Lotus Elan

How much fuel can your tank take?

PostPost by: rocket » Thu Jun 18, 2009 12:26 pm

My plus 2s fuel gauge seems pretty typical in that it reads 3/4 full when topped up but only goes down to 1/4 full and stops there..i always refill at 1/4 full reading which lets me put about 6 gallons in(uk).Is this as others find?

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PostPost by: pauljones » Thu Jun 18, 2009 5:43 pm

I have a plus 2,i could only get about 45ltrs of fuel in it at the very most,but 35 went in really quick,and it took an age to get the rest in.i ran out on the road when the gadge said 1/4 full so i would always fill before i made the same mistake. a friend said that they were never accurate and that was pretty normal,and also the breather system was possibly the cause of the slow refilling.this gave me about 200miles on the road.

hope it helps.
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PostPost by: richgilb » Thu Jun 18, 2009 6:08 pm

I don't think I have ever got more than 30 quids worth in mine and the guage will read half full. Weird, I have had a Smiths fuel guage on a Triumph and it was pretty good compared to this
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PostPost by: msd1107 » Thu Jun 18, 2009 9:56 pm

RE: Fuel gauge accuracy.

There is no reason for the fuel gauge to register improperly. The whole system is a variable resistance in the tank and a gauge to measure the current flow. It is the details that are important.

http://mgaguru.com/mgtech//electric/fg_06.htm
http://www.gomog.com/allmorgan/instrumentindex.html

have tutorials with pictures on calibrating the admittedly older fuel gauges on MGs and Morgans. The +2 is made more complicated in that two different sending units and fuel gauges were used. The electronics and math are not that difficult, so you should be able to come up with a reasonable callibration.

Also Google:
fuel sender ohms
smiths fuel gauge calibration

As far as filling the tank, +2s are renowned for this. You could try parking the car on a slight slope (experiment with nose up, nose down, and sideways to see what works best). Or bounce the car up and down to get the fuel sloshing around the tank so that the air pocket vents out.

There have been some posts on improving the venting of the tank, usually to get rid of the fuel smell but also to allow the tank to be completely filled.

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PostPost by: stugilmour » Fri Jun 19, 2009 3:20 am

David:

Thanks for the excellent links.

I think I will give the calibration technique a go and see how I make out. I have the tank and dash out of the car, so it should be straight forward.

A couple of (perhaps obvious) thoughts/details come to mind:

The Lotus Plus 2 fuel gauge is supplied from the voltage stabilizer, which knocks the gauge supply to ~10 volts. I expect this technique would work better using a 10v supply during calibration. It looks like this is the same in the MG, etc. that the author is describing, but he uses 12v supply on the bench testing.

Sounds like one should start by ensuring the gauge sender exhibits the spec'd 70 ohms at full fill. Seems if this is incorrect due extra resistance in the sender due to wear & age, the calibrated gauge won't match up when re-installed in the car. Does anyone know if this is the correct value for the Lotus sender? Maybe it doesn't really matter what it is; one just measures it, and adjusts the calibration resisters to suit?

Might have to dig up a bit more info on the Plus 2 sender to fully understand the second circuit mechanically that controls the low fuel lamp. On my car this presently works correctly but the gauge range is faulty as everyone else describes. Does anyone know if these are completely separate devises in the sender?
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PostPost by: msd1107 » Fri Jun 19, 2009 3:47 am

Stu,

I depends on the year of the car as to what the resistance values are.

I think (not completely sure) that the early cars (up through 68) used the GM 0-90 ohms sender value. The later cars used a 330-40 ohm (roughly) sender. There are several other sender values that have been used throughout the years by different manufacturers.

You will find out which one you have by using an ohm meter across your sender at full and empty.

On my stock gauge, I was lucky. A full tank registered slightly above "F". And the needle hit the "E" mark with a little more than 1 gallon remaining. Calibrating the low level so that the needle hits "E" with one gallon remaining substitutes for the usual low fuel warning light (a +0 has no low fuel warning light).

