Questionhi howard,
thanks for your quick reply last time, i wasn't sure how to link the questions, but you advised me to get the compression checked on my 75 mgb because it was back firing, anyway, i dropped it down to my local mechanic and asked him to check it out, as well as change oil and filter for me. when he was taking off the filter, the treads came with it, so we ordered a housing and had that fitted. i went to drive it home, and noticed the oil pressure was very high, i returned it to the mechanic and he said that he had not put the coreect oil filter on it and maybe that was the cause, the correct one is now on order. does this sound ok to you?? as you can imagine i wasn't very happy! thank you once again for your help.
dermot
AnswerHi Dermot,
When you change oil and filter it is normal for the oil pressure to be higher until the oil heats up which takes a little longer than the coolant in the engine. However, the pressure should not be too much higher for example, If you normally had 50 PSI and fresh oil may show you 60 PSI for a while. If you normally had 45 to 50 PSI and after a filter change of the wrong filter and you get 70 to 80 PSI, you should not drive it that way because that would not be normal.
The filter on an MG is a "Full Flow Filter" system meaning all the oil that goes to the bearings has to pass through the filter. This design would normally be dangerous to the engine if the filter were not changed and became clogged. So a filter design that has a pressure relief valve is used so just in case the filter gets clogged due to lack of changing, the relief valve would open and allow unfiltered oil to get to the bearings. Unfiltered oil is better than no oil.
BE SURE to tell the mechanic that the filter MUST have a bypass valve in it
The MGB is known for oil pressures to be between 50 PSI for a norm for that car and some as high as 80 PSI for a norm for that car. The pressure is built up because the oil can not get through the bearings. As the bearings ware, the oil pressure goes down. Another factor is a regulator valve in the block that prevents the system pressure from going too high. The oil pump has the capacity for pumping way over 400 PSI. The spin on type filters can only stand about 350 PSI before exploding. This is rare but can happen if the block regulator sticks closed and a filter was use that did not have a bypass valve.
When you don't change your oil the pressure goes down even though the filter may be getting clogged. The reason is that the oil gets diluted with unburned fuel that gets past the rings. This makes the oil get thinner and thus goes through the bearings faster which builds less pressure. When you do change the oil, the pressure may go up higher than expected, for example, A car that normally runs at 60 PSI goes until the oil is contaminated and sees 40 PSI. Then he changes oil and sees 65 PSI until the oil gets hot.
You didn't tell me what pressure you normally see and what pressure it went to and you didn't tell me what the mechanic's compression test readings were.
Howard
