328 Idle issues - AAV, vaccum leak etc

Having an issue with my 328 whereby it will start OK (hot or cold) but from cold the RPM is more or less around 11/1200, no more than that. Drive off down the road, come to junction or whatever, slow down, dip clutch & revs drop & engine dies unless I give it some gas. It'll keep doing this till its really warmed up at which point it starts behaving itself & more or less idling properly - although the revs will dip a bit below 1000 and then come back up.

I recently had the CO adjusted (on a warm engine) because we found it was running too lean at the annual inspection. It was done on a calibrated gas analyser thing & set to just over 1.1 .

I trawled through the archives here & found a number of references to similar problems & my first thought is to look for an air leak & to check the AAV. I have taken that out from under the expansion tank & done the usual stick it in the freezer, stick it in the oven thing & it opens pretty much fully when cold & closes and seals when hot - I couldn't blow through it when it was hot (after I stuck a bit of rubber hose on the thing to blow through to avoid burning my mouth!).

Tested the resistance across the two terminals & I got dead on 40 ohms. I also checked that there was voltage to the AAV connector when the engine was running & there is voltage there.

So the AAV appears to be OK

Next I have taken off all my vacuum hoses & the hoses to & from the AAV & checked them & as far as I can see they are all OK - but now they are off the car I will try to find some replacements anyway. The ghoses to & from the AAV are 15mm ID & the only stuff I can find that size is fuel hose - would that work? There's also some 10mm in the system - same problem I think. Most of the stuff I can find that's classed as vaccum hose is no more than 5 or 6mm ID.

What I did find is that the vacuum line between the plenum and the digiplex appears to be torn somewhere because its not holding air. Presumably this would allow additional air into the plenum & also might screw up the ignition advance I think (If I have understood what it does correctly). Might that also explain why I get lousy fuel economy (always have done). How hard is it to replace that line? Presumably I just need to take the wheel off & the inner arch to get to it (I hope!). Maybe the answer is just to cut it where its leaking & put a straight joint rather than trying to drag the thing out.

If replacing/fixing that doesn't cure it I'm guessing the next thing to look at would be the Warm up Regulator for which I think I need to get a fuel pressure test done.

Anyone have any thoughts?

 

Replaced my torn vacuum line (and it looks like its been torn for quite a while). Airflow sensor plate appears to be in the right place . Car is idling much better though I havn't had a chance to take it out & road test it (it started throwing it down with rain just after I finished). I'll get the mixture reset this week

Does anyone know/can anyone explain what exactly the digiplex vacuum does? Is it an adaption of the old fashined vacuum advance for engine timing or something else? I would have thought timing would have been managed by the digiplex via the crank sensors etc.

I wonder how much power/performance I've been losing with this thing.....

I.

 

Thanks for that - a great explanation.

I'm looking forward to seeing how much better it is - since I now think this has been like this for a long time, it might be the cheapest "upgrade" I ever did!

I.

 

CO level checked, & idle properly adjusted and for the first time in a loooong time my car is idling properly & also idling well through the warm up phase (around 13/1400 when cold). Very much smoother winding up through the rev range as well - I wouldn't say much more power, but the engine is clearly working better.

So that moral of that story is: Bad idle? Aside from the obvious electrics, plug extenders etc, check your vacuum hoses!

I am currently running the tanks dry to change a couple more fuel hoses but after that I shall be interested to see if the fuel consumption is any better - it should be in theory.

 

Good grief - spectacular thread resurrection!

Air leaks are the absolute enemy & these kind of issues often start & end there. There are some documented tests you can run on the AAV & also worth checking that the diaphragm inside the ignition box is still OK as well.

 

Mine will, but only if its very cold & even then it doesn't do it for long.