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Discussion Starter #1
I have a 390 in my 79 bronco and it is using a lot of oil. I checked the compression and it turned out good. It also has good oil pressure. It has around 90000 miles on a professional rebuild. It smokes when warm when you first start it but not while driving. I think it needs valve seals but why is it using so much oil. I would say it uses a quart around 800 to 1000 miles. Is it the valve seals or something else? It has all the power it ever had. Any ideas?
 

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House of Windsor 4ever!
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You need to get your hands on a compression leakdown tester (and a source of compressed air if you don't have one) and run a cylinder leakdown test. That, in combo with your compression test, will tell you the exact condition of the cylinders, and tell you if the rings are compromised. Ten or more percent leakdown is considered outside normal specs.
 

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You can pull the plugs and see if one or more is oil fouled. That will tell you which cylinder is the problem.
just FYI, I had a 390 that burned oil on two cylinders and even though the compression was good on all the cylinders, every cylinder passed the leak down test, it was still burning oil due to rings. Or maybe just the oil control ring on those two cylinders. It burned a valve and a set of rebuilt heads was put on it and the oil burning problem continued on the same two cylinders. The cylinder walls on the oil burning cylinders looked no different then the others so I has hoping the heads would fix the problem but no such luck. Point is the leak down test may not find your problem.
 

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Discussion Starter #6
Thanks for the info. All the plugs look great. They are a light grey color. You cant tell one plug from the others.
 

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House of Windsor 4ever!
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If they're all the same and that color, you're simply leaking the oil out at speed.
 

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I would guess valve seals/guides from the start up smoke. The high consumption, I would look at the rear main seal.
 

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is the pcv valve working?

agree with valve seals as smoke on start up/and high vacuum idle

does inside of tail pipe feel oily?
it may be burning at speed and not smoking enough to see it ?
 

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Discussion Starter #10
The rear main I replaced not long ago and the tail pipe is clean. I am sure the pvc is working but I will double check.
 
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