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Lower oil pressure after oil change?

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Lower oil pressure after oil change?
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1994ranger

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Location
Middle TN
Vehicle(s)
'79 CJ7, 360, T176, D300, 8.8 rear, D44 front, aspiring to swap a 5.3/4L60E!
I just did my first oil change (not just on the CJ, but ever) on my 79 CJ7 w/ an AMC 360. Oil that was in it was very black (no idea of weight) and filter was made by STP. I used 5qts of Lucas Hot Rod 10W-30 (non-synthetic with added zinc) and a K&N HP-2003 filter. I put a decent amount in the filter (near full) before screwing it on.

I ran the engine to operating temp (180F), shut it off, waited 20 minutes, and changed the oil. With old oil, my pressure was about 55 PSI at fast idle 42 PSI at normal idle. After the change, my pressure was about 45 PSI at fast idle, but dropped to 20PSI at normal idle. When I press the throttle, it jump up to 30PSI (cannot recall the RPM but it seemed to scale upwards accordingly).

I am in the middle TN area and its 90F today with high humidity. Given these conditions, my oil choice, and filter, does 20 PSI seem right? From my research, I think I am ok, but wanted to see what others thought.

Thanks everyone!
 
My old Mopar race engine manual says minimum 10psi for every 1000 rpm, but if I was less than 20 at idle I would be nervous as well


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Thank you both for the replies. I forgot some details. I am using a mechanical gauge and not the factory one. The sender is located near the oil filter in the block and is going to the gauge.

Warm idle is around 650 rpm, which is when the PSI is about 20. I'll play with it more tomorrow, but the PSI definitely increased as I gave it more throttle
 
Up your idle to 800rpm.
LG
 
I checked the oil level yesterday with a cold engine and it was halfway between the Add line and Full line on the dipstick. Just a few minutes ago, I went out and added more oil (cold engine again) to where it was right at or just under the Full line. Started the engine and cold start oil pressure was almost 60 PSI which was an improvement (was 45-48). However, as it warmed up and hit 180F, the pressure went back down to 20 PSI.

Adding oil didn't do much, but I suspected that. LG - I think you are right - I need 15W40 or something thicker than the 10W30 I put in it. Shucks. I'd rather get this right the first time, so would you say adding a blue bottle of STP would have the same effect as redoing it with 15W40? I'd think 15W40 would be better.

Also LG - I am going to have someone rebuild the carb for me (I'm not confident enough to do that yet). When they reinstall and tune, I'll have them adjust the idle to 800. Thank you for the suggestion!
 
Lumpy Grits said:
The STP will help. I run it, and Castrol 20W-50 hi-mileage oil.

You really need to learn the carb.

A site for you to bookmark.

Jeep Adventures Under the Hood



Carb info-

https://drive.google.com/file/d/1Y7ZOg0RGaeHSgP7hRDvrefTjiQxMna23/view



Just turn the idle speed screw 1/8-1/4 turn, and you should be GTG.



LG
Click to expand...



Dude that carter manual is cool [emoji41] first time I have seen anything other than the little adjustment sheet you get with the rebuild kit


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Thanks for the links LG, will dig through those.

Also, I've got 5qts of 10W30 in there now. Looks like the STP bottle is also a quart, and it says it should be used w/ 5qts of oil. Is this going to be too much oil (Haynes manual calls for 5qts for my 360)? I don't want to have issues with seals.

The oil I put in has added ZDDP and I see the STP has that too. Is there such a thing as too much ZDDP?

Thanks again.
 
Last edited:
Drain a QT out and add the STP.
LG
 
LG - I'm going to try this today and will report back. To confirm, this is the blue bottle you're talking about, correct?

https://www.walmart.com/ip/STP-Oil-Treatment-15-fluid-ounces-8262-Oil-Additives/16817398

Also, this just came to me and I should've asked - the Jeep is now putting out more blue smoke than I did before I put in the 10W30. Research tells me its burning oil. Would a thicker oil like the ones you recommended reduce this?

Thanks!
 
