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Old 12-15-2020, 10:00 PM   #1
spock345
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Default B18 low oil pressure

I finally got around to installing an oil pressure gauge in my 122. I fired up the engine as normal and got about 50psi cold with 15w40 delo. I thought that seemed OK and let the car warm up. It went down to 20psi at idle. I thought, OK, the oil pump is a bit tired. I drove it around a little bit, just up to the local university and back. It was down to below 10 on the gauge. With the old sender the warning light never came on.

I think my sender is bad given that the "warning light" function isn't working. I have a 3 bar 240 gauge and sender lying around and I am going to plug that in to verify my findings.

In the mean time is it worthwhile to order a new oil pump seal and replace that anyway even if it is just the gauge?

Update: with the 3 bar gauge it has started idling just below 2 bar.
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Old 12-15-2020, 10:32 PM   #2
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10 psi at what RPM? That's not unusual for a fully warmed engine at idle. Not so good if it's revved up a bit.

The warning lights only go on at close to zero psi. Does it work with the key on, engine off?
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Old 12-15-2020, 10:39 PM   #3
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Quote:
Originally Posted by JohnMc View Post
10 psi at what RPM? That's not unusual for a fully warmed engine at idle. Not so good if it's revved up a bit.

The warning lights only go on at close to zero psi. Does it work with the key on, engine off?
10 psi idling at 750-800 RPM. Almost registering nothing on the gauge at the same RPM when I got home.

At 2000 RPM when warmed I am getting around 30-35 psi.

The warning light works just fine. I haven't taken the car for a drive with the 240 gauge yet to see what it is like when I get the oil hot with that sender.

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Old 12-15-2020, 11:07 PM   #4
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The oil pressure spec for a B18 is 50-85 psi warmed up at 2000 rpm, with a new oil filter.

From the green book.
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Old 12-15-2020, 11:12 PM   #5
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It's a bit low, but not catastrophically low. Probably won't cause any real issues.

Could be the oil pump, could be the transfer tube seals, could be worn bearings.
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Old 12-16-2020, 12:32 AM   #6
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I've been meaning to check the bottom end in the motor anyway before moving forward with my performance plans. I'll check the oil pump and replace the seals and relief valve spring when I do that, if the pump and bearings are still good. I know the rings are good.

The sender for my new gauge is a no-name unit I found in a bin of parts I got for cheap. Met the ohm and PSI specifications for my gauge so I thought might as well run it. It did do some strange stuff with the warning light initially, which makes me doubt it. I ordered a NOS VDO unit to replace it.

The 3 bar sender I put on it has the gauge wire badly soldered on because the post broke. There is a reason it is my backup plan but I know it works. I assume that since 3 bar is 43-ish psi then if I can peg the gauge at 2000rpm then I am good.

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Old 12-16-2020, 01:12 AM   #7
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Copied this from the 700/900/850 Pocket Data Booklet PDF...




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Old 12-16-2020, 03:23 PM   #8
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I got the oil nice and hot today. The 3 bar gauge reads just above 1 bar at idle (800-ish rpm) and just below 3 bar at 2000 rpm. The gauge is pegged by 2500rpm.

Is this a "need to fix it now" sort of problem or just a bit low and I should take a look at the oil pump when I get check out the bottom end problem?

I have read some about weak relief valve springs in addition to bad seals on the pump tube causing low pressure.
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Old 12-16-2020, 03:40 PM   #9
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Could be a stuck relief valve bleeding off pressure in all situations. But generally, that will just trim the peak oil pressure down to whatever the spring can handle, but when the pressure is lower, it's not going to be letting oil by. Unless the relief valve is sticking open.

And it could be the transfer pipe seals, but to check those you have to pull the pump out, so you might as well check the pump over carefully at that point. 4 bolts (iirc) and it comes apart to check the condition inside. It runs in unfiltered oil, so it can have more wear than the rest of the engine if it was neglected n its past.

In general, it's not great to have oil pressure that low, but it's not critical either. I'd keep driving it until you're ready to work on it. No need to stop driving it, every thing is still getting oil.
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Old 12-16-2020, 07:19 PM   #10
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I'll probably order some seals (might as well replace at least those) and drop the pan over the holidays when I am off work.

If it is the oil pump are there any suggestions on replacements? OEM? Any good aftermarket options? I noticed IPD sells a "heavy duty" pump.
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Old 12-19-2020, 11:31 PM   #11
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Sound like a lot of worn parts, bearings,pump,etc. As general rule of thumb 10 lbs per 1000 RPM or the limit of the oil pressure valve. The oil pressure valve spring could be weak. 3 bars is about 45lbs? All the stuff that I work on is in LBS/SQ"
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Old 12-20-2020, 12:19 PM   #12
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Yep 1 bar is about 14.5 psi. So I am seeing around 40-ish at 2000rpm. The only reason I am using the gauge in bar is because I knew that one was reasonably accurate and my 100psi sender turned out to be faulty.

At this point I also want to know how the rod and main bearings look down there and check the clearances.
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