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Different Oil Pressure Problem

Discussion in '"Stock" Alpine' started by HPMotors, Dec 10, 2020.

  1. HPMotors

    HPMotors Gold Level Sponsor

    This issue has me completely baffled. Car is a 1968 Series V with correct 725. New engine rebuild with some mods, none of which should effect oil pressure.

    New bearings, (main, rod and cam) oil pump etc. Engine started easily - oil pressure at cold idle was about 40 psi. When engine got fully warmed up this fell to 10 and below.

    Modified the pressure relief valve to make it adjustable. Now cold pressure at idle is 60 to 80 - basically whatever you want to set it to. Now hot idle still drops to 10. Changed oil three times, the last time we put in old school Valvoline Racing 20-50. No real difference. When driving the car, pressure initially goes up to about 40. Then as the engine gets fully up to temp it drops down to 15-20 at 5,000 rpm. For a new engine this is really not good.

    I've built many engines over the past 50 years, however this is the first Alpine. Never experienced this issue before. Pulled the engine back out of the car and disassembled to check. Bearing clearances on mains and rods are good. Oil pump clearances are tight. I am stymied.

    Are there any secret oil galley plugs that I might have left out? Only one that was odd for me was the timing chain squirter and I even crimped the end leaving only the small hole to feed oil to the chain and gears. It has the small ball and spring installed as called for.

    Car owner has been extremely patient but I am at my wits end. Any members out there ever experienced this problem before?

    Henry Payne
  2. Bill Blue

    Bill Blue Platinum Level Sponsor

    Are these pressure figures from the dash gauge (bad idea) or a for real, known to be accurate gauge (excellent idea).

  3. Filister

    Filister Silver Level Sponsor

    I had the exact same problem with a gm v6. All new and in perfect condition. Drove me nuts. After a couple of teardowns I eventually found that a cam bearing had been installed incorrectly leaving one of the supply channels slightly exposed. It was hidden where it was not visible without mirrors.

    Probably not your problem, but I was able to find it by pressurizing the assembled engine with 100psi air and tracking down all the hissing of the escaping air. led me right to the cam bearing which I was then able to confirm with a mirror.

    Good luck!
    Shannon Boal likes this.
  4. Tom H

    Tom H Platinum Level Sponsor

    You modified the oil pressure relief valve to make it adjustable. But are you sure it is not sticking open after running a bit? Have you tried adjusting the valve when hot to see if you can get the pressure up? If so, it seems that would clearly indicate it's sticking open.

    Over the years I've had lots of issues with that valve. Occasionally I was even able to actually remove the valve and see that it was stuck open. Over time (and oil changes and cleaning of the valve) the problem has gone away. I have concluded that it only takes a tiny amount of grit to cause that OPR valve to stick open. Seems most likely to happen shortly after a rebuild. I have often thought about designing a valve that used a Ball instead of a Piston as the moving part, with less probability of sticking.

  5. RootesRooter

    RootesRooter Platinum Level Sponsor

    New cam bearings and you checked the rod/main gaps. That eliminates most of the likely suspects.

    Did you run this engine before the rebuild? What were the oil readings like? Any fluctuations?

    Does the psi go down immediately every time the engine warms up, or is there a semi-regular lag time? If you turn off the engine while the psi is low, and then restart almost immediately, does the psi temporarily rebound to normal before slacking off again after a few miles. The latter would be classic SV PRV sticking.

    If you have a PRV that is sticking open when hot, making it "adjustable" might not stop the sticking. I've had four PRV's that stuck open. But on the bench, there was no hint of sticking. No visible ledges or heavy scratches in the bores. Cleaned them all thoroughly and re-installed them. All four PRV's eventually started sticking open again when hot. It was only a matter of days or weeks.

    If you have access to a Series IV PRV (no dimple on the face of the bolt), install it and see what happens. Note: SIV's PRV's are rated at 40 psi max, vs 45 on SV's.
  6. hartmandm

    hartmandm Moderator Platinum Level Sponsor

    Two different OPRVs were used on series V. One was brass and one was steel. I think the consensus is the brass one sticks open more often than the steel one, which is why Rootes switched from the brass OPRV to the steel one.

    Pictures are below. The brass OPRV and steel OPRV go with different oil filter blocks, as you can see the OPRV threads are in different locations.

