Fun With Oil Pressure - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**pjmoreland** · 07-19-2021, 02:29 PM

Last week I was out driving my truck. As I was accelerating to go around someone, I heard my engine start to make a faint noise it doesn't usually make. I looked down at my oil pressure gauge, and it was at zero! I immediately let off the gas, and the pressure came back up to normal. I headed straight home from there, staring at the gauge the entire time. I noticed that if I revved to 3500 RPM or higher under no load, the oil pressure would drop off. I checked my oil level when I got home, and it was right at the full mark. I had my oil pressure drop off one time several months ago, but that time my oil level was down two quarts. That was before I replaced my heads, and I had a huge oil consumption problem due to severely worn valve guides (smoke in the exhaust at startup).

I found a post on another website from a guy who had the same thing happen to him where his oil pressure would drop to zero at higher revs. The cause in that case was an oil pump pickup that was set too high. I pulled my oil pan and took some measurements. Sure enough, my oil pickup was about 1" off the bottom of the pan instead of the correct 3/8". The pickup was welded to the pump, so it hadn't come loose. It was just poorly installed by whoever built this engine 20+ years ago. This pump also produces excessively high pressure during warmup (gauge pegged out of view). I decided to install a new Melling 10553ST pump. I installed the lower pressure spring that came with it. I've got the pickup tube set to 3/8" off the oil pan floor (confirmed with clay), and now my oil pressure is right at 60psi during warm-up, and it hovers around 50-55psi when warmed up and driving. It's at 20psi at idle, which is a little higher than with the old pump, which was down around 15psi. I installed a Pioneer 839061 pickup tube clamp to keep it in place.

One thing I'm confused about is why I didn't see this problem earlier. Maybe the hot weather has something to do with it? I also noticed that the pressure relief valve in the old pump did not slide freely. It was scrapy feeling in the bore, and took a fair amount of pressure to dislodge it. Maybe it was getting stuck partially open. It could also be that this problem has been happening all along, but I just wasn't looking at my oil pressure gauge while accelerating aggressively. I hope my bearings are ok.

**Steeveedee** · 07-19-2021, 03:55 PM

If you still have the engine out, take a cap off #5or #6 rod and check for scoring or bluing. These seem to be the first to go when oil pressure is too low.

**pjmoreland** · 07-19-2021, 04:15 PM

Quote:

Originally Posted by Steeveedee

If you still have the engine out, take a cap off #5or #6 rod and check for scoring or bluing. These seem to be the first to go when oil pressure is too low.

Too late. Dang it!

Caddylackn · 07-19-2021, 04:55 PM

Well, look at it this way.

The damage (if there is bearing damage) is already done. Try to drive it and if there is no noise, knock, or abnormal temp, or low oil pressure- you dodged a bullet.

If there is a knock, high temps, metal in the oil, or low oil pressure, take it back out, have a machine shop grind the crank, new oversized bearings and you are new again. You learned a valuable lesson. I don't think it was the excessive oil pan clearance but a cheap Chinese oil pump and bad oil pressure relief valve that caused this. Maybe the oil pump was shimmed incorrectly? It should not run that high at startup, something was up with that oil pump. Running it two quarts low didn't help it any with the oil pick up 5/8" too high.

**pjmoreland** · 07-19-2021, 05:13 PM

Quote:

Originally Posted by Caddylackn

Well, look at it this way.

The damage (if there is bearing damage) is already done. Try to drive it and if there is no noise, knock, or abnormal temp, or low oil pressure- you dodged a bullet.

If there is a knock, high temps, metal in the oil, or low oil pressure, take it back out, have a machine shop grind the crank, new oversized bearings and you are new again. You learned a valuable lesson. I don't think it was the excessive oil pan clearance but a cheap Chinese oil pump and bad oil pressure relief valve that caused this. Maybe the oil pump was shimmed incorrectly? It should not run that high at startup, something was up with that oil pump. Running it two quarts low didn't help it any with the oil pick up 5/8" too high.

