Possible low oil pressure issue in 74 road runner 318…Can someone help tell me if this aftermarket gauge is electric or mechanical?

[tonyp25]

If you’d like to skip my essay below I’m just looking to see if you can tell from my pictures if I have a mechanical or electric gauge. And if I can just swap in an autometer gauge or if I need to replace the whole setup….would I need to replace the oil sending unit too?

So I recently acquired a 74 roadrunner in a trade (previous posts about this) and the owner said that his gauge had dropped to zero when the oil got hot. He just never ran it after this and thought it needed new bearings.

When I got it the engine sounded fine. But the oil pressure gauge would drop to zero (though the gauge itself seemed terrible and had little plastic pieces broken in it). I’d shut the car off and tap the gauge which would be stuck at 15 and then drop to zero once the car is off.

Anyways I changed the oil to 20w-50 and ordered a new oil pump, pan gasket and oil pump picked tube. Once I changed the oil (which was extremely black) the engine still ran fine and the gauge still was all over the place but stayed above 20 and would got to 40 if I have it gas (never dropped back to zero).

Sorry for the long post but all this leads me to wanting to replace with a quality autometer gauge but I’m new to adding gauges to car. Can I just swap the two out and use the same wires? Can you tell from my pictures below if I have electronic or mechanical? I’ve included the original garage picture too but the oil and temp gauge don’t seem to move much

 

[1 Wild R/T]

The aftermarket gauge is mechanical, the dash mounted gauge is electrical... The sending unit is obviously electrical & I don't see a Tee with a line feeding the mechanical gauge.... But my eyes aren't as good as the use to be & the photo isn't great either.... Is there a Tee?


Okay, after re-reading the post, Why not get the OE gauge working.... First temporarily put a trusted test gauge on it & see what you really have for oil pressure, then just use the stock guage..

 

[1968rt]

You have both. Someone teed off the oil line from the factory sending unit and added a mechanical one. Sometimes you need to bleed the air out of the line. Be very careful if you decide to bleed it or you will have a big mess! A deep bucket filled with paper towels will prevent it from splashing. Do not start the car just crank it. Disconnect the distributor to coil wire.

 

[tonysrt]

I don't think bleeding the line will change the pressure reading. Any oil in the line will give you a true pressure reading. It's like an pressure line that has no liquid in it but still gives you a pressure reading.

 

[Fran Blacker]

Is there a manufacturers name on gauge.

 

[1 Wild R/T]

Is there a manufacturers name on gauge.

Sunpro...

 

[Don Frelier]

I wouldn't hesitate to take it out and install a good gauge.

 

[RemCharger]

Here's a fictional possible story,
Dude has car with low oil pressure. Dude clamps a ground wire to the sender (first I've seen) in hopes that the low oil pressure is a bad sender ground. Doesn't help.
Dude then installs an aftermarket gauge in hopes that the oil pressure will return. It does not.
Dude resigns to selling the car in desperation.
Chapter 2;
Dude B finds car for sale. Dude A explains it has no oil pressure...
Rinse, repeat

Remember this story is fictional

 

[DeltaV]

Anyways I changed the oil to 20w-50 and ordered a new oil pump, pan gasket and oil pump picked tube.

First, get yourself an oil pressure test kit and determine if you have an engine oil pressure problem. You can get an inexpensive kit from Harbor Freight or a local auto parts store for around $30 plus tax.

 

[tonyp25]

First, get yourself an oil pressure test kit and determine if you have an engine oil pressure problem. You can get an inexpensive kit from Harbor Freight or a local auto parts store for around $30 plus tax.

This is my plan at the moment...While I'm mechanically inclined I've never used an oil pressure test kit before and I'm not even sure where to connect it. I'm sure a few youtube videos can help with that though.

 

[tonyp25]

Thanks for all the responses and information everyone. I wound up getting this car for such a good price that even I needed to put a new engine in it I'd still be happy. It turned out to be much more solid than I expected. I'm almost positive that there isn't an oil pressure issue. I don't believe the previous owner did anything with the car (let alone put these gauges in). He just knew that he shouldn't run it if the gauge said 0. I think the gauge is trash. I'd love to just get the original gauge working but lets see if it truly has an oil pressure issue first. Off to learn how to use an oil pressure checking kit.

 

[DeltaV]

This is my plan at the moment...While I'm mechanically inclined I've never used an oil pressure test kit before and I'm not even sure where to connect it. I'm sure a few youtube videos can help with that though.

Unscrew the sending unit from the engine block and attach the test kit. Make sure the kit has the adapter you need...it isn't metric.

