4.2l oil pressure questions - JeepForum.com
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post #1 of 15 Old 07-15-2021, 06:33 AM Thread Starter
K5TBL
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4.2l oil pressure questions

My 88 YJ has the 4.2l in it and is having what I consider an oil pressure issue.

Initially it would have 60psi of oil pressure at start up and as the engine/oil temp came up oil pressure would decrease to less than 20psi at idle. To be on the safe side I changed the oil pump and now I get 80psi at start up and just over 20psi when at operating temp.

I know that the oil will thin with heat but I suspect another issue also. Chrysler motor build oil pressure at the oil filter while Ford and Chevy build pressure at the camshaft. A poorly spec'ed oil filter on a Chrysler will cause oil pressure issues.

Is that my issue here? Where does the 4.2l make its oil pressure?


John
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post #2 of 15 Old 07-15-2021, 06:35 AM Thread Starter
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For reference the motor has 115k miles on it. My 2001 Chevy 1500 has 220k on it and maintains 30psi of oil pressure after running 2 hrs down the road at 80mph.

John
1988 Jeep Wrangler Sport “Red Leader”
4.2l I6, BA10-5
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post #3 of 15 Old 07-15-2021, 12:48 PM
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I don't think you have a problem. If you go by the "rule of thumb" as it were for the 4.2 most people generally say about 10 psi per 1k rpm. If that's the case your idle is likely in the 750 range, 10 psi should be fine.... if you're just over 20 at idle I don't see a problem at all.

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post #4 of 15 Old 07-15-2021, 04:07 PM
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Im with Tim, 20 psi is fine.

This is a spec sheet from a 4.0 i just had rebuilt



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post #5 of 15 Old 07-16-2021, 04:26 AM
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Your oil pressure is fine. If anything the 80PSI cold is what is alarming. Oil pressure lights do not come on until around 6PSI. 10 at idle is very good.

Quote:
Chrysler motor build oil pressure at the oil filter while Ford and Chevy build pressure at the camshaft.
ALL engines build pressure at the oil pump and relief valve. They then maintain pressure throughout. Nobodies filters "build" pressure. ALL motors must maintain pressure at the camshaft or oil pressure will not make it to the next place it needs to be such as the lifters.

If you cannot fix it with a hammer then it has to be an electrical problem.
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post #6 of 15 Old 07-16-2021, 10:08 AM Thread Starter
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Quote:
Originally Posted by Boojo35 View Post

ALL engines build pressure at the oil pump and relief valve. They then maintain pressure throughout. Nobodies filters "build" pressure. ALL motors must maintain pressure at the camshaft or oil pressure will not make it to the next place it needs to be such as the lifters.
We will have to agree to disagree on this one. Pumps move volume, Pressure is created by a restriction of said volume. I spent 6 years working in a Chrysler Dealership and a heavy line mechanic, doing internal engine repairs, and found that the use of a less that precision made filter would reduce the oil pressure in the older 5.2l (318 cid) engines. Generally this was seen when the owner had been using FRAM filters. I tested the theory on my 1970 C-10 inline 6, my dad's 1988 F150 5.8l, my grandfathers 1988 Chevy 1500 350 cid and a friends 1979 Dodge D150 with a 318 cid. The use of a FRAM oil filter on the Ford and Chevy motors did not change the oil pressure, on the Dodge it did. I quit using FRAM filters after that.

In the 6 yrs I spent in the dealership I rebuilt no less that 10 engines all due to oil pressure issues cause by use of a sub-standard oil filter.

