mac55

I'm a newbie here and I'm hoping someonecan help me. I have a '90 Silverado with a 5.7L in it. Last week the oil pressure gauge went from 35psi, which is where its always been to down around 15psi. It spiked up and down a few times but now it only reads 15psi when the engine is running. I thought right away that it was the sending unit. Iinstalledanew sending unit on Saturday. What a pita that job was! After I got the sending unit installed I startedthe engine and the gauge still only reads 15psi.[]The engine still runs good and there is no tapping or knocking sounds in it. I want to install a mechanical gauge, but I don't want to mess with the stock sending unit or where the stock sending is located. Is there another place on the block that I can put the fittings for a mechanical oil pressure gauge? I have a pipe plug on the side of the oil filter housing. Can I install the fitting there and then run the tubing into the cab to the new gauge? TIA for all your help and/or suggestions.

cams 98 silverado

personally i dont think it would be your pressure gauge. 15 psi is still running pressure. not so good but still something. if you installed a new sending unit and it still isnt doing well then perhaps its more than a bad gauge. im unsure how to install a mechanical unit i cannot help there but when is the last time oil was changed? im guessing your using conventional. a friends t-bird had the crappy v-6 in it and he didnt change his oil because he's a moron and thinks its not needed often. his pressure decreased and then when he had my buddy pull his motor up a bit and drop the pan for a new pump install they found rod fragments and teh motor now serves as a weight so the car doesnt blow away. end result. he had almost no oil and the pressure was low then pump burned and the rods next
hope you find what your looking for. and i hope it isnt what i described because that would not be a cool thing

therewolf

Cams98 is right. Pressure drop-off like that is usually signalling time for an oil change. Mechanical pumps are OK, but the electrical ones are an improvement, IMO, less stuff to jam and clog. And if the mechanical gauge tube comes off the gauge under the dash while the engine's running, you're in for a special treat.

wellmax

Mac,

Most folks that install a mech setup tee off a pipe that is screwed into the sending unit outlet. Not much help there as I assume you had issues replacing the sending unit.

I was pushing my old 350 hard this last week pulling my travel trailer, it normally runs, oil pressure of about60 psi at speed and around 40 psi at idle. I noticed it was running around 50 psi at speed pulling the travel trailer.

I run 10w40 valvoline oil with one qt of Lucas Oil Stablizer and a wix filter. Change oil at 5K intervals, 80K on motor.

I would start by changing oil, filter, and add one qt. Lucas oil stablizer. Check oil psi, if it does not incease then you may want to install the mech gauge.

15 psi may be enough pressure, as you did not say how many miles on motor and how often you change oil

good luck,

mac55

Thanks for the replies guys. I should have mentioned in my first post the truck has 145k miles on it. I change the oil, Pennzoil 10-40w, and the filter, NAPA Gold, every 3,000 or 3 months. It's just about due for a change soon so I'll try that with the Lucas and see what happens. Changing out the sending unit is just a pain where it is located. I had to climb up in the engine compartment to get to it. Then you're hunched up and bent over trying to reach back there. I hear ya about the tube coming off the gauge in the cab, (roasted peanuts real fast). I told my son the same thing about the tube coming off the gauge. Two reasons why I was thinking about hooking up a mechanical gauge was one, to see what the oil pressure is and two, to see if the gauge in the cluster is bad. Do those gauges ever go bad?

therewolf

They probably do, but I personally never heard of an actual gauge malfunction. The sender or the wiring's much more likely to be the culprit.

mac55

One more question. I was lookingin my Chilton's manualat a wiring schematic for my truck. In the schematic there was an"Oil Pressure Switch" and a "Oil Pressure/Fuel Pump Sensor". I know that some of these manuals try to cover too many years and may not be accurate, but did I replace the wrong part? I replaced the part in the back of the engine right under the distributor.Is there another sending unit for oil pressure mounted somewhere else on the engine? As many times as I've been under and in the engine compartment, I don't recall seeing any other sending units. I knowthe part that I replaced is a three wire unit if that helps.[/align]

mac55

Update: I changed the oil, filter, and added Lucas. Started the engine and no change on the oil pressure gauge. Next, I removed the instrument cluster and sprayed the connector and the pins on the back of the cluster with CRC QD Electronic Cleaner. Put everything back together, started the engine and still no change on the oil pressure gauge. It reads just under 15 lbs on the instrument cluster gauge. I contacted Auto Meter to see if I could use the npt port on the oil filter flange to hook up a sending unit for an oil pressure gauge. They said it was OK to do so. Tonight I removed the pipe plug on the oil filter flange and install a pressure gauge right there at the port. I have the face of the gauge facing the front of the truck so I can see what the pressure is while standing in front of the truck. I wanted to see what the pressure was at that port. Started the engine, got out of the truck and looked at the gauge. 40 lbs of pressure!! I let the engine run till it reached about 160 deg and looked at the gauge again. 36 lbs of pressure. I guess the next step would be to check out the wiring. I know the sending unit has three wires. Does anyone know what color wire goes where? I think they are orange, gray and tan, but I'm not sure. I really would like to know the color wire that goes from the sending unit to the instrument cluster. I'd like to trace that wire to make sure it is OK. Is there anything else I should look at? Thanks everyone for your help.

mygreenihc

The oil leaves the oil pump and goes into the filter first before flowing up through the engine. What you have determined is that you have 36 to 40 psi at the oil pump befopre even entering the filter. Your dash gauge is measuring what you have at the top of the engine after it has passed through everything. You are not comparing apples to apples here. I would venture a guess that your dash gauge is correct. Does anyone know how much pressure a stock oem oil pump should have at the pump?

Brad