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Old 12-09-2007, 01:11 AM   #1
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Low oil pressure.

My dash gauge says I have about 20-23. When I got the truck this summer it would run at about 40 while driveing, then maybe 25 at red lights. I checked the oil, it was down about a 1/4 quart, filled it, still reads 20-23 cold or warm. The only oil leak is the valve stem seals, so it smokes when you start it, but thats it. I'm pretty sure it dropped yesterday, but no ealier than wednesday. I am hoping its a oil pressure sending unit going bad. Is there anyway to test it, and the dash gauge? Any thoughts or comments, as always are appreciated. Oh one more thing, on wednesday I started driveing it in drive around town 48 mph and under, then pop it in to overdrive at 49 mph and above. I started doing that because of a thread I read at this site.
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Old 12-10-2007, 06:35 AM   #2
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Re: Low oil pressure.

I forgot to mention that it is an 89 silverado 5.7.
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Old 12-10-2007, 07:15 AM   #3
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Re: Low oil pressure.

That would be my thoughts also, would be a sender going bad. Probably the cheaper of any other solution too.

I don't know of anyway to test them other than changing them out. They probably work on a resistance application, but I don't know what the measurements would be.
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Old 12-10-2007, 11:29 AM   #4
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Re: Low oil pressure.

Let me tell you a story and I have had this happen more than once on my 91 Silverado 5.7 motor.


Just about once a year, I usually get an indication of low oil pressure or the oil pressure gauge bounces up and down on the dash. I even hooked up a manual gauge to the engine and read out that the oil pressure was still good. I have purchased more than one oil pressure sending units and found that sometimes the problem would correct itself or sometimes not. I finally got mad and took the dash (Instrument panel only ) out and was going to find another dash unit at a salvage yard to replace it. I had to get the series number off of the instrument panel to match at the yard, so when I put the instrument panel back in to allow driving to the yard, the problem went away. I found that cleaning and reseating the dash electical plug (big copper slotted junction) the problem with my oil pressure dissapated. Take that as another possible avenue for your problem. I would however, try a new pressure sending unit, also.

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Old 12-10-2007, 12:34 PM   #5
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Re: Low oil pressure.

Well I'll wait a couple days and see if Chevytech reads this, maybe he'll know a test I can perform. I would just r&r the sender but the parts store wants $65 for a new one. I may pop the instrument cluster out and leave it out for a few and see what happens, but its running at 15 now, so I dont know. Thanks for the input guys.
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Old 12-10-2007, 01:14 PM   #6
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Re: Low oil pressure.

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Originally Posted by get me on the road View Post
Well I'll wait a couple days and see if Chevytech reads this, maybe he'll know a test I can perform. I would just r&r the sender but the parts store wants $65 for a new one. I may pop the instrument cluster out and leave it out for a few and see what happens, but its running at 15 now, so I dont know. Thanks for the input guys.
I think your parts guy is off just a little bit on price. A pressure sending unit for my truck is $29.45 at the local NAPA (just checked) and that is about what I have paid in the past. I am not sure what the part he looked up, but it sounds high.

As for troubleshooting oil pressure issues, I would try the el cheapo oil pressure gauge trick, connected to the same port as the oil pressure sending unit is screwed into and see if you really have low oil pressure. You won't have to drive with this, just start up the motor.

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Old 12-10-2007, 07:15 PM   #7
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Re: Low oil pressure.

Well I told the parts guy to look up an oil pressure sending unit. That is right, right? If my dash just had a light the unit would be $8, but if the dash has a gauge, which mine does, then it would be $65. There is a NAPA in the next town over, so I'll call Monday and see what they want for it.
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Old 12-10-2007, 07:20 PM   #8
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Re: Low oil pressure.

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Originally Posted by get me on the road View Post
Well I told the parts guy to look up an oil pressure sending unit. That is right, right? If my dash just had a light the unit would be $8, but if the dash has a gauge, which mine does, then it would be $65. There is a NAPA in the next town over, so I'll call Monday and see what they want for it.
My truck has a pressure gauge and the price was as I said $29.45 and in stock as of today. Take a look at your plug for the sending unit, some are three prong and others are two prong plugs. Mine is a three prong sending unit and wire plug.
This sending unit is at the back of the motor by the firewall, just behind the distributor.

