[Brendan]

The mechanical fuel pump on my 240Z is leaking oil from a small vent hole in the side of the pump body near where the pivot pin is. The pivot pin was sealed with silicone by the previous owner (a common remedy), but the vent hole is leaking oil into the prop-wash of the cooling fan and spraying the plug-side of the engine bay. I guess the vent hole was also sealed with silicone because it was fine for the first week of ownership, but now leaks with each drive.

I had a few gear shifts when the throttle jammed wide open :-O until I worked out that it was the aftermarket carpet fouling the pedal and not the throttle linkages. Maybe the 7500RPM blew/sucked the opening clean again?

My previous car, an ugly Datsun 120Y (B210 in US?) with an L16 SSS motor exhibited the same leakage/oil spray (the 7000RPM episodes were always intentional in that car so I guess its' a common problem in the L-series mechanical pumps.

Anyway, the vent hole is obviously intentional - but brand new Datsuns didn't spray oil all over the bay, so I guess the Positive Crankcase Ventilation setup is supposed to keep that hole sucking air not blowing oil?

Any ideas on how to stop the leak?

 

[Brendan]

> I had the same problem with my fule pump.
> Took me a while to locate that leak. I just
> bought a new pump for around $45 and fixed
> the problem. That was my fix.

After Brandon's reply (thanks) I checked out the whole positive crankcase ventilation system. There wasn't too much info in my Hayne's Manual about PCV, but I have a Datsun 180B (610) manual that better describes a similar system.

Without going into it, as long as the rubber seals are good on the Oil Filler Cap and the Dipstick, the crankcase will be under vacuum from either a) the hose going from the rocker cover to the air cleaner box or b) the larger hose going from the crankcase to the balance tube (post-carby inlet manifold). The normal flow actually reverses at full throttle. Anyway, there shouldn't be positive pressure in the crankcase even with a failed PCV valve, although I expect there is a lot of air movement between the cylinders underneath the moving pistons. I have an even 150 compression across all cylinders, so I shouldn't be getting sufficient blow-by to upset the system.

A closer look at the fuel pump exploded-diagram shows an internal oil seal around the diaphragm rod which sounds like the culprit. It looks like there are three regions in the fuel pump:

- the fuel chamber (fuel/wet)
- an airspace on the other side of the flexible diaphragm (air/dry)
- an opening into the crankcase that gets sprayed with oil to lubricate the rocker pivot (oil/moist

I suspect the air hole opens into the dry airspace to allow the diaphragm to move, which makes more sense than opening a hole into the crankcase where I first thought it went. I guess you shouldn't seal this hole with silicone!

Anyway, I'll verify all this next weekend when I dismantle it. The fuel pumps in Australia are about $AUD150 from Nissan (about $100 US) so I'll overhaul it if I can - or buy an electric pump instead.