Yeah, you're right. Time will tell. I didn't see any metal in the oil when I dropped and cleaned out the oil pan. I went for a 40-mile drive yesterday in the heat of the afternoon, and everything seemed normal. My fingers are crossed.

The old pump was a Melling M-55. One slightly odd thing I noticed was that the pickup screen seemed collapsed compared to the new one.

schovil69 · 07-19-2021, 03:59 PM

Good catch on that one!

If you're still in there, pulling a rod cap or two and main would help relieve any bearing wear worries.

Rich72C10 · 07-19-2021, 05:07 PM

Man, that would freak me out if that happened to me! Seems like you had some good luck on this one catching it like you did.

**pjmoreland** · 07-19-2021, 05:51 PM

Quote:

Originally Posted by Rich72C10

Man, that would freak me out if that happened to me! Seems like you had some good luck on this one catching it like you did.

I sure hope so. I just opened up my oil filter, and there was no metal in it.

**MySons68C20** · 07-19-2021, 06:42 PM

Well that's good news!

**pjmoreland** · 07-19-2021, 06:53 PM

Quote:

Originally Posted by MySons68C20

Well that's good news!

I agree!

**pjmoreland** · 08-11-2021, 11:47 AM

I have discovered that my oil pressure is still dropping above 4000 RPM. It is just dropping much more slowly than before, so I wasn't noticing it during quick acceleration where revs aren't high for very long. If I hold the engine at or above 4000 RPM, then the oil pressure drops at a steady rate. I have let it get as low as 30psi, but I suspect it would keep dropping below that if I let it.

I tried adding half a quart of oil, and this didn't help. I then drained a quart and a half so I was one quart low. This made the low pressure issue go away! It seems I have a windage issue.

I am running a Milodon 30700 stock-style 5-quart oil pan. It has internal baffling like a stock pan. When I do an oil change with a new filter, I add 5 quarts, and the oil level goes exactly to the high mark on the dipstick.

It seems like there must be something abnormal about my setup. I have read that windage can be a problem with 383 cranks, but I'm pretty sure I don't have a 383. There are no relief cuts in the block. Any ideas?

body bolt · 08-11-2021, 12:18 PM

Yeah, it sounds like aerated oil.

**pjmoreland** · 08-11-2021, 12:50 PM

Quote:

Originally Posted by body bolt

Yeah, it sounds like aerated oil.

Yep. I agree. I just don't understand why my engine is doing this when there are millions of stock 350s running around without this problem.

**pjmoreland** · 08-11-2021, 01:08 PM

Here are some photos of my engine's bottom end. Is this a stock crankshaft? Is there anything obvious in these photos that would be the cause of a windage issue?

**Steeveedee** · 08-11-2021, 02:52 PM

Is the tube on your sump screen metal? I'm not really sure what I see, there. Are pics 2 and 3 from the same engine? The sump screen and tubes are different.

**pjmoreland** · 08-11-2021, 02:56 PM

Quote:

Originally Posted by Steeveedee

Is the tube on your sump screen metal? I'm not really sure what I see, there.

It is metal. It's a Milodon 18600. It's the one that was recommended for use with my oil pan.

https://www.summitracing.com/parts/mil-18600

I used one of these tools to install it:

https://www.summitracing.com/parts/pro-66491

A tool that clamps onto the pickup tube would be better, but this one worked.

**Steeveedee** · 08-11-2021, 07:02 PM

OK I got nothing on the low oil pressure issue, nothing jumps out at me. I've installed dozens of pickup screens, but I never used anything that fauncy. I just used the right size of an old open end wrench.

**pjmoreland** · 08-11-2021, 07:04 PM

Quote:

Originally Posted by Steeveedee

OK I got nothing on the low oil pressure issue, nothing jumps out at me. I've installed dozens of pickup screens, but I never used anything that fauncy. I just used the right size of an old open end wrench.

Yep. I've done that in the past. I was just trying to be as careful as possible with this so I wouldn't question it later.

