AAR causing high idle

[richtbiscuits]

Hi all. I'm looking for a little advice regarding the auxiliary air valve. My 412 FI idles high, even with the idle screw right in. I've been all over the engine and carried out the testing in John Muir's book regarding components and have found nothing wrong with them, resistances are correct, vac on MAP sensor is good etc. The only thing I've found is the auxiliary air valve. When I block the air when cold the idle drops off to a nice even rpm. It does the same when hot. There's power to the connecting wire. Investigating all others hoses and connections has no effect on idle rpm and all the vac hoses were replaced no so long ago. I've tried warming the AAR body with a hair dryer and seeing if the valve closes. It doesn't. I've tried the other three AARs I have. Same with all, one partially closes. So it seems all are knackered. If you strike the valve body and listen two have a spring-like boing sound and two don't. I've tried soaking with carb cleaner before but that hasn't worked. I'm thinking to put them in the oven, albeit not too hot, to see if they close. Failing that I'm stuck. Does anyone have any ideas for freeing up stuck valves? What else goes wrong with them? Can I run the car with the AAR blanked off, accepting that it'll need some coaxing when warming up? Thanks in advance for all advice.

[937carrera]

Auxiliary air valve lets additional air in when the engine is cold, to go with the additional petrol to overcome higher friction.

If it isn't closing , then it's like the idle bypass is wide open, so high tickover.

The heating element (I think yours should have this) should have about 12.5 ohms resistance, have you checked that ?

Other than that, it's basically a bimetallic strip type arrangement, heat from either source gradually closes the port off. So I suspect you are looking at a mechanical problem - like a broken component.

If the diagnosis is correct, you can block off the pipe and it'll run fine when hot, but as you say, might be tricky to start, so fast idle may be achieved by pressing on the accelerator. Idle screw can also be returned to the normal position when hot.

It's a long time since I looked at one of these, can you disassemble it ?

[937carrera]

a quick google gets this

https://oldtimer.tips/en/d-jetronic/69- ... ryairvalve

So maybe the piston is just stuck. I would be tempted to fill spray some plus gas or diesel in there and leave it to dissolve whatever may be causing it to stick

[richtbiscuits]

Thanks for the reply. I did have a look at oldtimer earlier today as the chap that writes it seems very knowledgable but I got a little confused when he starts talking about water ingress, presumably on AARs on water cooled cars. I did give all four AARs a tap or two around the body then put them in the oven at 75-80 C. One fully closed (the one I took off the car), one nearly closed and the two 'boing' ones didn't move (suggesting broken element). I refitted the closed valve on the car once it had cooled a little (it did reopen) and idle rpm is much lower. The car had been parked up since November with only very occasional use in February so perhaps the valve had just stuck. I'll try soaking the other potentially working valve then maybe open one of the broken ones up to see what's inside.

[937carrera]

Yeah, water ingress is a foreign country for us guys, but it'll be like carb pre-heating or oil cooler / warmers on modern cars, the coolant acts to provide the heat rather than electricity. In that section I think he is showing pictures on MB parts.

Sounds like a plan, if you can get them moving freely with plusgas / diesel and heat that'll be a good outcome. (wd40 / carb cleaner not as good)