1985 burning oil
Thread Starter
Drifting
Joined: Mar 2017
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From: Nashville TN
2017 C4 of Year Finalist
1985 burning oil
so I got some time with my car today, fixed a few issues that had been bugging me, and checked the oil. It’s at “add” after about 1k miles. I checked it with the engine cold-dumb question but that’s the right time to check oil level, right?
I swapped per spark plugs when I did the oil last. They were a little black and gunked up. Now I’m thinking that may have been oil on them.
Ive not had any issues with the car driving-the engine pulls hard and performs just like I’d expect. I get some light colored smoke from the tailpipe when I cold start her, but she starts right up without issues and doesn’t smoke after warming up a little. There’s never any black smoke and no smells that concern me. Oil pressure is 50-60 psi usually while driving (usually around 51-52).
Coolant shows no signs of oil in it.
Thoughts? What should I check/do about this?
I swapped per spark plugs when I did the oil last. They were a little black and gunked up. Now I’m thinking that may have been oil on them.
Ive not had any issues with the car driving-the engine pulls hard and performs just like I’d expect. I get some light colored smoke from the tailpipe when I cold start her, but she starts right up without issues and doesn’t smoke after warming up a little. There’s never any black smoke and no smells that concern me. Oil pressure is 50-60 psi usually while driving (usually around 51-52).
Coolant shows no signs of oil in it.
Thoughts? What should I check/do about this?
EDIT to say that the oil is pretty dark for 1k miles. It was pretty dark before the oil change though too, so it could be leftover impurities being cleaned out of the system. Just thought it worth mentioning. Definitely still looks like driveable oil, just darker than I’d expect.
Last edited by Bfenty; Jan 28, 2018 at 04:29 PM.
Thread Starter
Drifting
Joined: Mar 2017
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From: Nashville TN
2017 C4 of Year Finalist
I doubt it’s a leak. No stains on the garage floor etc. I can do the compression/leak down test. What does that tell me, exactly? I’ve done the test before but everything came back good (different car) so I’m not sure how to interpret the results.
Safety Car
Joined: Mar 2017
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From: LeClaire Iowa
Low compression means a leak-bad seal of the rings or valves. Leak down test will tell if it is rings or valves. Oil burning is either rings or maybe valve guide seals. valve guide seals usually result in some smoke at start up that then stops. I would do the compression check first, But again I am not an expert and I hope the experts chime in. Dan
Thread Starter
Drifting
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From: Nashville TN
2017 C4 of Year Finalist
So it's "consuming" oil, not necessarily burning it. If you don't drive it much, oil consumption may be higher during warm-up, and if it's always in warm-up, it will use more oil. You could also try a different brand of oil, and even a higher viscosity.
I don't think I'd worry about it. Check it every now and then. And I generally don't add oil until it gets to the "ADD" mark. Some engines seek a level somewhere lower than the FULL mark. If you constantly add oil to keep it at the FULL, you will probably add more oil during a specific period than you would if you let it need a full quart each time.
I don't think I'd worry about it. Check it every now and then. And I generally don't add oil until it gets to the "ADD" mark. Some engines seek a level somewhere lower than the FULL mark. If you constantly add oil to keep it at the FULL, you will probably add more oil during a specific period than you would if you let it need a full quart each time.
This is kind of my plan right now. If there is another problem then I want to fix it. Adding oil every once in a while doesn’t concern me, and I monitor this car pretty closely. Since I’m an amateur mechanic I like to err on the side of caution and ask this group when I’m not sure.
Thread Starter
Drifting
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From: Nashville TN
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Safety Car
Joined: Aug 2009
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From: Melbourne, Victoria, Australia
First what oil are you using? 20W 50 mineral or 5W 30 synthetic oil?
Using synthetic or quite thin oil will increase consumption.
Do the valve guide seals and that will reduce the oil consumption.
Using synthetic or quite thin oil will increase consumption.
