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2G Odd Compression Result

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spyderdrifter

10+ Year Contributor
5,267
711
Jul 11, 2009
Somewhere in, Colorado
I'm posting this since currently, the search options seem to be broken and I can't search anything. Anyway, I ran a dry compression test just to get a basis of my engine's health. The results were as follows:

Cyl 1: 147 psi
Cyl 2: 143 psi
Cyl 3: 100 psi
Cyl 4: 150 psi

From there, you can all see the problem. But there's more. I didn't do a wet test, because I don't think cyl 3's reading is accurate. The other 3 cylinders are well within spec of one another, and they were higher than I had assumed they'd be, so I'm happy with those. However, while cranking the car over, cyl 3 would make a popping sound and wouldn't go above 100 psi. Every time yielded the same result with no variations. The only thing is I don't remember if the pop came before or after hitting 100 psi. I failed to note that. The car's power has been pretty good, but still at times it does kind of feel like it falls short. Not always, but on rare occasion. I don't know where this pop noise came from, and have no idea what could be causing it. The only idea I've been able to come up with, is if the pop happens right when it hits 100 psi, could one of the injector seals be popping and then re sealing? However, there's no leaking once it hits 100, and just simply won't go higher either. I'm a little lost on this one. The engine was warm as I had just returned fro ma test drive, and the only thing I forgot to do was open the throttle plate. Injectors were all unplugged and the plug wires were all pulled from the coils.
 
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Ok

The fuel injector is not inside the combustion chamber and has no effect on a compression test.

Bad valve seals also would cause a boost leak, and smoke at idle, but no effect on the compression numbers. (Fighting my valve seals now)

Valve guides or valve seats can cause the valve not to seal and to loose compression

I would try a different compression tester, it is more likely the spark plug threads on that cylinder are worn down and causing the tester not to seal completely.
 
Thanks for all the info everyone. Whether it pertains to my issue or not, if it seems related, I'll entertain the thought. Even though I hadn't asked about adjusting fuel trim, it might be useful in the future, so no harm. I too think I need to try another tester to be absolutely sure before moving on. I forgot to ask one of my friends about a compressor today since I was busy moving furniture and a bunch of car parts. Will ask tomorrow and hopefully be able to do this on Saturday. I'm pretty sure he has everything I'd need in his garage.

And I do intend to update this whenever there's new info to share. I hate it when members fail to do that.
 
I would try a different compression tester, it is more likely the spark plug threads on that cylinder are worn down and causing the tester not to seal completely.

+1 - I was thinking of that, it would explain the popping sound (escaping pressure) that you are getting while you're turning it over during the compression test. Take a good look at the seat of the spark plug well and like you said that the threads aren't gnarled and worn down. Does the threaded end of your compression tester thread all the way down to the combustion chamber (see my compression tester example)(pic below). Or does it only have a couple of threads on the end and is short and stubby? If the first couple threads of your spark plug well were damaged and you're using a compression tester with the short stubby threaded fitting it would explain why it's not sealing and your losing pressure.
 

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Well, I tried a different compression tester, and had the throttle plate open. All results were identical, except cyl 1 which rose to 152psi. Cyl 3 remained at 100psi. Ran a leak down on cyl 3 at tdc and it leaked (as we all figured). Only heard and felt air escaping througth the oil cap. No bubbles in the coolant, no air coming out the TB elbow, couldn't hear anything from the exhaust. The lowest of the other three cylinders was cyl 2 @ 140psi. Did the leakdown on it and it sealed good enough to turn the engine at 100psi.

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No plans yet. The only bad thing is I can't afford to go without my car. My wife and I have drastically different work schedules and she's in school full time again. So I may just build the 6 bolt I have in storage. But I'm not sure yet. I'm really wanting this to be a valve issue and not a piston issue.
 
I think there's a chance that a Seafoaming could help you out, for a little while anyways.
Also you might find out something from looking around the cyl #3 area underneath the valve cover. Looks like another member already suggested this in this thread. I don't see how a piston issue could create a pressure leak that ends up underneath the valve cover.
 