**pjmoreland** · 08-11-2021, 07:06 PM

I'm thinking I'll start adding oil back in, a little bit at a time, and see when the oil pressure problem comes back. If it works alright at half way between the low and full marks, then I'll just live with it.

body bolt · 08-11-2021, 07:56 PM

Do you know what depth your dipstick reaches? Maybe your overfilling?

**pjmoreland** · 08-11-2021, 08:03 PM

Quote:

Originally Posted by body bolt

Do you know what depth your dipstick reaches? Maybe your overfilling?

That's a good question. I haven't taken any measurements. All I know is when I add five quarts to an empty pan and empty filter, it ends up exactly at the full line on the dipstick. My oil pan is advertised as a "5 quart" pan. Let me see if I can figure out some way to measure the dipstick level. It's a Mr. Gasket dipstick & tube, and the tube is fully seated on the block.

**pjmoreland** · 08-11-2021, 08:58 PM

Quote:

Originally Posted by body bolt

Do you know what depth your dipstick reaches? Maybe your overfilling?

I took some measurements:

Block deck to bottom of dipstick hat = 6-1/4"
Dipstick full line to bottom of dipstick hat = 17"

Subtract one from the other to get the distance from the block deck to the full line:

17" - 6-1/4" = 10-3/4"

According to the website linked below, GM spec'd it at 11", so my fill line is 1/4" too high. The distance between the full and low lines is 7/8", so being 1/4" too high is slightly more than 1/4 of a quart (2/7 of a quart).

https://nastyz28.com/threads/check-y...-length.32460/

**pjmoreland** · 08-12-2021, 02:18 AM

This evening I tested different oil levels to see at what point the windage issue would come back. I started at the low line on the dipstick, which is 4 quarts, and added 1/4 of a quart for each test run. I saw no oil pressure drop at high rpm with 4-1/4 and 4-1/2. Then at 4-3/4 the pressure started dipping a little at 5000 rpm. Yesterday when I had 5-1/2 quarts in there, it would start dipping at 4000 rpm. Tomorrow I'm going to back it off to 4-1/2 quarts and drive it that way for a while to see how it does.

**pjmoreland** · 08-14-2021, 01:15 PM

Here's something both annoying and convenient. I drained 1/4 quart of oil out of my engine to get it to the level I found was the highest that wouldn't cause the oil prrssure to drop at high RPM (5500). This oil level was showing about 2/3 of a quart below the full line on the Mr. Gasket dipstick I've been using. I had another dipstick that I bought from Classic Parts last year. I tried it out, and the oil goes exactly to the full line. I guess I'll be using that dipstick from here on out. I no longer have the tube from the Classic Parts dipstick, so it's possible it was longer than the one currently installed. I think it's safe to say this Mr. Gasket dipstick reads low.

**geunther** · 08-14-2021, 03:43 PM

Getting ready to install new oil pump. Did you set the clearance 3/8" from the lowest part of the pickup?

07-19-2021, 02:29 PM	#1
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 6,039	Fun With Oil Pressure Last week I was out driving my truck. As I was accelerating to go around someone, I heard my engine start to make a faint noise it doesn't usually make. I looked down at my oil pressure gauge, and it was at zero! I immediately let off the gas, and the pressure came back up to normal. I headed straight home from there, staring at the gauge the entire time. I noticed that if I revved to 3500 RPM or higher under no load, the oil pressure would drop off. I checked my oil level when I got home, and it was right at the full mark. I had my oil pressure drop off one time several months ago, but that time my oil level was down two quarts. That was before I replaced my heads, and I had a huge oil consumption problem due to severely worn valve guides (smoke in the exhaust at startup). I found a post on another website from a guy who had the same thing happen to him where his oil pressure would drop to zero at higher revs. The cause in that case was an oil pump pickup that was set too high. I pulled my oil pan and took some measurements. Sure enough, my oil pickup was about 1" off the bottom of the pan instead of the correct 3/8". The pickup was welded to the pump, so it hadn't come loose. It was just poorly installed by whoever built this engine 20+ years ago. This pump also produces excessively high pressure during warmup (gauge pegged out of view). I decided to install a new Melling 10553ST pump. I installed the lower pressure spring that came with it. I've got the pickup tube set to 3/8" off the oil pan floor (confirmed with clay), and now my oil pressure is right at 60psi during warm-up, and it hovers around 50-55psi when warmed up and driving. It's at 20psi at idle, which is a little higher than with the old pump, which was down around 15psi. I installed a Pioneer 839061 pickup tube clamp to keep it in place. One thing I'm confused about is why I didn't see this problem earlier. Maybe the hot weather has something to do with it? I also noticed that the pressure relief valve in the old pump did not slide freely. It was scrapy feeling in the bore, and took a fair amount of pressure to dislodge it. Maybe it was getting stuck partially open. It could also be that this problem has been happening all along, but I just wasn't looking at my oil pressure gauge while accelerating aggressively. I hope my bearings are ok. Attached Images

