Oil pressure problem - JeepForum.com
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post #1 of 36 Old 02-25-2020, 05:10 AM Thread Starter
hodge5
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Oil pressure problem

1985 CJ7, 258, no idea of the life or miles on the engine, but it is tight. Uses no oil, runs and pulls great, no noises. If I had to guess, it's newer than the Jeep (Jeep has 109,500 miles on it).
I've owned the Jeep since the end of September- 5 months. Both the voltage and oil pressure needles bounce to some electrical interference, so I interpolated the oil pressure between the bounces, concluding that it was in the normal range . For whatever reason, the interference has lessened, so both needles stay stationary now.
Driving the Jeep the other day, I glanced at the oil pressure. Unlike what I normally saw, it was reading about 60, pulsing up to 70 PSI. I put a mechanical gauge on it, to see if the dash gauge was reading correctly- it was, and the mechanical gauge was also pulsing.
I changed the oil, replacing the filter; didn't change things. I checked the radiator and pulled the plugs; no evidence of a head gasket leaking.
I know that bad bearings can cause high pressure, but I don't believe that this is the issue. I can start the Jeep 10 times in a row, and it will do the same thing every time- for the first 30 seconds at idle, it will pull about 45 PSI. After that 30 seconds, it will surge up to 60, and then pulse between 60 and 70. Shut it off, let it sit a minute, start it up again, same thing. I had my mechanic here yesterday (picking up some steel that I had ordered for him), and let him check it over. We both believe that something is going on with the pressure relief valve in the pump. So, I've order the parts, and he'll replace it for me next week.
Anything we might have missed?

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post #2 of 36 Old 02-25-2020, 05:46 AM
jeepdaddy2000
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I agree with the pressure relief valve being the issue.


Worn bearings will cause low not high oil pressure.



On a tight engine the oiling capacity of the pump should always outstrip the leak down of the engine oiling system. The relief valve piston is designed to maintain a constant regulated pressure by relieving the excessive pressure via the pressure relief valve spring, which is usually in the 30 to 50 LB range. If the piston is sticking in the bore or there is damage to the spring, the valve won't work properly, which can cause high, fluctuating, or low/no pressure.


On the plus side, high pressure is always better than low/no pressure.

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post #3 of 36 Old 02-25-2020, 09:33 AM Thread Starter
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Thank you for the valuable input.
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post #4 of 36 Old 02-25-2020, 11:20 AM
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Looking at my 1980 Jeep Service Specifiations manual, the 258 oil pressure relief valve is set at 75 PSI. OP, what grade oil are you running?
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post #5 of 36 Old 02-25-2020, 11:57 AM
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On my replacement pump I just put in, the relief kicks in exactly at 75 psi. If I goose the throttle on the drill when testing It will go to 80 for a fraction of a second then read a steady 75. Its possible that something is slightly sticking in this valve. Once the jeep warms up do you get this fluctuation. If you only get it when its still cold and likely in that 75psi range I would agree the relief that is built into the pump is likely a culprit. The bypass valve where you screw the oil filter on should not have an effect on oil pressure at all so that is not it.
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post #6 of 36 Old 02-25-2020, 03:08 PM Thread Starter
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10w30 conventional, same cold or warmed up.
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post #7 of 36 Old 02-25-2020, 07:11 PM
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Quote:
Originally Posted by jeepdaddy2000 View Post



On the plus side, high pressure is always better than low/no pressure.
Most postings you see on the forum are in regards to low oil pressure. Not sure that you'll see any negative effects. My CJ-5 runs 60 psi cold and drops to 50-55 at operating temp. Hot idle is 25 PSI. I run 15W-40 oil. No negatives from oil pressure that I can see.
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post #8 of 36 Old 03-04-2020, 08:53 AM Thread Starter
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The oil pump has been replaced with a new Melling, along with a new pickup tube. Same oil pressure fluctuation.
Is there a oil bypass valve on these engines? I've found a replacement part, but I can't locate any info on replacing it, or the 258 having one.
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post #9 of 36 Old 03-04-2020, 02:02 PM Thread Starter
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I pulled the oil filter and removed the bypass valve, then reinstalled the filter. Oil pressure fluctuates the same. Could it be a weak bypass spring? If a new/stronger spring, or the blocking plate, is in place, I wonder if that would stop the fluctuation? Or, is it another issue?
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post #10 of 36 Old 03-05-2020, 10:10 AM
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Maybe I didn't read it or see it. What oil/filters are you using? How much oil?
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post #11 of 36 Old 03-05-2020, 11:29 AM Thread Starter
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Quote:
Originally Posted by dirtdudeaz View Post
Maybe I didn't read it or see it. What oil/filters are you using? How much oil?
Mobil 1 M1-107A filter, Castrol 10W-30 oil. I had 5 1/2 quarts in it, reading full on the dipstick.

