Bfenty

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?

Whaleman

I am sure not an expert. If it was me I would first rule out a leak. If no leak I would do a compression and leak down test. I would keep my fingers crossed. Dan

Bfenty

Whaleman

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

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VikingTrad3r

puff smoke on startup u r leaking past valve seals. ill bet they are original and will break aprt on removal. cheap and easy to fix. like a boss.

Bfenty

gerardvg

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.

Bfenty

PatternDayTrader

^^^ This.

Bfenty

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.

Mike Holmen

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.

vader86

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.

VikingTrad3r

Vader would the iron head 86 have the same forged pistons?

Mike, 2 strokes seems low but if that is what the manual says then ill redo my test. I read it was supposed to be 7!!

Bfenty

I can answer this one-no, the '86 has cast pistons not forged. One reason I like the '85

VikingTrad3r

interesting. i did look at porting cast iron heads because they can take the heat better (supposedly) but the porting work is crazy long.

vader86

84-85 have TRW forged, in 86 they went to cast.

Bfenty

What’s the rightPSI for oil pressure? I’m sitting at a constant 60psi which I thought was too high

vader86

That is good pressure, roughly where it should be. It should remain 45-65psi anytime your foot is on the gas, and idle should be about half of what it normally runs at.

Min rule of thumb is 10psi per 1000rpm.

Bfenty