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PostPost by: Robbie693 » Fri Jun 19, 2009 9:38 am

Mine seems fairly accurate, the fuel light comes on half way between 1/4 and empty. I usually put 40 litres in as this is the dose required for a bottle of Millers VSP. This gives me a reading of 3/4 full.

I remember with my previous Plus 2 that the only time I managed to get the gauge up to full was when I was at a garage with a sideways slope - the filler being on the high side, as David suggested.
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PostPost by: stugilmour » Fri Jun 19, 2009 6:14 pm

Thanks again David for the follow-up.

There may be a difference between model years and the Plus 0 and Plus 2 set-ups, but here's what I found today on my '71 Plus 2 with the older style gauges (four small gauges in dash). I have dash & tank out, so fairly simple to do. Took the sender unit right out of tank and manipulated it, and got the following approximate readings:

20 ohm when completely to full top stop.
180 to 195 ohm when totally empty. Sender rheostat goes to full open if totally against lower stop, hence approximate lower limit value.
164 ohm when low fuel lamp switch closes.
approx. 70 ohm when roughly 1/2 full on sender float arc.

I expect the 20 ohm resistance in the sender is causing the gauge to read just below the full mark when the sender is connected and pinned to the top stop. I can live with this, so did not open sender and will use as is. Note all testing with gauge & sender uses 10v stabilized supply.

As described in post above, my low fuel lamp lights half way between empty and 1/4 full on the gauge, which seems perfect.

Checked back of fuel gauge, and the adjustment holes described on the provided links and various goggled sites are sealed over, so assuming not easily adjustable to compensate for extra resistance in sender. I don't have the adjustment posts protruding from the gauge case.

Seems for my car at any rate, that the gauge would be reading low if the sender is in need of changing or if there are dirty connections, bad wiring, etc. creating additional resistance in the sender circuit. To correct misbehavior described in original post, suggest checking & cleaning all connections, followed by changing of sender if required; the fact that you's stops at 1/4 full might indicate the float lever is hanging up anyway. Looks like taking the gauge out of car to adjust may not work for you, but YMMV.

Sender terminals are:

Top middle = ground
Left marked "T" = to gauge
Right marked "W" = to low fuel lamp.

HTH
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PostPost by: peterako » Fri Jun 19, 2009 8:39 pm

45 ltrs Max, guage reads between 1/2 and 3/4.

Red light at 1/8 ish.

Empty = empty :)

I always carry a gallon in the boot just in case.

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PostPost by: msd1107 » Sat Jun 20, 2009 6:45 pm

I should have read my manuals before posting.

Smiths fuel gauges came in two styles. The early style (with prefix FG or X) was used until some time in '68. This was an instant reading type, with calibration screws and did not use the voltage regulator. The Elan used FG2530/71 until March '68. The corresponding sender was FT3334/00 until July '65 and FT3334/50 from Aug '65 til Mar '68.

The later style (with prefix BF) was used after about late '67. This had a damped movement and used the voltage stabilizer. The calibration holes were sealed off.

The Elan used BF2201/19 from after about Apr '68 with corresponding sender TB1114/023 from about Apr '68 until Apr '70 and TBS1114/010 from about May '70.

The +2 used gauge BF2201/13 for all years Oct '67 on with corresponding sender TBS1214/004. The +2S used gauge BF2209/01 From Nov '68 with corresponding sender TBS1214/004.

Reading the posts, it seems that many people find the low reading acceptable. If the reading does not go down to "E", it may be possible to bend the float arm to "calibrate" the gauge. Not reading "F" is probably a problem with fuel tank venting. All this assumes the wiring is good. Corrosion may affect either the reading or calibration.

http://www.google.com/search?hl=en&safe ... f&oq=&aqi=

should point to others with the problem and possible corrective actions. On my Elan, I would park with the nose up if possible, fill till the hose clicked off, then bounce the car, fill some more, bounce again, and then carefully fill until the gas level reached just below the gas cap. Then the tank was completely full and I could get over 400 miles/tank. Conversely, on my MB 280SEL, I parked nose down for the same effect. On a +2, it takes more time to do this unless you tilt the car sideways or fix the venting.

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