The bypass valve on 1995's (LT1) oil filter adapter seems to be stuck open. I have zero oil pressure at warm/hot idle, though great pressure once off idle. The pressure data has been verified with a mechanical gauge. The engine was recently rebuilt, which included a new oil pump. I have not been able to find a replacement adapter (new or used), so starting to look for alternatives. So far, I've come up with this:

- Replace housing with this generic SBC adapter. This can't be used as a stand alone solution because of oil filter to cat converter clearance.
- Install a relocation kit like this somewhere on the car.

Is there a reversible way to force the bypass closed to verify my diagnosis?

Thoughts/suggestions?

 

I would think that there are still LT1(VIN P) engines in salvage yards if you shopped Caprice. I'd suggest using something like car-part.com and searching using your zip code for possibilities near you. A quick search near me there looked to be many possibilities. There's maybe many 'mom-pop' salvage yards that could be possibilities. The 4.3 V8(L99 VIN W) I imagine used the same and they're out there also.

 

I tried some of the salvage search sites, like car-part.com, for my area with no luck. I sent some requests out to many salvage yards.. waiting to hear back. However, I didn't know about the Caprice and 4.3(L99 VIN W) cross over.

Thanks a lot!

 

I think you're looking at the wrong thing; I don't recall any bypass valve ever being in the adapter, its sole purpose is to relocate the oil filter. Oil pressure is built by backpressure caused by parts internal to the engine; bearings/etc. If the adapter was stuck open (which again, I'm like 99.999999999% sure there's zero valving/spring in the adapter), you should still have oil pressure because the oil feeds/clearances.

Edit: and the more I think about it, the less it would make sense for GM to put any valving in the adapter. The oil filter itself has a drain-back valve, and the oil pump has a spring which controls pressure. There's no reason to spend extra money to put yet another spring to control oil pressure in the adapter.

How long ago did you refresh? I'm -really- thinking something is going on with the pump/pickup.

 

I'm quite confident in the Caprice and the L99 but you could call a local GM dealer and have them confirm in the catalog that they're actually the same. There's maybe 3 potential part numbers. The Caprice LT1 & L99 would have to be with out KC4 oil cooler. Part numbers maybe 12556204, 12554305 or 10215674.

 

There's definitely a bypass in the filter housing. I was just playing with it again just now. It isn't stuck open though. If the bypass is 4 psi (read that somewhere), this spring feels about right. There's a bit of tension.. but definitely not just stuck open. I'd take a picture, but its back on the car with a filter attached. There's a good chance I'm wrong about the bypass though.

The rebuild was done about a year ago. There's about 1000 miles on it since then. The pressure has always been low at idle, but not zero. I don't recall what the idle pressure was before the rebuild. Maybe there is something off with pump/pickup. Here are my observations:

- Cold start, idle oil pressure is ~40 psi
- As oil temp rises, pressure falls.
- around 180 F, idle pressure is effectively zero (sometimes actually zero).
- I've tried 2 ACDelco oil filters and heavier weight oil. Issue remains
- Off idle, pressure is perfect. >=10 psi per 1k rrpm

I don't understand what internal conditions could exist to produce the behavior. What could be happening that produces tons of cold pressure idle pressure and zero at warm.... but great pressure otherwise. There's a bypass spring in the oil pump itself, but that's for high pressure.. right? Would poor machine shop work allow for good pressure everywhere but idle? The shop was difficult to work with, and did have some dopes working in it while they had my engine. The rebuild was over COVID, so personnel challenges.

 

If there truly is a bypass in it, the only thing it's bypassing would be the oil filter; it would still be going to the oil feeds internal to the engine which would be causing (back) pressure. The reason oil pressure drops as temp gets hotter is the viscosity of the oil. As oil get's hotter, it gets "thinner" (I use this term loosely, it really means it just flows easier . . such as out of passageways, reducing oil pressure). This is why multi-weight oil exists. Eg, for 10w-30, it has the viscosity of 10w at cold, but as it heats up, it tries to correct this decrease in viscosity by performing as a 30w oil.

