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Oil Pressure

Oil Pressure

I recently completed rebuilding the engine from my early 1955 XK-140. This early engine (before the revision at block G1906) was fitted with an oil pump that is not interchangeable with the later and much more available 140 pumps. So I had my machinist fabricate an adaptor plate to enable the installation of a more modern and much more available pump for 4.2 L engines. Now I have great oil pressure! I have adjusted the pressure relief valve to limit max pressure at about 60 lbs. When the engine is quite warm and running at idle, the pressure will stay above 40 lbs. So, what's the problem? Occasionally, while slowing down for sharp turn or upon comming to a halt at a traffic stop, the oil pressure will drop to or very near ZERO. Of course I freak out and stare at the pressure gauge for what seems like a very long time, about 3 seconds, and then the pressure gauge moves rapidly back to its operating range above 40 lbs.

Two theories have thus far been proposed, both related to my new high capacity oil pump.

1. The oil pump is pushing so much oil into the upper engine that it sucks the sump dry and the oil pressure drops until enough oil finds its way down the oil return passages and into the sump.

2. The oil pump is pushing so much oil that the oil pressure relief valve is almost always held open. When the rpm's drop quickly, as at a stop or slow turn, the oil pressure relief valve may be sticking open for a second or two, causing the oil pressure to drop at idle until the valve becomes unstuck.

Theory # 1 seems less likely to me because the oil pressure stays high when the engine is running at speed and the oil pump is pumping its maximum output. Does anyone have any experience with this type of problem? Are their some tests or other symptoms I could look into to diagnose this problem. Are there any other theories in addition to #'s 1 & 2 above? I would appreciate any advice. Thank you. - Jim Flack

The oil pump is not "pushing" any more oil than its predecessor. You are getting air in the oil pick-up system. Check the "O" rings in the pump to pipe interface. Why did you need an adapter plate? Just a different size oil pipe would suffice. Sounds like you may have out engineered Jaguars SAE certified engineers. - George Badger

Perhaps when you come to a stop or slow down the little oil that is left in the pan moves forward of the intake for the oil pump and the pump ends up trying to push air resulting in your pressure drop. - Dionys Murphy '59 xk-150s OTS

Is the pickup for the new pump at the same level in the sump as the original? If it is higher, you could have your oil shifting forward in the sump when you decelerate leaving the pickup high and dry. - Bruce Cunningham, '53 OTS

To test the high pickup theory, you could add a quart or two to the sump and see if the supply stays on during deceleration. Conversly, take a quart or two out and see if it gets worse. Normally, you should be able to pick up oil when you are way below the low mark on the dip stick. - Bruce Cunningham, '53 OTS

Hi all -- the story I've heard is the 12 quart sump was to keep racing Jags out of the pits, because the engine would show pressure 9 QUARTS LOW! Hope this helps -- Larry Martz

Oil Pressure The problem is similar to what happened in a MKVII I had years ago. It was an oil surge. There were no baffles in the pan and a hard right turn or stand on the breaks hard would result in a sudden drop in oil pressure. - Bill XK140 OTS

I can attest to the fact that the XK with a 12 qt. sump will still pump oil with only 4 quarts left. I know all XK's leak oil but mine leaks more than most. I lose about 3 oz. per gallon of gas with average driving and slightly less on a long trip. It loses a lot out the slinger when it is shut down because the engine has about 170,000 miles on it. (There is an up side to this - with fresh oil flowing into the sump regularly, the time between changes can be extended significantly.)

I understand some early motorcycles had "total loss" oil systems where an oil resevoir provided oil by gravity feed and it just dripped out on the road. Ah - the good old days. No oil filter to worry about. BTW - what does "dry sump lubrication" mean anyway? It seems almost like a contradiction. I have always been a bit puzzled when I read that term but never looked into it.

Until recently, I was trusting my perceptions regarding whether I had checked the oil level rather than being systematic about it. I occasionally got about six quarts low (still on the dip stick) before I thought about checking it. This has caused a couple of amusing reactions from service station attendants, by the way. I never fill it above ten quarts as it seems to leak even faster when full. I try to keep it at the low mark.

A couple of months ago, I noticed a loss of pressure when I took a left uphill curve under acceleration. One might wonder why I would be looking at the oil gauge under such circumstances and all I can say is it wasn't a conscious thing - it's just that the gauge is in my peripheral vision and there is something in my brain that triggers alarm when there is a sudden movement in that area. Anyway, I was not carrying a quart of oil in the boot and my home was about five miles away. Since I had pressure with conservative driving, I continued toward my source of Castrol 20-50 in my garage rather than turn around and go the same distance to the nearest convenience store. Unfortunately, the last half mile is up a steep private road where going up at constant speed resulted in no pressure so I had to get a running start and actually decelerate on the steepest parts.

The net of this is I was down to three quarts when the pickup was out of the oil under acceleration or hard cornering. While four quarts is full for a lot of modern engines, it is marginal for the XK. Three will apparently provide oil running level at constant speed.

I now add a quart every time I get gas and only occasionally don't check the level when I am in a hurry. Maybe I should always check it before I start it. I could have avoided the burn scar on my right hand that way. I now keep a pair of painters gloves in the door pocket for that task. - Bruce Cunningham, '53 OTS

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