Rev hang, a miracle on 147th St...

[johnl]

The dreaded and evil 'rev hang' (the work of the Devil), my car had it, and now it doesn't, hallelujah, praise the Lord, it's a blessed miracle...

I'll assume that people will know what the phenomenon of 'rev hang' is, and why this scourge exists with so many modern cars, for reasons related to emissions. Suffice to say that it is undesirable and affects many different makes and models of car. My car did it, and I had assumed it was a 'characteristic' of the version of the TS engine used in the 147. But a few days ago, for whatever reason, the engine just stopped doing it. One day it was there, next day it wasn't, and so far it's still not doing it.

Overnight the throttle behaviour became much more 'normal', like an 'old fashioned' car that doesn't give a toss about NOx emissions. Instead of the rpm 'hanging' or even 'jumping' slightly when the throttle pedal was lifted and the clutch pedal depressed, the rpm now just start to drop almost immediately. Instead of the engine 'running on' for about half a second to about one second as the driver backs off the throttle in gear, the car can be felt to start decelerating pretty much straight away.

This has resulted in the gear shifts becoming faster and smoother, without any need to modify gear changing technique (i.e. slowing, with a pause inserted between pedal and throttle movements). The engine also seems somewhat more responsive as the throttle is opened, not more powerful, just 'snappier' as the throttle is cracked open. 'Blipping' the throttle for downshifts now elicits a much more predictable response from the engine, so no more 'under-blipping' or over-blipping'. The throttle pedal now feels much more directly 'connected' to the engine response to it, I no longer feel as if I'm having to lodge a request form at the ECU desk, and take a seat waiting for a reply...

This has magically transformed the way the car drives, much more enjoyable. It's a marked difference between before and after, definitely not a wishful figment of my imagination. The problem is that I have no idea as to a) why it has stopped 'rev hanging', b) why it was doing it in the first place, and c) will it at some point go back to its old bad habit...

Any ideas? Perhaps a sticky butterfly that has 'unstuck' itself? Something electronic? Act of God?

Regards,
John.

[bazzbazz]

No, this is a common fault on the TS & JTS engines, it is caused by an air leak after the Throttle Body.

If not an actual leak in a seal or a crack in the air passage it is caused by one of the following:

The USUAL culprit is a stuck PCV valve (Positive Crankcase Valve), the pipe that goes from the camshaft cover to just behind the Throttle Body has a small plastic cylinder at the end. Within this is a small spring loaded "Golf Tee" valve, it can get stuck open due to clogging or the spring weakening. It is only supposed to open at higher RPM, if it is open at low rpm or idle it is the same as an air leak behind the Throttle Body, thus the raised RPM.

The other cause is a cracked or broken Brake Boost Vacuum line, it is right next to the PCV valve behind the Throttle Body.

Baz

[johnl]

Thank you Baz.

The rpm are and were not not raised at idle (i.e. no change between before and after the miracle), which led me away from suspecting a vacuum leak (though it did cross my mind, as did the PCR as a possibility). I suppose if the 'leak' were small enough then at idle the ECU might be able to compensate for it, keeping the idle speed in spec by closing the butterfly (or other idle speed compensating valve, not sure how the TS ECU and FBW throttle body deals with this) more than it otherwise would...?

I also suppose that a PCV could (partially?) stick only under fairly specific circumstance, or at least be slow to close...? I'll have a look at it, after I've ripped my sons' Prelude starter motor apart to see if I can figure out why it won't work (my job for the rest of today I think...).

Regards,
John.

[johnl]

I've finally found the time and headspace to have a look at my PCV valve. It appears clean and seems to be operating correctly (assumption). Idle rpm is quite normal, not unstable or raised at all. I blanked it off as a test, and all this seemed to do was make the engine response a bit 'doughy' and it felt that there might have been a tiny power loss (totally subjective from one test drive). I've reinstated the PCVV and am now less suspicious of it as a possible cause of the previous 'rev hanging' behaviour, though of course it could have previously been sticking and now isn't. If the 'rev hanging' comes back then I'll revisit the PCVV.

Baz said:
It is only supposed to open at higher RPM, if it is open at low rpm or idle it is the same as an air leak behind the Throttle Body, thus the raised RPM.

Baz, did you mean to imply that at idle / low rpm the PCV valve does not permit any gas flow at all? If so then I would have to disagree, as it goes against my understanding of the purpose and method of PCV valve function. As I understand it (FWIW), when an engine is running the PCV valve is open almost all the time (effectively all the time), to lesser and greater degree depending on the pressure differential between the inlet manifold and the crankcase. I can see no reason why Alfa would not use a PCV valve in the conventional manner, and plenty of reasons why they should.

