Inlet, exhaust cam position problem, maybe?

[spannerrash]

Hi all

I apologise in advance if this seems a bit of a ramble but I think you'll need all the information in order to help me out.

5 years or so ago I replaced the timing belt, chains and chain tensioners on the car. I used the paint a mark on the chains and cams method, only referring to counting the links to double check my work. All went well and the car has run perfectly for the last 5 years. That said I have only covered approx 3000mls in that time.

A month or so ago I started to notice a missfire at tick over. It wasn't enough to register any faults or even trigger the misfire counters but I could feel it through the seat. She was just more shakey than normal.

My first thought was a vacuum leak but I couldn't find anything. I have previously renewed all the vacuum pipes a couple of years ago.

Now, I'm not one for firing the parts cannon but I have hanging around in my shed a full set of new spark plugs and a full set of oem ignition coils. Being very easy to swap I thought I'd give it a try. I then had another thought. I've never done a compression test on this car so while I had the plugs out I thought I'd give it a go. This is where things got interesting.

Bank 1 was constantly reading 165psi give or take 5psi across all 4 cylinders.

Bank 2 was constantly reading 187psi give or take a couple of psi across all 4 cylinders.

How can this be?

I've probably gone a bit ott with this but because I have the time on my hands I went for it.

I've removed the inlet manifold just incase there was anything going on in there with the variators. The compression this time was slightly lower on both banks. I guess the manifold does have a slight ram air ability.

The timing of the belt is bang on. TDC and the camshaft locking bar fits perfectly.

I'm not even sure at this point that this is the cause of my slight missfire. I guess it could be but I could have just stumbled across something else.

So, leaking valves? I guess I'll have to get hold of a leak down tester but I don't understand why all 4 bank 1 cylinders would be low, unless I have bent valves. There was nothing wrong with the original chain tensioners by the way. I changed them because of the startup rattle but the plastic guides were all intact. So I have no reason to suspect bent valves.

I have used an endoscope to check inside cylinders 1 to 4 and can see no piston or valve damage.

I guess my question is, how possible is it to get the chain timing 1 tooth out, not set any codes and the car seem to run perfectly fine for 3000mls?

I'll sort out a leak down test and let you know how it goes but does anyone have any other ideas?

I've included some pics of the chain tensioner as of today when I took the bank 1 cover off. The engine is set to cylinder 1 TDC.

Thanks Frank

[Ronin]

Since 5 years have passed it's time for another belt, so you might want to do it all properly this time.

I have seen compression issues being caused by infrequent oil changes or lack of them all together causing stiff piston rings not sealing.

Engine oil flush like Tec2000 can be used to remedy this, I use it on my car myself.

[spannerrash]

I completely agree that the timing belt is coming due for doing again l don't think this has anything to do with my problem. The timing is bang on. I don't think changing it will fix the problem although I might do it anyway.

I change the oil every 6 months regardless of millage. Unfortunately I can't speak for previous owners.

[MikkiJayne]

A thought - do you have oil pressure doing the test? If one bank was on the cold-start pin and one had pressure then the relative cam positioning could be different.

I'd suggest pulling all the spark plugs and spinning the engine on the starter in 20-30 second bursts until all the valve train quietens down and everything has pressure and then try the compression test again. Give each cylinder the same amount of revolutions when testing so that oil pressure is consistent.

[spannerrash]

Hi MJ, thanks for the reply.

I have done the compression test on all 8 cylinders one after the other so there has been plenty of time for oil pressure to build. I did about 10 cycles for each cylinder.

If the chain sprocket was out by one tooth would it throw a fault? It all looks OK but with all bank 1 cylinders the same it is clearly a common fault to that bank.

The only thing I can think of to progress this at the moment is to do a leak down test with each cylinder at TDC. I guess I could do a smoke test at the spark plug with the cylinder at TDC, see if any smoke escapes via the inlet ports or through the exhaust.

[MikkiJayne]

But did you have oil pressure before the test? If you get pressure during the test it will change the results for subsequent cylinders as it changes the overlap of the inlet and exhaust valves. I've seen it before. I would repeat the test in the reverse order before committing to mechanical issues.

If the chain is out by a tooth it runs like crap. You would 100% be able to tell the first time you fired it up if this was the case.

[spannerrash]

On reflection, a leakdown or smoke test will only tell me if the cylinders are leaking. Neither test will help me understand if there is a valve timing issue. I think for that I'm gonna need a pressure transducer in the spark plug port connected to a good scope. Only by comparison with bank 2 will I know if the engine has a breathing issue.

I did wonder if an exhaust restriction could have something to do with this but at a cranking speed of only 120rpm it would have to be one hell of a restriction.

Something seems to be affecting the volumetric efficacy of bank 1.

[spannerrash]

Thanks once again MJ.

I will do the compression test again tomorrow but I doubt it will change the result. I've been round all cylinders multiple times as I was struggling to believe the results. There are no spark plugs fitted at the moment nor during any of the previous tests so cranking speed is quite high. I have already bounce back and forth between cylinders 1 & 5 to see if continually repeating the test would change the results but it never has.

It may seem a little counter intuitive but I'll do the test again tomorrow with the spark plugs in, obviously apart from the one I am testing. It will be interesting to see if I still get the same discrepancy at a slower cranking speed.

[ainarssems]

Did you change the belt tensioner/shock absorber when doing the belt last time?

[spannerrash]

Yes, all of the components of the timing system were changed. All with either oem or good quality units.

Remember the belt timing is OK though. Everything lines up perfectly.

After giving it a lot of thought last night I think I need to check somehow the operation of the timing chain adjuser. MJ illuded to the fact that if the oil pressure wasn't building enough to lift the adjuser then the correlation between the inlet and exhaust camshafts will be out. Excessive valve overlap will have an affect on the air charge in the cylinders as MJ has already mentioned.

Is it that because bank 2 tensioner is the other way up it doesn't show the same issues?

I could be chasing a ghost here. It could be that when the engine fires up and oil pressure builds the tensioner takes its correct position and everything is fine.

I really would like to test it out though before rebuilding everything. Let's not forget I then need to find my real misfiring issue.

I think I'll also have to do an oil pressure test. My gut feeling tells me that turning the engine on the starter motor with no plugs in at all, should generate enough pressure to activate the tensioner. This is what I have done consistently and there is no chain rattle at all.

Is there any way to check the height of the tensioner to see if it is in the correct position?