07-19-2021, 03:55 PM	#2
Steeveedee Who Changed This? Join Date: Jan 2013 Location: Simi Valley, CA Posts: 10,746	Re: Fun With Oil Pressure If you still have the engine out, take a cap off #5or #6 rod and check for scoring or bluing. These seem to be the first to go when oil pressure is too low. __________________ ~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA

07-19-2021, 04:55 PM	#4
Caddylackn Registered User Join Date: Jun 2016 Location: Bremerton, WA Posts: 171	Re: Fun With Oil Pressure Well, look at it this way. The damage (if there is bearing damage) is already done. Try to drive it and if there is no noise, knock, or abnormal temp, or low oil pressure- you dodged a bullet. If there is a knock, high temps, metal in the oil, or low oil pressure, take it back out, have a machine shop grind the crank, new oversized bearings and you are new again. You learned a valuable lesson. I don't think it was the excessive oil pan clearance but a cheap Chinese oil pump and bad oil pressure relief valve that caused this. Maybe the oil pump was shimmed incorrectly? It should not run that high at startup, something was up with that oil pump. Running it two quarts low didn't help it any with the oil pick up 5/8" too high. __________________ 1969 Custom Camper C20; Factree Air, 350/TH400/Dana 70U with C30 wheel cylinders, Disk brakes, H4 conversion, headlight relay mod, 3G 135 amp alternator. 7500 GVW

07-19-2021, 05:07 PM	#7
Rich72C10 Mr. Cheyenne Join Date: Jun 2020 Location: Kyle, Texas Posts: 1,687	Re: Fun With Oil Pressure Man, that would freak me out if that happened to me! Seems like you had some good luck on this one catching it like you did. __________________ Richard *'72 Chevy C10 Cheyenne Super WHT/MED BRONZE/WHT SBC 350 \| TH350 \| LWB \| AC \| Buddy Seats Upgrades:** Tilt \| Front Sway Bar \| Radio \| Sliding Window \| Bed Spare w/Lock Bed Rails \| Full LEDs \| Courtesy Lights \| Tachometer \| Glovebox Lock & Light \| Head Light Relays \| *Upgraded Fully Super Trim

07-19-2021, 06:42 PM	#9
MySons68C20 Senior Member Join Date: Sep 2014 Location: Wenatchee, Wa Posts: 940	Re: Fun With Oil Pressure Well that's good news!

07-19-2021, 03:59 PM	#6
schovil69 Registered User Join Date: Oct 2013 Location: Lincoln, NE Posts: 117	Re: Fun With Oil Pressure Good catch on that one! If you're still in there, pulling a rod cap or two and main would help relieve any bearing wear worries.

