Problem: The oil pressure light stays on (ran it for about 15 seconds). Just replaced 110 engine (Engine# 110994-20-000514 transmission # 720109-20-004743.

History: I was driving on the motorway when the oil pressure light came on. I had just picked the car up from the independent shop which is reputed to be the best German repair shop in Auckland where they were supposed to have fixed a delayed starter problem. Having spent over $1,000 for all sorts of new parts, it still did not work (it turned out that the problem was caused by the rear oil seal leaking, eventually filling the starter solenoid with oil and required the oil cook off before it would engage the starter… a simple $200, one hour repair once I removed the starter and noticed the mess), thus I was driving home, most frustrated.

I do not know how long the light stayed on, but long enough to starve the main bearings which eventually self-destructed. When the light came on, I had it taken back to the same shop. This time they decided the oil filter was blocked, thus charging me $120 for an oil change, pronouncing the problem solved. The oil pressure light did not come on again, but the thunk and then howl from the bearings when I got back on the motorway suggested I had a far more serious problem. I limped home, eventually down to below 20 mph to keep it from seizing, and like a faithful but wounded steed, it made it, although the last mile was a symphony of steel on steel turning the crank into an elliptical orbit which would make Haley's Comet proud.

Having lost faith in the repair shop, and because the engine had over 300,000 miles on it, I decided it would be cheaper to re-power than to rebuild the existing one. I located an identical engine, and replaced the dead one. I decided to do the replacement myself over the Christmas holidays.

Re-powering: I did not remove the oil filter housing (the aluminium unit attached to the block which holds the filter canister, and to which the oil pressure sending unit and the two oil cooler hoses are attached) from the replacement engine, which proved to be a mistake. When I started it in the vehicle, the housing gasket on the block leaked (but I should mention, the red oil pressure light did go off). So I took the housing off the dead engine, cleaned it up and took it to MB to make sure I would get the right gasket. Because it was cleaner, I installed this older housing unit, not the younger one that came with the replacement engine. I installed a new copper gasket in the oil pressure sending unit, and did clean up the sending unit before doing so (this could be a clue). I also installed an MB original oil filter.

When I put everything back together and started the replacement engine, the oil light stayed on. After about 15 seconds I shut the engine down, as I do not want a replay of the last self-destruct. I checked the oil level, added oil since it now had filled the filter and oil cooler. Repeated the process for 10 seconds, and the light stayed on, so I then quit and turned to the Internet for help. I'll spend the rest of the day cleaning the workshop and cutting out the rust holes so it can pass WOF (warrant of fitness), waiting for advice from one of you.

I see the following possibilities:

1. In cleaning the oil pressure sending unit (including inserting a pin in the unit sensor hole to make sure no dirt was clogging it), I either damaged it, or created an air pocket so it thinks there is no pressure when there really is.

2. Bad luck, the sending unit decided to pack it in and I really have oil pressure but no way to tell.

3. The housing from the old engine actually has something wrong with it that intermittently causes it to block the oil flow, and I have no oil pressure and could easily trash the engine.

4. Coincidentally, the oil pump died (which makes little sense under the circumstances).

Access to the housing is difficult, especially without a car hoist. Access to the sending unit requires I either unbolt the housing from below (hard because of the cooler hoses), or remove the fuel injection manifold (which is what I had to do to replace the gasket… a long job).

One option might be to cut into the rubber hose which runs from the housing to the radiator oil cooler and install a brass Tee off of which I could hook an oil pressure gauge to get an actual pressure reading. But I do not know how accurate this would be, given the location. Access is easy, and the return (bottom) rubber hose is new.

Any thoughts and any ideas? The engine is a standard 110 engine with an automatic transmission, but I hesitate to mention the vehicle MB installed it in, because few on this forum know much about Mercedes' army jeep, the G-wagon. I have posted an enquiry on the G-forum, but the replies to date are from owners rather than a technical person who might know how the oil pressure system actually works and what goes wrong.

Happy New Year to all.

Toka