What is the oil pressure thing on the back of the block that everyone says is a pain to get to? What does it do?

I'm still having an oil issue and I don't want to go throwing money and random parts to try and figure it out. My oil light still comes on after highway driving but I now know it's not due to low RPMs. The light came on yesterday and the RPMs were slightly higher than when the light would come on before. I have the clacking at start up until the pressure comes up. I've only recently had this. My pressure on the gauge looks no different than it ever has. And I am now getting a slight clacking noise (not spark knock) just before a shift. When I was on the highway Friday it made a noise similar to it when the gas pedal was in a certain spot.

If the thing on the back of the block could cause any of this, I'd rather replace it than a new oil pump or whatever misc parts.

 

oil pressure sending unit.

part link
http://www.rockauto.com/catalog/x,ca...,parttype,4588

pic
http://info.rockauto.com/SMP/SMPDetail2.html?PS291.jpg

 

What is the oil pressure at startup?
Normal running?
Hot idle or when light comes on?
How many miles are on the truck and has the timing chain been replaced.

 

What is the oil pressure at startup? - It takes about a second or two for it to come up and then is just to the right of the center line at just over 40.
Normal running? - right at the center line at 40.
Hot idle or when light comes on? - it covers the left side of the 4 of 40 but is still well to the right of the left mark. About half way between the left mark and the center but stilll within the center sweep area.
How many miles are on the truck and has the timing chain been replaced. - 174k. No.
Those pressure numbers are all normal for mine. I don't know if the later years increased oil pressure or not but mine has never been in the 50 - 60 range.
What does the Oil Pressure Sending Unit do?

 

your pressures are exactly like mine (and normal) - a little over 40 at rpm, and a little under 40 at idle. the sending unit has a diaphram on the surface that sits inside the oil passage. its measuring the pressure that the oil pump is putting out. that pressure reading is converted to a voltage level, and sent to the pcm via the electrical connection. the pcm then decides what to do with it, including displaying a reading on the gauge and triggering a light. note that the gauge is NOT a direct reading from the sending unit, like it was in the days before the computers.

do a few basic things. check the oil level and make sure its not low. change the oil and filter, as a clogged up filter can affect pressure. i recently used a gunk brand oil flush just prior to an oil change. its a solvent type stuff you pour in and idle the engine for awhile and then change oil. i don't know if it did any good, but i don't think it did any harm. it supposed to remove any sludge that might be blocking oil passages. it didn't cause my oil pressure to either go up nor down.
you might also experiment with thicker oil to see it it makes a difference, either 10w40, 40w, or 20w50. its not very popular anymore, but i used stp in my old truck. its a '76 and still running strong.

 

Replacing the sending unit might be a cheap check, but the gauge reads ok so hard to say.
The reason I asked about the mileage/timing cain is that as they wear out, the plastic coating and teeth wear down and all the pieces end up in the oil pan. The oil pump pick up is like a shirt pocket and the pieces end up in there and start to restrict the flow.
It seems that your pressure is OK so this isn't happening yet, hopefully.
Anyone with a lot of miles should consider this thought. If replacing the chain and a lot of material is worn off, lower the oil pan and clean out the pick up and pan.

 

hmmm. I just put on a new filter cause I thought the last one was doing all this. The last one was the first time I had used the Napa Spinflow. Every time I had taken the filter off before, I'd end up with oil all over the place cause there's so much at the filter. With the Napa one, none leaked out. But it was full of oil. So, I put on my regular brand and I'll see here in about 800 miles I think. I'm at a loss. If the pressure is right and the reading is right... what's causing start up clatter and the oil light only after some highway driving?

 

Well the one i am working on would not run without holding the gas pedal and it ran rough at that. So we changed the tps and it ran by itself but then it would bog down on acceleration . so i then pulled the exaust off of the manifolds and ran good no bog . then oil pressure would be about 70 at start then drop to 40 then drop to 0 so we thought oil pump and changed it and the pickup tube because of the sludge and i made the comment if there is some there its gonna be under the valve covers and intake manifold . Hehe more work so we pulled the motor apart and cleaned about 1 1/2 gallons of sludge out of top and also changed the timing chain lots of slack in it .During the cleaning of the motor we used diesel and a shop vacuumalso we pulled the oil pan off again cleaned the crumbs out and then fresh cheap oil and a can of flush it started missin during the flush operation and we killed the motor and then changed oil to a good brand which we used valvolineand runs damn good oil pressure at cold 75 psi hotabout 38 driving back up toabout 60 at highway speeds . runs damn good with new chain still need to do one more pan drop and more new oil just to be safe of the crumbs . stock pick up was a small hole in the bottom new one all of screen is exposed . but anyway good luck on it i believe this one could go alot further than the 140k on it right now i bet over 200k if my friend changes the oil regularily .