Triumph Repair: 1976 tr6 oil pressure gauge, oil pressure gauge, british sports cars

Question
Hi Howard, I'm hoping you can help with a question I have about my 1976 2.5 6 cyl. no overdrive, sub model Triumph TR6 I recently purchased. It has sat in a garage for a long period of time(2000 miles in 16 years. 0 miles in the last 2 years). It has 105,000 miles on the odometer. It runs strong but the oil pressure gauge tells me something else. When I start the car cold the pressure reads fine (about 55 pounds) when I hit the road she falls back down to about 5 or less. There are no other problems. It sounds great, the motor runs great.(no noises,clangs, or anything like that). The oil checks about 3/4" above normal. I just had the oil changed it was slightly overfilled. Some of the motor work it has had over the last few years; timing chain,front seal,major tune-up,head gasket,valve springs,water pump. These repairs were all done in 2006. So my question is what could cause the gauge to read this way and what are some of the things I need to do or not do in fixing this problem?? Thank you Mike

Answer
Hi Mike,

55 lb oil pressure is ok and you are correct that 5 PSI is not. There can be one or more problems so you need to address every one as it can be a combination.

First what grade oil are you using? Do not use the new 5W X ? oils that many new cars use. I usually recommend 10w 40 for most of these old British sports cars for summer driving and 10w 30 or 10w 20 for winter. The grade # is the viscosity of the oil and the multi-grade means it is used for several different oil clearance parts of the engine. Often the oil does not quote it's viscosity index. That is it's ability to maintain a viscosity when heated.

In the early 70's I had a chance to talk to an oil company engineer and he related and confirmed things I had read about oil. Penn state crude oil had one of the highest rating for viscosity index. However now many oil companies purchase crude oil from all over the world and often they end up with a soup mix so they put additives in for many different features they want for their oil including a viscosity index booster (STP). They don't call it STP as that is a brand name index booster. But many use a booster of their own design. Don't be fooled by it's thick appearance, as it will enter a oil clearance area that grade 5 oil can't enter.

Also don't run the engine with it over filled. There are several reasons for that.

That is just on the oil now look at the car. First get another hydraulic oil pressure gauge from any auto parts store. They are not expensive and is a good tool to keep handy for other engine problems. I have seen experienced mechanics waste a lot of time chasing a low oil pressure problem when it was just a gauge problem.

After you confirm low pressure with heated engine. Check the oil pressure regulator valve. Look closely at the sides of the spring for excessive ware and check the free length at 1.53" or 40mm. and examine the end of the valve to see that it is seating well.

Oil pressure is built up because the oil can't get through the bearing clearances. The oil pump has the capabilities of pumping over 400 PSI so thus the regulator valve. It is very rare to see an oil pressure problem caused by an oil pump in any of these old British sports cars.

The loss of oil pressure due to excessive clearance in bearings is the more common cause. However, you should remove the valve cover to watch the oil at the rockers as this is easy to do and you don't want to condemn rods and mains due to a loss of oil pressure in the rocker area. Start the engine with the valve cover off and if there is a problem it will be very evident. Normally you just want to see a dribble of oil from the rocker arms.

If all of that is correct then you should consider removing the pan and a few rod caps and use Plasti-gauge (available at most auto parts store that mechanics use.) They can explain how to use it if you don't know.

Let me know.

Howard