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Discussion Starter · #1 ·
Hi,

I am still experiencing low oil pressure on my 258. I have checked the electricel part of the gauge and it works ok.
I haven't check the sending unit yet (don't know how to do it).
I was wondering if there was any way to check that my relief valve is not stuck open ?
Thank you

 

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If your pressure guage is working and the relief valve is stuck, open or closed, the oil pressure you have will be directly proportional to engine speed.

If the valve is stuck closed, pressure will be low at idle and just keep going up as engine speed increases. I suppose it might get to 100 PSI or so, at which point there is a danger of blowing out engine seals.

If the valve is stuck open, pressure will be almost zero at idle, and come up steadily as engine speed increases. It should hit about 40 PSI at a very fast idle, and go up from there.

In either case, the pressure guage will act like a tachometer, exactly matching engine speed.

FYI, the oil pump in my '78 258 went peculiar at a very young age - maybe 4 years and 20,000 miles (33,000 kilometers). Idle oil pressure was very low until I replaced it.


I might have been born at night, but it wasn't LAST night.
 

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One more thing; if the relief valve is working, but your low oil pressure reading is due to another cause, the pressure will come up with increasing engine speed, but at some point will stop rising with additional engine speed.

Also a relief valve can still function but do strange things. If the spring gets weak, it will open at lower pressure. If the valve or bore get contaminated with deposits, it can stick closed until the pressure gets higher than normal, then suddenly pop open and let the pressure drop down.

The factory manual says the valve is set for 75PSI maximum in my engine. I think mine is lower than that; maybe I need a new pump. The valve is an integral part of the pump, by the way.

Good luck.



I might have been born at night, but it wasn't LAST night.
 

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The relief valve is not a part of the pump.
You can actually see it when you take off your oil filter.
You should be able to eyeball the fiber glass button with the aid of a mirror.
You can test it's function by pushing in (opening) the valve with a small screwdriver and you can easily replace the button valve and spring on your next oil filter change. These inexpensive parts can be had from your Jeep dealer.
As for your oil sender, complete diagnostics for it can be found on my "CJ Gauge & Sender Diagnostics" article http://www.monsterslayer.com/jeep/Gauges.htm



JAF
http://www.monsterslayer.com/jeep
 

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Discussion Starter · #5 ·
Thanks for your help.

Now I know it's not my relief valve because when the engine is cold I get 10/20 psi and when it's hot i have 0 psi at idle and close to 10 psi at high rpm. but i never get more than 20 psi.
Could it be linked to the bypass valve, the one close to the oil filter, then ?

 

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The pressure relief valve IS part of the oil pump. What you're thinking of is the bypass valve. It's job is to let oil flow bypass the filter if it gets too clogged to let sufficient oil pass. It will not cause lowered pressure because no oil is dumped back to the sump; it just bypasses the filter, that's all.

I think your problem is very likely the relief valve in the pump, especially if it came on suddenly. Some other possibilities are completely worn out main and cam bearings, completely worn out oil pump, blockage of the intake screen or pickup tube, and of course, the obvious; not enough oil.

I don't think it's a guage problem because the gauge would still register the opening of the relief valve, even it it showed that happening at 5 PSI.

Attached is a scan of the pertinent page from the '78 Jeep shop manual.

Whatevery you do, don't drive it until it's fixed.
Let us know what you find.

I might have been born at night, but it wasn't LAST night.
 

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The pressure relief valve IS part of the oil pump. What you're thinking of is the bypass valve. It's job is to let oil flow bypass the filter if it gets too clogged to let sufficient oil pass. It will not cause lowered pressure because no oil is dumped back to the sump; it just bypasses the filter, that's all.

I think your problem is very likely the relief valve in the pump, especially if it came on suddenly. Some other possibilities are completely worn out main and cam bearings, completely worn out oil pump, blockage of the intake screen or pickup tube, and of course, the obvious; not enough oil.

I don't think it's a guage problem because the gauge would still register the opening of the relief valve, even it it showed that happening at 5 PSI.

Attached is a scan of the pertinent page from the '78 Jeep shop manual.

Whatevery you do, don't drive it until it's fixed.
Let us know what you find.

I might have been born at night, but it wasn't LAST night.
 

Attachments

G

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Just in case you want to go this way, I have an extra oil pump of a 73 CJ5 258. Worked great when I removed it so that I could go to a high pressure pump for my new cam. I'll let it go REAL cheap if you want it.

Jon
84 Toy,72 CJ5,91 RM125
[email protected]
 

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Discussion Starter · #9 ·
Jim, I am a bit confused : you said that if the relief valve was stuck, the gauge should move with the rpm right ?
It is absolutely not my case, as even if I rev up the engine high I won't get more than 10 to 20 psi. So why do you still think it could be my relief valve ?
You said it is part of the oil pump : do you mean I have to replace the complete oil pump assy to fix it ?
Thanks again for you help !

 

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Discussion Starter · #10 ·
Thanks for your offer, but since I live in France I am afraid that shipping cost will more expensive than a new pump here ..
Thanks anyway

 

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If the relief valve is stuck open, whatever pressure you have will be proportional to engine speed. You're showing zero at low speeds, but obviously have a little, just not enough to register. As engine speed comes up you get a little pressure. A little faster, a little more pressure, until you get to 20 PSI, and common sense takes over and you quit thrashing it.

