cylinder #2 low compression, leaking cylinder. Wheres it going?

Hello to all,
I've been lurking around this forum since I found it several years ago. It has provided me great insight and has always been spot on with the issues I have had with our 4 prius'.

I had an issue on Friday while commuting back from work. My 2007 Prius (245,000 miles) suddenly began shuddering on the freeway. It would do it for about 30 seconds then even out for a minute or two before doing it again. The check engine light began flashing and I noticed the hybrid system was not charging as good as it should. I got it home and let it sit for the night. Next day pulled the codes and it showed cylinder 2 misfire. No problem; I had a new set of coils and plugs so I replaced all cylinders and that did not fix the problem. I pulled the injectors and tested them. The spray patterns were acceptable so I replaced them. I put everything back together and it was still missing, and knocking badly like they do when a cylinder is acting up. I then did a compression test and the results were 167-90-160-165. I repeated the test 3 times and they always tested within 1-2psi of variance. Next I did a leak down test and the results where:

Cylinder #1- 13% loss with no audible sound except normal crank case evacuation
Cylinder #2- 90% loss with evacuation through spark plug holes in cylinder 3 and 4
Cylinder #3- 26% loss with a bit of evacuation through throttle body, and normal crank case evacuation
Cylinder #4- 10% loss with only the slightest sound of normal crank case evacuation

When I replaced the spark plugs in cylinders 3 and 4 the cylinder #2 leak down test remained the same, only the air was no longer leaking through the spark plug holes in those cylinders. Also, there was not leaking from Cylinder #2 into cylinder #1 at all.

So my question is, Where do I go from here? Do I maybe have a valve that got something in it and not letting it close? It does tend to consume oil and my scope did let me get a good view of the excess carbon and perhaps oil on the top of the piston in cylinder #2. Or, are my tests erroneous as I may not have deployed the right tools/tests for the problem? I'm a bit stuck on this one, though I can say that I really don't understand these engines nearly as much as the 401cid in my AMC Javelin.

Any help and/or advice would be greatly appreciated. Thanks in advance Priuschat.

--Will

 

Thanks to everyone who contributed questions, comments or advice to this thread.

So, initially I was thinking head gasket, or more likely a stuck valve but I can't confirm this with the tests I did. I did check the coolant for exhaust gases and there was none. I don't see evidence of water being vaporized in the cylinder using the scope. It does not use water, and the coolant level has always been the same.

The only codes I have received are "cylinder 2 misfire, cylinder 3 misfire, and multiple misfires detected" but they don't come all at once and it takes a while for any of the codes to be thrown, even though the car is knocking like crazy (like they do when a cylinder is misfiring.

To be sure I was testing in the correct orientation, I did the leak down test at TDC of each cylinder, I tested the cylinders by setting TDC on compression stroke, then for the heck of it, I rotated the cylinder once and tested TDC again. Each time the cylinder that was not on TDC compression stroke would evacuate through the throttle body, but on compression stroke, they gave the readings that I posted origionally. I know if was not needed to test each cylinder in both TDC positions, I just wanted to confirm to myself that I was doing the first test correctly.

When I placed the spark plugs back in the holes of cylinders 1,3 and 4 while doing the leak down test in #2 cylinder, I could hear air still evacuating, but it did not change the intensity of the air getting passed the rings, through the PCV, the throttle body or the exhaust. I removed the radiator cap and watched it for several minutes and not one bubble came up.

I did watch Eric the car guys' video that was posted here. His video reminded me that I had a leak down tested and prompted me to do the test. I have a different tester than his, and my results seemed solid, and repeatable.

I think I will remove the valve cover and take a look at the top side of the valves to see if I can determine if the valves are opening/closing properly.

Please feel free to suggest anything further and I am happy to explore it. Thanks again everyone. I appreciate your assistance on this issue.

 

Hello all,
It took me a bit to get back to the thread here as work as been quite busy. Yesterday I had the time to take off the valve cover to get a better idea of what's happening. With better access, I once again did a leak down test. I was able to see the position of the cams so I knew they were not opening the valves themselves. I had the same 90% loss of air, but this time I was able to better trace the loss and it turns out it is escaping through the exhaust pipe. I could not hear any, but I put one of my rubber gloves over the exhaust pipe and it filled with air instantly.
I don't know these specific motors very well, but if it were on my AMC Javelin, I would say that I have a burnt or stuck exhaust valve. In the Javelin, it would be easy to see if the valve was stuck as it would be in a chronically low state, but the Prius motor seems to have a cap, with what I am assuming a spring underneath it to keep contact with the cam.
My question is, does the cap (which has direct contact with the cam lobe) push on a valve stem or what goes on underneath it? I quickly tried searching for an answer on the net, but did not get enough time to really dive into it. I'm mainly trying to see if this is something I can explore further from the top side or if it will just be necessary for me to remove the head.
Thanks again for all who take the time to read and/or respond to my questions. I really appreciate the help. Have a good day everyone.

-Will

 

Hello and thank you for your input. My thought now is that the air evacuation between cylinders 2,3 and 4 where due to a stuck or burnt valve in cylinder 2 which was evacuating air into the exhaust manifold and then it was leaking back up into the cylinders where the cam lobes were in the normally open position for exhaust stroke for cylinders 3 and 4. I don't think the air was traveling through the gasket.
Please let me know if my thinking is flawed for this scenario. Thanks again to everyone.

 

Yes, that is my thought exactly. I feel that I am more than capable to remove, service and replace the head, but I'm just trying to see if there have been know issues similar to my own that did not require a head removal to remedy its cause. This was a sudden event so I would not expect a burnt valve, but it does seem to me the most likely scenario is a valve seat that is partially obstructed.
If the popular consensus is to remove the head, I will make it my Sunday project.
Thanks again to everyone who has put some thought into this.

 

That would be a nice tool to have. I think I have decided to remove the head tomorrow so I can get a good look at the issue. It looks pretty straight forward. Thanks again for your help.