I know a lot of machine shops that are of the mindset "It's good out of the box!" when it comes to oil pumps but in building motors, I've found brand new ones that were out of spec. The other aspect of this is considering what tolerances your motor was built to. I like to build my engines on the "loose" side, going towards mid range on the crank mains and towards the top of clearance on the rods to promote oil flow to the rods. This requires me to run a high volume pump (also running a Derale cooler).

Looking at this: Plugging the oil filter adapter bypass? Has anyone done it and how? - CorvetteForum - Chevrolet Corvette Forum Discussion

This just reaffirms my position - If the bypass is stuck open, it just skips the oil filter and your bearings/oil feeds should be causing enough of a restriction to build oil pressure. If the adapter is stuck closed, it's just going to force oil through the filter. You should still have pressure from the oil feeds in the engine regardless.

 

I think you're on the right path. I put a new standard volume (Melling?) oil pump in. I'm considering installing a higher volume pump now. I don't want to disassemble the engine to take measurements. Should I be able to tell how close to spec the tolerances are based on this data:





I understand the terms.. not sure of my ability to compare to OEM specs. Thanks a lot for all the help.

 

I have this pump in there now. High pressure/standard volume. Assuming the tolerances are the problem... should I be looking at high pressure/high volume, or standard pressure/high volume given that my current high pressure/standard volume isn't performing well enough. It seems at some point, there's a potential to drain the pan.

 

There is absolute -zero- reason to run a high pressure pump unless very specific build considerations. The fact that you're only getting 10psi/1krpm with a high pressure pump says either you have a bad reading (you said you already verified with a mechanical gauge) or something is very wrong/out of spec internally, or bad oil pump.

The "general" rule of thumb is 0.001" clearance per 1" of journal. Checking service manual for an LT1, different journals have different limits.

Mains (in inches)
#1: 0.001-0.002
#2,#3,#4: 0.0010-0.0025
#5: 0.0015-0.0030

Rod journals: 0.0010-0.0030

I like to get my crank journals to around ~0.0018 for all (which is within limit) and 0.0025 for rod journals. So you seem to be within spec except for your #1. . . but I don't think two tenths is going to cause that issue. The one thing I don't see on your work is an align hone on the mains. Generally, they're pretty good in spec, I just do it for insurance, but if you don't, I could imagine those clearances opening up a bit. Have you seen a bunch of glitter in your oil?

 

I haven't seen any glitter in the oil, or found anything on the magnet I run through drained oil. I'll pull the engine again, and see what I've got going on in there. Maybe a bad oil pump (like you mentioned)... or something else.

Thanks again for getting me off my wild goose chase this started with.

 

Just spitballing some idea's; I know the stock coupler/sleeve b/w the distributor and the oil pump is plastic. I instantly ditch those for a metal unit. Could yours be damaged? Did you -ever- have higher oil pressure values after the rebuild?

 

The current oil pump had the sleeve built into the shaft... no plastic coupler.
I don't recall the oil pressure after the rebuild, but I replaced the standard pressure oil pump with high pressure while the engine was out to replace timing cover gaskets. So I suspect the pressure was always low after the rebuild. That also means its on its second new Melling oil pump too. :-(

This car/engine has been kind of a back burner project. I wish I kept better notes.

 

I was always under the impression the filter bypass was intergal to the oil filter. This allowed the filter to bypass when it got dirty and clogged up. What is the point of another bypass? Dan

 

There's a bypass spring in the pump that controls the pressure output of the pump. I have had this get stuck open and produce low oil pressure at idle in an old Pontiac V8.

 

I’ll admit I didn’t read the thread in its entirety but will offer a few thoughts.

The filter adapter in an LT1 does contain a bypass, however, whether that bypass is open or closed shouldn’t effect oil pressure, rather it’s impact is on the volume of oil passing through the filter versus going around it - the oil filter adapter bypass does not return oil to the pan, so I too think you may be looking in the wrong place.

The oil PUMP bypass does return excess oil to the pan, so if that is stuck open it could be your issue. Or, and I would think this more likely, your pickup tube is loose, fallen off, or not set to the correct depth and the pump is sucking air.

Pulling the oil pan isn’t much work once the cats and starter have been removed, I’d start with a good look at the pump and pickup.