Long term lambda and base idle

[Dalamar]

To leave things be or not?

I've just finished changing the valley gasket, cleaned all the usual bits from the inlet manifold upwards and have just given it a drive tonight.

Drives fine, no petrol leaks (touch wood), vacuum connections all OK, no water leaks and hooked up Rovergauge when I got back.

Short terms are cycling nicely around zero, MAF values fine, TPS fine and the only thing I noticed that was different from before was the long term lambdas were higher around -95% on the odd and -60% on the even. Before they were closer to zero (I can't remember the exact value). This is all after an ECU reset (battery was disconnected for a few weeks) and the car was up to temp and idling for a few mins to set the long term trims.

One thing I was wondering about was whether or not I need to adjust the base idle screw after cleaning that area when I cleaned up the plenum. When I took it out to clean, I counted the number of turns out and did the same when putting it back in. The car idles fine and drives great.

Shall I just leave things be or go through the set up process for the base idle screw?

[Steve_D]

If the stepper idle valve was reading high or low then a base idle could be the problem. If stepper is sitting pretty and coping then base idle is OK.

Steve

[Dalamar]

Thanks - stepper was fine.

I couldn't resist tweaking the base idle screw and found it was 2 turns out from the bottom. I wound it back out 1.5 turns and went for a run. Back to garage, let it idle on and off the fans for a few mins (>3 mins) while I put the roof back on and then gave the accelerator a kick to lift the revs above 2k. The long term trim then reduced to more respectable levels -40% and -30%.

Car still idles fine and drives great.

What have I learnt? The base idle screw does effect long term trim values even though the car idles fine.

[spitfire4v8]

Dalamar said:

What have I learnt? The base idle screw does effect long term trim values even though the car idles fine.

that's not what youve learnt at all. if you look at your screenshots you will see that the rpm is about the same in both, the stepper position is identical at 10 percent, and the maf output is within 2 percent of each other. Your base idle adjustment has made no difference in real terms. Anyway the maf doesnt know if the airflow is more through the base idle screw or the stepper, all it measures is the total airflow, which for any given target tickover will be about the same for any blend of base idle / stepper position which gives that target idle speed.

all you are seeing in the long term change is the natural variation you get in trimming at hot idle.

you will never have one value of long term trim that remains the same, it always changes when it's resetting itself at hot idle

leave it alone. it's fine.

[Dalamar]

Thanks - definitely won't mess with it again.