Do the valve guide seals and that will reduce the oil consumption.
Last edited by gerardvg; Jan 29, 2018 at 03:05 AM.
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Thread Starter
Drifting
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From: Nashville TN
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10w30 Mobil 1. Anyone have a good guide to doing valve guide seals? I can google but I tend to like having an endorsement of a guide 
Race Director
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From: Lansing MI
Thread Starter
Drifting
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From: Nashville TN
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Been poking around, looks like this is a full weekend job and I might as well do the valve springs as well, is that right? Anything else that is a 'might as well' part to replace? I've got close to 90k on this car and I drive it pretty regularly (I drive it 15 miles to work on sunny days, essentially). Want to keep her running in good condition for a long time to come.
Burning Brakes
Joined: Nov 2015
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From: Airdrie Alberta
Here what the 85 corvette manual states for doing a compression check.
You should have at least 100 psi of compression in four compression cycles per cylinder (four complete crank rotations). Typically you should get full cylinder pressure in one to two crank rotations, if your into three to four to get to 100 psi, the engine has issues. Without a leak down test your only guessing to what the issue is. Compression tools are cheap.
To do a leakdown test you have to bring the cylinder up tdc (both intake/exhaust valve have to be closed) and pressure up the cylinder with compressed air (100-140psi). A leakdown tester will have a flow gauge on them to measure actual air loss, but with a good ear you don't need a flow gauge. Any air out of the intake/exhaust means you have valves leakage issues. You also see if there is any air bubbles in the coolant system (head gasket). Air coming out the crankcase, means worn rings.
I typically my limit is 120 psi and in two compression events.
You can use a some of compression tester tool for a leak down test. Just remove the gauge and check valve, only use the air chuck that goes into the cylinder head and attach an air hose.
You should have at least 100 psi of compression in four compression cycles per cylinder (four complete crank rotations). Typically you should get full cylinder pressure in one to two crank rotations, if your into three to four to get to 100 psi, the engine has issues. Without a leak down test your only guessing to what the issue is. Compression tools are cheap.
To do a leakdown test you have to bring the cylinder up tdc (both intake/exhaust valve have to be closed) and pressure up the cylinder with compressed air (100-140psi). A leakdown tester will have a flow gauge on them to measure actual air loss, but with a good ear you don't need a flow gauge. Any air out of the intake/exhaust means you have valves leakage issues. You also see if there is any air bubbles in the coolant system (head gasket). Air coming out the crankcase, means worn rings.
I typically my limit is 120 psi and in two compression events.
You can use a some of compression tester tool for a leak down test. Just remove the gauge and check valve, only use the air chuck that goes into the cylinder head and attach an air hose.
Last edited by Mike Holmen; Jan 29, 2018 at 11:01 AM.
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85 has forged pistons which will have a bigger cyl to piston gap as they expand different from later pistons, so they can burn more at startup. Plus, they're ancient as are the valve seals. It doesnt shock me that youre burning up oil. Try the seals and see if it improves.
If its almost black that can be just that its pulling a lot of gunk out of the engine or youre running rich enough that fuel is just getting into the bottom of the engine. One or both can happen.
If its almost black that can be just that its pulling a lot of gunk out of the engine or youre running rich enough that fuel is just getting into the bottom of the engine. One or both can happen.
Thread Starter
Drifting
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Team Owner
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From: Athens AL
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Thread Starter
Drifting
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From: Nashville TN
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What’s the rightPSI for oil pressure? I’m sitting at a constant 60psi which I thought was too high
Edit: that’s at idle. It fluctuates a bit on acceleration but never goes below 50.
Last edited by Bfenty; Jan 29, 2018 at 01:18 PM.
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Min rule of thumb is 10psi per 1000rpm.
Thread Starter
Drifting
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From: Nashville TN
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Ok. Thing is that it doesn’t really go down at idle. Not sure what that means.