New rebuild, lost oil pressure

(Joe Frazier) #1

Finished a complete rebuild. Everything cleaned, heads machined, new valves and guided. Reassembly went as planned. Mated engine to transmission on a test bed. Hooked up starter to battery and a start button (no spare plugs). The goal is to get oil moving periodically while awaiting completion of body restoration.

I was pleased to see 55psi on a mechanical gauge on the first four weekly cranks. Compression is also solid. However, I lost pressure. I decided to replace the stock oil filter with a new spin-on filter adapter (SNG). Oil looked clean. Some fine particles in oil, but I would expect some on initial startup with new rings and bearings. Crank turns smooth using breaker bar. Still no oil pressure with new filter adapter.

Question, what is the best next step in my search for oil pressure?

(Ian) #2

Could fine particles be in the Mechanical gauge , check the gauge out first , fit one you know is good !

(Aristides Balanos) #3

Indeed, something is happening there… you can’t just loose oil pressure like that, unless a bearing popped or something… maybe try with the car’s oil pressure sender ? You could just measure variation in resistance, I think it will give you a clue.


(Joe Frazier) #4

I actually removed the pressure sender and left the oil feed pipe open while cranking. No oil getting to the rear. I can’t hook up the electric sender; gauge cluster not wired up. I trust the mechanical sender is functioning. I plan to work my way to the front of the engine. Maybe the first step is to unscrew the new filter to see if oil if moving to that point. I would be surprised if the pickup tube is clogged given the new rebuild and fresh break-in oil.

Please give me your thoughts as I work through this. I will keep you up to date as I am able to work on this.

(Joe Frazier) #5

Unscrewed the oil filter…dry as a bone! What is the most likely cause? Pump failure after supplying solid pressure? Oil delivery plumbing failure? Is there a bypass that may be open before getting to the filter?

(Joe Frazier) #7

Thanks for the heads-up Wiggles! Just what I looking for, incremental steps before getting too invasive. Just for future reference, can to timing chain cover be removed without pulling off the heads?

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #8

I’m not sure what this has to do with the lack of oil pressure.

I’m not sure what “drive dog” we’re talking about. In the Jaguar V12, the oil pump is driven by a key on the crank. That might have sheared, I suppose, but I don’t recall a case where it did.

This does seem an unusual case. Offhand, I’m wondering if there’s anything wrong at all. The oil pump intake is well above the level of the oil, and apparently it’s just being turned over on the starter. Perhaps it’s just lost suction. It might need to turn closer to idle speed to actually draw oil out of the sump.

I hasten to point out that if the oil filter was dry, it has never pumped oil. That filter fills up and holds oil. The earlier indications of pressure were either in error, or it was measuring air pressure.

(Paul Wigton) #9

OOPs! Didnt see this as a V12…never mind…:wink:

(Joe Frazier) #10

After I lost pressure I removed the filter and it was full of oil. I took that opportunity to change to a spin-on adapter. the pressure has never returned. that is why I check the filter again and found a dry filter. The oil pump was most definitely pumping. The starter is turning the engine a 50-60 RPMs, well below idle. It was picking up oil initially. Why would it quite? Keep the ideas coming. I will continue to report back as I evaluate the engines actual condition.

(Paul Wigton) #11


Go back to stock…bet THAT fixes it.,

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #12

Suction on this type of pump is very iffy at low rpm. It might have drawn oil to begin with because there was assembly lube on the pump that sealed it up and allowed it to suck oil up the pickup tube, and later on the lube was gone and it sucked too much air to draw oil any more.

If you’re not willing to actually start the car and see if it develops oil pressure, I’d suggest three other ideas. One is to overfill the sump so the oil level is higher. Another is to jack up the rear end of the car, tilting the oil toward the pump. Third would be to use a hand pump to fill the galleys and backflow into the pump, and see if it pumps for a short time after that.

(Ian) #13

When I fitted XJ6 engine in my MK2 , I measured out the new oil , to make a new line on the dip stick , then I cranked the engine for a few moment’s , no oil pressure was on the gauge , but soon as I startead the engine pressure was fine , so like said , before you start stripping , start the engine !!

(Guy Fontaine) #14

I went trough same problem on a V12 e-type.
Oil pump unprimed. My quick fix was to back prime by removing oil pressure sender and injecting with a large seringe motor oil until full
(overflowing). Patience required. Put the sender back in and voilà ! Oil pressure at starter speed ! Hope it work for you. P.S. the V12 use a centrifuge type.pump wich is very hard to prime.
Suction is very low.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #15

It’s not a centrifuge pump, it’s a gear pump. But it’s on the crank, which means it’s a ways above the level of the oil and therefore has issues priming. Back in the day, oil pumps were on the bottom end of the distributor drive shaft and hence right down in the oil for easy priming.

(Paul Wigton) #16

One of the miracles of a Rover P6 pump: they are at the same level as the block deck!

Never seemed like a great place for an oil pump, but only knew of one that failed.

MINE, in 2015…:disappointed:

(Nigelplug) #17

I’ve rebuilt V12s that won’t show oil pressure with plugs out at cranking speed.
Suggest you try to start it and see if that cures the problem?

(Paul Wigton) #18

Key paragraph, from OPer. Bolding mine.

(Joe Frazier) #19

Thanks for your continued discussion. Very helpful. Questions: How long can I safely run the engine without coolant? I will need to wire the fuel and ignition systems while on the test bed. Can this be done without messing with the wiring harness?

(Paul Wigton) #20

Not a worry to run it 10-20 seconds.

(Ian) #21

Could you not block the bottom hose outlet , and just fill the engine with water , better then nothing !!