c4c5 - Low oil pressure on cold start

[lrrp 51]

The oil pressure on my 98 takes a LONG time to come up after an overnight soak. After the initial lag it runs OK.

The two things that I can think of that may cause the problem are:

1. The anti drainback feature in the oil filter isn't working well.
2. The oil filter bypass valve in the pan is stuck open.

Either one would permit the oil to siphon out of the main gallery back into the pan.

The service manual isn't very clear on the configuration of the bypass valve (no pictures).

The question is - can the bypass valve be checked out and serviced without yanking the pan?

 

[c4c5specialist]

Hi there,
First, what kind of filter do you have on the car?????
Second, please reference the diagram attached, as this is your lubrication flow path that the oil takes.
If you look directly above the oil filter, in the oil galley of the block, you will see a small bypass valve, directly inline of the oil galley, where the oil takes a downturn, to go into the filter.
Honestly, I would say to change your oil and filter, using a high quality filter, like ACDELCO, or Fram. Use a synthetic oil, meeting GM spec 4718, in the 5w30 grade.
Then let us know how you make out.
Allthebest, c4c5

 

[lrrp 51]

The filter is an AC Delco PF44F. It was installed 2000 miles ago by the local Chevrolet dealer (www.corvetteking.com) along with 6-1/2 quarts of Mobile 1.

I went ahead and had the oil and filter changed again in order to rule out a bad drainback valve in the filter. The used filter looked good as far as the anti drainback feature function.

I was not able to check the oil filter bypass valve myself while the car was up in the air (insurance regulations etc.) but one of the Technicians told me that it appeared to be sealing because no oil was dripping down through it. (of course there might not have been any oil above the valve to leak out).

Right now it takes 10-15 seconds for the oil pressure to move off zero on a cold start (oil temp around 20F) after sitting for 8-9 hours. I get a low oil pressure warning so apparently the PCM is expecting pressure to come up a lot quicker than it does.

If the oil pump relief valve was stuck open it might siphon the lower main gallery but that would show up as low oil pressure under stabilized conditions.

Do you happen to know what the oil pump relief valve is set to?
Also, what's a new engine spec for oil pressure at a given rpm? (I know it should be a lot higher than the minimums listed in the repair manual).

With the main bearings being the last in line for oil I'm starting to worry a bit. Any thoughts you might have will be greatly appreciated.

 

[c4c5specialist]

Hi there,
Before you worry, I would have a cold start oil pressure check done. We install a gauge on the oil filter adapter at the engine, and then let it sit overnight. Using a stopwatch, time from when you crank the engine, till the oil pressure builds on the gauge, and then till it builds on the IPC. You could simply have a bad sensor for the oil pressure.
Honestly, variable abound on oil pressure when cold, and I would stand by the pressures listed in the service manual.
Do the cold start pressure check first, and then I would proceed before worrying about it.
Allthebest, c4c5

 

[motorman]

paul could it be a cut "O" ring on the pickup causing a slow oil prime?

 

[lrrp 51]

Motorman, you may be right on.

I also am suspicious that the problem may be on the suction side. The warmer it gets here, the faster the pressure comes up. If the gallery was draining, it would probably take nearly the same time to refill regardless of the oil temperature.

Also, a couple weeks ago when the outside temp was around zero, the oil pressure would actually drop about 8-10 psi during a cold accel (that really gets your attention) until the oil warmed up a bit. That may indicate a leak or obstruction in the pickup tube.

I think the sender is OK because if I let the pressure come up, shut it off and do a restart, the gage comes right up like it should.

Does anyone have any thoughts about how I could confirm that the suction side is leaking before I have to drop a fortune for Mr. Goodwrench to pull the front suspension and pan to take a look? Based on the above, I probably already know the answer but I can only hope.......:cry

 

[c4c5specialist]

Hi there,
Motorman has brought up a very good possibility. However, if it has not always been this way, then I would regress.
Anyway, not really on the cheap way to diagnose, but like I said earlier, using a oil adapter, and checking to see if the pressure is accurate, as guessing should not be an option.
Allthebest, c4c5

 

[offshore]

In the early 80s iI worhed for ford. I" think" it was penzoil that picked up the cost for over 100,000 engines because after 10.F the oil would gel and not reliquifie it stayed jello I personally replaced 50 engines at there exp. 10 years later I did an engine on a dodge mini van. the night I finished it i took it home for the test drive. That night it got down to zero! the next morning I went ti go to work in the mini started it up (and of course the first thing I did was look at oil psi) new motor i just built I don't want any trouble and if I do it'sw going to happen BEFORE I give it to the customer.( I'd rather have it blow on me than them) anyway it took 1min 30 sec for oil psi to come up. I didn't know if it was my prob. or what.I checed every thing no prob. The next night I left the oil pan outside with same oil in in it guess what. JELLO. sence then my winter oil (after breakin) is SYN. 0w-30. just opinion( engine builder 20+years) NO I REMBER NOW IT "WAS" QUAKER STATE" and they paid for EVERY engine.