    The Parts Manual shows the VIN switch from brass OPRV back to steel OPRV occurred at VIN B395012220. (Your vehicle may not be a 1968 model. Look up the VIN at http://sunbeamalpine.org/production-build-dates/)


  7. RootesRooter

    RootesRooter Platinum Level Sponsor

    The later steel PRV is definitely the way to go, but you'll need a matching base to screw it into. In the meantime, a SIV PRV will fit your existing base.
  8. Tom H

    Tom H Platinum Level Sponsor

    Rooter, your experience is similar to mine, except I WAS once able to "catch it red-handed" open by removing it carefully while still hot. Each time it appeared clean, no burrs or scratches. But also each time you could kinda tell it was just a touch of dirt or grit that caused it to stick. When the issue finally went away I concluded that when I rebuilt the engine, in spite of vigorous cleaning, there must have remained some tiny specs of dirt in the oil passages.
  9. husky drvr

    husky drvr Gold Level Sponsor


    Just a question of curiosity, did you check the side clearances for the rods at the crank?
  10. RootesRacer

    RootesRacer Platinum Level Sponsor

    I too have caught my OPR stuck open (visually after a LOP excursion).
    The steel OPRs (late models) seem to develop a burr on the inside of the tube in the area where the dump hole exists. I had to go in with a deburring tool to remove the burr, I also converted the valve to to an adjustable at the same time though this is not a solution to this type of problem IMO.
    I dont think that a spherical valve is the solution as this will alter the dump valves transfer function, I do think that a radiused piston made of a softer material is a better solution. I believe that the piston rattles in the OPR bore and forms the burr that eventually causes it to hang open.
    Shannon Boal likes this.
  11. DanR

    DanR Diamond Level Sponsor

    Interesting point! What would be the solution?
  12. hartmandm

    hartmandm Moderator Platinum Level Sponsor

    We haven't established which OPRV Henry has yet. However, I thought the design of the series 4 steel OPRV was the same as the late series 5 steel OPRV. So a series 4 steel OPRV won't go into a series 5 oil filter base designed for the brass OPRV. Maybe I am missing something.

  13. HPMotors

    HPMotors Gold Level Sponsor

    Wow! I am overwhelmed by the responses! Just got back online as my old brain forgot my password!
    I'll try to go over the various ideas/comments in order:

    Bill: Oil pressure numbers were from the stock gauge. We verified the readings by putting a "test" gauge in the system. Numbers correlated

    Fillster: Interesting idea. At the moment the engine is back out of the car and apart so will try the air idea if nothing else comes up. I watched my machine shop guy install the cam bearings and he very carefully aligned the ports in the bearing with the galley hole in the block.

    Tom H: Pretty sure the PRV is not sticking open. I used the older steel version and installed a screw in the head that increases the pressure on the PRV spring. My son found this idea via Google and I forgot the gentleman who posted the idea. Very simple and elegant solution and I thought we had it licked as we were getting 40 psi hot idle. Then the engine got hotter when driven and it went below 10. I have opened the engine back up and found no signs of any grit or machining debris, especially in in the PRV

    RootesRooter: Did not run the engine before I took it apart. Had been sitting a long time, carbs gunked up, fuel pump and tank crap. Owner wishes to go Vintage racing so we installed a NASCAR fuel cell in the trunk. You are right that the adjustable bit on the PRV may not be the cureall - but when the bolt is run all the way in the pressure on the spring is so high that it must be going in. Backing this idea is that if it has been run hot to the point of no pressure, with the PRV bolt ALL the way in, turn engine off and let it sit. On cold start up the gauge is pegged beyond the end of the scale, which is 80 on this car. The changes in pressure are gradually down as the engine warms over 10 - 15 minutes. Jumps up on initial start up. On hot restart, the pressure remains low.

    Mike: We are using the steel PRV. I have several of three different designs and the conventional wisdom was to use the one we had on the engine.

    Husky drvr: No, didn't check the side clearances beyond the fact that the rods looked good, no evidence of side contact. We used the best 4 rods out of about 40 to use to balance them.

    RootesRacer: The PRV, or OPR, used looked very good with no visible scratches or burrs on the piston or in the bore.

