Speed 25 oil pressure

On the Scandinavian Tour the oil pressure on my Speed 25 suddenly halved. Whereas it relieves at 40psi and normally runs around 35 hot, overnight it started running at 15 to 18 hot. First thoughts were that one of the gaskets on the pressure manifold had blown. Have now dropped the sump (nothing untoward in the oil) and removed the pressure manifold. All the gaskets were in good order. Am about to investigate the oil pump - but it occurred to me there may be a known cause of sudden pressure loss in this engine. Any advice welcome. Posted by rogergooding 22/08/2012 21:56:32

Hi Roger,  when suffering similar issues with my Jensen 541, it was suggested that I plug the feed to the rocker assembly (the head was off at the time) with my finger holding a small shaped cork or rubber bung and see whether this suggested an issue with the lower half or the upper half of the engine. It should be possible with the head still on, to remove the oil feeds to the rockers on the Sp25 and, without starting the engine (ie ignition and fuel disconnected), turn it over on the starter and watch the pressure gauge.   Also have you tried a different pressure gauge and feed pipe to rule that out?   Have you changed the oil recently?  it might be past its best - I guess this will happen as part of removing the sump!  all the best  Dave Posted by DaveT 23/08/2012 09:51:07

Thanks Dave. Oil is about 1500 miles old. The gauge is OK. As soon as the problem occurred the engine was run with the rocker cover off. Everything seemed normal. Have looked inside the pump now. Seems OK. I'll try pressure testing in sections next. Whether compressed air will show the problem up remains to be seen. I can imagine hisses coming from everywhere! Roger Posted by rogergooding 23/08/2012 19:07:55

Hi Roger, Is there an oil pressure relief valve on these engines? If so, is it possible that there's something stuck in it or a broken spring stopping it closing properly? Paul. Posted by pryantcc 23/08/2012 20:41:10

Hi Paul, Had a look at that straight away. The relief valve is very simple and on this occasion is not the culprit. Thanks for the suggestion. Roger Posted by rogergooding 23/08/2012 21:55:17

Hi Roger - I had this problem once and found that two of the valve rocker solder plugs had blown out - very easy to spot with the engine running and the valve cover off (wear an apron!). The solder plugs seal the lateral oil feed drillings in each rocker. Failing this, a bearing surface may have started to break up somewhere - big ends, mains or could be the timing chain tensioning device. Hope its not too expensive..... Posted by Nickalvis 24/08/2012 20:01:28

FYIO, on my SP 25, PSI is 47 cold, and drops to 21 at 2,800 RPM/62 MPH when really hot in South Florida weather.  Down to about 12 at idle. Been like this since I  bought the car in 2002.  Your engine is probably in much better nick than mine! Posted by johnlayzell 27/08/2012 17:09:31

Thanks everyone for your help. The oil pressure is back to normal - and Paul gets the prize. All I could find wrong was wear on one of the three guides on the relief valve. This had apparently been caused by the return spring tilting in the rather larger tube for many years. I turned the spring through 180 degrees, cleaned it up and stuck it back. Have since done 400 miles and the problem has gone. Posted by rogergooding 02/10/2012 20:21:48

[quote="rogergooding":3jmwy1h2]Thanks everyone for your help. The oil pressure is back to normal - and Paul gets the prize. All I could find wrong was wear on one of the three guides on the relief valve. This had apparently been caused by the return spring tilting in the rather larger tube for many years. I turned the spring through 180 degrees, cleaned it up and stuck it back. Have since done 400 miles and the problem has gone.[/quote:3jmwy1h2]   Great news Roger, I love fixes like that! I hope that you didn't go to too much trouble or expense before you came to this conclusion. You must be relieved not to have to undertake major engine surgery. Posted by pryantcc 03/10/2012 07:48:49