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HI my 93 gmc 3500 6.5L has low oil pressure, I changed to sendind unit and nothing so I changed the oil pump and still nothing when the engines cold I have 40psi at idle when it warms up I have like 20psi and when Im driving and going down a hill and let off the gas the pressure drops so low the light comes on. I just bought this truck so im not sure on the history. please can anyone help
 

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While you had the pan down to change the pump did you drop any of the rod caps or journals to check for wear? Sounds like something has excessive wear and you are loosing pressure. I don't know where the sending unit is on your truck, but if it is forward you may want to put a guage on the oil line someplace closer to the oil filter and check the pressure there just to see if you can locate the lose. If the pump is good, the sender is a ways downstream, and it is wear you should see a pressure differance across the engine. If you got it from a dealer do you have any leverage with them? Often there are plugs at each end of the camshaft oil line that you can get to.Edited by: HowieE
 

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There is another possible reason for low oil pressure other than excessive wear. If the oil line is clogged you would get a pressure drop across the restriction. If a filter become clogged they have installed a bypass valve to overcome this problem. I don't know how to tell you to check this possibility out. Maybe someone else will chime in.
 

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Kuilima, I had an oil cooler line blow and I had to bypass the cooler, when I did this my pressure went from 45-50 hot @ highway speed & 35 or so @ idle to 35-40/highway & 15-20 @ idle. You might want to check the cooler & plumbing for a restriction, It may be wishful thinking, but you may find the problem there. When I reconnected my cooler my usual pressure returned. Hope this helps.
 

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Discussion Starter #7
Thanks for the advise, Ill check to see if thiers any thing with the cooler and lines. So you can not bypass the oil cooler? if so the pressure will drop? my oil cooler is part of the radiator is thier any other type I could install.
 

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What's the temp there?

If it's not too cold you could try to use straight 40 wt oil and see if it helps.
 

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I'm no mechanic, but my manual says that SAE 40 can be used above certain temps. Of course this will not fix anything but if the pressure comes up it will at least let you know how bad the wear is. If it does not come up I would not run it like that.

I would think that if it comes up higher it would indicate wear, but if it drops lower that would indicate a blockage.Edited by: DavidPhillips
 

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kuilima said:
HI my 93 gmc 3500 6.5L has low oil pressure, I changed to sendind unit and nothing so I changed the oil pump and still nothing when the engines cold I have 40psi at idle when it warms up I have like 20psi and when Im driving and going down a hill and let off the gas the pressure drops so low the light comes on. I just bought this truck so im not sure on the history. please can anyone help

Kuilima,


Unfortunately, I have the same exact issue! Have you learned abything new? Does this sound familiar:


I recently purchased a 93 Sierra 6.5TD. When the truck is initially started, the oil pressure is steady around 40 psi and raises up to about 50psi at 2500rpm. I have noticed after driving for about 30 miles, the oil pressure drops suddenly from about 40psi to about 25psi at 2500RPM/70mph. Every once in a while (about once every 10 minutes) it will "click back" to 40psi but will only say there for a few moments.


When the truck comes to a stop and idles at about 600RPM, the oil pressure will settle in around 10psi (a little to the left of the 1st white hash mark) and it will do the same thing as at steady spead in the way of "clicking" up to about 25psi every few moments and then go back to the lower reading. If you park the truck for an hour or so and start driving again, the above process begins all over. I checked the connection on the oil pressure sending switch and it was secure and dry. I also changed the oil to the suggested wt.


I am going to try and install an aftermarket oil pressure swith to verify the pressure is falling.
 

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The 97 and up 6.5 with the oil cooled pistons has a different oil pump that increase volume. Pressure is the resistance to flow and the greater the flow the higher the pressure. I put one of these pumps on my 94 to improve pressure when towing and noticed a 5 -10 lb increase at operating temperatures. It's been a while and I can't remember if I changed the pickup as well, nor can I find what the part # is. It won't correct any wear problems in the engine, but could step up the pressure a little.
 

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A comment on toyboxrv"s posting.


For a given output from a pump the greater the flow the Lower the Pressure. Yes if you increase the pump capicity in GPM in the same engine you may see a higher pressure but that is just hidding the underlying problem.


Pressure becomes lower as wear takes it's toll on an engine.
 

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Howie, I know they make high flow high pressure oil pumps and they make high flow low pressure oil pumps for gas motors
 

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Sounds like the main bearings are getting loose, How many miles do you have on the engine?
 
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