308 Compression/Leakdown Result - Help?

I am a big fan and believer in the durability in the 4 valve engines. I think you should be able to go many miles with it with out serious problems. I would however pull the valve cover and check the valve clearances to be sure the problem is not just a tight valve.

Up until recently I had not taken a head off a 308 4 valve for quite a few years. We did one a few weeks ago. Bad news. That car is 20 years old and chances are the heads have never been off. When the day comes the heads do have to come off it may be a nightmare. There will be 20 years of corrosion between the heads (aluminum) and the studs (steel). The old 2 valve motors are quite a bit easier when they get stuck. You can make tools to bolt to the head to press against the studs to remove the heads. Can't do on a 4 valve, the studs are tucked away under the cams. We had more time in removing the heads than we did removing and stripping the motor putting it back together and reinstalling it.

You may not have to go through that, but if you do you should be aware of the possibilities. 84/85 308's are great cars but like all of us they are getting old.

 

Just saw the leak down numbers from the other thread. 16+18 % are getting pretty high.

 

For what it's worth, my 84 308:


Front:

Compression Leakage
1-170 15%
2-175 18%
3-170 15%
4-175 15%

Rear

1-180 15%
2-175 20%
3-175 12%
4-175 10%


The test date was 9/15/1997. So far I have had 35,000 trouble free miles, two major services and LOTS and LOTS of hard track days too.

308 QV's are pretty good cars. She has 92,000 on the clock now.

 

Tommy,
It would be interesting to know what the numbers are like today. Better, worse or the same.

Dave

 

Thats good to know. Those motors were so little trouble ever we hardly ever checked them unless someone insisted. I hope we are going in that direction again.

 

That's how I got these #'s; by insisting during a PPI (the car was running just fine). Seven months later I had the valves adjusted. It turned out that all the valves were in spec except 2 and they were out exactly in reverse. We just swapped the shims between the exhaust and intake to correct it. It was as if the previous mechanic had the correct shims in his hand but simply put them on the wrong valve accidentally.

To be honest I am actually afraid to check it now. I mean the car runs hard and strong and doesn't smoke. But, It does have nearly 100,000 miles and an engine rebuild isn't cheap. I am scared that I'll see a bad number somewhere and that's all I'll be able to think about from then on. I'll never sell this car so since an engine rebuild is the only real solution at this point anyway, I may as well forget about it and just drive it until it acts like it needs it when something finally gives. Otherwise, what's the point in checking it and/or tearing it down now? None that I can figure.

 

I just had compression test on my 85 QV in Dec,
and I had a 130 on all 8 cyl.

Not to hijack but are these good ?

 

Thats just the point. Ferrari's of that era generally ran and ran and ran. No smoke and good power so we just never checked them, the right pedal worked great so what was the point? Thats the reason I have so little as a data base for those cars. For years one of our least used tools was a compression tester but then a series of car came along that had issues and serious sensitivities in that area and so in those we just can't ignore numbers like that. But on a 308QV, hey I take it back, drive it. Like I said earlier the one I just had apart had 105k and came apart for head gaskets (water pouring out), car ran VERY strong.

 

Personally, I would not buy the car. It wouldn't really bother me that much to see numbers like on Tommy's car. They are all about the same, the compression is very good, the leak down is a high,but honestly with those compression numbers I suspect the leak down test method...my engine give numbers in the 10-20% range when cold, but 0-2% when warm.

The car you're looking at clearly has 2 cylinder that are not like the others and if you can hear the air leaking out the exhaust valves then there is a problem. It might just be the valve adjustment or it might be that at some point it broke a belt and bent a couple valves.

Then throw the repaint and redye on top and it's not a very good car IMO. But If you really want it, I would think the right price would be in the $20k-$25k range.

FYI-I bought my car without a PPI. It was leaking everywhere so I decided to pull the engine and re-gasket it. Before I did I compression and leak down tested it and got numbers like yours. I checked the valves and they were perfect. When I cracked the engine it had 2 bent valves and little dings on the front 4 pistons. I had to rebuild it, just the parts ran about $4k and it took many many hours or time.

 

Like Mark,
I don't lik the numbers. You've aso got to know how the test was done - warm or cold because the readings will be different.

My 77 308 with 40K odd miles on it tested as 6 - 8% on leakdown on 7 of the 8 and 12% on the last. Since then we've only run a compression test and that was 185 +/- 10 if I recall correctly.

One benchmark.
Philip

 

That engine needs to come apart......

 

It's not as if the 308QV is a rare car. Unless you're getting a STEAL, go find another car. If you still want this car, beat the seller up badly. A rebuild on the motor could possibly cost 30-50% of the value of the car.

 

I agree. There are plenty of cars out there that don't need rebuilding yet. Focus on the engine, not the color, etc. that you want. All things being equal it's much cheaper to repaint then to rebuild. Don't get too caught up in the difficulty with locating that mechanically sound marrone colored 308. If it has never been wrecked and the mechanics are perfect and the price is right, who cares if it's lime green? Buy the car and paint it whatever color you want later. Don't misunderstand me, search for what you want first but be realistic since the last one was build 20 years ago this year.