Last edited:
1994ranger said:
LG - I'm going to try this today and will report back. To confirm, this is the blue bottle you're talking about, correct?

https://www.walmart.com/ip/STP-Oil-Treatment-15-fluid-ounces-8262-Oil-Additives/16817398

Also, this just came to me and I should've asked - the Jeep is now putting out more blue smoke than I did before I put in the 10W30. Research tells me its burning oil. Would a thicker oil like the ones you recommended reduce this?

Thanks!
Click to expand...

With the smoke deal-
Look at a V8 size can or Restore in place of the STP.
https://www.walmart.com/ip/Restore-...MIsrOQ84Xv4wIVCv_jBx01bgM_EAQYAiABEgL1Q_D_BwE
BTW-That was the STP I spoke of in your link.
 
Lumpy Grits said:
With the smoke deal-
Look at a V8 size can or Restore in place of the STP.
https://www.walmart.com/ip/Restore-...MIsrOQ84Xv4wIVCv_jBx01bgM_EAQYAiABEgL1Q_D_BwE
BTW-That was the STP I spoke of in your link.
Click to expand...

Thank LG, I just ran a can of V8-sized Restore in it. I added it to the 5qts and let the engine run for about 15 minutes. Took just under 3 seconds to build pressure to 60PSI. The blue smoke level seems to be closer to what it use to be, which is good.

My oil pressure, however, is still at 20PSI at 180F warm idle. So I resolved one issue and still need to figure out the other.

What would you do at this point? Would you recommend going with something like 15W40 to boost pressure + a can of Restore? Middle TN likely gets colder than the Mohave where you are (occasionally single digits here), so I don't know how well 5W20 would do in the winter.

And thanks for sticking with me on this!
 
We will see the 20's and sometimes the low 'teens' here. It's 105* now.:eek:
I run 20W-50 year round.
I suggest you drive it for a spell and the PSI may well increase.
Also-up the idle to 800rpm.............
If you're not sure which screw is for 'curb-idle. Post a picture of'em..
LG
 
Y'all got me wondering about my oil now. I don't think the gauge works as it reads 20-30 when I'm driving. I was planning on replacing it but I guess I'll try a heavier oil first.

Lumpy Grits said:
We will see the 20's and sometimes the low 'teens' here. It's 105* now.:eek:
I run 20W-50 year round.
I suggest you drive it for a spell and the PSI may well increase.
Also-up the idle to 800rpm.............
If you're not sure which screw is for 'curb-idle. Post a picture of'em..
LG
Click to expand...
 
Work has kept me busy. I ran the Jeep for about 20 minutes yesterday. I did not go far (couple of miles). At 2k RPM, oil pressure was just under 40 PSI, but towards the end of my drive, at idle, it was at 15 PSI.

It seemed to run fine and I didn't hear any new noises (I've had a lifter tick for a while), but that low pressure worried me, so I brought it back home.

LG - would you continue to drive this and see if things improve, or is 15 PSI at idle just too low?

I failed to mention I have a Holley Truck Avenger carb (670cfm) and an Edelbrock RPM Air Gap intake. Based on the documentation I've seen, I think this is the curb idle adjustment screw:
xJqjuAY.webp
 
Up your idle to 800-850rpm, and see what the gauge shows then.
LG
 
So I upped my idle. Wasn't as hard as I made it out to be in my mind, and it runs a bit better too. Thanks for that suggestion. Its now between 800 - 850rpm. Ran the engine for a half hour and drove a couple of miles. RPMs never went past 3k.

After it warmed up (180F), pressure was still about 20 PSI, maybe a shade higher. Pressures looked good when driving (still 40ish PSI @ 2k RPM).

Sadly bumping the idle didn't impact my pressure much at all but I am still glad I did it. What would you do next? Swap oil or are there more things I can try?
 
Drive it and enjoy.......
What's your hot oil pressure at 65mph.
40psi at 2K rpm is fine.
LG
 
Thanks LG, will do. Never had the Jeep up to 65mph. Without sway bars I'll probably keep it around 50.

The pressure at 2k looks good, but I am concerned about 15-20 PSI at idle. I'll drive it for a bit and see how things go. Thanks for all the help!
 
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