 

[ZZRider]

Any news about your low oil pressure cause and fix?

I am having a low oil pressure problem also, but mine is a little different. My start up pressure is lower than usual (~27), but as the car warms up and as I drive it drops down to ~15 where it stays without variation. No change with idle vs increased rpm. It makes no noise, sounds fine. Does not appear to smoke or be burning oil.

I changed the wicks filter, no difference. Changed the sending unit, no difference. Replacing the oil pump next. Wondering if the spring/relief valve is broken. If that doesn't work maybe pick up tube obstruced, a gallery plug blew out, ?.

Any thoughts Irrp 51, c4c5specialist, motorman, or others?? Thanks.

 

[c4c5specialist]

Hi there,
It sounds to me like your bypass valve is stuck, not allowing a buildup of pressure.
You will change this when you change your oil pump, as the bypass valve is inside the pump assembly.
Allthebest, c4c5

 

[lrrp 51]

I haven't fixed mine yet because as the weather has gotten warmer here, the oil pressure comes up in about 5 seconds after a soak. The warmer it gets, the faster the pressure comes up so it makes a leaky pickup tube a likely candidate. The gage seems to work fine on all but the first start after a soak.

ZZ, I agree with c4c5. If your pickup tube was obstructed you could actually get lower pressure as the RPM goes up.

I believe the pressure relief valve is serviceable. You'll need to inspect and measure the plunger bore (it needs to be absolutely perfect) before replacing the spring/plunger. That might save you a couple bucks on the pump but if you're at all suspicious, swap the pump.

c4c5. My car came from Texas. This is probably it's first hard winter so I'm not sure whether it's always been that way or not. Maybe it's just getting even with me for exposing it to this crap weather we have here.........

 

[ZZRider]

Thanks much guys!!!!

We are going to have a new pump here just in case today, but with no noise at all, I think you may be right on with the bypass valve being stuck. I haven't heard much about that happening before, but sure sounds plausible. I'll let you know what we find when we get to it over the next few days.

Irrp, are you up in Detroit? I could see how a Texas car could want to get back at you for bringing her up here to the upper midwest. I know what you mean. My car doesn't like the cold Iowa weather here either, the big cam would be much happier if we were in Texas on some of our cold mornings.


Thanks again!!

 

[ZZRider]

UPDATE: Oil Filter Bypass Valve stuck open.

I/we did not see this the first time I changed the filter. Before my tech tore into the engine further, they used a manual pressure tester and found the oil filter bypass valve suspect. Took it apart and it was stuck open with some debris. Repaired/replaced, new filter, now she runs 40's up with rpm and varies idle/rpm.

Thanks guys!!! I'm smilin' again now.



Good luck with yours Irrp51.

 

[lrrp 51]

Thanks ZZ,

That's very interesting that an oil filter bypass valve would affect your stabilized oil pressure.

The only mechanism I can come up with would be if the oil filter bypass valve is stuck it somehow partially blocks the main oil gallery causing the pressure relief valve in the oil pump to open. That would explain why your pressure stayed constant with RPM. The pressure relief valve on the oil pump would maintain a nearly constant pressure that would be reduced by the restriction at the oil filter bypass valve.

Good information. That's one that everybody can put in their toolkit.

I guess I'll have to take a lot closer look at that valve..........

 

[lrrp 51]

Well, I was able to look at my car with the pan down and the oil pump out. All of the pieces are in good shape. No stuck valves, debris or broken springs. At first glance, everything looks good.

However, there are witness marks inside the pan indicating that the screen and cover assembly was actually in contact with the pan. It's actually touching in three places so there couldn't be more than a millimeter or so clearance. It's apparently a misbuilt pickup tube that located the oil inlet too close to the pan and nearly shut off the oil flow into the pickup.

Thankfully, there was enough flow to keep from smoking the bearings.

 

[ZZRider]

Interesting. Have you corrected the problem and how are your pressures? If not yet, please keep us posted, I'd be interested to hear how she turns out.

As for mine, still running well with oil pressure 42-45 and normal rise as I get on it.

 

[lrrp 51]

ZZ,
First, thanks for posting your oil pressure problem. Under normal city driving conditions I really didn't notice a problem other than the slow prime. Your post inspired me to check out my pressures at higher RPM and much to my surprise, at about 4000 RPM the pressure dropped rapidly. I took the car into the shop the next day.

In city conditions now there isn't a great deal of difference in my stabilized oil pressures from previously. However, pressures respond much more quickly to changes in RPM and on cold starts pressure comes right up.

Now it all makes sense. The colder it got the harder it was to suck the oil through the small cross section between the oil pickup and pan. At higher RPM, increased suction would cause the oil to "boil" and cavitate at the pump inlet.

Thanks again.

 

[ZZRider]

Glad to hear that she's running well for you again!