Having the PCV valve closed at idle / low rpm would result in no crankcase scavenging during running conditions in which very many engines spent a great deal of their working life (urban traffic, and also often short trips where the oil spends little if any time at an adequate operating temperature). This would result in accelerated oil degradation from combustion contaminants because they are not being purged from the crankcase with clean air drawn in from the induction tube. It probably wouldn't result in excessive crankcase pressure at idle / low rpm, because blow-by gas production will be relatively low and could still escape via the breather tube (attached at the induction tube before the throttle body).

If you mean that the PCV valve will be more restrictive at idle / low rpm than at higher rpm / load, then I do agree. I don't think this means that the PCV valve is ever fully closed while the engine is running, other than in the event of an inlet manifold backfire (or if the engine is supercharged), in which case the PCV valve will prevent pressurised gasses exiting the manifold and entering the crankcase.

To prove that the PCV valve is at least somewhat open at idle; with the engine idling take off the oil filler cap and hold a small sheet of thin cardboard against the orifice. As you lower the cardboard toward the orifice it will initially be pushed away by the pressure pulsations emanating from the crankcase (caused by 'windage' inside the case, mostly due to the rise and fall of the pistons), but when the cardboard is almost against the orifice it will suddenly get 'sucked' onto it and stay there (pressed on by the difference between atmospheric and crankcase pressure). This demonstrates a slight net negative pressure in the crankcase (relative to atmospheric pressure), which can only be created by manifold vacuum acting through the PCV valve, ergo the valve must be open. I've tried this 'test' on a number of engines, and it's always the same result.

If you then give the engine a rev, the cardboard will get blown off by an increase in crankcase pressure caused by an increase in blow-by gas quantity, even though the manifold vacuum will decrease as the throttle is opened and the PCV valve opens further (i.e. the case becomes more exposed to whatever degree of lesser vacuum that exists in the manifold). Manifold pressure rises, case pressure also rises, but manifold pressure rises more than case pressure, so manifold 'suction' effectively decreases as the throttle is opened, even though the PCVV is more open. Under heavier load the blow-by production and case pressure will rise enough that not all of the blow-by can escape through the PCVV (regardless of how open it might be, it's restriction even when fully open), and so the cardboard gets blown off by increased case pressure.

On the road, even at only somewhat higher speed and load this means that blow-by gas exits the case through the PCVV, and flow through the case to induction tube breather hose reverses. Blow-by gasses then also enter the induction tract via the breather hose (entering before the throttle body, which is why they get dirty with oil vapour and combustion by-products).
If at idle the cardboard never gets 'sucked' onto the oil filler orifice then this suggests that blow-by is quite excessive even at idle (bad rings), or that the PCV valve is blocked with gunk or stuck closed.

Note that while the PCV valve is variably open 99.99% of the time when the engine is running (all the time until / if there is a backfire), it is closed when the engine is not running (i.e. when there is no pressure differential between the manifold and the case), because the little internal spring lightly holds it shut. It can do this because in effect there are two physically connected but functionally separate valves inside the PCV valve. So, it could possibly become stuck closed with gluey deposits while the engine is shut down...

Both of these valves are simultaneously acted upon by the internal spring, because they are connected to each other by the internal 'rod'. The first of these valves is a one way valve that permits gas to flow into the manifold with no restriction, but won't allow gas to flow from the manifold into the case. This is the valve that is held closed by the spring when the engine isn't running (and prevents gas from exiting the manifold). As soon as pressure in the manifold falls below case pressure this valve opens, but it has no affect on metering the case gas entering the manifold.

This is the job of the second valve (the tapered end of the internal rod to which both valves are 'attached'). This second valve is variable because it is a tapered rod (i.e. one end of the internal rod) that moves in a fixed orifice to change its' flow area, and only does anything when the first valve is open. It opens variably according to the pressure differential acting against the internal spring. The second valve opens less when the pressure differential is higher and more when the pressure differential is lower, until the first valve closes and stops any flow through the PCVV (when the pressure differential reaches zero, or very near to it). That is, as the pressure differential decreases as the throttle is increasingly opened the second valve increasingly opens until the point at which the first valve shuts off all flow through the PCVV (really this means when the engine is turned off and the pressure differential disappears).

I'm not sure if the second valve in the PCVV (i.e. the tapered gas metering valve) can ever completely close or not, say if the pressure differential becomes very great (as occurs on the over-run at higher rpm with a closed throttle). Or, whether it will still allow some flow at its' most restricted position. I suspect it's a bit of a non issue anyway because under such engine conditions (extreme manifold vacuum) the production of blow-by gasses would be minimal.

This was supposed to be just a short note when I started writing it. Apologies for verbosity and possibly less than clear explanation...

Regards.
John.