 

[1 Wild R/T]

Unscrew the sending unit & attach your test gauge there...

 

[HawkRod]

Your engine seems like it is tired. If it has very low oil pressure with regular oil, but then increases to 20 with 20W-50 oil, then that is telling you something.

To test the gauge and see the health of your engine: Start your car. Forget about the electrical oil pressure gauge in the dash - it's too slow. Look at the mechanical oil pressure gauge. Rev the engine. The oil pressure needle should move like the tach needle moves. It should immediately increase pressure as you blip the throttle. If it does, that is good. If it doesn't, then you have other issues, either the gauge or the engine.

If the oil pressure gauge DOES blip upwards when you blip the throttle, then you are doing OK. You mains are likely worn and your engine needs the thicker 20W-50 oil to run OK. Run the thick oil until you can rebuild it.

 

[tonyp25]

Your engine seems like it is tired. If it has very low oil pressure with regular oil, but then increases to 20 with 20W-50 oil, then that is telling you something.

To test the gauge and see the health of your engine: Start your car. Forget about the electrical oil pressure gauge in the dash - it's too slow. Look at the mechanical oil pressure gauge. Rev the engine. The oil pressure needle should move like the tach needle moves. It should immediately increase pressure as you blip the throttle. If it does, that is good. If it doesn't, then you have other issues, either the gauge or the engine.

If the oil pressure gauge DOES blip upwards when you blip the throttle, then you are doing OK. You mains are likely worn and your engine needs the thicker 20W-50 oil to run OK. Run the thick oil until you can rebuild it.

Thanks. This makes a lot of sense. I’ll see what the pressure kit shows. At idle with the regular oil or 20w 50 it’s right around 20-25 in the mechanical gauge. When I give it some gas it gets up to about 40 and then back down to 20 at idle. The gauge itself has broken pieces of plastic in the lens. After awhile the gauge needle seems to just stop working and will drop to 0. I’ll shut the car off and it’s stuck at 15, I tap it and it drops to 0.

 

[HawkRod]

At idle with the regular oil or 20w 50 it’s right around 20-25 in the mechanical gauge.

I misunderstood earlier posts then. I thought it was zero with regular oil and 20 with 20w 50.

When I give it some gas it gets up to about 40 and then back down to 20 at idle.

That's good - that is exactly the behavior you want.

The gauge itself has broken pieces of plastic in the lens. After awhile the gauge needle seems to just stop working and will drop to 0. I’ll shut the car off and it’s stuck at 15, I tap it and it drops to 0.

OK, so based on this (and maybe I missed it earlier - sorry) then, yes, it seems to be your gauge. but when the mechanical gauge drops to zero, what does the slow dash gauge do? does it drop to near zero too?

Regardless, get a replacement gauge and see how it behaves. Your engine might be just fine.

 

[moparsaver]

I'd love to just get the original gauge working

If you need one…

 

[tonyp25]

Unscrew the sending unit from the engine block and attach the test kit. Make sure the kit has the adapter you need...it isn't metric.

Unscrew the sending unit & attach your test gauge there...

So I removed the sending unit but the T and mechanical gauge are still hooked up. Should I remove that as well or do think I’ll still get an accurate reading running the test gauge through the T?

 

[HawkRod]

So I removed the sending unit but the T and mechanical gauge are still hooked up. Should I remove that as well or do think I’ll still get an accurate reading running the test gauge through the T?

View attachment 1489575

The T shouldn't cause any problems unless it is plugged. As long as the orifice of the T is close to the ID of the tube feeding the gauge, there won't be any delay.

 

[tonyp25]

The T shouldn't cause any problems unless it is plugged. As long as the orifice of the T is close to the ID of the tube feeding the gauge, there won't be any delay.

Alright so I hooked it up to the T. So I had both the harbor freight test gauge and cheap mechanical sunpro gauge hooked up (just ordered a nice autometer). It seemed like the mechanical one in the car wasn’t on very well so I tightened it down.

Anyways on start both gauges were above 45 psi. After it warmed up I let it sit and idle for about 30-40 minutes, the harbor freight was solid at 12 psi and the sunpro was solid at 18psi. If I gave it some gas it would jump to 25-35 psi.

I feel like the harbor freight being in the 10-15 psi range is low at idle when hot (though I read something online that said Plymouth/dodge originally stated it should be below 6-8 psi). The sunpro after it was tighten down was in the 20 range the whole time.

I guess we’ll see after I get the autometer. I’m still tempted to pull the T out and hook the gauge up so it’s the only thing connected. Not sure what you think of the current numbers it’s giving me. But it never dropped to 0 like the guy I bought it from was saying.