oil pressure question
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Senior User
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oil pressure question
Guys, tonight I cranked my truck (302) and was letting her warm up a little and I noticed that I did not have any oil pressure. I have installed VDO gauges and last night I was running about 50 lbs of oil pressure at idle. Tonight my gauge is not moving. I checked my ground wire to my gauge (good). and I ohmed out my oil sending unit wire to my gauge (good, no shorts to frame). The engine seems to be running fine and the oil pump is new. I wiped the oil off my dipstick and checked it oil is fine. I cranked it back up for about a minute and shut her down. I checked the oil stick again and oil has been pushed back up my stick past the full line like it normally does. Questions: Is the oil backing up the stick a normal verification of oil pressure? Also, does anyone know how to check a VDO oil sending unit? Would it read like a diode or rectifier? Like I said until tonight, it had been working fine. Any help would be appreciated.
Posting Guru
Joined: Jun 1999
Posts: 2,415
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From: Comox Valley, Canada
I would expect to drop from 50psi to zero in one day would be a major failure that I am sure you would hear as it happened 
but for peace of mind hook up a direct reading gauge to make sure all is well. You could have a plugged oil pickup or some other prob. Sorry I do not know about VDO guages but normally if you disconnect and ground the sending unit wire it reads full one way and when you leave it hanging in the breeze it reads full the other way meaning the guage and wires are good. Good luck hope it is a simple fix.
but for peace of mind hook up a direct reading gauge to make sure all is well. You could have a plugged oil pickup or some other prob. Sorry I do not know about VDO guages but normally if you disconnect and ground the sending unit wire it reads full one way and when you leave it hanging in the breeze it reads full the other way meaning the guage and wires are good. Good luck hope it is a simple fix.
Postmaster
Joined: Sep 2005
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It is an electrical or mechanical gauge?
I am guessing here, but I would assume a mechanical gauge can get air trapped in the line and mess up the reading? Mine is mechanical and has a small capillary tube that the oil follows up to the gauge.
Electrical...I would not know what to check other than what you already have.
I am guessing here, but I would assume a mechanical gauge can get air trapped in the line and mess up the reading? Mine is mechanical and has a small capillary tube that the oil follows up to the gauge.
Electrical...I would not know what to check other than what you already have.
Fleet Owner
Joined: Jul 2004
Posts: 27,290
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From: NM
Did you by any chance pull the distributor between the two runs?
The distributor drives the oil pump via a hex-shaped shaft. If you pull the distributor, the shaft can fall into the pan. The shaft can also snap if you are using a HD oil pump, or the pump seizes, etc.
Sparky is correct about testing an electric gauge. Air in the capillary line to a mechanical gauge won't affect its readings. The oil getting up past "Full" on the dipstick just means the crank is splashing oil around, not a reliable indication of anything.
I would get a mechanical gauge on it ASAP! better safe than sorry.
The distributor drives the oil pump via a hex-shaped shaft. If you pull the distributor, the shaft can fall into the pan. The shaft can also snap if you are using a HD oil pump, or the pump seizes, etc.
Sparky is correct about testing an electric gauge. Air in the capillary line to a mechanical gauge won't affect its readings. The oil getting up past "Full" on the dipstick just means the crank is splashing oil around, not a reliable indication of anything.
I would get a mechanical gauge on it ASAP! better safe than sorry.
Moderator
Joined: Dec 2003
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From: Saskatoon, Saskatchewan
Like what has already been said a good and a known reliable mechanical guage is the only one I would trust.
When that says no oil pressure then it`s time to worry and not before.
It doesn`t take long to hook one up and really find out what the motor really
has for oil pressure.
If there isn`t ANY oil pressure there would be lots of engine noise, certainly rattling lifters for a start.
When that says no oil pressure then it`s time to worry and not before.
It doesn`t take long to hook one up and really find out what the motor really
has for oil pressure.
If there isn`t ANY oil pressure there would be lots of engine noise, certainly rattling lifters for a start.
Junior User
Joined: Feb 2007
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I have VDO Gauges on my 55 truck and they vary the resistance as a function of Oil pressure which the gauge senses and turns the needle. To check the sending unit disconnect the gauge and put an ohm meter between the point that normally goes to the gauge and ground. I can't remeber what the range is but it should read either a few ohms with the engine off or somewhere in the hundreds. With the engine ON and with pressure the gauge should read a different value either higher or lower depending on its design. If you need to check the function of the gauge independent of the sending unit let me know and I will post that as well.
Good luck
Good luck
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Senior User
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Thanks guys, it is a electric gauge. I will ohm out that sending unit after I check it with a direct line gauge. I rather make sure before i have it running anymore. I didn't touch the engine between runs.
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Laughing Gas
Joined: Jun 2007
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From: Richland, WA.
Did you do anything to the truck, change oil maybe? or like Ross said pull disributor. How high does oil go when motor shut off? Full mark on dipstick is when motor has been run then shut down. you want oil filter full. This I'm sure you know but, did you know that a motor overfull on oil can drown an oil pump. It will make a vortex in the pan instead of pumping. Most common when oil pan goes over framerail and has drain plug on both sides of frame rail. I worked with a guy who did this and it cost us an engine. You could always pull valve cover or oil sender on running engine, with someone else present to shut down engine. Kurt G. Y-blocks rule.
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Senior User
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I haven't had a chance to troubleshoot since I posted this but I did come up with a mechanical gauge to verify oil pressure and I bet it will something stupid with the wiring since I am in the process of wiring the truck. I just want to sure about my pressure before I start pulling wires again. I have my wiring going through jack and pin connections and this wiring is not the final secure process, since I still have to paint the truck. Plus I had been drinking that night but I usually work the best when daddy is a little calm. I will be on vacation this weekend, but I promise to report what I did stupid on this.
The oil level is fine and engine component have not been touch in over 2 months.
The oil level is fine and engine component have not been touch in over 2 months.
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Senior User
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Okay the wiring problem bets win. I have not found the problem yet but I have verified oil pressure(50 lbs with mech gauge), the sending unit(about 8 ohms) and the gauge. I just have to trace out the wiring and hunt down the problem. I should get that done tonight and I will let ya'll know what I have done stupid. Could it be the knuter valve?