I think there's a chance that a Seafoaming could help you out, for a little while anyways.
Also you might find out something from looking around the cyl #3 area underneath the valve cover. Looks like another member already suggested this in this thread. I don't see how a piston issue could create a pressure leak that ends up underneath the valve cover.

The pressure leak is going past the piston and rings into the crankcase and coming back up to escape to atmosphere through the valve cover oil cap. When doing a leak down test if you hear a considerable amount air pressure coming out of the oil cap, dipstick tube or PCV system you know there is a piston/ring/cylinder wall problem.
 
I'm leaning more toward a valvetrain related problem. I'm only saying this because back in 2013, I had low compression on my old spyder's 6 bolt. The wet test indicated the rings were bad. After I tore it all apart, I had two broken pistons (ring lands broke), but I still had compression #'s well above 100 psi. Even those 2 cylinders had I think at least 125psi. So from that experience, I'd like to assume the same would apply here. I didn't pull the valve cover since I was short on time at my friend's shop, but will look soon. We just got a little snow today and the temp dropped to the 30s, so I really don't want to be outside pulling my car apart in that.


This is the only one I have left, but both pistons were broken like this, and I still had enough compression....

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I'm leaning more toward a valvetrain related problem. I'm only saying this because back in 2013, I had low compression on my old spyder's 6 bolt. The wet test indicated the rings were bad. After I tore it all apart, I had two broken pistons (ring lands broke), but I still had compression #'s well above 100 psi. Even those 2 cylinders had I think at least 125psi. So from that experience, I'd like to assume the same would apply here. I didn't pull the valve cover since I was short on time at my friend's shop, but will look soon. We just got a little snow today and the temp dropped to the 30s, so I really don't want to be outside pulling my car apart in that.


This is the only one I have left, but both pistons were broken like this, and I still had enough compression....

I know you're hoping for it to be valve train related. But you said it yourself that you didn't hear any air coming out of the intake thru the throttle body or out of the exhaust. You only had air coming out of the crankcase and I'm guessing it was quite a bit more than the other cylinders that you leak down tested. Or did you only test #3? I guess what I'm trying to say is is that if you didn't hear any air escaping through the intake or exhaust valves on #3 then there are only a couple of options, #1 it's the Piston #2 Rings #3 cylinder wall damage or #4 head gasket leaking into another cylinder adjacent to #3 cylinder (or a combination). The last one #4 you would figure out if you pulled #2 and #4 spark plugs when you were doing the leak down test on #3. Just because you had a failure before and you were getting better results compression wise doesn't mean the same failure couldn't happen on this engine and have worse compression results. Good luck man
 
The only other cylinder that got leak tested was cyl 2 since it had the next lowest compression and it was sealed. So how would air be coming back up if it were piston or ring related? The oil drain holes from the cyl head?
 
So if I have the time available and I drop the oil pan, remove the piston for inspection, and the bearings aren't bad, would I be able to put the piston assembly back in until I can fix the issue? Kinda need my car running for now. Or just wait?
 
So if I have the time available and I drop the oil pan, remove the piston for inspection, and the bearings aren't bad, would I be able to put the piston assembly back in until I can fix the issue? Kinda need my car running for now. Or just wait?

I would just fix it once and fix it right. You don't know what you're getting into until you open it up. And I guess you could wait but you run the risk of it getting worse and doing more damage depending on what the problem is.
 
Just keep driving it. I see no signs of immediate danger. It's a daily driver, swap to some rotella 5w40 and some restore compression repair additive.

If the car is running fine then don't worry too hard about it. Just check your fluids regularly, every time you get gas at the pump.
 
Just keep driving it. I see no signs of immediate danger. It's a daily driver, swap to some rotella 5w40 and some restore compression repair additive.

If the car is running fine then don't worry too hard about it. Just check your fluids regularly, every time you get gas at the pump.

For diesel engines?
 
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