We were contacted by a client with an elusive problem:
MY 1998 AUDI A6 ESTATE V6 2.5 TDI AUTO RUNS FINE FOR SEVERAL HOURS THEN GOING UP A HILL OR AFTER A STOP IT SUDDENLY GOES INTO LOW POWER LIMP HOME MODE (PEDAL TO THE METAL 5000RPM TO REACH ONLY 30MPH). THIS IS QUITE DANGEROUS PULLING INTO TRAFFIC WITH NO POWER. I HAVE CHECKED AND CLEANED THE TURBO AND EGR AND BOTH SEEM FINE AND I CANNOT FIND ANY AIR VACUUM LINE LEAKS TO THE TURBO. 246,000 MILES. 4TH CAM BELT. NORMALLY IT RUNS AT 80MPH AT 38MPG. JUST SOMETIMES IT GOES SCARY LOW POWER MODE ..... ANY IDEAS?
This engine was very advanced when it first appeared on the scene. It was used by Audi and also VW in the Passat Estate. 140BHP and yet 38+mpg. Lots of pulling power for caravan or trailer. Powerful yet economical. One of the innovative features was a sophisticated engine management system. The amount of air passing through the engine was established by measuring the difference between two wires lodged in the airflow just after the air filter. One wire carried current at ambient temperature and one wire was heated so that the resistance of the wire as the air flow increased could be measured as the wire cooled, and hence the ecu could estimate the air flow passing into the turbo. It is called a mass air flow sensor otherwise known as a MAF and although the air filter removes leaves, sand and dust, microscopic pollen does manage to worm through the air filter. These tiny particles of plant matter promptly stick to the hot wire and over about 50,000 miles the MAF hot wire gets clogged up with the pollen which insulates it and then tells the ecu to overfuel. This then tells the ecu that there is a a problem and the ecu shuts you down just as you pull out of a junction in front of a big truck coming at you at 60mph. Scary indeed.
Fortunately, a new MAF for a VW Audi vehicle is not expensive. The dealer will exchange and install a unit for about £120 or if you can manage a crosshead screwdriver, put the vehicle details into ebay and a replacement (I think this audi A6 2.5 tdi 1998 uses a genuine bosch PBT-GF30) is under GBP£30. Instant full power.
It is located just after the air filter and looks like a black drain pipe with some wires coming out of it. The insides can be spray cleaned with electrical spray but the problem comes back so i would recommend a replacement every 3 yrs paid for in fuel savings and of course safety is a concern going at 20 mph up a hill.
You can change the entire unit yourself with a crosshead and a flat screwdriver. The sensor itself can be removed but it is very fragile and you would need a special torx screwdriver so it is easier to replace the entire MAF unit and also replace the air filter at the same time.
Volvo also use the MAF concept but as far as I can work out their MAF is self cleaning and that is why they cost £700+ but never seem to go wrong. Hope this helps and have a great day.