Low oil pressure.

[get me on the road]

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.

[get me on the road]

I forgot to mention that it is an 89 silverado 5.7.

[Palf70Step]

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.

[piecesparts]

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.

[get me on the road]

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.

[piecesparts]

Originally Posted by get me on the road

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.

[get me on the road]

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.

[piecesparts]

Originally Posted by get me on the road

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.

[ChevyTech]

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.

[ChevyTech]

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.

[get me on the road]

I unplugged it, key on engine off, and the gauge went all the way up. So should I R&R that sender? So if that is called oil pressure sender, what is the two wire one by the distributer(tor) called? I did plug it back in and start the truck and it still reads 15 so that eliminates a bad/dirty connection.

[ChevyTech]

So should I R&R that sender?

Yes!
Replace it.

So if that is called oil pressure sender, what is the two wire one by the distributer(tor) called?

It is easy to get lost in the terminology.

As for the one in your photo:
Some people will a sender
Some call it a sending unit
Some call it a sensor

It changes its resistance depending on the pressure applied to it.

As for the two wire one by the distributor, it is technically a switch. It is for the redundant circuit for supplying power to the electric fuel pump. When there is oil pressure, power flows through the switch for the fuel pump, and if it is working ok the truck would keep running if the fuel pump relay was removed after the truck was already running. If the fuel pump relay was left out the engine would have to crank until it had oil pressure, before there would be power supplied to the fuel pump for the truck to start.

[get me on the road]

ChevyTech or anyone keeping up on this thread, I cant get the sender loose, but the pipe that its sticking out of is loose, so before I take my air hack saw and cut enough off the sender to be able to spin that pipe out (and get the rest of the sender out once I get the pipe out) do you have any suggestions? I sprayed the sender threads with penetrating oil so maybe that will help. Also should I use thread tape on that pipe where it goes into the block?

[piecesparts]

If you have decided to replace the sending unit, then I would have the new one in hand before taking the old one out. Just in case you need the truck to move. Now, it surprises me that the old one doesn't want to move, but then again it is metal and metal does weird things when it is old. If you have to destroy the old sender, then that is a choice that you can make. After the pipe is out check it over for any fatigue or cracks, before coating it with a teflon dope. If you use tape, then use it on the last few threads (threads up towards the pipe elbow, not the tip of the threads) that are engaged into the block. That way you get the sealing effect, and also the grounding effect of metal to metal contact for the sender to work properly. (this is important)

IF you try to take the sending unit out of the pipe elbow, the try as you can to use anothe wrench or small vice grip on the pipe to support the piece---don't want to break anything now.

Did you ever find a sending unit for less than $65.00? That sounds awful expensive.


Just my .02......

[get me on the road]

Well I got a replacement sender for that one I took a picture of for $21 w/tax at AZ. But as for the $65 one at Advance, they didnt have a picture on their computer or one in stock so I couldnt tell you if that was even the right one. Thanks for the info on the thread tape, that thing you said about grounding had never even crossed my mind. I guess you really do learn something new everyday.

[get me on the road]

Thanks guys, especially you ChevyTech, I really appreciate your help. The first picture is for you ChevyTech and Pieces. The second picture is for you Pieces, I had to drill out most of the old sender that was left in the pipe and use that thing (cant remember what their called right now).

[ChevyTech]

You’re welcome. A thank you is always appreciated.

Good to hear you were successful.

Nice extractor and nice job of using it.

[piecesparts]

The word you may be looking for is an "extractor" or even and "Easy-out". The thing appears to be working now. Great. Y'all be welcome.

[get me on the road]

Originally Posted by piecesparts

The word you may be looking for is an "extractor" or even and "Easy-out". The thing appears to be working now. Great. Y'all be welcome.

It was kinda late, and I was tired. I was thinking "easy out" but when I did a picture of a chain come along was popping into my head which created some confusion, so I just said I couldnt think of the name.

[mfg15]

I realize this is a ten year old thread, but I just found it useful for reference as I dealt with an identical problem, so here's my .02 in case someone else finds this thread again:

Mine is an '89 GMC 5.7, had similar oil pressure issues, which I traced to the sender down by the filter. Couldn't get the thing out of the brass connector. Tried going at the nut at the top with a deep socket, but that ended up making the sensor housing spin upon the base, still not freeing the lower threads from the brass connector. Was worried I'd have to cut it off and remove the brass fitting (which I didn't want to do because that fitting wasn't loose as with the previous poster).

Final solution, which worked great, was applying some heat. The sealant they put on the threads is quite responsive to heat I think. I have a small torch with a fine, directional flame, put that directly on the connection for awhile, and then it loosened up pretty well.