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Old 12-10-2007, 08:02 PM   #9
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Re: Low oil pressure.

I went out and looked pieces, my sender has a 2 wire plug, and the sender is sticking out of a 90 degree (looks like brass) elbow. So if I want to take that elbow out and clean it which I do, I may have to remove the intake. Which would give me one of those cheap/sorta long jobs to do, and I am all for that.
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Old 12-10-2007, 08:45 PM   #10
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Re: Low oil pressure.

Quote:
Well I'll wait a couple days and see if Chevytech reads this, maybe he'll know a test I can perform.
Unplug the connector from the oil pressure sending unit.
Turn the key to the on position.
The oil gauge should go all the way high.

If it did go high you can either put a mechanical test gauge on it to read the pressure or replace the oil pressure sender.

When the oil pressure is very high, the oil sending unit has approximately 90 ohms resistance.

Quote:
I went out and looked pieces, my sender has a 2 wire plug, and the sender is sticking out of a 90 degree (looks like brass) elbow. So if I want to take that elbow out and clean it which I do, I may have to remove the intake. Which would give me one of those cheap/sorta long jobs to do, and I am all for that
Be careful. It is very easy to break off that brass elbow. Unscew the sender from the elbow but you may want to support the elbow so you do not exert any force on the lower end of the elbow. Then you can unscrew the elbow if you want to take it out. No need to pull the intake manifold.

I will say it one more time. It is VERY easy to break off that brass elbow.
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Old 12-10-2007, 09:08 PM   #11
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Re: Low oil pressure.

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Originally Posted by ChevyTech View Post
Unplug the connector from the oil pressure sending unit.
Turn the key to the on position.
The oil gauge should go all the way high.

If it did go high you can either put a mechanical test gauge on it to read the pressure or replace the oil pressure sender.

When the oil pressure is very high, the oil sending unit has approximately 90 ohms resistance.

Be careful. It is very easy to break off that brass elbow. Unscew the sender from the elbow but you may want to support the elbow so you do not exert any force on the lower end of the elbow. Then you can unscrew the elbow if you want to take it out. No need to pull the intake manifold.

I will say it one more time. It is VERY easy to break off that brass elbow.


The oil pressure connection "Tree" on my 91 truck is not brass, but I agree with Chevytech, these fittings can be broken easily. On my 84 and my son's 86 they are brass, and sit down in the corner of the intake manifold. In all essence, I believe that taking it off may not be the best of ideas. Changing the sending unit is not a bad thing, but leave the connection point alone.
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Old 12-10-2007, 09:17 PM   #12
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Re: Low oil pressure.

DUT DA DA DAAAAAAA......... ChevyTech to the rescue, thanks for the reply. I will check that tonight. As for the sender, theres no way to test it with the engine off? I dont think it will start or run with the sender unplugged, but then again I dont know. Does it read lbs of vacuum or pressure, so could I take the sender off and hook my mighty vac to it or put my air nozzle from the compressor to it and put a little air pressure to it? Can I safely clean the sender?
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Old 12-10-2007, 09:54 PM   #13
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Re: Low oil pressure.

Unplugged the sender, turned the key on(run position), gauge did not budge. Hooked the sender back up, started the truck, gauge reads 15 then 16 when I rev the engine. So I'm guessing I should start with R&R'ing the gauge? Can I do that myself and be very careful about the static thing and replace the trip gauge while I'm in there? Or should it be done by the dealer?
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Old 12-10-2007, 09:58 PM   #14
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Re: Low oil pressure.

I logged off and I went to the kitchen to get a snack. I got a good idea the next question is going to be, how do you get the broken elbow out of the engine block?

Another thought is – have you checked to see is the oil is diluted with fuel? Has the truck flooded lately?

22 to 25 PSI of oil pressure is enough sitting at a red light with a warm engine. You could drive it for years this way. At least until it is warm out again. The high temp here today was 6 F. Maybe it is warm where you are, or you have a heated garage, but if you break off the elbow the job just got a bit bigger.

