I recently replaced the oil level sender in one of my cars, due to a leaking O-ring. Only the O-ring needed replacement, but I figured since the sending unit was 30 years old, it was worth the $50 for a new one (which includes new O-rings). After draining the oil and removing the oil pan, I went out of town for a few days, and completed the job when I returned. Total time with the oil pan removed was about 1 week.
Filled up with new oil, started the engine, and... no oil pressure. Uh-oh. Triple checked everything, tried again, still nothing.
Well, that's not good. I've done this job multiple times on other M119's and never had this issue. I thought in the past I had left the pan off for days, a week, or longer... but can't say for sure. At any rate, the oil pump lost its prime and wasn't sucking anything out of the pan. The FSM says to fill a new pump with oil prior to installing, but I was hoping to avoid dropping the pan again, and wasn't crazy about removing the pump either.
At first I thought maybe this could be rectified by filling the filter housing, but NOPE. The check valve in the housing prevents oil from flowing back to the pump. If you fill the filter housing, this only speeds up pressurized oil getting to the oil galleys in the engine. This will not fix a situation where the pump itself has lost prime and is not sucking oil out of the sump.
Anyway - @jhodg5ck to the rescue!
He suggested connecting a shop vacuum in reverse, so it would blow air into the crankcase, to pressurize it. You also must disconnect and plug the PCV hoses from both valve covers. Not all shop vacs have an "exhaust" connection where the hose will plug into. I have 2 shop vacs, one had the outlet, the other did not. The vac sucks dirt in but the exhaust is filtered air, assuming the vac filter is present and intact. Wouldn't hurt to check/clean the filter before proceeding. I also wrapped a clean rag around the end of the hose where it stuffed into the oil filler cap, to catch any debris from blowing into the engine, and help seal the hose to the valve cover.
Fired up the shop vac and cranked the engine (with CKP disconnected, so it wouldn't fire)... still no oil pressure. I didn't time how long I cranked but it was enough that I was concerned about the battery draining. I'd guess 15-20 seconds. Not looking promising. I wondered if the cranking RPM was too slow. As a last resort, I decided to start the engine and let it idle with the vac pressurizing the crankcase. I figured 10 seconds was a reasonable time, if oil pressure didn't build within 10 seconds, I'd shut it off. Turned on the vac, started the engine, and about 5-6 seconds later - OIL PRESSURE!
Crisis averted.
Note that the oil level on the dipstick was approximately between the midpoint and MAX marks. Another possibility would be adding oil to the MAX level, or slightly above MAX... this may help get the oil level closer to the pump mechanism so there's less air to move. I couldn't try this because I did not have any "extra" Red Line oil of the same viscosity handy, and I didn't want to mix oils. Remember once the oil filter housing fills up, the level in the sump will drop ~0.5 quart. The overfill would be temporary and should result in a level at or below MAX once the pump primes and fills the filter.
Top Gear Top Tip: If you have the oil pan empty (or removed) overnight or longer, and oil pressure does not build within about 5 seconds after filling/starting the engine... immediately skip to the methods above to reprime the pump. Don't keep cranking or idling the engine, the pump will not prime itself. See note below from FSM job 18-2100.
Thanks again to Jono for the assistance!
Filled up with new oil, started the engine, and... no oil pressure. Uh-oh. Triple checked everything, tried again, still nothing.
Well, that's not good. I've done this job multiple times on other M119's and never had this issue. I thought in the past I had left the pan off for days, a week, or longer... but can't say for sure. At any rate, the oil pump lost its prime and wasn't sucking anything out of the pan. The FSM says to fill a new pump with oil prior to installing, but I was hoping to avoid dropping the pan again, and wasn't crazy about removing the pump either.At first I thought maybe this could be rectified by filling the filter housing, but NOPE. The check valve in the housing prevents oil from flowing back to the pump. If you fill the filter housing, this only speeds up pressurized oil getting to the oil galleys in the engine. This will not fix a situation where the pump itself has lost prime and is not sucking oil out of the sump.
Anyway - @jhodg5ck to the rescue!
He suggested connecting a shop vacuum in reverse, so it would blow air into the crankcase, to pressurize it. You also must disconnect and plug the PCV hoses from both valve covers. Not all shop vacs have an "exhaust" connection where the hose will plug into. I have 2 shop vacs, one had the outlet, the other did not. The vac sucks dirt in but the exhaust is filtered air, assuming the vac filter is present and intact. Wouldn't hurt to check/clean the filter before proceeding. I also wrapped a clean rag around the end of the hose where it stuffed into the oil filler cap, to catch any debris from blowing into the engine, and help seal the hose to the valve cover.Fired up the shop vac and cranked the engine (with CKP disconnected, so it wouldn't fire)... still no oil pressure. I didn't time how long I cranked but it was enough that I was concerned about the battery draining. I'd guess 15-20 seconds. Not looking promising. I wondered if the cranking RPM was too slow. As a last resort, I decided to start the engine and let it idle with the vac pressurizing the crankcase. I figured 10 seconds was a reasonable time, if oil pressure didn't build within 10 seconds, I'd shut it off. Turned on the vac, started the engine, and about 5-6 seconds later - OIL PRESSURE!
Crisis averted.Note that the oil level on the dipstick was approximately between the midpoint and MAX marks. Another possibility would be adding oil to the MAX level, or slightly above MAX... this may help get the oil level closer to the pump mechanism so there's less air to move. I couldn't try this because I did not have any "extra" Red Line oil of the same viscosity handy, and I didn't want to mix oils. Remember once the oil filter housing fills up, the level in the sump will drop ~0.5 quart. The overfill would be temporary and should result in a level at or below MAX once the pump primes and fills the filter.
Top Gear Top Tip: If you have the oil pan empty (or removed) overnight or longer, and oil pressure does not build within about 5 seconds after filling/starting the engine... immediately skip to the methods above to reprime the pump. Don't keep cranking or idling the engine, the pump will not prime itself. See note below from FSM job 18-2100.
Thanks again to Jono for the assistance!
