Fuel Problems on Mazda B2500
 

Hi Guys I have a fuel problem.

I drive a Mazda B2500 dual-cab pickup.

It has a Mazda WL-31 naturally aspirated diesel, coupled to a 5 speed manual transmission.

The engine is unusual in that it is a 4 cylinder, overhead cam design with 12 valves - that is 3 valves per cylinder - 2 intake and 1 exhaust.

Bore and stroke are slightly oversquare and the compression ratio is 21.5:1.

Engine displacement is 2.5 liters. Maximum RPM is surprisingly high with a redline at 5000 RPM.

Another strange feature is that the camshaft and injection pump rotate at half speed in opposite direction to the crankshaft - they are driven by a set of 3 gears off the crank and are coupled together via a toothed rubber belt.

The injection pump is a licensed built version of the ubiquitous Bosch VE series of distributor pumps.

Fuel is fed from the tank immediately into a water separator unit (that has a drain) and then to the main fuel filter before the injection pump.

The original fuel filter had no hand pump and could not be bled of air whenever you replaced the fuel filter (every 10 000 kilometers).

Whenever you replaced the fuel filter or did any work around the injection pump and fuel lines you had to crank the motor over continuously until enough fuel was drawn through the fuel system to start.

That drained the battery considerably and took a long time to boot.

Still I felt it retained a considerable amount of air in the fuel lines and filter.

So the first thing I did was to replace the stock fuel filter assembly with one that had a hand pump and air bleed screw so that whenever I touched the fuel system I could pump it full of fuel and bleed of the air.

Another thing I noticed was that the small thimble-sized fuel strainer inside the injection pump would fill with dirt and then the engine would lose power and sometimes even shut off.

I was able to download and read the Service Manual for the Bosch VE series injection pumps.

It said that the pump drew in fuel via a vane-type rotary pump built into the back of the pump body near the drive.

Fuel was drawn in and fills the interior of the pump body.

Fuel Pressure rises with rising engine rpm, controlling the rate of injection (and hence ignition) advance.

Pressure inside the pump is regulated by a pressure valve and excess pressure is bled off to the return line to the fuel tank - same one used by the injectors.

During my last filter change and strainer clean out - I experimented by pouring clean diesel into the fuel line to fill the pump body before attaching the fuel lines.

I noticed that I couldn't fill the thing up - it would slowly drain out.

But if I understand the Bosch VE manual correctly this should NOT happen.

It should fill up and be able to maintain the fuel at that level - how else would it be able to maintain the internal pressure via the pressure regulating valve?

What is your opinion on this?

Thanks a bunch in advance.