John
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post #7 of 15 Old 07-16-2021, 05:26 PM
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Quote:
Originally Posted by K5TBL View Post
We will have to agree to disagree on this one. Pumps move volume, Pressure is created by a restriction of said volume. I spent 6 years working in a Chrysler Dealership and a heavy line mechanic, doing internal engine repairs, and found that the use of a less that precision made filter would reduce the oil pressure in the older 5.2l (318 cid) engines. Generally this was seen when the owner had been using FRAM filters. I tested the theory on my 1970 C-10 inline 6, my dad's 1988 F150 5.8l, my grandfathers 1988 Chevy 1500 350 cid and a friends 1979 Dodge D150 with a 318 cid. The use of a FRAM oil filter on the Ford and Chevy motors did not change the oil pressure, on the Dodge it did. I quit using FRAM filters after that.

In the 6 yrs I spent in the dealership I rebuilt no less that 10 engines all due to oil pressure issues cause by use of a sub-standard oil filter.
Quote:
Chrysler motor build oil pressure at the oil filter while Ford and Chevy build pressure at the camshaft.
Let me start with this. It makes no sense. Maybe you mean that Chrysler READS it oil pressure at the oil filter and Chevy READS its oil pressure at the camshaft.

You are 100% correct that the engine clearances provide the restriction, thus make the pressure.

You are proud of your resume. Maybe you should inquire sometime about mine. JS. If you gave up after 6 years you were just another one of those guys who just came and went in my eyes. I have nearly done your career 7 times over.

If you cannot fix it with a hammer then it has to be an electrical problem.
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post #8 of 15 Old 07-16-2021, 10:48 PM
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I'm not a professional mechanic and I've never played one on TV, I have however stayed at a Holiday Inn Express. I think you guys are sorta arguing semantics. If you have a well pump pumping 100 gallons an hour (I say 100 just in case someone really wants to do the math) into a open ended 10" pipe you'll obviously have less "pressure" than the same pump feeding a 1/2" pipe but the pipe hasn't created anything anymore than putting a spigot at the end of the pipe and saying it created the pressure, the pump stops so does the volume and the pressure.

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post #9 of 15 Old 07-17-2021, 12:15 AM
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If you want more oil pressure at hot idle,
  • Use a thicker oil
  • An oil cooler
  • Both the above

Oil pressure is controlled by wear, amongst other things.

Wear is controlled by
  1. The types of oil filter used over the years, amongst other things, look the chart
  2. The types and qualities of oil used over the years.
  3. How often oil was changed over the years.
  4. How often the oil filter was changed over the years.
.
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post #10 of 15 Old 07-18-2021, 03:49 AM
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I have seen sludge build up where the oil pressure sender is threaded in.
Make sure the passage is clean so the pressure sender can get a good reading.

Is the engine sludged up?

Assuming correct clean oil is used, low pressure can be from excessive clearance in main, rod (and or) cam bearings.
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post #11 of 15 Old 07-18-2021, 01:38 PM
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Don't they "read" the oil pressure at wherever the oil pressure sending unit happens to be on a given design?
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post #12 of 15 Old 07-18-2021, 01:58 PM
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Quote:
Originally Posted by cas6969 View Post
Don't they "read" the oil pressure at wherever the oil pressure sending unit happens to be on a given design?
Yes. The LS motors is top rear behind the intake or near the camshaft journal feed. Many Chryslers are near the oil filter.
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If you cannot fix it with a hammer then it has to be an electrical problem.
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post #13 of 15 Old 07-20-2021, 06:35 PM
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FWIW... my 88 2.5, I finally remembered to ditch the crap Fram filter and put in a... well... middle of the road Bosch. (wish there were more options for this engine)

Anyway I gained 10-15 pounds of oil pressure, if the gauge is to be believed.

Probably meaning no gain in oil flow, just more resistance to pump through the filter. I assume.

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post #14 of 15 Old 07-22-2021, 07:53 AM
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Well dang, I use Fram for al my cars lol...time to switch?
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post #15 of 15 Old 07-22-2021, 08:20 AM
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Go kill 15minutes (to 7 1/2 hours) on Youtube and watch people cut them open, they're sad inside. I think the only ones worse are the Wal-Mart brand, who probably has them made by Fram. lol

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