 

Interesting. My criteria was the opposite. Having had an MGB of old (2 of them actually), I was most worried about body rust and I found once the moth had set it, it is darn near impossible to eradicate. With the GTB, I figured that most mechanical stuff could be fixed, at a price. All that said, if the motor needs to come apart (as it sounds), I'd budget $15K. If the seller will meet you part/most of the way, I'd do the deal. That way you know the motor will be pristine.
Just my 2c.
Philip

 

I second some of the other voices, you have a bad motor. While it is "possible" it might come back, as of today that motor is suspect, and as such the price should be negotiated respectively. Nobody I know has a crystal ball, and nobody can see the rings short of disassembling the engine. With one cylinder showing ring trouble, with a tear down it already needs rings, so probably pistons as well, and as its leaking at an exhaust valve, we have to assume it needs a few valves as well. You absolutely have to figure worst case scenario when your buying one of these. And forget knocking off for labor thinking your doing it yourself. Deduct exactly what a dealer would charge for r&r the engine with a total overhaul. If you deduct less, your only shooting yourself if the engine goes south. If the owner wont negotiate, walk away. Its just a car man, and there are others available. If you get a deal you cant walk away from, and the motor heals itself, you win.

 

My opinion only, to add to the others.

Use the test results as a lever on the price but is there really a need to worry? Buying any car is a risk and an engine's performance is not constant through it's life. Take it for a hard run and the results would probably be different. The 308's apparently have an excellent reputation for longevity. If the 30000 is genuine, service history good and the price is right, the chances are the engine will not be a problem, so why not go for it.

 

18% leakage past an exhaust valve? If I were doing an annual or 100 hour inspection on an airplane, the FAA inspector would ground the plane until the problem was resolved. We use 80 psi on airplanes because the big pistons are hard to hold back with a propellor blade. 72 over 80 is about the bottom end limit, about 10% leakage. But it depends on whats leaking. Rings are not as big a problem as a valve, and an intake isnt as critical as an exhaust. Exhaust leaking past the valve can turn it white hot and melt it right off the stem. If I recall, I think we were only allowed 6% leakage on an exhaust valve. Maybe less. That problem needs to be resolved before miles are put on, or it can and will do serious engine damage. There is every reason to worry, IMO.

 

Sorry for Hijaking but a quick question.

How long do you test each cylinder for in a leakdown test.

The % loss must be time dependent - the longer you leave it the bigger the leakage.

I have access to the kit for a leak down test but I don't know how long to leave each cylinder for comparison.

Anyone got a full procedure, or know how long I should leave each cylinder leak?

 

No, it is not time dependent. There always flow into the cylinder through an orifice. What you are actually measuring is the pressure drop across the orifice, which is a flow rate or leak rate really I guess, but for some reason it is referred to as % loss.

 

Ah, it is not a static presure drop test!,

It is a constriction orifice plate pressure drop! as a measure of air flow.

I never did one before now I know I need to find someone with the right equipment.

So much for me thinking I could do it by myself.

Thanks for the reply.

 

$69.95 for summit racing, but you need an air compressor

 

Do it on a hot or cold engine?

Why do some people test them cold?

 

Some will argue differently, but oh well. I was taught by an FAA inspector and a whole bunch of grey hair aircraft mechanics. We are talking pistons with rings in a cylinder, and valves. Basically an engine is an engine. The leakage is based on one air pressure regulator set to one setting, and seeing how much of that pressure will hold inside the combustion chamber. If our first set was at 100 psi, and we could maintain 90psi inside the cylinder, we could say that is a 10% loss, The cool thing about a leakage test, is that the engine is static, and you can listen for the air leaking out.

You listen at the exhaust, the intake, and the oil cap. leakage past rings will be heard at the oil cap, leakage past the intake valve will be heard at the intake manifold or aircleaner, etc. You will virtually never see a cylinder hold 100%, but they can get awfully close.

On aircraft, we would turn the engine with the prop, all plugs out, and use the prop to hold the motor with the prop at TDC with air pressure applied through the leakage tester. Some of those airplane engines have big pistons, and if you put 100 psi inside it most guys could never hope to hold it back if it went over center. So we use 80 psi on out leakage tester, and get a reading we would call 80/78 for example. This would be very good, and was what we wanted to see, on occasion we would see a 80/79, lots of 80/77, but as it fell past 76 it was considered worn if it was past the rings.

Why warm? Because the piston swells with heat and forms itself to fit the cylinder fully. The rings expand and close thier gaps up. So the general opinion is that you would get a more accurate reading of leakage at running temp. But you can do it cold, and you should still have minimal leakage.

One area where the test differs from the general way cars are done, was that while we held the propellor, and while pressure was applied to the cylinder, we would walk the prop back and forth to bring the piston back and forth over TDC. Two reasons. One, on almost every cylinder in just about every engine I ever tested, and I have done hundreds, the rings could be heard to make an audible snapping sound, with a noticable increase in holding pressure and a reduction of loss. In other words, the rings wernt seating until we shoved the piston back and forth under pressure. The second reason for doing it, is that a cylinder can wear drastically in the very last few thousandths in the top of the cylinder, and you can see a noticable increase in held air pressure, just below TDC. This would indicate the cylinder is dying and would need service very soon.

Why cold? Sometimes you need to determine the condition of the engine before attempting to run it. If you noticed a large loss, say something over 20% from either valve or the rings, one could reasonably conclude it was needing help. 18% loss past an exhaust valve is just not acceptable, IMO