Mikhail (mozolin@phoenix.cs.uga.edu) writes:
I wonder if somebody has or had an American built Diesel Rabbit 82 ( with square headlights ). If you do could you tell me where the fuel expansion tank is supposed to be installed? I could not find it anywhere on my car, so I suspect it is missing, which makes my fuel consumption a way too high -- 35 MPG. I did all the checks suggested by Bentley, including injection pump replacement. Nothing seems to affect MPG.On the 1985 and up diesels, the expansion tank is an integral part of the fuel tank, and is located in the wheel well as part of the filler neck. In this case, however, I do not think this is the source of poor fuel economy. The injection pump has an integrated positive displacement low pressure vane pump, which supplies an excess of fuel to the high pressure pump gallery. The excess fuel is simply returned to the fuel tank. Therefore, if the engine runs properly, then the low pressure pump is drawing fuel adequately and is returning fuel properly. The expansion tank simply allows for the contraction and expansion of the fuel as temperature changes, which where I am can be as much as 40 deg. C. in a single day.
I assume that the injection pump has been calibrated recently, and that it is properly timed, as is the cam shaft, and that all the injectors have been tested and are properly set. If the compression is good and there are no blockages in the intake system (I have seen pictures of Diesel Rabbits with acorns in the air cleaner box stashed away by squirrels!), then I would look elsewhere for the cause. An obvious cause of poor fuel economy is loss of transmission of power. The clutch may be worn just enough to drop your fuel economy, but not significantly affect drivability. The clutch may also be soaked with oil leaking from the main bearing seal on the engine. The rolling resistance may have increased if it is a high-milage car (I would guess that the 1982 Rabbit is close to 200k miles) from worn cv joints, front bearings, poor alignment, improper or worn tires, or brake callipers trailing. All of these can add up to a significant amount. The exhaust system, if not a true VW system, may have become plugged with soot, since an aftermarket generic muffler was probably not designed for diesel exhaust.
I hope this sheds some light on the problem. I have a 1990 Jetta Turbo-Diesel which I have managed to accumulate 253,000km in the past five years, and a Mercedes diesel with 400,000km which I restored (except the engine which is fine!). I also maintain two other 1988 Jettas in the family, one a diesel with 289,000km and the other with 98,000km. We typically get anywhere from 45mpg in the city to 72mgp on the highway in the naturally aspirated diesel Jetta (289k). (BTW the gallons are imperial, 4.54 litres). We have found that it is necessary to recalibrate the fuel pumps at each timing belt change (about 150,000km) and that the injector nozzles are usually worn out. At that time, the glow plugs are also replaced. The most time consuming aspect is retiming the pump to the engine. With the proper tools and lots of practice, it still takes me 45 minutes just to set the injection pump plunger to within 0.005mm of the nominal specified value. This is absolutely crucial for fuel economy and cold weather performance. Our cars stay outside and must start, without assistance (block heater, etc) at -30 deg C.
Best of luck!
-- Dent Harrison, P.Eng. Chelsea, Quebec, Canada cn992@freenet.carleton.ca