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MG MGB Technical - At my wits end with fuel consumption
|OK I replaced the fuel tank and yes that did improve matters with my fuel consumption. I put 15 litres into the tank (3.3 UK gallons) and I was aiming for at least 75 miles for that fuel. Although the fuel isnt finished yet Im not going to make it to 75 miles and I've been driving like a granny to eek as much as I can out of it. The engine is running fine. I mean the car has to do better than 25mpg (UK) and its not even getting that when Im driving it light and not in heavy traffic either. There must be a leak somewhere. Aside from anything else, the clue for me would be in the fact that when the engine is off but the ignition is on, the pump is pulsing once every 2 or 3 seconds. I cant see anywhere the fuel might be leaking from. Is there a way of checking for leaks aside from just looking, cause there's nothing obvious and Im losing my patience.|
|Ross You put 15 liters in the tank but the gauge usually does not read the first few liters. Also the fuel pickup will not pickup the last couple of liters. The only way to check the consumption is to fill it to the top, drive until its low then fill it again. Noting the miles of-cause.|
At one time you mentioned that you intended to put your car on a gas analyzer where you work, and you also mentioned buying a gas analyzer.
Have you checked the hydrocarbon (HC) emissions with a gas analyzer to see if it is overfueling?
For a point of comparison, I was on the open road enough a few weeks ago to check my mpg. Since my speedometer doesn't work, it is only easy to check on the open road. It came in right at 30 mpg, US gallons. That was about 90% open road miles on a trip of around 360 miles with overdrive, and most of it in the 65 to 75 mph range.
Another thought, what is your exhaust system like? I once put a cheap "will fit" muffler on a Peugeot 404 and dropped the mileage from about 30 mpg to about 20 mpg. Could have been coincidence, but it was the only thing intentionally changed.
|C R Huff|
|Yeah I did both in fact. I have an exhaust gas analyser and the mixture is spot on at 3% CO on that. The engine runs well. There's the usual slight lumpiness at idle but it idles quietly and pulls well across the range. The exhaust is a stainless steel exhaust that looks to be very good quality. I dont know what make it is though. So you can see why I have my doubts that the engine is actually using this amount of fuel. It just doesnt seem plausible.|
with the ignition on and the engine stopped, there should be a clicking from the SU pump in ~25 to 30 Sec. intervals, as long as there are no leaks anywhere.
I would check the whole fuel lines from the tank to the pump and from the pump to the carbs. If there are any suspect areas in the system i would renew the whole piping. Keep in mind, your car is 30 years old and so are the lines, i suggest.
The next point to check should be the float levels and, if running on HIF carbs, the choke valves and their cable and linkages too.
|I'll add my vote with Ralph, check the float level in the carbs. If they're a bit high you could be running a bit rich as well as possibly losing some fuel out the overflow.|
|I don't know if this was discussed earlier in another thread, but I solved this problem by replacing my fuel intake needle valve and seat with a neoprene tipped needle. I had overflow from my HIF4 carbs that I couldn't solve with several other needle/seat combinations and float adjustments. The neoprene tipped needles immediately and completely solved the problem.|
I even ran for a little while with a plastic bag wrapping the overflow tube (under the car....away from the exhaust) to make sure I was not getting any more fuel bypass occurring.
|No I checked to see if the float was too high. Its perfect. I rebuilt the carbs recently and they seem fine now. Even when I disconnect the fuel line into the front carb and plug it and turn on the ignition the pump still pulses every 2 or 3 seconds. I get smell of petrol every so often when Im driving too.|
|Ross. In addition to what Ralph has suggested, and Bill seconded, consider what BH has mentioned--some form of recovery system for the fuel overflow vents. A hose from the two vents to a temporary catchment system (plastic pop bottle or something of that nature). That will tell you if you are losing fuel out of the overflow vents. Does little to tell you about rich mixture caused by too high a level in the fuel bowl though. |
One test for fuel system leaks (drips) would be to use a large sheet of cardboard, or taped together newspapers, on a flat surface. Drive the car over it and park there. Remove and insulate the wire(s) going to the coil on the input side, then turn the ignition switch to the "run" position and leave it there for fifteen minutes. Then, turn off the ignition, reconnect the wire(s) to the coil and push the car off the ground covering. If there are slow leaks, you should see drips of fuel under the area(s) where they are located.
You might also want to check out the brakes to see if they are dragging, another problem that can result in low fuel economy. Jack the car up and put it on jack stands, make sure the parking brake is off, and rotate the wheels looking for one, or more, which do not rotate freely.
As to the CO setting, the factory specification, on the North American specification cars was 5% plus or minus 0.5%. CO (carbon monoxide) and HC (unburned hydrocarbons) have a fairly direct relationship with the HC dropping as the CO increases--at least within the normal range of operation. You might fill up the tank, take a drive to determine the fuel efficiency, then, top up the tank and make the same test with the CO set to 4.5% and see if there is any change in fuel efficiency.
|Ross, I'm not overly concerned with the occasion tic from the fuel pump, a leaky valve there could cause that and do no harm otherwise. The pump does have a vent that is normally run up into the boot area, perhaps you're getting some leakage there. It's a stretch, but it would explain the odor you sometimes detect.|
|If it were me, I would do what Denis suggested and check the mileage for several full tanks, not partial tanks, before doing a bunch of other things. You also don't note the type of driving you do - city versus highway versus some combination?|
Bill stated an interesting fact. The valves in the pump might be a reason for the fast clicking of the pump, but if there is a leaking diaphragm inside the pump, you should have a look upon the vent pipe of the pump housing. On late B's ( 1974 1/2 until end of production), the vent pipe was directed into the rear frame of the right hand side (in Ire drivers side).
If there is a leakage it would not be visuable from outside and will flood the rear right hand chassis leg.
Simply take off the vent pipe from the pump and follow the information given by Les will show you if there is something wrong inside the pump and it's diaphragm.
|Hmmm now there's a thing. I never knew there was a vent pipe on the pump. I must check that. I'll use all the other reccomendations as well to try to get to the bottom of this. I know this car can do better mileage. My driving is mainly combined. Id expect poor mileage in city driving but I havent been driving in the city at all recently.|
|75 miles on 3.3 gals of gas seems abit high if all your driving is around town. When I drive intown only I get about 20mpg, on Hyway only about 30. This seems OK to me especially considering my in town use is ussually about 5 miles each time. You might check a couple of things that can effect it. Is your alinement in good order? Add air the the tires 33-35lbs is what I run these days. Of course handling is not as good but those two thins can bring down rolling resistance. Bob|
This thread was discussed on 13/06/2008
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