Quote:
theres no way to test it with the engine off? I dont think it will start or run with the sender unplugged, but then again I dont know.
The test I wanted you to do was “Turn the key to the on position” NOT run the engine.

Quote:
Does it read lbs of vacuum or pressure, so could I take the sender off and hook my mighty vac to it or put my air nozzle from the compressor to it and put a little air pressure to it?
Now I got an image in my mind of you with an oil sender in one hand, an air hose in your other hand, and your face covered with oil.

Oil pressure changes the resistance of the sending unit. With low oil pressure it has a low ohm reading. With high oil pressure, somewhere around 80 PSI (Pounds per square Inch), it should have approximately 90 ohms of resistance.

I can picture the three wire oil sender circuit in my mind but I am having a little trouble remembering a two wire oil sender and where the redundant fuel supply circuit gets its power.

Quote:
Can I safely clean the sender?
I have never though of even trying that. I don’t know what is inside them. They are CRAP and are known high failure part.
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Old 12-10-2007, 10:04 PM   #15
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Re: Low oil pressure.

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Originally Posted by get me on the road View Post
Unplugged the sender, turned the key on(run position), gauge did not budge. Hooked the sender back up, started the truck, gauge reads 15 then 16 when I rev the engine. So I'm guessing I should start with R&R'ing the gauge? Can I do that myself and be very careful about the static thing and replace the trip gauge while I'm in there? Or should it be done by the dealer?
Static does not worry me as much on a dash like yours, but take precautions.


Now I am wondering if the test I gave you is valid. I just can't picture in my mind the wire schematic of your truck.

Before you tear into it let me see if I can find a schematic for your exact truck.

Last edited by ChevyTech; 12-10-2007 at 10:21 PM.
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Old 12-10-2007, 10:16 PM   #16
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Re: Low oil pressure.

I have not found a schematic yet but I am thinking the item you unplugged is the redundant circuit for the fuel supply if it has two wires and yours may have a separate sender for the oil gauge.

Is it a little switch you unplugged or a sending unit that is a few inches long?

If I remember correctly the early TBI trucks like the 1987 model had an oil sender down by the oil filter in addition to the one by the distributor.

Last edited by ChevyTech; 12-10-2007 at 10:25 PM.
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Old 12-10-2007, 10:29 PM   #17
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Re: Low oil pressure.

SCREWED THIS ONE UP, BUT DID MY BEST TO FIX IT>

YOU
Quote:
Originally Posted by ChevyTech View Post
I logged off and I went to the kitchen to get a snack. I got a good idea the next question is going to be, how do you get the broken elbow out of the engine block?

ME My dad told me never "man handle stuff", so I dont. You warned me twice to be careful, so if I do remove it I will be.

YOU Another thought is – have you checked to see is the oil is diluted with fuel? Has the truck flooded lately?

ME The oil smells fine and looks good, and no, the truck has not flooded.


YOU 22 to 25 PSI of oil pressure is enough sitting at a red light with a warm engine. You could drive it for years this way. At least until it is warm out again. The high temp here today was 6 F. Maybe it is warm where you are, or you have a heated garage, but if you break off the elbow the job just got a bit bigger.

ME But it was running at 22 going down the road, now its at 15 -16 going down the road. The temp here today was high 30's low 40's then 60 by tomorrow.

YOU The test I wanted you to do was “Turn the key to the on position” NOT run the engine.

ME Do you have dislexia like me(I think that is what its called where you mix words or letters up while you read) or are you tired and have blurred vision, I did not run the engine, I turned the key on to the run position, no crank, no run.

YOU Now I got an image in my mind of you with an oil sender in one hand, an air hose in your other hand, and your face covered with oil.

ME No comment.

YOU Oil pressure changes the resistance of the sending unit. With low oil pressure it has a low ohm reading. With high oil pressure, somewhere around 80 PSI (Pounds per square Inch), it should have approximately 90 ohms of resistance.

ME Will the truck run with the sender disconnected? Or should I ohm from the back of the plugged in plug, while the truck is running?


YOU I can picture the three wire oil sender circuit in my mind but I am having a little trouble remembering a two wire oil sender and where the redundant fuel supply circuit gets its power.

