I need some opinions on an oil pressure drop on my 350 Chev. When the engine was first built about 10yrs ago,the startup oil pressure was 60-70PSI, hot around40. This year I noticed its around 40PSI cold and about 20 hot. I double checked my dash gage and its ok. I don't know if its the pump dying or if the motor is finally broken in. It has maybe 10 thousand mi on it. Any thoughts?
I checked the dip stick oil -no sparkles- but I will check the oil filter also. I hope its just clearances but I will be watching closely. I cant detect any knocking or noises so Im hoping its not too serious.
You seem to have a base line that the pump would produce at one time at least 60 psi. That doesn't mean there was any oil at all getting to the motor but when the relief would open and dump any excess back to the pan. the drop from cold to hot is due to viscosity the thickness of the oil,that seem to be consistent , if the pump is working correctly you should see a increase in pressure as the rpm increase. Psi = force/ area the area never changes the fluid just flows past easier from hot to cold. Gpm =CIR x rpm /231. As rpm increases more oil is packed into the same area = more resistance making the pressure rise . If you have 20 psi at 800 rpm and 20 psi at 4000 rpm and you know at one time you could get 60psi I would suspect wear in the pump
You seem to have a base line that the pump would produce at one time at least 60 psi. That doesn't mean there was any oil at all getting to the motor but when the relief would open and dump any excess back to the pan. the drop from cold to hot is due to viscosity the thickness of the oil,that seem to be consistent , if the pump is working correctly you should see a increase in pressure as the rpm increase. Psi = force/ area the area never changes the fluid just flows past easier from hot to cold. Gpm =CIR x rpm /231. As rpm increases more oil is packed into the same area = more resistance making the pressure rise . If you have 20 psi at 800 rpm and 20 psi at 4000 rpm and you know at one time you could get 60psi I would suspect wear in the pump
Yes Slim ,that was my first thought. When I rev the engine, the pressure drops a bit,maybe 3-4 PSI when hot. I am suspicious of the pump,maybe the bypass. It is an after market hi volume pump. I think I will replace it before the racing season.
I need some opinions on an oil pressure drop on my 350 Chev. When the engine was first built about 10yrs ago,the startup oil pressure was 60-70PSI, hot around40. This year I noticed its around 40PSI cold and about 20 hot. I double checked my dash gage and its ok. I don't know if its the pump dying or if the motor is finally broken in. It has maybe 10 thousand mi on it. Any thoughts?
When I rev the engine, the pressure drops a bit,maybe 3-4 PSI when hot. I am suspicious of the pump,maybe the bypass. It is an after market hi volume pump. I think I will replace it before the racing season.
WHAT ????????
Are you saying that the engine has 20 lbs at idle, when warm then DROPS (????) 3-4 pounds as the engine is revved, and STAYS below 20lbs, regardless of what rpm the engine is revved to???????
If I'm reading this correctly, you DEFINITELY have a problem.
Sometimes more is not always a good thing. I could see the high volume pump if you built the motor to race specs.... loose.... Were you need to replace the volume created by more clearance . Oils main purpose it to lubricate and cool the parts it comes in contact with . When the relief / bypass opens that extra oil is doing nothing but go through the pump and dump back into the pan. I shouldn't say nothing as the excess is transformed into heat at the cost of hp. As I said before volume increases with rpm so if you are sitting at maximum pressure at idle Increasing rpm just adds more heat to the oil with no benefit to the system . If you look at the new cars you will notice a trend that they are using thinner oil 5 or 0/ 20. One the oil today is better especially synthetic Two the lighter oil flow through easier doing the job it was meant to do without the lose in hp. Myself I like to run a light enough oil that you get a decent base line pressure at idle and the pressure will max out with the top rpm I also like synthetic . many won't agree as it is a total 180 to what we where taught .
I understand the loss of pressure with a loose or broken-in engine but I find it strange that it drops slightly when reved under load.I seem to remember reading that there is a gasket between the pump and block and if it blows out you loose pressure. I will look into that as I don't know if its true.
Friend had issues with oil pressure .... turns out he used teflon tape on the sending unit (a no-no) and a "flap" of teflon tape was sometimes covering the hole and causing the gauge to act up. Have you used a known-to-be-good gauge to double check the pressure?
