Oil pressure drop! is it the pump?

Hi,

I’ve got an 89 GMC 2500 4x4 with 150K. Recently I noticed my oil pressure dropping down to near zero when then engine is hot. When I first start the motor cold the pressure is up around 30 psi then begins to drop back to zero as the engine warms up. Even at higher speeds (60 mph) it stays low but still around 10 psi with it occasionally dropping; revving the engine brings it up but then it drops again. I’m assuming it may be my oil pump?! Anyone with any ideas or thoughts?

If it is the oil pump can I drop the oil pan without too many problems. There only looks to be about 4 inches above the 4-whell drive IFS. Is that enough room to get the pan out to get to the pump?

Otherwise truck runs great!

first I would check it with a mechanical gauge and find out if the sending unit is really saying what the pump is doing before I go and drop the pan off.

That's good advise to check with a mechanical gauge first. But from the sounds for things I don't think it is your oil pump. Oil pumps typicaly fail at high rpms so if you rev the engine and the pressure comes up I don't think it is your pump.

Now that being said when is the last time you changed your oil? And what are you useing 10W-30 or 10W-40? You may want to try a bit thicker oil. If that does not help then it may very well be time for a rebuild on your engine.

The oil could also be getting too hot and thinning out, do you have a remote oil cooler? May be covered in mud. Has the engine coolant temp come up since this started? You could have another problem that is causing this.

I usually change my oil every 3-5 but it is getting close to 5k and I normally use 10-30. I'll try thicker.

I also thought the oil was thinning out, but under normal conditions I didn't think it should start so suddenly. Again, thicker should help. No, I don't have an oil cooler but it could help.

I've been trying to keep from needing a rebuild or new engine but I know it will inevitably come.

Could it be the switch under the distributor that might be wigging out? I figure it would be more erratic and not just when the engine gets warm.

Thanks for the help! Any other advice would be great!
Jason

Oh, also no the temp has not come up, but I have also not been driving my truck much, just the couple of miles to work and back. So, the temp barely gets up to normal range, if at all, but just in those couple of miles the pressure drops from 25/30 psi (just after starting) down to near zero with the check gauges light on. At lunch the check engine light also came on while at a stop light! I also noticed that applying the brake or stopping tends to cause pressure to drop, then as I accelerate it increases up to about 10-15 psi then drops again when I stop accelerating.

mine did this and i have to ask a stupid question here. Is it full of oil? I had that problem and I found out that I was three quarts low on oil. :(

3K is the longest you should ever go, especially with an older engine, unless we're talking kilometers. Unless you pull a trailer regularly, you probably don't need a cooler, that was just a thought I had. I wouldn't think it's the oil pressure sender, for the reason you said.

So the problem started after several thousand miles on an oil change? If so, I'd think just an oil change would solve the problem. As for making your engine last, I've got an '86 Silverado w/350 ci engine with 159,000 miles and it runs like new. I change the oil every 2,500 miles, but I beat the hell out of it. You should have many more miles in your truck.

Well good news and bad news. The good news is that I changed my oil, new filter and put in thicker oil (Rotella 15-40). Unfortunately, the bad news, is that it didn't seem to help. I put in two gallons (6 quarts) just to be sure, and still the same thing occurred. Started it up, jumped up to 25 or 30 psi and progressively dropped back to 0/5 psi. Again when I apply the brake or stop and idle the pressure is low (near 5psi). Then as I accelerate it increases up to about 15 or 20 then levels off around 10 or 15 at driving speed.

It has always had plenty of oil, which was my first thought and it was near the full line but I added another quart just to be sure when all this started.

When I was at the parts house I forgot to ask about the mechanical gauge. They should have one correct? If so, how do I install it? I've never encountered a problem where I was stumped like this. I'm fairly savvy around mechanics but I wouldn't call myself a mechanic!

Any other thoughts or suggestions?

You should be able to install it in the same port the you oil pressure sending unit is in. It should be on the block near the oil filter. And yes the parts store should have one. Another note is to make sure you do not over tighten the nut on the plastic hose you can crimp it shut.

I put my manual op gauge in the spot for the switch (I think it is a fuel pump kill switch) behind the distributor. I had to pull the dist. out though.

...I put in two gallons (6 quarts)...

2 gallons is 8 quarts, if you put 2 gallons in, drain out 3 or you'll blow a seal out. I don't think your pump is bad, or you wouldn't have the good pressure when you first start it. My guess would be the pressure dump off valve. Unfourtunately, it's no easier to replace than the oil pump.

Did jmalaney ever find out what was causing this? My 1996 Chevy Z-71 with approx. 150,000 miles is now doing the exact same thing. It all started with a problem with my starter about 4 weeks ago. We got a new starter, then swapped out the plugs and wires. The plenum had a leak, so we had that resealed and the fuel system flushed. Now I have the oil pressure drop problem that jmalaney had. I have never had problems with oil pressure.

