Soooo...I did a compression test today and here are the results. I think I know where this is going, but I wanted some other thoughts/interpretations of the results. Plugs were all over the map too...oil, carbon deposits and some high speed glazing, sometimes on the same plug

At least my 9 y/o daughter got a lesson in engine compression....she was my key turner

Any help is appreciated

Oh yeah...engine is L48 with an alleged 120K on the clock

1- 65 Did not go up after injecting oil
2- 130
3- 130
4- 135
5- 125 Dropped off quickly
6- 125
7- 120 Dropped off quickly
8- 125

 

time for a valve job at a minimum, good time to upgrade to better heads

 

Time to pull motor with that mileage total rebuild

 

It's tired, wounded, and needs some love. Unless you're determined to keep it stock, Oldalaskaman's suggestion to get better heads is a good one. You'd probably spend as much to go through your original heads as you would to get some modern ones that will make a bunch more power.

 

Obviously, #1 is low. But a compression test is kind of incomplete without all the data.
What was the compression reading on the first stroke?
IS the total number of strokes the same on all cylinders?

And after that, why did 2 cylinders drop off? Does your tester not have a check valve that holds the pressure?

Granted this is a high mile engine, but there could be other problems that might be there that aren't so visible. Giving #1 a shot of oil proves the rings, but is that being masked by a valve problem?
Not picking on your work, but there is a bit more information that a simple compression test can give up, and help make a better informed decision about what you need to do.

 

A leak-down test is next - don't make decisions on what needs to be done without it, as you don't know where the problem is...a leak-down will tell you exactly.

 

Did u block open the carb throttle plate? I would try another gauge too for verification.
120k mi is over due for vlv job and a new cam too. But your bottom end has another 200k mi to go. save your back and $$$ until u find the problem.

Good luck,
cardo0

 

You're quite the optimist- the main bearings, rod bearings, the entire rotating assembly, pistons, and rings are all good for 320,000 miles? In a performance car like a Corvette? You can run whatever risks you want to, but that would not happen in my Corvette.

 

Those are not very good numbers, but the main thing is that you have one cylinder that is way, way off from the rest. That's not good. Now you need to figure out if it's the rings, or the valves. That means a leakdown test. Hopefully, it's a head problem, might be just the excuse to chuck a set of vortecs on there!


Keep the shiny side up!
Scott

 

Tim- Test was performed per the GM service manual. My guage does have a check valve. And I tested 1 ,5 and 7 three times to verify the results were at least consistent.

Card0- yes throttle plates were blocked open. I am considering doing the test again with another guage, just to verify consistency.

Billa- I have read a little on performing the leak down test, but have never done one. Is there anything special I need instrumentation wise?

One item I am interested in knowing more about is why 5 and 7 were the ones to drop pressure. Correct me if I am wrong, but they are next to each other.

Also based on what I saw on the plugs and the general performance as of late, I knew at a minimum the valve seals were worn. But this test proves there is more going on than just a few oil seals

All, thanks for all of the responses. I knew this was leading toward at least a head issue, if not a complete rebuild. Was just hoping for another year before I had to go that route.

Going forward, I am not looking a keeping the drive train original. Probably a roller block ( Have not decided CID) and a 700R4 swap. I will be working with a local machine shop on the build. I know crates are a favored way to go, but I want a little more control over what is in the engine.

 

Yes; you need a leak-down tester. They can vary dramatically in quality...but the cheaper ones are fine for occasional use.

http://www.harborfreight.com/cylinde...ter-94190.html

Roller-cam engine is the right foundation; good on you for taking control of the design and build.

 

excellent plan on the roller block and od trans.

 

With the insturments you presently have, you performed something like a "leakdown" test (if the valves were closed in the cylinders you tested); you watched the gauge as the internal pressure bled off and recorded that some leaked quickly and some did not. Now, you need a gauge with a one-way check valve and a release button to determine the maximum pressure that can build up in those cylinders. With the info you now have, you can easily see that there is some significant cylinder leakage. But, whether you do a full rebuild of the engine, only change/rebuild cylinder heads, or do nothing...depends on whether you have short-term or long-term interests in the car. If you intend to keep it for several years, rebuild the engine or replace it with a 'crate' engine.

 

To the op, dont worry about any more testing, tools, your plan is good, save your money and time for the upgrades. Even if you were going to rebuild or have that one rebuilt you're at the point where its time to start pulling parts. Further testing will only tell you what you already know. its old and tired. If you're doing the work yourself, good used roller blocks are cheap. I'm in the process of doing the same thing only with a 2004r

 

As an example, about two weeks ago I completed a leak-down of a fairly tired 355 with about 120K miles on it. Cranking compression testing had shown that one cylinder was in the low 60s, but a leakdown showed that it was definitely at the intake valve. I "staked" the valve, and the cylinder came up to within a few pounds of the other cylinders.

30 minutes and one light thwack with a mallet...and everything's good, at least for a bit

Troubleshooting and diagnosis is the core of being a mechanic. No question the beast is old and tired, but it's still worth investing in finding out where the issues are - which will also provide some guidance on your overhaul plan.

As an aside, leak-down testing isn't about "maximum pressure" - it's about determining the percentage of leakage and where it's leaking from. It compares what the cylinder can hold to the inlet pressure -and then your ear will tell you where it's leaking

 

This thread has been good for me since I have a 78 L-82 4 Speed with 66,000 miles on it that is stock except for the exhaust-everything internal to the engine is OEM. The engine produced on 2 different occassions on the dyno 225-233RWHP but is blowing oil on the last dyno run a lot of smoke through the passenger valve cover breather. I would think that the motor has to be in decent shape for to be making somewhere between 40-50 HP over the the NET rating of 220hp. I am hoping that all the smoke through the valve cover at WOT and 6,000 RPM is from the heads leaking at the valves and not the rings. I am going to do a leak down test and compression test soon when I have time. All the plugs looked good with just a couple with very little black spots on the electrode but no real signs of oil in the combustion chamber. The smoke through the breather is mostly white in color-not coolant though.

 

I missed this previously; they are side-by-side and could be an indication of a head gasket leak

 

I have 400,000 on my bone stock, untouched, original engine in my daily-driver Vette. No valve job. No new bearings. No problem. No reason to tear into a bottom-end after only 120,000 unless it's been run out of oil...

Leak-down test is the only true measure and evaluation that gives you some real data about the problem.

Lars

 

yer a bit different then the average car owner, lars

 

That's impressive Lars but I'm sure you've maintained your car very well and if it's lasted that long you definitely haven't abused it. While the OP may have had his car since new, the odds are pretty slim. More likely, he doesn't know the levels of abuse or maintenance that preceded his ownership. He, and you, may be different from me- I really don't know. I don't beat on my motors but I build them to make as much power as is realistic for a street motor and I want to know that if I decide to run them hard through the gears they're up to the task with a minimal chance of failure. I wouldn't have that confidence given his situation.