An idle problem today - F.I. Car (Solved I Think?)

[adrians]

Fiat 2000 F.I. – Bosch L Jet with additional switches for the old turbo setup.

Hi All,

Well, I had an interesting drive home this afternoon after the Fiat Club Xmas Party (some photos in another post)

This morning, I started the Spider, it sat in the driveway for about 5 minutes and was idling at about 1000 rpm. No problems.

We drove to the event at about 11:00am - no problems a quick spirited run - 46 kms from Southern Sydney to the Inner West- again no problems.

At 3:30pm we got in the car and I noticed a high idle about 1800-2000 rpm, didn't think much of it since it was a hot day.

As we drove, the idle didn't drop, instead it went higher. At about halfway home, the car was now revving at 3000 rpm while idling at the traffic lights. Finally I pulled over and adjusted the idle bolt and then the throttle mixture screw - got the car to idle at 1800 and not stall.

We continued on ....wife making some comments about - random problems ..... then the car went back to the high idle at 3000 rpm. As we got closer to home, the idle started heading towards 4000 rpm, I pulled over again and adjusted the idle bolt. - got it home still idling at 3000 rpm. The temp gauge stayed under 190 all the way home.

Now, just went out to the garage some 5 hours later. Started the car - straight up to 3000 rpm, - adjusted the idle bolt and got it to idle at 800rpm no problems.

Any ideas – sticky AFM flap, some other sensor ???? I checked the accelerator cable - no kinks Any comments welcome !

[RRoller123]

Something is telling the ECU that there is a lot of air coming in and to add gas and get the engine howling, and that would likely be the AFM flap sticking far open, at random spots. An air leak would be unlikely to cause this symptom I think.

[wizard124]

I would look at the throttle plate. Could be sticking or misaligned inside the tube. Sticking throttle cable but you say this was checked out all ready.

If the AFM flap was sticking, then it would be out-of-synch with the throttle plate. This would make a too rich or too lean condition. Seems to me this would make the engine run poorly or stall.

[vandor]

Must be getting air from somewhere. I think even if the idle screw was backed out all the way one would not get a 4000 rpm idle, so air must be getting into the plenum after the throttle plate (assuming the throttle plate closes all the way). Booster hose and AAV to manifold hoses come to mind.

[RRoller123]

With the throttle plate closed normally and the AFM flap not sticking open, I would think that all that extra air would stall the engine, not make it rev up to 4000. Seems something is telling the ECU to dump a lot of fuel in with the air? With the TPS closed and assumed working, it would tell the ECU that the engine doesn't need much fuel (idle condition signal to injectors)?

[vandor]

Hm, you are right. Maybe the Aux air valve has gone bonkers?

[RRoller123]

I bet the AFM is sticking open and telling the ECU to rev up the engine. Suggest that the top of the AFM be removed and see what is happening when the engine is running?

[adrians]

Thanks for all the suggestion to date.

The AAV is a new unit from Mark Allison, the Throttle switch is also a new unit (Bosch original).
At the time when I was adjusting the idle on the side of the road I also tapped (banged) heavily on the AFM housing - in case the flap was jammed open, no change.

It will be the weekend before I get time to pull out the AFM and air filter box and inspect it.

Just thinking could it be the cold start injector - what triggers that during normal cold starting ??? Could that be stuck open ? It's also a new unit.

[RRoller123]

Just another thought... If the TPS is not set correctly, then a large air leak would allow the ECU to open the injectors and raise the rpm. So the AFM could be working properly and this could result from the TPS being set incorrectly(?). I would suggest working through Brad Artigue's FI Manual step by step and I bet you find the problem pretty quickly.

[bradartigue]

Hello-

Any high idle condition in L-Jet is from air. If it just delivers more fuel - and not more air - then the idle won't go up much if at all...fuel needs air, lots of air, 14.7x more air than fuel. So you look for air leaks first. Take some kind of spray cleaner, like the cheapest "carburetor cleaner" you can find at the auto parts store, start your car, and spray it lightly all around the air hoses and connections. When the engine revs go up then stop, you found a leak.

Start there, report back.

BTW It is absolutely not the oxygen sensor (yours is likely ruined from the way it is running, but it does not increase/decrease idle speeds). After you get the thing diagnosed and fixed, replace the O2 sensor. There are only a few things it can be that cause a 3,000 RPM idle, fortunately.

[adrians]

Thanks Brad

Yes, speaking with the local Fiat mechanics their first thoughts was air as well, I replaced the ''big air hose" two months ago as it had developed a crack under the outlet for the AAV. So I'll check the rubber breather hose and the vacuum hoses and do the check with the carby cleaner or the 'Start Ya Bastard' spray.

I pulled the AFM out last night and the flap appears to work and seal find with a reasonable amount of spring/tension on the flap door. The bypass screw ( hex headed screw ) on the AFM is missing the sealing plug, would that make any difference ? It appears my AFM has been opened at sometime in the pass and the top resealed.

I have a spare CO2 sensor - so will replace it in due course. - this car initial ran without one until I replaced the exhaust manifold with the FI version.

BTW - The throttle butterfly looks okay and is sealing correctly.

Thanks

[bradartigue]

Sealing plug makes no difference if the screw is intact.

[adrians]

Well I think I found the problem ?

I discovered that the bolt that holds the 'double return spring'' bracket in place had come loose. (corner of the Plenum chamber) This allowed the spring to move around and I suspect when I adjusted the idle bolt I actually made they little difference to setup.

I think that when I pressed the accelerator pedal the entire mechanism moved around and settled back at a high idle position.

I've replaced the bolt holding the bracket, rechecked all the hoses ( no cracks or leaks ) and tested the car , idled perfectly !

Still need to do a road test !

So thanks everyone for your help.

[bradartigue]

That would do it. In your case the throttle wasn't closing completely.

[wizard124]

Simple fix
Too often we think it is something worse than it is!