Compression numbers

I had a 355 arrive at the shop with a consistent miss fire in number one cylinder. Compression was excellent at 180 psi. Leakdown showed approx 5%. Hummmm..obviously this points to either a plug or injector. Swapped out injectors with a good cylinder and plug. Same thing, miss fire.

So I decided to do a running compression test. With the gauge plugged into the cylinder, I started up the car. With the engine running, the cylinder was only pumping 40psi..that's it.

So here we have a car that passes leak down,and compression tests with flying colors, but there was internal engine issues.

A compression and leak down, is not the be all and end all to tell you and engine is in good condition. It's a good gauge, but it will not show any future problems. A 355 engine can go from good to bad very very quickly.

 

I have never known a tech who knew how to do that or ever do that. Good for you!

 

Tell me about it, and the 2 other shops the car went to before it got to me. Never seen it before.

 

Possible explanation. Valve in compression tester is too slow to response at running speeds. I.E. doesn't open fast enough to build pressure and doesn't close fast enough to hold pressure.

 

Yes, it could be a valve spring issue. However I wasn't given the go ahead to pull the car apart. So it is now sold (to a fellow fchater). I honestly can't wait to see what he finds out when he starts getting into it.

 

I did not mean an engine valve spring issue. The valve in the compression tester.

 

My compression tester works perfectly. I even put a new valve in it to confirm the result.

 

Actually it makes perfect sense. Traditional is higher than idle about double. Running idle that low implys engine damage, for example valve guides. The tip off that more advanced diagnosis was needed is passing the compression/leakdown and verifying typical causes of misfire are not there yet the misfire persists. So the next logical step is the running compression.

Can you imagine purchasing a 355 with a very simple innocent misfire CEL thinking you got something simple like old plugs for $8 bucks only to find you get to do a 5 figure valve job? I have always said 355 purchase is like advanced Ferraring. That is not a put down on the model. It is a great car if well sorted. You have 3 choices. 1) know what you are doing, or 2) trusted PPI from someone who knows what they are doing, or 3) Be lucky. Having a bit of all 3 does not hurt either.

 

You are not understanding what I am saying. No problem. I'm not going to bother to try explaining further because it was just speculation. No big deal.

 

"No big deal" is a great phrase to read these days as civility is setting in. Thanks to all.

Sometimes it's hard to be clear on the forum. If we were all standing around the car with drinks... It would be Crystal clear.

Sorry we are all so far A. W. A. Y.

Robb

 

With drinks is key when working on a 355. Sometimes it makes you think outside of the box. If figured out it after an evening in the hot tub with my wife and a couple of glasses of wine..

 

When you get your 355 make sure you hook up with 355rockit in SD and come join the Stooges in Redondo Beach. You will learn a lot about these cars. There are several very advanced DIY guys that always show.

The answer to your question is just auto mechanics 101. That is just how it works. It's like putting a smaller pulley on your waterpump thinking you are increasing the flow of coolant only to find you have cavitated your pump.

 

I can imagine
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don't get caught up in the tests as snapshots in time. Standard compression test is very gentle 4 to 6 cranks of a starter motor. That hugely different from running compression or the explosion that goes on at idle or the mayhem in the cylinder as wot. There are some who only value the FAA leakdown protocol and others who are all about low pressure testing. You get into a huge dissertation on valuation methods and each tells a little different story. Think about all the things that can happen if you had valves rattling around in valve guides and how that interacts with seats but look at the whole system valve springs etc and how different compression and leakdown tests and methods can play with your mind. That is one way see what you are getting.

What are you racing and who with? Did you do SCCA runoffs this year? Im racing SCCA T2 in a Z06 corvette. The National championship race at Laguna was the most fun I ever had racing.

 

Oscilloscope cranking compression tests are pretty cool also

 

Did you happen to test any other cylinders compression while running? (and only at idle or did you go WOT) It will of course be lower at a running idle than cranking at WOT.

Although the misfire didn't follow the plug or injector I assume you verified the injector/coil was firing? Header tube temp?

Just curious!

 

Yes..a few times over, as I didn't believe what I was seein either.