The Stovebolt Site Forums The Shop Area Engine Shop '49 1/2-Ton Oil Pump

Has anybody experienced an oil pump that is slow to prime and/or develop initial oil pressure? This issue seems to occur only at initial start-up after the truck has been sitting idle. The pump loses its prime. Once the pump is primed and operating, I get plenty of pressure and constant pressure.

For example, yesterday I started the engine and it ran for several seconds without oil pressure. I promptly shut the engine down because I don't what to damage it. I removed the distributor and used an electric drill to turn the pump and quickly got pressure and plenty of oil from the rockers. I then replaced the distributor and re-started the engine and instantly saw oil pressure. The pressure stayed high for 20 minutes while I ran the engine. I stopped and re-started the engine and again saw pressure instantly. However, the following morning, same issue. No pressure at initial start-up and I'm afraid to let the engine idle for any length of time to see if it will prime itself.

So my question is, how quickly should I be seeing pressure at a cold start-up? Is this normal, or should I replace the pump? What keeps the pump primed? Is there a foot valve or seal that is leaking? Thanks!

Hy norcal49er, there is no foot valve on an automotive engine oil pump. Under normal circumstances the pump pickup is submerged in oil, even part way up the tube, if the pump is not primed on morning startups, the most likely cause is excessive clearances in the pump itself, or the obvious, low oil level in the oil pan.

just a thought, could the line to the gauge leaking cause this?

When you turned the oil pump with the drill did the drill maintain the same speed and counter-rotating pressure or did you experience a considerable drag down on the drill as it started pumping? If there was no change it is likely the pump was fully primed. If the pump was not fully primed you should feel a significant difference in the force on the drill once it starts to pump oil.

If the pump appears to be primed I would temporarily attach another oil pressure gauge as close to the block as possible and see what that says. If you want to try that and do not have a gauge let me know and I will try to find one you can use.

Chuck

First, I need to correct my earlier post. I have a '49 truck, but the engine is a '59 full pressure 235.

I believe my oil pressure gauge is working properly. To check it, I removed the sender connection at the block and blew compressed air into the fitting. The gauge is accurately measuring the air pressure in the air tank.

I don't know if my electric drill spins the pump faster or slower than the starter motor on the truck. Probably faster. When I use the drill, initially it spins the oil pump quickly, but I can definitely feel it slow down and load when the pump primes and the oil begins to flow.

Here is a follow-up question... With the key turned off, should I be seeing oil pressure on the gauge by turning the engine over with the starter - or does the oil pressure only show up after the truck is running?

If none of you is experiencing delayed priming/pumping at a cold start, I'm guessing my oil pump is just worn out. The pan and pump will come out without removing the engine from the truck, right?

Are you running an oil filter? If so does the line for it come off the same spot as the oil pressure line? Maybe the oil filter needs to be filled each time after sitting a while before the oil pressure will show on the gauge. With the drill spinning the oil pump if it just takes a few spins to get resistance It is probably not the pump. If it takes 5-10 seconds then it does sound like you are loosing prime. If this were my truck I would sure rather confirm the situation with another oil pressure gauge before removing the pan and pump. Just blowing air into the gauge doesn't tell you a lot.

Chuck

I agree that if you have an oil filter, you should check it out. Some canisters have a very small restriction on the outlet and some others have it in a fitting. If you have a bypass filter you could plug the pressure feed and try it that way. If it makes a difference then you have found the culprit.

Hookalatch - There's no oil filter on my truck, but, nevertheless, you have a good point. Even without a filter, it still takes time to fill the oil galleries after they drain back to the pan. I suppose this could explain the slow pressure after the truck has been sitting idle. It's not a priming issue, but rather it just takes time to fill the oil passages before I see pressure at the gauge.

The oil passages are very clean. The engine was completely re-built except for the oil pump (my mistake!). I cleaned and inspected the pump before re-installing it in the engine. The pump basically looked so simple and bullet-proof I didn't suspect any problems.

I certainly don't want to replace the pump unless I know it's defective, but I'm also terrified of running the engine for more than 5 to 10 seconds without oil pressure. Maybe a few others will respond and let me know how long it takes to develop pressure in their engines.

Hy norcal49er, I am a retired mechanic, just so you know where my comments are coming from. To rebuild an engine, and then install an old oil pump is just plain foolish in my estimation, the oil pump is the heart of the engine. I have installed a lot of new pumps in old engines just to keep those engines running. Most of the engines I have rebuilt will show the oil pressure coming up as the engine is being cranked, that is if it doesn't start right away. I would replace the pump!