thin gas tank...sending unit not accurate...adjustments??

[KenJr]

I have a 90 Crestliner Fish & Ski with a gas tank under the floorboard (luckily with easy access to the sending unit via a piece of plastic that pops up). I did some troubleshooting and determined that the sending unit was bad. I've done quite a bit of electrical reworking since I bought the boat a few months ago, but I'm not comfortable playing with electricity on top of the gas tank. d

At any rate, I took it to my local mechanic who verified the SU was bad and replaced. However, the gas gauge isn't reading accurately (shows 1/3 tank even though I filled it up the day I took it in and was only on the water a couple hours and hardly used the gas engine before dropping it off) and I'm guessing because that tank is so thin.

They used a 'universal' sending unit that they had in stock and apparently had to make some minor adjustments to get that to work properly in such a thin tank. (I'm not sure what the total capacity is - maybe 15-20 gallons)

I'm curious if there are some specific adjustments that can be made to get this a little more accurate. I don't care if it's dead on just as long as when it's around 1/4 tank left...that's the real story. d

Any ideas?

 

[Silvertip]

Re: thin gas tank...sending unit not accurate...adjustments??

You have a common problem. Long skinny tanks means the fuel level and therefore the sending unit float position varies greatly depending on the attitude of the boat. At the dock the boat will be level and the gauge will probably provide one reading. Get under way with the bow a little high and it will read totally different. There is no way to adjust for both attitudes. The best adjustment you can make is to run the fuel level down until there is about 2 or 3 gallons left. Adjust the float for for an EMPTY reading at that level (either under way or at the dock). That reading will then be your reference. You want to know for sure when you are low on fuel. You don't know that with any other calibration. Better to know your about to run out rather than full -- after all, you just filled it so you now that. Right??

 

[rickdb1boat]

Re: thin gas tank...sending unit not accurate...adjustments??

Not cheap, but this may be an option:

http://www.livorsi.com/catalog/waterproof.htm#SENDERS_ ] Senders [/url]

 

[Silvertip]

Re: thin gas tank...sending unit not accurate...adjustments??

In my view, a Navman (or similar) fuel flow monitor is the best solution. It not only tells you what your current flow rate is in Gallons/hour, but how much remains in the tank. It's calibrated at the time of installation. You tell it how large the tank is. From flow rate vs time it calculates the remaining fuel. I will install one before my next week long fishing trip where I burn a bunch of fuel.

 

[KenJr]

Re: thin gas tank...sending unit not accurate...adjustments??

In my view, a Navman (or similar) fuel flow monitor is the best solution. It not only tells you what your current flow rate is in Gallons/hour, but how much remains in the tank. It's calibrated at the time of installation. You tell it how large the tank is. From flow rate vs time it calculates the remaining fuel. I will install one before my next week long fishing trip where I burn a bunch of fuel.

While I agree that a fuel flow monitor is probably a better solution, it's way more expensive and all I need to know is when I'm a few gallons away from turning back to the boat ramp. Hell, I don't care if the gauge says full all the time and then drops to emtpy when I've hit that point - I just don't want to have to fill up every time I head to the lake because I have no clue (other than a good guess) how much gas I have left in the tank.

Is there a cheap fuel flow monitor somewhere? Everything I've seen is pretty pricey (compared to sending units and gas gauge combo's).

 

[Silvertip]

Re: thin gas tank...sending unit not accurate...adjustments??

My Navman 2100 was $116.00. Prices vary all over the map. With gas prices today, a fuel flow monitor also provides an indication of where the "sweet spot" is that provides the best fuel economy.

 

[KenJr]

Re: thin gas tank...sending unit not accurate...adjustments??

My Navman 2100 was $116.00. Prices vary all over the map. With gas prices today, a fuel flow monitor also provides an indication of where the "sweet spot" is that provides the best fuel economy.

NM - posted asking how to hook that up...

http://www.navman.com/upload/Marine/Internet_Manuals/2000/2100_fuel_MN000207A_eng_web.pdf

So, could you hook this thing up right by the engine? It says it has to be after the fuel filter, which I'm assuming is very close to the exit of the fuel tank (can't see mine). If it's mountable say within a foot or two of where the fuel cable enters the outboard that would be perfect (and easy to install). If it needs to be closer to to the gas tank/fuel filter then it's going to be a pain to install on my boat.

 

[Silvertip]

Re: thin gas tank...sending unit not accurate...adjustments??

The transducer installs anywhere in the fuel line between the tank and the engine. Wires run up to the gauge at the console.

 

[KenJr]

Re: thin gas tank...sending unit not accurate...adjustments??

The transducer installs anywhere in the fuel line between the tank and the engine. Wires run up to the gauge at the console.

Do you think it matters if it's before or after the primer bulb?

 

[Silvertip]

Re: thin gas tank...sending unit not accurate...adjustments??

No -- but you don't want the transducer bouncing around so it should be installed so its somewhat protected and not just dangling. It should also be mounted vertically. With this in mind, it would be installed before the primer bulb. It is also recommended that another filter be installed between the tank and transducer so debris doesn't get into the transducer (it has a small trubine wheel inside and would be jammed or damaged by debris).

 

[rickdb1boat]

Re: thin gas tank...sending unit not accurate...adjustments??

Could you put one after a fuel/water separating filter between the tank and the bulb? Kinda like this:

 

[Boomyal]

Re: thin gas tank...sending unit not accurate...adjustments??

My Navman 2100 was $116.00.

Where'd you find that Silvertip. I checked a while ago and found prices from 150/160$.