08-11-2021, 11:47 AM	#11
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 6,039	Re: Fun With Oil Pressure I have discovered that my oil pressure is still dropping above 4000 RPM. It is just dropping much more slowly than before, so I wasn't noticing it during quick acceleration where revs aren't high for very long. If I hold the engine at or above 4000 RPM, then the oil pressure drops at a steady rate. I have let it get as low as 30psi, but I suspect it would keep dropping below that if I let it. I tried adding half a quart of oil, and this didn't help. I then drained a quart and a half so I was one quart low. This made the low pressure issue go away! It seems I have a windage issue. I am running a Milodon 30700 stock-style 5-quart oil pan. It has internal baffling like a stock pan. When I do an oil change with a new filter, I add 5 quarts, and the oil level goes exactly to the high mark on the dipstick. It seems like there must be something abnormal about my setup. I have read that windage can be a problem with 383 cranks, but I'm pretty sure I don't have a 383. There are no relief cuts in the block. Any ideas?

08-11-2021, 12:18 PM	#12
body bolt Registered User Join Date: Oct 2020 Location: Albuquerque, New Mexico Posts: 474	Re: Fun With Oil Pressure Yeah, it sounds like aerated oil.

08-11-2021, 01:08 PM	#14
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 6,039	Re: Fun With Oil Pressure Here are some photos of my engine's bottom end. Is this a stock crankshaft? Is there anything obvious in these photos that would be the cause of a windage issue? Attached Images Last edited by pjmoreland; 08-11-2021 at 01:31 PM.

08-11-2021, 02:52 PM	#15
Steeveedee Who Changed This? Join Date: Jan 2013 Location: Simi Valley, CA Posts: 10,746	Re: Fun With Oil Pressure Is the tube on your sump screen metal? I'm not really sure what I see, there. Are pics 2 and 3 from the same engine? The sump screen and tubes are different. __________________ ~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA

08-11-2021, 07:02 PM	#17
Steeveedee Who Changed This? Join Date: Jan 2013 Location: Simi Valley, CA Posts: 10,746	Re: Fun With Oil Pressure OK I got nothing on the low oil pressure issue, nothing jumps out at me. I've installed dozens of pickup screens, but I never used anything that fauncy. I just used the right size of an old open end wrench. __________________ ~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA

08-11-2021, 07:06 PM	#19
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 6,039	Re: Fun With Oil Pressure I'm thinking I'll start adding oil back in, a little bit at a time, and see when the oil pressure problem comes back. If it works alright at half way between the low and full marks, then I'll just live with it.

08-11-2021, 07:56 PM	#20
body bolt Registered User Join Date: Oct 2020 Location: Albuquerque, New Mexico Posts: 474	Re: Fun With Oil Pressure Do you know what depth your dipstick reaches? Maybe your overfilling?

08-12-2021, 02:18 AM	#23
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 6,039	Re: Fun With Oil Pressure This evening I tested different oil levels to see at what point the windage issue would come back. I started at the low line on the dipstick, which is 4 quarts, and added 1/4 of a quart for each test run. I saw no oil pressure drop at high rpm with 4-1/4 and 4-1/2. Then at 4-3/4 the pressure started dipping a little at 5000 rpm. Yesterday when I had 5-1/2 quarts in there, it would start dipping at 4000 rpm. Tomorrow I'm going to back it off to 4-1/2 quarts and drive it that way for a while to see how it does.

08-14-2021, 01:15 PM	#24
pjmoreland Senior Member Join Date: Jan 2005 Location: CA Posts: 6,039	Re: Fun With Oil Pressure Here's something both annoying and convenient. I drained 1/4 quart of oil out of my engine to get it to the level I found was the highest that wouldn't cause the oil prrssure to drop at high RPM (5500). This oil level was showing about 2/3 of a quart below the full line on the Mr. Gasket dipstick I've been using. I had another dipstick that I bought from Classic Parts last year. I tried it out, and the oil goes exactly to the full line. I guess I'll be using that dipstick from here on out. I no longer have the tube from the Classic Parts dipstick, so it's possible it was longer than the one currently installed. I think it's safe to say this Mr. Gasket dipstick reads low. Attached Images



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08-14-2021, 03:43 PM	#25
geunther Senior Member Join Date: Apr 2005 Location: Cheyenne, WY Posts: 740	Re: Fun With Oil Pressure Getting ready to install new oil pump. Did you set the clearance 3/8" from the lowest part of the pickup?

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