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At first sight you wouldn’t think that this would be a problem, but on firing up the Cup again after realising that the problem with the carburettor was actually the fancy BTH magneto’s timing slipping (should have stuck to the tried & tested BLIC), I noticed that the oil pressure was only 6psi, from starting, with cold oil. Tried changing the pressure gauge – no change. Let it warm up gently - still 6psi, and it seems to stick to 6psi regardless of engine speed or temperature. Do I have a really effective oil pressure relief valve, or can anyone think of a better explanation for this, before I start pulling things apart?
Previously the oil pressure behaved in a normal Austin 7 fashion, off the scale when cold and a few psi when warm.
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Perhaps a stuck oil vane or broken spring?
How long are you running it for when testing?
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Could be dirt under the pressure relief valve
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12-06-2021, 10:54 PM
(This post was last modified: 13-06-2021, 07:43 AM by Colin Morgan.)
Yes, that is strange behaviour. Trying to think a bit about what Ruairidh has suggested above - both possibilities. Presumably a broken spring would mean that the pressure seen might not go above 6psi on cold start-up but then it would be expected to come down below this once the oil is warmer, especially on tick-over? And a stuck oil vane might still be expected to give a variation with engine speed? As you know the pressure is generated as 'back pressure' from the resistance created by the oil being pushed through the jets - seems unlikely that this would be constant with changing speed/temperatures if oil is circulating and the gauge is showing the true back pressure? Given you have tried two gauges and they both read the same, this is a puzzle. Sorry - I don't know - hope you soon have it sorted out.
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Hi David,
Oil starvation due to floating debris at pump inlet?
Russell.
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13-06-2021, 06:05 PM
(This post was last modified: 23-06-2021, 01:24 PM by Chris KC.)
One possibility you might consider David is a 'foreign particle' obstructing the feed to the oil pressure gauge.
n.b. with hindsight, far more likely to cause constant zero pressure, but...
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Hi - Did you manage to resolve this unusual situation, please?
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My guess is dirt under the ball in the relief gallery is a likely culprit.
Anyway it is an engine out job.
It is then easy to check the various suggestions.
Hope that is helpful.
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23-06-2021, 10:20 PM
(This post was last modified: 23-06-2021, 10:20 PM by David Cochrane.)
Many thanks for all the helpful suggestions, and apologies for the lack of response (mainly caused by taking the RP on holiday to Norfolk last week). The next things to be done are checking the oil jets, blowing back through the oil pressure pipe, dropping the sump and checking the oil pump vanes & springs, then trying to get at the oil pressure relief ball & spring without completely removing the engine (not sure if this will be possible until I try it). The engine has a full-flow oil filter and a magnetic sump drain plug, so I think it's unlikely that there's any debris floating about. I didn't time how long it was running when testing it, but probably about 5 minutes.