My tank is very difficult to moniter. It is rectangular, flat on the top with 4" side walls and a bottom that angles down to about 8" at the bottom of the Vee. In addition to the bottom profile, the sending unit insertion point is about 2/3 rd's of the way up from the deepest part of the tank.

Assuming that you have a float sending unit that can reasonably follow the height of the gas in the tank (really impossible) the last portion of gas in the Vee is used up very quickly, leaving you with no way to extrapolate your time on tank. Very disconcerting.

 

[KenJr]

Re: thin gas tank...sending unit not accurate...adjustments??

My Navman 2100 was $116.00.

Where'd you find that Silvertip. I checked a while ago and found prices from 150/160$.

My tank is very difficult to moniter. It is rectangular, flat on the top with 4" side walls and a bottom that angles down to about 8" at the bottom of the Vee. In addition to the bottom profile, the sending unit insertion point is about 2/3 rd's of the way up from the deepest part of the tank.

Assuming that you have a float sending unit that can reasonably follow the height of the gas in the tank (really impossible) the last portion of gas in the Vee is used up very quickly, leaving you with no way to extrapolate your time on tank. Very disconcerting.

I found it here for $113:

http://www.anchorexpress.com/aa004551r.html

 

[Boomyal]

Re: thin gas tank...sending unit not accurate...adjustments??

Ken. what motor are you using it on? Earlier studies that I made were confusing as to the range of consumption. I don't remember all the numbers now but it would be good to hear some experience.

 

[KenJr]

Re: thin gas tank...sending unit not accurate...adjustments??

Ken. what motor are you using it on? Earlier studies that I made were confusing as to the range of consumption. I don't remember all the numbers now but it would be good to hear some experience.

Well, I'm not using it yet - but my motor is a 1990 70HP Evinrude with VRO. I have Crestliner Fish & Ski and probably have the same type of tank as one of the previous posters was referring to. I had a local mechanic fix my sending unit and I know there has to be at least 3/4 of tank left - he's reporting that it's only showing 1/3 full sitting on the trailer. So, I'm afraid that is never going to be an accurate measure (and the previous boat owner confirmed that when it was working for him it wasn't ever really that accurate - when it got to 1/4 tank, he'd fill it up).

The problem for me is that while either the factory or a previous owner was nice enough to cut out the floor and put a nice plastic cover right over the top of the sending unit - I don't think I can see another inch of the gas line until the connector on the transom. From there I have about 3 feet of hose exposed with the bulb and of course the connection to the motor. Even if there's a place where I can see the hose under that back seat where my batteries are - it's defintely not going to be a vertical mount...

 

[rickdb1boat]

Re: thin gas tank...sending unit not accurate...adjustments??

You know Ken, there are still some adjustments that you can make to the sender to make it more accurate if you have access to it. It's really just a float set-up in which the arm can be bent to achieve close to correct readings. Might give it a try. Run her down until you have 2 or 3 gallons left in the tank and adjust the arm so it just reads empty...Here's some reading: Click on Fuel Sender on the right side....

http://www.teleflexmarine.com/cgi-bin/frameset2.cgi?site=gauges&type=us&link2=1047 ] Fuel Sender[/url]

 

[Silvertip]

Re: thin gas tank...sending unit not accurate...adjustments??

Yes - if you already have a water separating filter, the Navman transducer would fit between it and the bulb. Cant post the link but I got mine from J and H Products. $116.00 plus $9.00 shipping. Many sites that advertise low prices are also out of stock so check that first. Some also have verrrrrry high shipping costs. I've seen as high as $32.00. Rick is right about adjusting your float arm. You can get it to read accurately at empty. Or you can make it accurate at half full or any other setting you like. You just can't make it accurate across the full scale. You can only pick one point.

 

[Chris1956]

Re: thin gas tank...sending unit not accurate...adjustments??

Ken, I am with Rick. First make sure the float is pointing to the back of the boat. If you needed to move it, now check to see how it works. If not good, use the bending of the float arm described. Also, in a boat that size, it doesn't hurt to fill it up before each long trip, or day on the water.8)

 

[KenJr]

Re: thin gas tank...sending unit not accurate...adjustments??

Ken, I am with Rick. First make sure the float is pointing to the back of the boat. If you needed to move it, now check to see how it works. If not good, use the bending of the float arm described. Also, in a boat that size, it doesn't hurt to fill it up before each long trip, or day on the water.8)

Thanks! I got the sending unit fixed but even with the tank completely full it only shows slightly above half. That's OK I guess as long as it takes a while to move from where it is down to slightly less than 1/3rd as that represented about 11 gallons of gas when I filled up today. Anyone have a rough guesstimate as to what a 70HP 3cyl Rude burns an hour? I know that's a loaded question...burns at idle...burns at WOT? I'm really going for WOT. I know boats generally get more like gallons per mile and not miles per gallon but wasn't sure if that applied to the smaller outboards as well...

I figure I'm pretty safe on the lake for the day with a full tank. Especially since I'm fishing a lot and mostly on the troller and normally use the gas engine to get me from spot to spot.

I'll sit on the fuel flow monitor idea though - I may take that on as a spring project next year before I put the boat back in the water.

 

[Chris1956]

Re: thin gas tank...sending unit not accurate...adjustments??

Ken: the rule of thumb is your motor will burn 10% of the HP in gallons/hour, wide open(7 gal/hr). I would guess that you run in the 2/3 throttle range, so figure 4-5 Gal/hr. Now if you take frequent beer breaks, your mileage will vary!8)