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Discussion Starter · #1 ·
I rebuilt my '78 304 about 1500 miles ago. It has always registered high oil pressure on start up (80 PSI), then lowers to 20 - 30 PSI at idle after driving it and warming up the engine. I ran Rotella 10-30 wt conventional oil and a WIX 51522 oil filter with the break-in oil additive.

About a week ago, on a cold morning (30 degrees), the oil filter seal let go and sprayed oil all over. I cleaned up the mess, put on a "standard" oil filter and added 10-30 oil. The new filter immediately blew out the seal. Again I cleaned everything up, went back to the smaller diameter 51522 WIX filter - and everything seems to be holding together. The only difference is that I switched to 5 - 30 wt oil. The smaller case size seems to support the oil filter seal better.

The oil pressure sender stuck and maxed out the oil pressure gauge (it now pegs out when the key is turned on). I have a new oil pressure sending switch on order.

Anyone else have this happen?
 

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The pressure relief spring in the oil pump is stuck closed. I would start by removing the relief assembly and cleaning everything out and make sure that it moves freely when you put the plunger back in before reinstalling the spring and plug.
 

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Stock, tight engines with stock pumps should be in the 40 PSI range and should hold within 5 LB of running pressure at idle.

After fixing the bypass valve, I would replace the oil with the 10/30 (COLD winter). I like 10 or 15/40 for most winter and all summer use. 5/30 IMHO, is a bit thin.
 

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Discussion Starter · #4 ·
I removed the oil pressure relief cap screw and spring. The piston is stuck at the of the bore. I tried a magnet to pull it down - no dice. I replaced the cap screw without the spring and cranked the engine to force the piston down out of the bore. That didn't work. Any ideas as to how ro free up that stuck relief piston?
 

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Same thing happened to me on my oil pressure relief assy. What I did was using a few different angled picks that would stick up and go in the bore of the piston so you do NOT score the walls and worked it out. Mine was pretty stuck but using the different angles that were in the Harbor Freight pick set(orange handle) cheap ones. When you stick it in the bore of the piston hold pressure against the piston wall and work it sideways and in/out. Take you time it will come out. I got it out and put all new in, had to use a bit of scotch brite but it ended up going in smooth and back out with no binding.
 

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Discussion Starter · #6 ·
I've got a new oil pressure sender - should be here Monday. Gonna put it in and drain the 5-30 oil and refill with fresh 10-30. Gets pretty cold where I live (Eastern Colorado). I've always used 10-40. I did a complete rebuild on my 304 and it runs great - but eats gas like crazy with my auto trans and Quadratrac. I have the Mile Marker transfer case conversion kit with lockout hubs and still only get about 10 MPG. I put everything back to stock and am running 235 Wrangler tires. Makes me miss my old CJ5s with the 225 V6 or the inline 6 cylinder engines - they all averaged 18 MPG.
 

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A properly tuned V8, with the proper gearing should get highway mileage in the 15+ range.

Dirty air filters, retarded timing, sticking choke, blown power valves, etc can force mileage down.
 

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Discussion Starter · #8 ·
A properly tuned V8, with the proper gearing should get highway mileage in the 15+ range.

Dirty air filters, retarded timing, sticking choke, blown power valves, etc can force mileage down.
I expected 15 MPG - but I'm not getting close to it. New plugs at rebuild, new (every) filters, timing set at 10 degrees (no pinging - starts and runs great), no stuck choke, running 2 1/2" exhaust. Elevation here is about 6000 feet, I run mid-grade fuel. This is my first Jeep auto tranny, so I'm guessing this has a bunch to do with the MPG issue.
 

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Discussion Starter · #9 ·
Update: I went back to 10-30 oil. I FINALLY managed to get the darn oil pressure relief piston free - it now actually moves inside of the cylinder. It was stuck at the top and after 3 hours of fighting it I finally managed to free it and clean everything up. When I had the over-pressure (and ruptured the oil filter gasket) it seems that I also toasted my oil pressure gauge. I've replaced the oil pressure sending unit twice - but the oil pressure gauge jumps to max as soon as the ignition key is powered. Have a new gauge and another (3rd) sending unit on the way.

Thanks to all for the help and support!
 

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Update: I went back to 10-30 oil. I FINALLY managed to get the darn oil pressure relief piston free - it now actually moves inside of the cylinder. It was stuck at the top and after 3 hours of fighting it I finally managed to free it and clean everything up. When I had the over-pressure (and ruptured the oil filter gasket) it seems that I also toasted my oil pressure gauge. I've replaced the oil pressure sending unit twice - but the oil pressure gauge jumps to max as soon as the ignition key is powered. Have a new gauge and another (3rd) sending unit on the way.

Thanks to all for the help and support!
Classic symptom of a grounded gauge, usually a problem with the grounding wire or sending unit.
I would start by removing the wire from the gauge. If it drops, then something is shorted (grounded) between the gauge and the sending unit.
Reinstall the wire to the gauge and pull the wire at the sending unit (since this is where you were working) and work my way back. If the gauge drops, then the unit is bad. If not, then there is a short between the sending unit and the gauge.
 

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Discussion Starter · #11 ·
Classic symptom of a grounded gauge, usually a problem with the grounding wire or sending unit.
I would start by removing the wire from the gauge. If it drops, then something is shorted (grounded) between the gauge and the sending unit.
Reinstall the wire to the gauge and pull the wire at the sending unit (since this is where you were working) and work my way back. If the gauge drops, then the unit is bad. If not, then there is a short between the sending unit and the gauge.
When I replaced the sending unit, I did disconnect the wire and powered the electric system. The gauge didn't move. I connected the wire to the sending unit and it maxed out even before I cranked the engine. That is why I'm suspecting that I fried the gauge. Lord only knows how much pressure the oil system produced when the pressure relief valve was stuck closed...

As far as 'bad sending units' - e-v-e-r-y-t-h-i-n-g today is made in China. This is why I'm suspecting the new sending unit is bad. I ordered a new 0 - 100 PSI gauge that includes a new sending unit. If this doesn't solve the problem I will just go with a mechanical gauge.
 

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Discussion Starter · #12 ·
Just installed that (4th) made in China sending unit. Tested it with the old oil pressure gauge before swapping that gauge for the new one. Don't have to swap gauges now. The gauge reads like it is supposed to. Three bad sending units in a row. Go figure. I sure wish we still made stuff in the U.S.A.

Lets Go Brandon.
 

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A god way to test the gauge/sending unit is to connect a test light to the wire at the sending unit and turn on the ignition. The test light should blink and the gauge should rise slowly.
I should have mentioned this in my reply but feeble minds are at work here........
 
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