XU10J4 seemingly working ACAV but lack of mid-range torque

[Tseize 0]

Hi,

 

After 10 years without 405 Mi16 (phase 2), I bought one some months ago. It is exactly like mine ex-car (11 years of ownership).

So I am quite familiar with the working of the ACAV system. But this case is really strange for me. All the elements of the ACAV seemingly work very well from outside (of course I checked it only when the car was standing). I mean the vacum diaphragm units pull their connecting rods immediately and at the moment they should do it (first at around 1800 rpm). Despite of this the engine that is very torquey at lower rpms, is totally weak between 2500-4000. It regains its power from 4500-5000 rpm. So the symptoms are exactly same as if the ACAV would not work properly. 

My only guess could be that what if the butterfly valves of the inlet manifold are missing somehow, and thus they the short inlet tracts cannot be closed. 

There is one more additional info: when I bought the car one of the connecting rods was moving but it was not connected to the butterfly valves. I thought that this was the problem and I repaired it. It seemed to be an easy win. But the torque has not changed at all.

Do you have any clue for this phenomenon?

 

[welshpug 1,657]

Cam timing is easy to get wrong with these due to the dual timing belt tensioners, I bought a 405 a very long time ago that the previous owner had put on a rolling road and only made 135 bhp, he later changed the timing belt and found the cam timing way off, I don't recall which way but most likely super advanced.

 

 subsequently I had it on two different rolling roads recording 160 bhp with no other changes.

[Tseize 0]

Thank you for your comment, Welshpug!

 

Today I resolved the 1st part of the "mystery". The connection rods are moved by the dipagrams, but they go only to half way.

I sticked the butterfly valves to fully closed, and then voila the mid-range torque arrived. So the next step is to find which part is the guilty for this problem. 

 

 

[monty69 6]

i had an 306 s16 (with working acav) that was gutless midrange with a decat fitted. I replaced it with a genuine cat and the difference was night and day. ive never noticed a gti6 decat cause a noticeable loss of torque but on the s16 it ruined it.  I seem to remember the acav was big/short on idle, switched to small/long tracts off idle and switched back to big/short at 5000rpm.