Thanks in advance,
Jen

umm... i heard that oil pressure as low as 5psi at idle is comepletely ok. that whole thing about the oil pressure dropping as the engine warms up is just the oil is thicker when its cold and thinner after it gets hot. my truck frequently idles with the needle just a tad bit above zero and at about 60 it is anywhere from 20 to 60 psi.

yeah i would definately put no more than the amount the manual sez to put in for oil. so the reason not to overfill the oil is to not break a seal? all this time i thought that it would just make froth if it was too full because the crank would be hitting the oil.

yeah i would check the guage for sure and maybe its just normal unless it used to always be higher...

The oil pressure in my 92 5.0L would drop to about 5-10 psi or so when it was hot at idle on dino juice 5w-30. For the heck of it i used maxlife synthetic 5w-30 at the next oil change. Now it never goes below 20 psi at idle.

As it turns out I guess I had a faulty gauge. I went to the local parts house, and purchased a $25 nice chrome manual gauge. I installed mine just behind the distributor cap, drilled a small hole in the firewall behind the dash, and mounted the new gauge on the bottom of the dash in plain view. I still had 40 - 60 psi while running down the highway and over 20 at idle. However, even with my old oem dash gauge unhooked it still is giving me a read of 20 psi on start and dropping down to 0 after about 5 minutes. I can't figure out how to make it stop but my oil pressure is fine. You can also take the oil cap off and look inside the valve cover with a light to see if you see oil being pumped but a manual gauge is the only true way to know what your pressure is.

Hope this helps.

Jason

my 87 has 224,000 plus miles on it and my psi stays at 55-60 psi at all times -hot....cold....thicker oil....thinner oil it dosent make a diff. , and its been like this for 4 1/2 years ( as long as ive owned the truck).

Your 89 has a sending unit behind the distributer for the computer. It has another one for the gauge on the drivers side at the rear of the engine above the oil filter behind the exhaust manifold. It usually has a black cage or heat sheild covering it. I have replaced these on several of my 88 chevy trucks.

j-ri is most likly correct. the press. relief valve is
probably stuck. try 3qts. of rizlone with 2qts. of
5w30. run it at a high idle (1800rpm) & block the
radiator with cardboard to warm it up to 210deg.
NO HOTTER. Remove the cardboard while it is
still running & bring it down to normal temp.
You may need to do it several times. If you
keep an eye on the guage you MAY see the
valve un-stick. Change the oil using 5qts.
If that doesn't work you'll have to remove the
pump & either fix the valve (part of the pump)
or replace the pump. While you have it apart
ck the screen, it may be clogged causing your
problem. Either way replace both, they're
real cheap when you are lying on your back.
please post back & let us know how you
made out.

I may be a little late on this one, I have an 89 K1500, Im the original owner, about 10 years ago I was having the same problem with my oil pressure, when it was cold it was ok, as it warmed up it would drop, and if I come to a stop it would drop to zero, Thought it was the oil pump, so I went and bought one and got everything ready, dropped the oil pan and guess what was in the bottom of it, the oil pickup tube, needless to say I was pissed, anyway I went ahead and installed the new pump with a new (oil pickup), but that was causing my problem, and I havent had any more problems with it, with no pickup in the pump it will still be able to pump oil and give you pressure but as it warms up and you move or stop, the oil will move about in the pan and the pressure will drop. you may have the same problem.

I believe oil pressure should be @ 60 psi when you start the motor (cold). I want to add something kinda off topic right here.....when I start my truck up, I watch my oil pressure gauge AND WILL NOT drive until my oil pressure drops down to about 45 (atleast). When the motor is at normal operating tempreture, I belive oil pressure should be @ 30 psi and should never go below this. Now, it is normal for the oil pressure to rise as you accelerate and will range anywhere from 30 to 60 psi depending on the RPM's your running at. Now when you start to slow down, say for a light, pressure should begin to drop as the RPMs are decreasing. Thus at a red light, should read 30psi.
This is to my understanding. If I'm wrong, please correct me so that I may learn something as well. But, if I'm right, this should be the normal behavior of oil pressure, As for what I was noting "kinda off topic", waiting for motor to warm up when you first start it until your oil pressure guage to drop down atleast 15 psi is VERY crucial to your engine life and the need for premature maintenance. By no means should you ever start it up, pop it in gear, and go. I understand people are in a hurry first thing in the mornings but how about waking up 10 min later to let your mode of transportation that you rely on "wake up" as well?? If your running late already, whats 5 or 10 more minutes??

GM says 10 pounds of pressure per 1000 rpm is sufficient for a chevy 350.

do u know what itsays about a 262(?) aka 4.3 v6?

A 4.3 is a 350 with two cylinders chopped off -10 pounds per 1000 rpm would be sufficient for it. Also 72 rs you probably need a new sending unit for your truck if you have never repaced it. What you said about the pressure always being the same is generally a sign that the sending unit has gone kaput.

"A 4.3 is a 350 with two cylinders chopped off" haha, i never get tired of hearing that phrase. :biggrin: