XPAG Oil pressure fluctuation

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IanF
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XPAG Oil pressure fluctuation

Post by IanF » Tue Jul 16, 2019 4:39 am

Having just got back my TC after a major engine rebuild - everything new! - I am a little worried by the apparent oil pressure fluctuations. The engine has been away for over two years so it's been a while.

All dash gauges have been recently renovated as well and as such, temperature-wise, the engine runs at 75 - 80 degrees but the oil pressure gauge (also renovated) shows a wide variation in oil pressure when driving.
Cold the oil pressure is 65 and for the first 20 miles or so and holds this pressure slowly decreasing to about 50 psi. However after half an hour or so the pressure drops away to about 40 psi at 40 mph and 30 at 30 mph. Scarily it goes down to 10 to 15 on tickover whilst idling! The pressure does pick up on revs but does fluctuate significantly with a hot engine. I'm using top quality 20/50 engine oil and additionally gently trying to run-in/bed down the engine not exceeding 2500 revs. No harsh acceleration etc. etc.

So need to ask the question of all - is this fluctuation normal or do I have problem !

Discussions so far have centred on the oil pump needing a refurb, but this was looked at when the engine was apart, or is it the bottom end shells?

I would certainly welcome any guidance/comments as I do not want another expensive engine rebuild !

Many thanks!

Tom Lange, MGT Repair
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Re: XPAG Oil pressure fluctuation

Post by Tom Lange, MGT Repair » Tue Jul 16, 2019 7:03 am

The way your OP is fluctuating speaks to worn components. As the oil heats up and gets thinner, and is leaking past worn areas.

What was done to the engine - how deep was the rebuild? Turned crank? New bearings? What about the rocker gear - new shaft and bushings?

10 lbs pressure per thousand revolutions is my low end of the OP scale.

Tom Lange
MGT Repair

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Steve Simmons
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Re: XPAG Oil pressure fluctuation

Post by Steve Simmons » Tue Jul 16, 2019 8:21 am

I wouldn't expect a lot of pressure at under 2500 RPM, but that is a bit of a swing. I would be tempted to remove and inspect the bypass valve to ensure oil isn't leaking past the ball. Hopefully the oil pump was rebuilt by someone who knows what they are doing. The body and cap should be surface ground and clearances set precisely. There is no gasket or sealer under the cap.
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IanF
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Re: XPAG Oil pressure fluctuation

Post by IanF » Tue Jul 16, 2019 10:48 am

Hi Tom

The engine has been fully rebuilt from top to bottom. The car has a long racing career and whilst the engine has been rebored at some stage we have tried to return it to a more stock unit to try and build in some reliability for touring etc. It should make for a fast tourer I guess!

Starting at the top it has a new rocker shaft, new valves, valve springs, pushrods, followers ,new camshaft, pistons conrods, shells, chain etc etc.

The engine runs quite sweetly and is using no oil or water at all. Perhaps, naively, because it has been rebored it affects the oil pressure readings ? I don't know!

The by-pass ball has been changed together with the spring and as said there is decent oil pressure when cold but when warm.....

Ian.

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Re: XPAG Oil pressure fluctuation

Post by frenchblatter » Tue Jul 16, 2019 11:51 am

Ian, assuming the gauge is OK there is something wrong. I would say it is with the pump as it'd difficult to get the bearing shells wrong. Does the engine builder know XPAG engines? I rebuilt mine and get 60 psi cold and 50 psi hot at 2000 RPM.
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stephen stierman
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Re: XPAG Oil pressure fluctuation

Post by stephen stierman » Tue Jul 16, 2019 12:52 pm

Can we assume they put new cam bearings in the motor? I am reading 60lbs hot at road speed and 40 at idle and this after about 17,500 miles of driving on this engine.

Tom Lange, MGT Repair
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Re: XPAG Oil pressure fluctuation

Post by Tom Lange, MGT Repair » Wed Jul 17, 2019 3:58 am

If you are not burning oil, you must be leaking it past worn or incorrectly-fitted components. I would remove and have the oil pump checked over by someone else. You also don't mention replacing and fitting new rocker bushes, which, when worn, can certainly contribute to low oil pressure.

Tom Lange
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Re: XPAG Oil pressure fluctuation

Post by frenchblatter » Wed Jul 17, 2019 7:39 am

Tom, I was going to say that if it had a new rocker shaft it would have had new rocker bushes... but then I thought about a friends YT that had a full engine rebuild and the old timing chain, sprockets and tensioner were refitted. You could hear it 1/2 mile away. Strange what some folk do.
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IanF
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Re: XPAG Oil pressure fluctuation

Post by IanF » Wed Jul 17, 2019 10:26 am

Firstly thanks to all. Initially the oil pump and the bottom end shells were the main topics of conversation before posting this thread but now certainly going to have a look at those bits further up the engine. Cam bearings and rocker bushes now on the list to look at.

The car goes away again in two weeks for the engine rebuild and from this discussion thread we should hopefully get to the bottom of the problem.

When the problem has been revealed/solved I'll post the findings - if only so that the findings can add to the knowledge.

Again thanks to all.

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