If the relief were working but the gauge was reading, say, one fourth of real pressure, you would be showing maybe 5 PSI at idle, just a little increase, to fast idle speed, whould bring pressure up to the maximum reading. Any further increase in engine speed would cause no corresponding increase in the pressure reading.

A normally functioning system will build up enough pressure at maybe 1200 or 1500 RPM to open the relief valve. Any higher speed will not increase pressure. Lower speeds will drop the pressure. If one were to graph it, the line would start low, rise steeply in a straight line until the valve opens, then go flat at that level.

With the valve stuck closed, the line would just keep climbing. With the valve stuck open, the line would start much lower, near zero, and rise at a shallow pitch, but it would be straight. A gauge registering a fraction of the real pressure would graph the same as the normal system, with the increase and then flattening out, but at lower pressures all the way.

Regarding servicing the relief valve, my first guess would be a broken spring. The factory manual does not give any specs for the spring, but it can be replaced. The only challenge will be to find a spring of the right size and strength. In that page I attached you can see the setup; a conical valve, followed by the spring and a spring retainer cup, and held in place by a cotter pin.

Which suggests that the cotter pin could have broken or fallen out, but I would think that would completely kill any pressure. Another possible problem besides what I mentioned earlier is a loose pickup tube. The service manual says "If [the] tube is moved within the pump body, a replacement tube and screen assembly must be installed to assure an airtight seal."

In school years ago there were a dozen various engines on stands that were taken apart and reassembled by the students every semester. When the GMC V6 truck engine started up after I got it back together it had very low pressure. I took the pan off and found that the pickup screen was almost completely covered with a red felt-like substance. It was the collected lint from hundreds of shop towels used over the years to wipe things off. Some darn fool (me) hadn't inspected the screen, or else some darn fool (me) had left a piece of a shop towel in the sump to get thrashed apart by the crank.

If you want I will scann the next page and send it to you. It might be better to e-mail it directly because of the 100k file size limit on the board.

Jim

I might have been born at night, but it wasn't LAST night.
 

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Discussion Starter · #12 ·
Thanks for your patience I am learning a lot of things here !

From what you are describing I have a normaly functioning system : as I rev the engine, pressure build up until it reaches a top 20 psi cold or 10 hot and stop going up even if I keep pushing the engine to higher rpm. It is just that both my idling pressure or reving pressure are now 20 psi below what it used to be.

So I think it's not my relief valve.
I am left with :

1. A faulty sending unit.
2.A faulty screen and pick up assy.
3.A worn out engine.

Am I correct ? If yes, I will test my compression tomorow morning to try to eliminate one option, then try to change my sending unit, and then change my sreen/pick up assy and maybe oil pump.

Is changing the oil pump a simple bolt off / bolt on or is it harder ?


 

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If it hits its terminal pressure at a fairly low speed - fast idle range - and then levels off it means that the relief valve is working, not stuck open.

It's not the other two possibilities you mentioned because neither of them will make the relief valve open at a low pressure. It might take 3000RPM to get enough flow to open the valve, but it still won't open at a lower pressure.

That leaves you with two likely possibilities:

1 The pressure is good but the gauge is registering it at about 1/3 of reality.

2 The spring in the relief valve has broken, a couple of coils from the end, or lost its tension.

I suspect it's not 1, because you say the maximum pressure hot is half of the maximum cold pressure. Were the system functioning properly the idle pressure would be different between hot and cold, but the max would be the same, maybe coming in at a little higher speed hot, but the relief valve is still going to open at the same pressure.

On the other hand, if the spring has been shortened by breaking a couple of coils off, it could be bypassing some oil at all times, but more as pressure increases. By the time the valve body is well off the seat, the only restriction left is around the valve body, and hot oil will get by easier than cold.

It still could be the gauge, because it's got to be difficult to analyze what's going on when you only have a few degrees of needle movement to look at. Also it sounds like you've been running it quite a bit like this, and it hasn't spun a bearing, which also points to a gauge problem.

Now, I think it's time to stop bench racing this and get practical. It's not a very good idea to keep running the engine and trying to figure this out. If the oil pressure is that low you're going to toast the poor thing running tests and gathering data.

It's not that bad a job to drop the pan and pull the pump. On my '78 CJ the tough part is getting to the pan, which requires dropping the starter, supporting the engine and removing the right engine mount. I forgot what model you have, so it might be different. That's the good news. The bad news is that, according to the manual, you can't get to the relief valve without first moving the inlet tube out of the way. Then the book says to replace the inlet tube. As I recall, I reused the old pickup when I replaced the pump, and it's still running, so I don't know if I'd fret too much over that.

The only other reasonable course would be to find a known-good pressure gauge and install it in place of the one you have, just to see what's really going on. That shouldn't be that difficult. I think one can get a cheap mechanical gauge in a parts house for a reasonable sum

In view of the relative ease of that, I might be tempted to try it first. Maybe you want to get a good mechanical gauge to permantly replace the stock electric. Mechanicals tend to be more reliable. The only negative is that it's possible to develop a leak in the plumbing, and have oil dripping on your shoe. I replaced mine before the Jeep was out of warranty, just on general principal.


I might have been born at night, but it wasn't LAST night.
 
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