2009 Acura TSX with 75k miles purchased from only other owner. Issue was deemed (by Acura dealership) to be a stretched timing chain. Engine would turn over but not start. Coworker didn't want to deal with it so sold at quite a low price. Changed timing chain, tensioner, guides, and VTEC actuator with genuine Honda parts.

Now the engine starts and runs ok (initially) but has a tap or tick that sounds like its on the exhaust side of cylinder 3. It sounds & feels like an exhaust pipe. Also, cylinder 3 has 50-70 psi lower compression that 1, 2, or 4. It is still in spec (~130 psi). It runs & drives ok for a few miles with no CEL. Then the CEL comes on & sometimes is accompanied by a "check emissions" warning. After a few more miles, the CEL begins blinking and it goes into what appears to be limp mode, i.e. low power & will not go > 3000 rpm. The only code is for cylinder 3 misfire. Even when it's running badly, the #3 coil can be disconnected, & it makes a difference so it is actually firing.

Also noticed that when the oil filler cap is removed there is a lot of pressure.

Any thoughts - bad valve, piston, rings? Two issues? What causes higher crankcase pressure like that?

Thanks.

Possible valve kissed the piston and damage occurred. Maybe a cheap harbor freight bore scope can see in there and confirm.

I hope you bought very cheap. May need an engine or at least a rebuild.

Rod

Adjust the valve clearance's to spec, Perform Cam-Crank Correlation procedure via a capable scan tool.....Hail Mary attempt but it may fix the issue.
I would suggest a leak-down test after running the overhead, But few have a leak-down tester?

Was the head pulled? Valves checked? My money is on a bent valve. I would perform a leak down test on your weak cylinder and you will find out pretty quickly where that compression went.

That is awfully low mileage for the chain to stretch, do you know what caused it?

Lack of maintenance? I've done more K24 timing chains than any other Japanese engine with the Toyota 1GR-FE being second. Japanese makes being immune to timing chain stretch/wear is a myth......Not surprising given that owners will sweep it under the rug vs screaming from the hill tops on a Domestic & European make.

I don't see many internet complaints on 2AZ-FE head gaskets/Cylinder block threads pulling out......Anything to save face!

My money is on a bent valve as well. Timing chain issues aren't unheard of with this engine, but the jury is out on if the VTC actuator is the cause or not.

Sorry I can't help with the issue, but my 06' Acura has 225K on it and it runs like the day I bought it. The only thing i put in it is new oil and alternators. Lack of maintenance is the most likely culprit with your stretched chain. The timing chain in these things love nice, clean oil. Never paid attention to the OLM since day one. New synthetic every 4-5K. Good luck and please keep us posted.

The thing that really scared me is the high crankcase pressure. Bent valves do not cause high crankcase pressure.

only massive blowby. That is not a good sign. This is why I suggested a cheap harbor freight bore scope
Or just pull the heads, shame to waste all the parts though.
Rod

Check the spool valve and filter and related components. I know little about this particular engine buy have seen bad spool valves on the V6 cause similar problems.
The spool valves from Standard were reboxed OE (the last one I bought for a V6) I don't know if they still are but they are priced right.

Edit: Rock has spool valve and filter for this one.

https://www.rockauto.com/en/moreinfo.php?pk=8479932&cc=1442648&jsn=373

My indy mechanic gets a few where owners decide to overstretch the oil changes to the 7.5k - 10k level and the chain gets ruined. He has done a few K24 engines over

I would say there is piston damage if there is that much pressure out of the oil cap. First thing i would do is a cylinder leakage test to confirm or a boroscope down the plug hole.

this is like reading a book, looking for the ending-solution

I'm the subject TSX owner. I have borescoped and saw no obvious damage to the piston. I would like to try a leak-down test but do not have the equipment. I assumed it was a valve but the crankcase pressure has me worried.

You can do a leak down test of limited merit, take a sued correct size plug and break out all the ceramic. Then epoxy in a hose fitting of some appropriate size and pressurize the offending cylinder with at least 50 psi air. If you have air escaping out the oil fill port, then you have some sort of ring seal problem on that cylinder. For a reality test you can compare to one of the good cylinders. You can listen at the intake and tail pipe for air escaping, that confirms the diagnosis.

This does not give you any number so it is subjective. Nothing wrong with that, just some folks demand a number of some sort.

Rod