Still trying to identify a misfire on no.5, I am doing a leakdown test and getting tremendous leakage past the inlet valves on no.5 and no.4 (just to verify no.5 is usual or not).

Here's the thing though, if I remove the camshaft the leak rates are very low, so the valves are being held open by the cams and lifters.

This is a rebuilt engine after valve damage, however on the 4,5,6 bank none of the inlet valves were damaged. The car runs very strong but has a lumpy idle and P0305 code. Injector clicks like the others, I've switched and checked coilpacks so compression is the only other possibilty.

The question is - why would the valves be open when they should be closed? If I remove all but one lifter and put the cam back in, the leak will be present regardless of which lifter I install.

Please help!!

 

It sounds like the cam shaft pulley on bank 2 is retarded or advanced too much causing it to hold the valves open slightly. Whoever put the timing belt back on didn't use the cam bar correctly and got it a couple teeth off im guessing.
The chain tensioner on the back of the head can also be suspect. If you take the valve cover off make sure everything is in time with 16 chain links between the timing marks like the pic below.

 

Thanks for the suggestion on timing. I was checking by visually making sure that the cam lobes were well past the lifters, then performing the leakdown test. I will double check that I am on TDC on compression stroke just in case the cam profile has more to it than meets the eye.

The timing chain is at 16 links, the notches line up very nicely with the cam bearing caps and the cam bar slides on easily when the harmonic balancer mark is aligned with the arrow.

The only explanation I have is valve seat wear, but the car has only 99K miles so I wouldn't expect that much wear yet.

I don't have a Bentley manual yet - is there a gap specified for lobe to lifter? My smallest feeler gauge is 0.2mm and it won't fit, Haynes seem to think that 0.2mm is the limit for worn lifters.

 

If the cam chain tensioner marks are lined up and the cam bar slides on nicely then i would have to agree that its a mechanical problem with the cylinder head.
You say this is a re-built engine? Did they just replace valves or did they put a new cylinder head on it?

 

I rebuilt the heads myself, just as needed. I checked for bent valves and other damage and replaced as necessary. On this bank, there was no damage to the intake valves, just the exhaust valves so everything on the intake went back together as I found it.

The leakdown test flows air out of the throttle body only - no sign of airflow at the exhaust. The previous owner tells me that all was well with the car until the belt broke and I have no reason to doubt that, with 99K miles it shouldn't have any serious wear and tear.

 

No, its not wear, something is wrong with the intake valves, the valve guide may be cracked, slightly warped etc. Its definately gonna require pulling the head back off to find out.

 

What are you getting on cylinders 1, 2, 3 and 6?

 

I haven't pulled the valve cover on 1,2,3 yet - trying to minimize work here....

4 and 6 read less than 20% leak, which my gauge considers low and I tend to agree.

Through trial and error I have narrowed it down to the center valve on no.5. If I rotate the engine and check it will leak at 60% until I up the pressure then it seals up and leaks like 4 and 6, so for some reason the valve isn't going all the way home unless pressure is high enough.

So, I think it can be either a mangled valve and/or guide or a weak spring. A bent valve stem would have trouble sealing unless the guide has failed. I'm really hoping for a weak/broken spring! I can fix that with the head on.

Thanks for all the input, I'll update when things are sorted.

 

20% is not a low leakage rate on an audi, audi's typically range from 7-10% on a CLT test, even with an engine with 200k on the clock.

 

We're talking Harbor Freight gauges here, so I take the readings with a pinch of salt. Given the get up and go that the car has and no smoke, I think everything is pretty sound.

Here's the latest: I came across the term 'valve stick' which was a new one for me. Most people remedy using Seafoam to remove carbon build-up on the valves. I remember there being pretty heavy carbon build-up on the intake valves so I gave it a shot. Guess what?? It now idles very smoothly!! I am not going to reset me CEL, I'll let the ECU do that for me so I'll know it is ready for an inspection (only have a few days left since has to be done within 10 days of registration).

I have never used this stuff before but you have to believe me when I say that the idle is very smooth now whereas before it shook the car. I might be inclined to do it again soon because there was a lot of carbon on the valves.