oil pressure sudden drop
 

My 400 bigblk has never had low oil pressure. The gauge has no numbers so ill refer to it like the gas gauge: Idle: 1/4 full, Driving (highway): 3/4 full. Until today... This morning, after accumulating about 2 inches of snow in AUSTIN, TX last night, oil pressure was fine. Then when I left work around 2:30, oil pressure was at 1/4 full while driving and all the way to empty while idling.

Should I be fretting over this? I recently switched to 20w/50 oil. Could this be the problem?

I've read a few posts about this issue, I'm just a little concerned about the sudden drop, that's all.

Re: oil pressure sudden drop
 

Did you check the oil after it dropped? No oil pressure would freak me out and I personally wouldn't drive it until it's fixed...even if it is just the gauge/sending unit. The damage risk is too high and my pockets aren't that deep.

Re: oil pressure sudden drop
 

ditto /\

Did you rev it right after start up? With the heavier oil, the cam might be wiped due to having insufficient pressure due to the cold temps.

Get a mechanical gauge on it to check for oil pressure. Hopefully it's just a gauge problem.

Re: oil pressure sudden drop
 

Sounds like at best low oil level, worst bearing going way south

Re: oil pressure sudden drop
 

When my oil pressure dropped like that, the cam was wiped and the lifters, one lifter had a hole worn right through the bottom. I would take a valve cover off and check the rocker arms and or push rods are going up. If no or little movement I would assume the same that happened to me. Not trying to scare you but its easy to check. By the way if it is, rebuild the whole engine. I tried to just replace the cam and lifters and ended up blowing it in a couple of months anyway. Time and money wasted.

Re: oil pressure sudden drop
 

The oil is full, I just drove around the block, this is what happened:

Park: it's around "1/4 full"
Idle in gear: sits just above the "low" line
Driving 30mph: sits around "1/4-1/2"
Driving 60mph: sits at "1/2" and goes up with mph from there.

Re: oil pressure sudden drop
 

i'd look at where the oil line for the gauge comes out of the block and see if it maybe got kinked or broken maybe leaking

but with those readings i really wouldn't b overly concerned

Re: oil pressure sudden drop
 

I've just experienced the same problem on my 1990 TBI truck. I built the motor 3 years ago and it's been a great engine. Usually my oil pressure is 30 PSI at an idle and much more under throttle. A couple days ago, I noticed a significant drop in pressure. It's gotten worse the last couple days. At first I thought it might possibly be the 0-20 degree temperatures were experiencing here, but it has gotten worse. Today, I had 7-8 PSI at an idle and 15 PSI under throttle. Not good. It's parked until I've got time to fix it.
One side of my exhaust has been making a "chirping" sound for the last several months. I have no exhaust leaks anywhere. No brackets that are loose or vibrating. I'm thinking it might be in the head or the glass pack has somehow ruptured. Maybe the chirping and the PSI drop are related somehow. Luckily, I have a spare 4x4.

Re: oil pressure sudden drop
 

btw, the truck is running fine. My carb needs to be overhauled, but other than that.... I haven't noticed any difference in performance. I'm going to get a mechanical gauge tomorrow and check it out.

Re: oil pressure sudden drop
 

I agree with putting a different guage on it before jumping to any conclusions as long as you have no unusual clatter.. Also with the cold could the viscosity be a little high?

Re: oil pressure sudden drop
 

you know, when i pulled the dip stick to check the oil, it actually seemed kinda of thin... Though I had just gotten off the highway. I am using 20w50.

What sounds/performance issues should I be looking for, if this is a bigger problem?

Re: oil pressure sudden drop
 

Sound wise for cam/lifters it will pop back through the carb.

Re: oil pressure sudden drop
 

never had that issue with any of my wiped cams.


I'd change the oil, I'd also ditch that thick stuff too. All you're doing is working the pump harder than you need to, and it just takes that much longer for the engine to get oil when you get it fired up.

Re: oil pressure sudden drop
 

I've never wiped a cam, but to me the other quote about popping back thru the carb seems like a timing issue..

Re: oil pressure sudden drop
 

If you lose an exhaust lobe to the point that it barely opens, you can get a backfire through the carburetor....but like Andy said, they don't all sound like that.

Re: oil pressure sudden drop
 

Here are some pics of my cam,lifters and what I think was the culprit for the premature wear.http://i967.photobucket.com/albums/a...mk1/cam002.jpg[IMG]http://i967.photobucket.com/albums/a...mk1/cam007.jpg[/IMG] The last pic is a oil galley plug from front of the block I found in the oil pan. I guess it was'nt properly staked.