    Not having seen anything amiss so far I am pulling at straws. One suggestion given to me was that the oil cooler (it has the stock unit on it) could be the issue as they have an internal regulating valve. Others say this is not so, as it is just a straight tube.

    Another idea is to install an oil temperature gauge as the fact that the oil pressure slowly decreases with temperature (?) could be an oil thinning at high temps? Water gauge remains mid range though.

    Thanks to all of the folks for the time to respond.

  14. RootesRooter

    RootesRooter Platinum Level Sponsor

    Series IV was brass. I'm gonna have to look up those part numbers cited. I didn't think the steel PRV was introduced until the Arrow Range. There's a Service Bulletin from early '67 instructing dealers to deal with low oil pressure complaints by replacing the brass SV PRV with the SIV part number.

    Hartmandn, if you post another photo of a brass PRV, please show the top of the bolt where the SV "dimple" is. Those are the ones to be wary of.
  15. RootesRooter

    RootesRooter Platinum Level Sponsor

    So you already have a steel PRV? Yikes.

    My suggestion for your next step is still to swap your current PRV for a SIV brass one (easier to find than steel), but you'll need a matching base (easy to find). That's the easiest way to eliminate the PRV as the source of your problem.

    Frankly, except for extreme racing applications like yours, I don't like the idea of an adjustable PRV. If someone uses it to compensate for a low oil pressure reading, they're just putting lipstick on a pig. They still have a worn engine that isn't going to fix itself. The oil pressure gauge tells me a lot about the condition of the engine over time, but that cornerstone, that reference point is lost for good once you start moving the goalposts.
    Last edited: Dec 10, 2020
  16. husky drvr

    husky drvr Gold Level Sponsor

    upload_2020-12-10_23-30-57.png upload_2020-12-10_23-33-12.png

    The WSM 145 states 0.0075" to 0.0125" for the side clearance on the rods and bearing clearance of 0.0015" to 0.0020". Just depends on how you and the machinist wanted to set up clearances.

    The oil cooler is just a straight tube. The valve is in the oil cooler adapter block to which the supply and return lines attach. It is a simple spool valve which directs oil flow simply by the pressure differential between the supply and return ports in the adapter block. If you've been using multi-vis oil and in warmer weather, I wouldn't think it had needed to function much, maybe at start-up. It's not a part that has been discussed as an issue very often. If it's assembled correctly and free to move, it should be OK.

    On the list of causes, I guess the faulty pipe and/or unions, or possible leaky gaskets between the block, adapter, and filter base are the only things not actually touched on yet.


    Last edited: Dec 10, 2020
  17. puff4

    puff4 Gold Level Sponsor

    British sand cast blocks and heads are notorious for having bits of sand and detritus slowly leech out of them over time. For this reason, in addition to ease of cleanup on next tear-down, I always epoxy the inside of my motors when I rebuild them. It absolutely stops any such debris, plus when you drain the oil it makes it easier to get all the sludge out as the inside surface is now very slippery and it doesn't tend to get 'glued on' inside the motor.

  18. spmdr

    spmdr Donation Time

    I have seen an OPRV mod that is thought to be a PRESSURE adjustment but it is NOT.

    This mod uses a small diameter machine screw to be installed in the center of the head of the OPRV.

    What it does is JUST limit the TRAVEL of the OPRV piston.

    It does NOT change the PRESSURE of the OPRV piston SPRING.

    An OPRV with this adjustable piston travel LIMIT could cause the problems seen.

    What is the diameter of the bolt used for the OPRV mod?
    Last edited: Dec 11, 2020
  19. Shannon Boal

    Shannon Boal Bronze Level Sponsor

    I think the oil pressure is leaking away somewhere, and reads low when the oil thins out......Regarding main bearing clearance; was the shaft checked with an outside micrometer, and then the bearing inside diameter checked with a bore/snap/inside mic? Were the cam bearing clearances checked this way? I would rule out the rod bearing and rod side clearances because the oil to the rods goes first to the mains.....plasti-gauge is probably OK for main bearing checking, but you can't use it to check cam bearing clearance. I wrote all that to say this: if the crank mains were ground under spec by say, 0.002", it could behave this way
  20. Shannon Boal

    Shannon Boal Bronze Level Sponsor

    Suction leak? Anything wonky with the oil pick up screen, pipe and joints in that system? Grasping at straws....

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