As far as I see it, the pan needs to come off. Drop the pan, check the bearings (Plastigauge), you can also make sure the pickup tube is where it needs to be (they can loosen and fall off). With the pan off, at that point I'd just toss on a new pump (cheap enough). Also gives you the perfect opportunity to see what kind of stuff (if any) is laying on the bottom of the pan.
If you oil pressure is 20 at idle/warm, and never rises with rpm ... you have issues.
If you think its something other then the pump . drop the pan extend the pickup with a hose and run the pump with a old distributer and a drill. watch to see were the oil is pouring out . Bit messy but effective . A bucket or pail to the pickup may be less hassle.
-- Edited by slim on Sunday 15th of February 2015 06:12:52 PM
Friend had issues with oil pressure .... turns out he used teflon tape on the sending unit (a no-no) and a "flap" of teflon tathat I have 2 bad gages.I will double check with a new gage beforepe was sometimes covering the hole and causing the gauge to act up. Have you used a known-to-be-good gauge to double check the pressure?
As far as I see it, the pan needs to come off. Drop the pan, check the bearings (Plastigauge), you can also make sure the pickup tube is where it needs to be (they can loosen and fall off). With the pan off, at that point I'd just toss on a new pump (cheap enough). Also gives you the perfect opportunity to see what kind of stuff (if any) is laying on the bottom of the pan.
If you oil pressure is 20 at idle/warm, and never rises with rpm ... you have issues.
I have a T on my oil sensing tube so I can cross check my dash gage with my bench gage . It is possible that I have 2 bad gages so I will double check with a new gage before I tear into the engine
Good tip,Slim. I will try all the basic stuff before I drop the pan. I have to lift the front of the engine to get the pan off. Pain in the butt. Getting too old for this.
You already posted but I added a bucket or pail may be easier for the pickup . I think it highly unlikely two gages are wrong. SBC are not my thing but as i understand you need a special tool or an old distributer as part of the oil passage runs past the distributer body.
I understand you need a special tool or an old distributer as part of the oil passage runs past the distributer body.
Yes, this is true. I made a pre-oiler out of an old HEI. Just grind off the teeth that mate to the cam. The dist body needs to be in place in order for both sides of the engine to get oil.
Wonder if one of the oil galley plugs behind the timing chain popped out. Not sure what that would show up as on a gauge though.
If you need an oil pump primer or pre-oiler, I got one you can use. I'm in Simcoe, so not too far away. I can even lend you a hand, if your shop is warm...Lol
-- Edited by Moobster on Sunday 15th of February 2015 09:13:20 PM
On mine the pressure was acting funny up and down till I added another 1/2 quart of oil. I have a high volume pump and stock pan and was sucking the pan dry and getting some foaming in the oil. Was an easy fix for me.
Moobster-thanks for the offer,the shop is warm but crowded. I will probably start working on the 39 (small block) in a month or so. Im doing an engine swap on the Caddy now and that's a whole different story.2 heads are better than 1.Thanks everyone for all the tips. I will try all of them -the easy ones first of course!
Moobster-thanks for the offer,the shop is warm but crowded. I will probably start working on the 39 (small block) in a month or so. Im doing an engine swap on the Caddy now and that's a whole different story.2 heads are better than 1.Thanks everyone for all the tips. I will try all of them -the easy ones first of course!
The oil pump primer is here when you need it. Cheers
-- Edited by Moobster on Monday 16th of February 2015 09:08:06 AM
What is your oil pressure at hot idle, and at 2500rpm? At 2500rpm you should be on the bypass, even with a worn out pump.
20psi at idle is no problem, but should rise with RPM until it hits enough pressure to overcome the bypass spring. Given that you had 60-70psi before with cold oil, I would say that you have the Z28 spring in the pump, so by 2500rpm you should be seeing 60psi or so. If it does this, I would not worry about it - the clearances have just opened up. If the pressure does not rise, then I would check the pump for a stuck bypass, or a cracked casting - not uncommon with the Melling M55 pumps. If these check out ok, I would tear down the motor as you might have a serious problem.
Thanks Racer Rick- I may have to pull the engine to change the pump. I am going to run a few more tests before I do anything but I still suspect the pump.
