• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Oil pressure

Share this page

Morris

Morris

Yoda
Offline
I don't really want to know the answer to the question I am about to ask, but I guess I better get to the bottom of it before I drive the car anymore.

I installed an oil gauge recently, and was happy to discover after relying on my oil light for years, that my oil pressure is quite good. 60#s at cold idle 20#s after it warms up a bit. On the road with a warm engine I get 5 to 10 more than my rpm if the rule is 10psi per 1000rpm. In other words, when I am cruising down the highway at 4000rpm, my pressure is between 45 and 50 psi.

So here's the gotcha... After the car has been running for a while and things are getting really hot, my idle pressure drops to almost nothing. If I put the gas on, pressure climbs right up to where it should be. But there is nothing at idle. BUT, my idiot light does not come on.

What do you guys think? Is this a failure of the gauge I bought at Harbor Junk? (why do I keep going there?!?!?) A bad oil pump? Worn bearings? (my engine is freshly rebuilt, so if you want to see me cry, tell me its my bearings)

To confuse the diagnosis further, I am also addressing a weird cooling problem and just corrected a lean condition at idle (which may correct the cooling problem, I don't know yet).

So there you go... The game is afoot!
 
Time for a blown small block chevy just like your avatar.
/bcforum/images/%%GRAEMLIN_URL%%/wink.gif
 
Mine drops to around 10-15 when the engine gets really warmed up and the idle drops to around 800 or less, but the cold idle pressure is around 20. The only time it got much lower (I won't say how low) at hot idle was when I had 10W-30 in it in summer.
 
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]Time for a blown small block chevy just like your avatar.[/QUOTE]

NEVER!
 
If I was you(have experienced similar in my first 6) I would purchase an oil pressure relief valve spring(and washer, depending on the design you may need a new sealing washer). I found that the spring like all British steel in torsion, settles over time. It brought the pressure up where I was happy with it. 15-20 at hot idle.
 
I am pretty sure I replaced that when I rebuilt. I will have to look over my receipts. That portion of the project happened a long time ago. Of course, replacement parts being what they are these days, "new" does not necessarily mean "good."
 
"""""" that my oil pressure is quite good. 60#s at cold idle 20#s after it warms up a bit. """"""

"""""""and things are getting really hot, my idle pressure drops to almost nothing. """"""

A drop in oil pressure as the oil gets hotter is normal. A BIG drop...as in 60# cold at idle to "almost nothing at idle.....when really hot" means that your oil is up to temp.....about 200* and just motoring to the drugstore down the street the oil might only get to 20# as the oil wont be at more than about 120*..

What this means is the oil flows a lot easier when its quite hot.

This is an indication then, of excessive clearances or clearances on the high side of rod and main bearings, rod big end side clearances, camshft bearings and rocker bushings....among others.

All of the above assumes that you are using about a 20-40 oil and the idle speed is correct. The pressure relief spring and bullet is linear and not progressive.....you dont want to screw more spring in there as the cold oil pressure will also increase. The only band aid in that department is to use a thinner oil for lower cold start pressure and then use a tighter spring. I certainly would not want cold oil to have more than 40 psi at idle.

fix your cooling system also.....thats not a good thing either.
 
Yeap, dropping to 'almost nothing' is bad. I'm almost in agreement that its internal to the engine. However, a cheap double check is to replace the guage with another & then if they both do the same thing.........
 
Idle speed is about 900. I am running 10w-40 with a bottle of STP.

Clearances in my motor are definitely on the high side but within spec. I made a bad choice when I picked the machine shop that did my work.
 
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]I am running 10w-40 with a bottle of STP.[/QUOTE]

UH-uh....20W50 with no additives...change & see what happens.
 
The 10w-40 and STP is just intended to break in my new lifters. I was only planning on running it for a few hundred miles before switching back to good ol Castrol 20-50. How long do I need to leave the STP in there before my lifters have soaked up enough ZDDP?
 
The gauge might not be accurate. My Sprite with 20w50 in it and a freshly rebuilt engine does up to 80psi when cold. When hot, it idles at 20psi, runs down the road at about 50psi.

My other sprite has less oil pressure, but has never been rebuilt and I'm using 15w40 diesel oil in it.

Tullamore on here has a midget and a Sprite and they both have different oil pressure readings while running.

I never worried about oil pressure so much until I got these cars with actual needles telling you how much you have. Every other car I've ever owned has simply had an idiot light /bcforum/images/%%GRAEMLIN_URL%%/wink.gif

Zero PSI in reality is obviously 'bad' so you should at the very least check into trying another gauge. That or adjust your idle higher, since it sounds like you're fine while running down the road.
 
I don't think it's literally 0, because my idiot light is not coming on. A non-chinese crap gauge is definitely in my future.
 
Here's an update.

I bought a good oil pressure guage and determined the real situation. My idle pressure is around 10psi and my cruise around 45 psi. However, when I have been driving for a very long time, those numbers can fall a bit. This worries me. I replaced oil with 20-50 and a bottle of STP(for my new lifters)and I shimmed my pressure by pass a bit. Now my cruise pressure is around 55psi, but my idle is rougly 11-12 psi.

I bet dollars to donuts its the stupid no-bearing camshaft. I new I should have replaced it with an early Spit cam when I had it out, but I could not afford it.

Oh well, I am going to keep driving it until the oil pressure situation gets worse, then I will worry about it.
 
Did you replace the oil pump? I am sure mine with the same symptions is that.
 
I did not. I checked it and it was within spec, so I reused it. I figured it was not the oil pump since I am getting such good high side numbers. Am I wrong?
 
I used an old pump as the new one would not fit, you've heard that prob before. So............

I have 70 lbs cold at start, 40 lbs at idle cold.

All is ok till engine gets up to 180 temp where oil thins and now I have 25 or so at run and 10 at idle. It is much the same with 20-50 oil or 30W. The 30 is a couple pounds better.

Engine bearings and cam bearings are all new, etc, etc. I am just sure in my mind that it is the darn pump. No telling how old it is but had never had a pump prob before.

Now have two new pumps on my bench, both the same so both will not quite fit but when it cools next month I will pull the engine and modify the backing plate as necessary to make em fit. Hap and others here say the new pumps are just not quite like the older ones and a bit of grinding is needed.

Of course I will open up the old pump and check clearances and see what the heck is going on in there. Hap says I should even see the wear as it should be real bad.

I have a hard time beleiving that my new bearings, etc, etc are the problem.

However, that said if with a new pump I still have problems it is pull the engine again and take it up to Hap for a look see inside. At that point I will know that I have done all I know how to do and beleive me this is not my first rebuild on an A40 engine and I was more careful with this one than ever before.
 
That's tough Jack. I was not nearly as meticulous with my rebuild, so I figure I am getting what I had comin'.
 
Ive had your situation for 4 yrs. 10 at idle 50 cruising (hot). I use to worry about it but stopped. The engine runs smooth still adequate power no noises or rattles. Why me worry...
Don M
 
Yea I know still within spec but at the bottom. Just scares the heck out of me though. Something is wrong and I will find it. Hate it when something is wrong.
 
Back
Top