Re: oil pressure sudden drop
 

Sorry here is other pichttp://i967.photobucket.com/albums/a...mk1/cam004.jpghttp://i967.photobucket.com/albums/a...mk1/cam002.jpg

Re: oil pressure sudden drop
 

I replaced the oil pressure switch on my truck today and my pressure is back to normal. That's a relief.

Re: oil pressure sudden drop
 

I'm trying to change the oil pressure sending unit, but I can't find it! Gah lee....

Re: oil pressure sudden drop
 

It's just to the right (driver's side) of the distributor at the back of the block. They can be hard to get to. You might have to pull your distributor for clearance. I barely had enough room, but I have a small cap distributor. Also, the sending unit is threaded into a 90 or sometimes a 45 degree brass elbow fitting. Be careful not to twist it off!

Re: oil pressure sudden drop
 

I found the hole on the back of the block, under the distributer. There's nothing there. THen I followed to copper tubing from the firewall and it leads directly above the oil filter. No where, is there an oil pressure sending unit..... I swear i've looked everywhere, but now it's dark and it'll have to wait until tomorrow.

Re: oil pressure sudden drop
 

If you have a copper wire (tube actually) hanging dowm by the oil filter and a oil pressure guage in the dash then the copper pipe should go into a hole in the back of the block just behind the distributor, cant remember if its on top or behind the back of the block, something tells me its on the top. Or there may be a different oil port hole somewhere down by the filter ( cant remember exactly on a 400 were talking about) But that said the copper tube would go into a brass fitting that is screwed into the block and should have a ferrell ring around the tube that the female threadded cap would be held on by.

If you have a copper tube then you have a mechanical pressure guage.

An oil pressure sending unit would be on an electrical pressure guage and use electrical wires.

Re: oil pressure sudden drop
 

ok, so i have a mechanical gauge because the copper tubing goes straight down and connects to a port just above the oil filter. Should I replace the gauge then, to see if I get a better pressure reading? How about replacing the tubing as well?

Re: oil pressure sudden drop
 

I'd just buy a different guage and screw it into the block for test purposes to see if your stock guage is bad or not. Most pressure guages come with a new tube but i think you can get them without, regardless the cost of a tube wouldn't be much more. EDIT: to add, its very opossible the threads on the new guage may not be the same as the stocker so ya may need that tube and fittings anyway.

Its possible the tube could be clogged but not too likely, all the oil gallies recive oil after it passes thru the filter, the only exception to that would be if there was a clogged filter then the oil would flow thru the bypass.

You can if you want unhook the tube from the pressure guage and secure it into a containor to see if oil pumps out when you turn the engine over, but i'd just unhook the coil and bump the starter, otherwise it could get messy if your not carful. just remember that when you unscrew the tube to the guage under the dash its gonna dribble oil allover so have a bunch of rags around.

Re: oil pressure sudden drop
 

On a big block (which the OP has) the standard place for the oil pressure sending unit, or tubing for a mechanical gauge, is on the side of the block just above the oil filter.

Re: oil pressure sudden drop
 

Thanks Captin, wouldn't doute you for a minute, i've never owned a big block 400

Re: oil pressure sudden drop
 

I swear, this truck is an enigma. This morning the oil pressure was acting normal or "like it always has"... idle= 1/4, driving 1/2-3/4. I'm still going to hook up another gauge to check the pressure, maybe my gauge is f'd somehow.... I don't know. Loving all the participation!

Re: oil pressure sudden drop
 

If you do get another gauge, and it is a mechanical gauge, spend a little extra and get the copper line for it. The typical plastic line can become brittle and break at any time. Especially from the heat of the headers/manifolds on a big block. Don't ask me how I know......:o

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Where do you get such a line? I had to get a new sending unit when I bought my blazer and the only one I could find was a plastic piece.

Re: oil pressure sudden drop
 

i have an 85. with what i was told is a 5.7 liter engine. not really sure about what engine is in truck, but i notice sometimes my gauge works and sometimes it stays near 0. how can i check to see if my gauge is going or something is wrong with connection.
sometimes im driving and it reads a little over 0, some times it dont move and others it moves.
thanks for any advice. and let me know if i should be starting a thread for this. if so, sorry for posting.

Re: oil pressure sudden drop
 

The best way to check would be to install directly to the oil port on the block a known good pressure guage, fire up the truck and read the pressure, see i'm pretty sure on that year it would probably have a sending unit and electrical guage but not positive...

Re: oil pressure sudden drop
 

The one I've bought before is made by Autometer.

http://www.summitracing.com/parts/ATM-3224/