound/leaky system
This is the easiest of the tests. For an airbound system all you need is the clear vinyl line and a couple of barbs. You do not need the gauge at this point. Splice the clear test line (and I say TEST LINE, because this is for testing purposes only, NO BOAT, should ever be used with vinyl line outside of TESTING PURPOSES!!)
Start the boat up and run it. Look for bubbles in the clear line. A small bubble here or there every 20 or 30 seconds isn't a big deal, but if you get a somewhat steady stream of bubbles then you have an air leak. At this point shut the motor down and go through every clamp (most fittings should be double clamped) every fuel fitting, nipple, barb... etc all the way to the tank. Metal connectors and threads should be disassembled, cleaned and pipe doped. Do not use teflon tape, it has no place on a boat. But pipe dope with teflon works fine. Start up and retest. If you still have bubbles you missed something.... go back through your work.

Vacuum Testing Fuel System.... In other words... testing the BOAT SIDE fuel system for a restriction.

To be 100% accurate.... This test must be performed with engine under load; which means running in gear on the water(not muffs) or in gear connected to a dynamometer.(which your probably not going to have... so do it on the water)
Ensure that all fuel line connections are leak free, as per step 1.

However... this test can also be TRIED (and I can't stress the word TRIED enough) on the muffs, as long as you don't exceed 1500 rpms for more than a minute with good water supply.
This is CaptJason's rule.... Don S. may have a different rule.
I say you can TRY it because on a lot of boats you'll see the gauge indicating a problem immediately. If the gauge does not indicate a problem immediatly... you should still go through the test with the engine under a load as previously stated.

1. Install Vacuum and Fuel Pressure Gauge such as Snap-on? Tools MT311JB or equivalent, and 8 in. (20,3 cm) of clear, fuel resistant vinyl hose into the fuel line at the filter fuel pump inlet fitting. (NOTE: On some engines there is a filter hard plumbed to the fuel pump, these engines should have the gauge attached to the fuel filter and not the pump itself. )
2. Start engine and allow it to reach normal operating temperature. Run engine at full throttle (NOTE: As noted above, this is done under a load, not on muffs, if you do this on muffs, or in neutral, or not on a dyno... you take the chance of over revving, having a runaway, or just plain blowing up your engine... and possibly yourself!!!) for at least 2 minutes; observe vacuum gauge reading and check clear hose for air or vapor bubbles. Gauge reading must not exceed 4 in. of mercury (Hg) at any time, and there should be no bubbles visible in the clear hose.
If you are getting bubbles check for.
1. All metal fittings
2. Wrong type hose clamp used (Use mini clamps not standard size clamps, double clamps on everything in between the pump and the boat)
3. Filter seal leaking
4. Pickup tube in fuel tank defective.
5. Loose, cracked, corroded fittings in the system (including the tank and fuel filter or fuel line)
Repair or replace suspect part, then make another vacuum test to verify repair.

Gauge reads 0-4 in. of mercury (Hg) (vacuum)

1. If you have air bubbles.... then you still have an airbound system. And you need to repeat step 1.
2. 0 to 4 is normal, but no more than 4 Hg. If you are on the high side (close to 4) Then you either have a really really big boat with big gas tanks and that's okay....... or you have a problem/restriction causing the vacuum reading to read on the high side. If you have more than 4 Hg, then you have a restriction.
If you fall in the latter. The 3 major things, most common things are a....
1. Clogged fuel filter/fuel water separator
2. stuck anti siphon valve
3. clogged fuel tank vent
If you are Joe the Boater with a smaller/mid size boat, start with replacing the filters first and checking the tank vent and retesting.... then move on to the anti siphon valve if need be and retest. Ideally you'd like to have the vacuum restriction as low as possible.
If you still have a high reading. Its possible that you have a screen on the pickup in the fuel tank that is getting clogged. You can try forcing some compressed air down the fuel line to knock the stuff off (MAKE SURE IF YOU DO THIS THAT YOU TAKE THE FUEL TANK FILL CAP OFF!!!) Unfortunately though, if you have that much junk in your tank (and older boats will, especially steel tanks) the problem will likely resurface.
If you still have a high reading, replace your fuel lines. It isn't common to have collapsed fuel lines, but it happens once in a blue moon. What is your favorite color moon? Make sure you replace lines with Coast Guard approved type A1 fuel line Your going to have to get that at a marine store... the townie auto parts store is not going to have it.

To refresh
Gauge reading exceeds 4 in. of mercury (Hg)
Supply side of fuel system has a restriction. Check points of possible
failure as noted below.
1. Water or debris in fuel/fuel filter
2. Fuel pickup tube and screen blockage
3. Fuel tank vent blockage (this one can be a biggie...... i swear... a mud doggers only purpose in life is to build nests in tank vents)
4. Plugged external canister or carburetor fuel filters
5. Inoperative, restricted or incorrectly sized anti-siphon valve
Repair or replace suspect part, then make another vacuum test to verify repair.
6. Bad fuel lines(uncommon)

Testing the fuel pumps ability to create a vacuum
The next biggest boat side (but semi-engine side) test is to make sure your pump is actually capable of pulling a vacuum. A fuel pump has to create suction in order to work. It is fairly common to have a pump that is worn out or clogged enough not to generate enough suction.... And the most common symptom is lack of WOT or WOT surging in carbed engines.
This is another good test to do because alot of times when your testing the fuel system all you see is air in the clear line, as in no fuel, just air. If all you see is air in the line, then you either have a major air leak in the boats fuel system, or you have a fuel pump that's not capable of creating a vacuum.

What I (captjason) does is this...
1. I hook up my test rig in between the fuel pump and fuel(fuel/water seperator) filter.
2. I start the boat up and let it warm up for a minute or 2.
While the engine is running I pinch fuel line shut in between the FUEL TANK and the COMPOUND GAUGE. What should happen is the engine should run for a bit and the gauge should register that the fuel pump is TRYING to pull through the restriction that you made with the pliers.


You need to have the pliers in the right spot as stated before. I use self locking hose clamp pliers, but any old pliers will work as long as they are not so serrated they will puncture/mar the fuel line on the outside.

I have a "35 69 10" rule (Don S's) rule may vary.
-What I mean by my rule is this. If an OUTBOARD fuel pump can not generate 3 to 5 inches of Vacuum at idle or high idle over the course of 10 seconds, then the fuel pump needs to be rebuilt/replace.
-On a sterndrive with a conventional mechanical type, or suction type electrical fuel pump, If it can not generate 6 to 9 Hg of vacuum over 10 seconds then it needs to be replaced. A bit under the my rule isn't a problem as long as the engine hits WOT properly. If it doesn't, then the fuel pump is an item of consideration.
This test is mandatory if all your seeing is air (no fuel) in the test line. It is the only way to know if the fuel pump is trying to bring fuel in and cannot because of an air leak, or if it is incapable of bringing fuel in.

Here is what the face of the gauge looks like...

Good luck guys and gals!!!


Vice Admiral
Aug 27, 2007
Re: HOW TO: Testing your boats fuel system.

Original post from Don S. Edited by captjason

Don S

Honorary Moderator Emeritus
Aug 31, 2004
Re: HOW TO: Testing your boats fuel system.

Thanks Jason, The extra details and explanations helps my original post a lot.
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