I've ordered a block off plate for the bypass valve, and the M1-107A is a bypass oil filter. This morning, I checked the level again- showed full. I checked the owner's manual, which called for 6 quarts. So, I added a half. Now, the needle still jumps, but it's going from 40 to 50 constant, where before it would pull 40 during the first 30 seconds, and then jump up to 60, bouncing to 70- and stay that way until shut off and restart.
Two things- the dipstick handle is bright, shiny yellow, so I think that the previous owner replaced it- and it isn't correct. That's no biggie- I'll make a new full mark for 6 quarts. I'm wondering if, with a weak spring in the bypass, that it is the culprit for the fluctuating pressure. It's never been low- it goes between 40 and 70. I'm hoping that capping the bypass in the block, and the correct oil level, will make the difference.
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post #12 of 36 Old 03-05-2020, 12:19 PM
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How fast does your mechanical gauge pulse? I think you have been following along with my oil pressure pressure problem on my rebuilt engine. I also got some good information watching it pulse along with my phone in highspeed mode. (https://www.jeepforum.com/forum/f8/c...motor-4379347/) . I got a lot of information by running a distributor without the cam gear. I used a drill with a zip tie until I got to around 30 psi. Go exactly 2 revolutions on the crank and watch the gauge. Mine was +- 3 psi the entire revolution, except the low spot which is 10% of normal pressure. I would expect oil pressure to be constant through the entire rotation. Mine unfortunately is not. I have isolated it to something with the cam bearings but I have not gotten the engine fully out yet to inspect those. I hope to get that tonight.
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post #13 of 36 Old 03-05-2020, 12:23 PM
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Its also my understanding that the oil pressure valve that you bought the bypass valve is for if the oil filter gets clogged. Having that in or out or malfunctioning should not effect oil pressure at all. If its not working it only effects how much of the oil gets filtered each time it passes through. I dont expect this to change your pressure at all. The only valve in the jeeps' system that can relieve oil pressure is the one that is built into the pump itself. You have already eliminated that by replacing the pump and still getting the same result.
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post #14 of 36 Old 03-06-2020, 05:11 AM Thread Starter
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The plug is to do away with the block mounted bypass valve, and rely on the bypass inside the filter.
The pulsing of oil pressure varies. Sometimes, it reads constant. Sometimes, it varies up and down. It will start out constant, no bounce, for the first 30 seconds or so. Then, the needle will jump to 50 or 60 PSI, and then start pulsing up and down to 65 or 70. After I increased the volume of oil (from 5 1/2 quarts to 6), the pattern was the same, but the pressures changed- now, once it starts pulsing, it's from 40-55 PSI. I have not added more oil to see if it made another change. I'm waiting for the blocking plate, to see what change, if any that creates.
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post #15 of 36 Old 03-09-2020, 08:27 AM
dirtdudeaz
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Quote:
Originally Posted by hodge5 View Post
Mobil 1 M1-107A filter, Castrol 10W-30 oil. I had 5 1/2 quarts in it, reading full on the dipstick.

I've ordered a block off plate for the bypass valve, and the M1-107A is a bypass oil filter. This morning, I checked the level again- showed full. I checked the owner's manual, which called for 6 quarts. So, I added a half. Now, the needle still jumps, but it's going from 40 to 50 constant, where before it would pull 40 during the first 30 seconds, and then jump up to 60, bouncing to 70- and stay that way until shut off and restart.
Two things- the dipstick handle is bright, shiny yellow, so I think that the previous owner replaced it- and it isn't correct. That's no biggie- I'll make a new full mark for 6 quarts. I'm wondering if, with a weak spring in the bypass, that it is the culprit for the fluctuating pressure. It's never been low- it goes between 40 and 70. I'm hoping that capping the bypass in the block, and the correct oil level, will make the difference.

The old style long dipsticks are $$$ from Mopar, so a lot of guys will find a 4.0 xj/yj dipstick. The tube is the same, and the bracket to mount fits the block, the only difference is the stick length, so you are 100% right to need to remark. I did the same dipstick and put in 1-2 quarts until the level came up onto the stick a little (marked that as low), and then added a full 6 (marked as full).
  1. For your new oil pump, did you replace the same PN, or do you know? Is it a high output or regular?
  2. Other question I am curious on, have you checked that all your galley plugs are in? This one may be hard to check as they are under the timing cover and valve cover, and then at the back of the motor behind the bellhousing.
  3. Does your engine surge as far as rpm? Is it a constant RPM, or are you loping at 600-800rpm?
  4. The last thing that came to mind was some sort of air in your gauge system. I know you said you were using an electric sender and now mechanical. Either one could get air which would act as a shock absorber and cause pulsing. It wouldn't hurt to ensure your line to the gauge has no air either, or something weird stuck in the block where it threads in.
As mentioned before, maybe it's a nuisance thing, but 8psi at hot idle is good enough, and 10psi for every 1000rpm at speed as a rule of thumb. Your pressure is good, so I guess it's a matter of how big of a +- range do you want to get the fluctuations down to.
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