I've just installed a new oil pressure guage in my '47 Merc stock engine of unknown pedigree (and miles). I'm pretty sure the big ends and/or mains are a little tired due to the occassional knock I hear from the bottom end somewhere. However I'm not in the mood (nor have the dollars) for tearing yet another engine down right now so what I wanted to know was whether or not the pressure readings I am seeing are normal for a motor that's been round the block a few times. Pressures are as follows: Cold engine normal idle speed 30PSI. Cold engine fast idle speed 40PSI. Hot engine normal idle speed 15PSI. Hot engine mid-rev range 18-20PSI. I know the old pumps didn't run a lot of pressure, so could these readings be OK?
Oil pressures?
- Thread starter 66miles
- Start date
Old Henry
Well-known member
Those pressures are certainly OK and actually higher than I think most get. Of course, if you're reading those pressures from your stock dashboard gauge and/or sender they could be off a little or a lot. But, any pressure is enough pressure in my opinion so you don't have any worries in my book.
Old Henry
Well-known member
Thanks. I'm just putting together the story of the road trip I took to Grand Canyon last weekend with Pepe. Hope to post it tomorrow once I get the videos done.
Can't wait! Pepe's kinda cool. He sort of adds a little something somehow. My mom had a l'il dog called Pepe too. He used to ride up the front while she drove. I'll show her some of ya pics next time I see her. She'll laugh for hours. Keep up the good work mate! Miles from NZ.
Don't sound bad, what weight oil" 20/50 will help increase pressure if you are not already using that weight. G.M.
Sorry for the late reply. Prob getting late in your part of the world now (only late afternoon here but a day ahead). I was under the dash mounting the ugly new-world pressure guage where it doesn't show too much. Yeah, I got 20/50 weight oil in her right now. New detergent oil. I,m gonna pull the oil pan drain plug prob tomorrow and see if any big globs of sludge have ended up near the pickup. I've heard guys say that once an old motor has run new detergent oil for a while it will eventually clean the block right out, and from then on new oil is just fine.
Old Henry
Well-known member
I added a mechanical temperature gauge and put it in my glove box to be out of sight until I need to take a peek.
flatjack9
Well-known member
Detergent oil will not clean out and old engine. Old wives tale.
flatjack9
Well-known member
Because lots of guys don't know what they are talking about. Things get repeated over and over and all of a sudden it is gospel.
Ronnie
Well-known member
Like I said in my last post your pressures don't look to bad. You said " the old pumps didn't ran a lot of pressure" This isn't true unless they have warn gears or end plate clearance is to much. In almost all cases of low oil pressure on engines with minimal parts wear the low pressure is a result of the relief valve in the later pumps or the valve in the front of the valve valley. I test EVERY pump new or old before putting it in an engine. Most old pumps I tested will put out 80 or 90 lbs. Since I made the tester 15 years ago I never needed a new pump. I do have a supply of old ones to fall back on if I did find a bad one. DON"T be fooled by putting the pump pick up in oil and turning by hand. Skip rebuild his 37 engine years ago not long after I built the pump tester. He put the pump in a drain pan with a quart of oil and turned the drive gear by hand, After 2 or 3 turns it pumped out a big stream of oil. He said this is a good one and I said since we have the tester lets check it any way. We remove the gear and drive the pump with a 1/2 drill motor. It pumped a good stream but as soon as we put the slightest restriction on the output simulating the restriction of the internal engine parts the pressure dropped down to almost nothing. We grabbed an other used pump and it was up in the 80 or 90 lb range with a big restriction on the output. Always remove the plugs, lube the cylinders a little and with a mechanical gauge installed crank the engine with 12 volts even on a 6 volt starter. The oil pressure you see will be the pressure driving down the road. This can be preformed with the engine in the car or better with the engine sitting on the floor before installing. Easier to find and fix a problem then after it is in the car. How many of these problems have we read on this forum? The test fixture is a simple plastic tube assemble with the ends bored out to fit tight over the pump OD's that fit into the block. A fitting in the center of the plastic tube for a gauge and outlet plastic hose The plastic hose can have a valve or simply be bent to get resistance and the pressure is seen on the gauge. The pumps with the relief valve can have the valve spring shimmed to get the desired pressure, DON"T stretch the spring. The front valley spring can be shimmed with the engine spinning with the starter as described above. G.M.