Poor Mileage - Air bubbles and Internal Pressure

[b1rdjx88]

The last tank in my 1994 golf aaz only lasted 650km, the lowest I have observed for this car.  My best ever tank was 900km.  I understand some people are getting over 1000km with these cars, and I'm hoping with a few adjustments mine will achieve the same.  I will be sure to look into issues of alignment, tire pressure, driving habits, etc, but for now I would like to make sure the fuel system is optimized for economy.  

    My injectors were recently rebuilt, and timing is set at 1.00.  I measured the internal pressure on my pump today by tapping a pressure guage into the return line banjo fitting bolt on the injection pump. :arrow:    
               





The pressure at 1000rpm (engine speed) was about 40psi, and 70psi at 2000rpm.  Should I adjust this?  I see that for the 1.6 engine the pressures should be 43.5 and 75.4 psi at 1000 and 2000rpm, but is this the same for the aaz?  Are there any other adjustments I could make?  


I have also noticed a lot of bubbles in my inlet fuel line, which I have determined are originating at the white plastic return fitting on the fuel filter.  What exactly does this fitting do?  Could it be replaced by a tee into the inlet line upstream of the filter with a check valve to prevent air from being drawn in from the tank return line?

Thanks

[Vincent Waldon]

Can't accurately comment on your pressures... haven't measured mine yet...  (although they look close enough to not be causing you any major problems.. yes a bit low which will impact dynamic advance a bit), but I do know that cold temperatures play hell with mpg in a diesel... as I recall there's a bunch of reasons:  winter diesel has less BTUs, cold engine gets less power since compression has more heating to do, much more friction in the entire car which our diesels are particularly sensitive to thanks to less than stellar hp, etc.

A couple of my friends with a 1998 TDI tracked their mileage religiously... and all things being equal they've seen up to 200 km difference per tank, with the same daily 1 hour commute from the farm... winter to summer.  This pattern has repeated itself for the last couple of years so I'm thinking it's probably real.

Not saying nothing's going on... but if your last tank was in the bitter Canadian cold, and your best tank was during our wonderful 3 weeks of summer... the temperature might be a factor to consider.

Having said that... air in the intake line is never good... usual suspects are the various fittings and the o-ring on the fuel filter.

[jtanguay]

temperature does have quite a bit of impact.  so much that the TDI's measure fuel temp, and some even have fuel coolers/warmers.  don't forget the added drag resistance of the denser air  :wink:

the internal pressure of the pump has a lot to do with the dynamic timing advance system.  maybe someone can chime in on the proper spec pressures?

[Quantum TD]

I think that your timing is too far advanced. The 1.0mm setting is ideal for the old 1.6, but for the 1.9 AAZ, Bentley recommends a mere .80 +/- .02. If you're too far advanced, you'll eat more gas. How long has the pump timing been set to 1.0mm?

[jtanguay]

i think they recommend the timing reduction for reduced emissions