ME Please do.


YOU I have never though of even trying that. I don’t know what is inside them. They are CRAP and are known high failure part.
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Old 12-10-2007, 10:50 PM   #18
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Re: Low oil pressure.

I checked and I don’t have a schematic for your truck. The Autozone site, which is handy, does not cover that new of a Chevy pickup. I don’t want to do any more guessing and screw you up.

I expect the gauge to go full scale when the sender is unplugged. The location of things changes from year to year but the fundamentals of GM gauges are more timeless. Look down by the oil filter on the side of the block for an additional sending unit.

Quote:
Do you have dislexia like me(I think that is what its called where you mix words or letters up while you read) or are you tired and have blurred vision, I did not run the engine, I turned the key on to the run position, no crank, no run.
I am just tired and keep having to edit my post to fix them.

Quote:
Will the truck run with the sender disconnected? Or should I ohm from the back of the plugged in plug, while the truck is running?
It is in more of the useless information category. If you knew it had 80PSI of oil pressure you could take an ohm reading at the sender with the connector off it and the engine running, but I have never done this.

I would just put a sender in it as it has a very good chance of being the problem and so many of them fail. If it still had a problem with the new sender I would put a mechanical testing pressure gauge on it before I considered going into the dash.
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Old 12-11-2007, 12:32 AM   #19
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Re: Low oil pressure.

Quote:
Originally Posted by ChevyTech View Post

I am just tired and keep having to edit my post to fix them.

.
oh isee, well this is what I have, cant read diagrams very well, but maybe you can its kinda small print but this is the best I could get after 10 min of trying
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Old 12-11-2007, 12:39 AM   #20
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Re: Low oil pressure.

How about this?
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Old 12-11-2007, 09:17 AM   #21
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Re: Low oil pressure.

The print is hard to read but I can see “typical 1988 through 1994 gauges indicators and engine warning systems” which looks to me like a Haynes or Chilton manual.

I use GM vehicle specific manuals because the information is more accurate but even the GM manuals have errors.

Last night I looked in a 1993 GM schematic which I have and it shows a three wire oil pressure sending unit.

If you have a way of taking a digital photo and uploading it to web photo hosting site so you can attach it to your thread it would help. Then we could see what sending unit you have.
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Old 12-11-2007, 10:59 AM   #22
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Re: Low oil pressure.

There are differences between 1993 and 1994 wiring harnesses and any generic diagram that covers several years can not be trusted.

Here are links to some photos and illustrations they may help me understand what you are seeing.

Here is an illustration that shows a three wire type sending unit on a 1991. Is your sender long like this one?
http://autorepair.about.com/library/...bl283a-lib.htm


Long oil pressure sender
http://www.ssdieselsupply.com/images...0730192316.jpg

Short oil pressure switch links
http://www.jeep4x4center.com/cheroke...20Cherokee.htm

http://static.summitracing.com/globa.../sum-g1438.jpg


Older style oil pressure sender
http://di1.shopping.com/images/di/77...00x100-0-0.jpg
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Old 12-11-2007, 12:08 PM   #23
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Re: Low oil pressure.

ChevyTech this is what I found after you told me to check by the oil filter. I know you cant see it good, but that tube that is sticking out over the oil filter, which is what this screws into, the PO used what looks like liquid teflon tape to seal the threads on this thing. So if that pipe is stripped where this unit screws in I may have to R&R it. I'll even help paint an image in your mind of what it will look like if that pipe breaks off, it involves me on cold cement and at least 1 mirror. Do you want me to pop that connector off and turn the key to the on position and check the dash gauge?
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Old 12-11-2007, 12:54 PM   #24
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Re: Low oil pressure.

Yes!
The sender in the picture by the exhaust is an oil pressure sender.
Unplug it and turn the key to the on position, engine OFF, and the gauge should go off the top end of the scale.
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Old 12-11-2007, 01:03 PM   #25
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Re: Low oil pressure.

Quote:
the PO used what looks like liquid teflon tape to seal the threads on this thing.
Many OEM Gm sensors / sending units come with the sealant already on the threads.
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