Erratic Idle on Alfa 90

[Rocco166]

Hi all,

further to my previous posts about my freshly rejuvenated 90 it has developed an idling problem 
my mechanic suspects that it's a problem with the Bosche air mass meter (he used a difffrenet term for it but it's the gadget that sits in-line behind the air filter to the engine) and has suggested I get it reconditioned locally (for about $200). 

The problem is this: when pulling up at traffic lights the engine is idling at 1500 rpm, after a few seconds it suddenly drops to 900 rpm and then slowly builds up to 1500 rpm again - it does this constantly..
this is just the latest "issue" to crop up - I am tempted to cut my losses with this mechanic as I am quickly losing faith in his ability to repair the car properly - every time it seems ready to go another problem crops up. It means a return to my regular Alfa specialist and another big cash outlay to get the car sorted out so I can ENJOY driving it at the moment it runs/drives like it's on it's last legs.

[Typhoon90]

Does it jump up and down rpms very quickly, like someone throwing a switch, or does it waver up and down?

Regards, Andrew.

[stradale]

First thing to look for is an air leak, a solution of dish washing liquid and water in a spray bottle, spray onto and around all hoses and clamping points in the induction system including the plenum, listen for a change in idle.
Second check the auxiliary air device, this is basically a valve that allows more air into the induction system during start and just after startup as the engine heats up the valve closes ,sometimes these valve's gets stuck . If you can, take it off and clean it with Throttle body / carbie cleaner.Put it in the freezer for 15 mins and check to see if it opens, it will only open a couple of mm's then run it under hot water as the valve heats up it should then close again
 

[Typhoon90]

I think it's the throttle switch or overrun valve personally. AAV's typically cause a constant bad idle.
I had a Daimler with L jet that would surge just like this when coasting with no throttle, adjusting the throttle switch cured it. I also just fixed the horrible idle on my 90 by adjusting this switch as well. It tells the ECU to go into idle mode.

Regards, Andrew.

[Sheldon McIntosh]

That's good advice from Sportiva and Typhoon90 there.  Unfortunately an irregular idle, or the start-and-then-die syndrome can and often does afflict the V6, and there are about ten possible reasons for it, all the way up to a faulty computer.  Always start with air-leaks though, they're cheap and easy to fix.

Get your mechanic to have a read of this.... http://www.hiperformancestore.com/Ljetronic.htm

[aggie57]

The system shuts off fuel when the throttle is closed and revs are above 1300-1500 rpm.  The idle speed is essentially set by how much air the engine is getting, hence things like the idle speed and cold start are managed by simple mechanism's that modify the airflow based on adjustment or external measurements.

As the others say it sounds like you have some sort of air leak (could be any number of places around the plenum on the engine side of the butterfly) allowing air to get into the system, revs to slowly rise and then when they hit 1300-1500 the fuel cuts out and the cycle begins again.

Can't be the throttle body itself because it's the micro switch on the butterfly that sends the message to the ECU to tell it the throttle is closed (you can hear the click as you open / close the butterfly) and doesn't sound like a problem with the AFM.  From what you describe it sounds like it's doing it's job nicely.

[Typhoon90]

I agree, one of the downfalls of the flapper style AFMs is if the idle becomes erratic, airflow through the flapper speeds up and slows down and because the flap takes a relatively long time to adjust, this can cause a feedback cycle in the whole system as the ECU tries to chase the AFM, because something else is causing a bad idle.
I also wonder if the large hose between the AFM and throttle body has come loose? I can't see a vacuum leak magically appearing, if the car has been running well since it's return, but this is one area it could allow unmetered air in.
Either way, the mechanic should be shot. I work in the automotive industry and nothing burns me up more than these hero mechanics who take lots and lots of people's money and deliver back a car that is totally useless! Maybe I am one of the few with a conscience, but if I'd delivered a car back to someone like that, I'd want it back and we'd fix it ASAP, even if that meant weekend work for me.
Rocco, I think you should go through this checklist yourself:
http://www.hiperformancestore.com/Ljetronic.htm
There's nothing to difficult there, even for complete novices and the tricky bits are using  a multimeter and timing light, if you are not confident there, a mate with those skills could be bribed with alcoholic beverages and tall stories to help. The L Jet system is very basic and straightforward, it's a good system to work on.

Regards, Andrew.

[Rocco166]

Thanks guys for your excellent advice,

I'll go through the checklist link on the weekend and see what I can find myself, and Typhoon you're right about the mechanic, I trusted him to sort everything out over the 7 months he had the car at his workshop and yet every time I drive it a new problem arises - it's totally disheartening to have spent so much money and gotten back such a half-baked car.    I'm still waiting on the final bill for the most recent work on the 90 too which will be a bitter pill to swallow.

I'll chalk this one up to experience and in the future take my 90 to the guys who service my 166.

[Typhoon90]

Thanks guys for your excellent advice,

I'll go through the checklist link on the weekend and see what I can find myself, and Typhoon you're right about the mechanic, I trusted him to sort everything out over the 7 months he had the car at his workshop and yet every time I drive it a new problem arises - it's totally disheartening to have spent so much money and gotten back such a half-baked car.    I'm still waiting on the final bill for the most recent work on the 90 too which will be a bitter pill to swallow.

I'll chalk this one up to experience and in the future take my 90 to the guys who service my 166.

Personally, I would not pay the final bill and lodge a complaint with your state's motor trading body. They're just not complex cars to work on, nor are they difficult to source parts for.
Anyone with even basic experience with L jet will know how to sort these cars out, I suspect that mechanic is either too young to know or too old to change their ways.
If you were nearby I'd say drop it off for a week!

Regards, Andrew.

[stradale]

Hi all

Anyone with L jetronic problems should read Greg Gordons page's , through the link Sheldon has provided.
Greg tells you from the start he not a mechanic but his experience lets him a write a detailed description of L jetronic, how it works and how to trouble shoot it. I have a printed copy I keep with my service manuals.
It's easy to read and to follow, it's an excellent reference