Very lean at idle RPM/MAP problem

[jnorris]

This problem is on a 1998 Z28 A4 with a 230/230 cam, ported heads, and LT headers.


The lean condition only exists when on the SD tune. With the MAF I do not have this problem.

On cold starts my AFR will be in the 16 – 18:1 range for 5 to 10 min even though the OLAF table has an AFR of 13.3:1 at the idle RPM/MAP. With the scan tool I have confirmed a commanded AFR of 13.3 but my WB shows very lean and the car idles like it is very lean.

As an experiment I richened up the AFR in the OLAF table to 11.0x at 20c and tapering it up to 14.63 at 70c. This corrected the cold start lean problem. Commanding an AFR of 11.2:1 netted a real AFR of 13.2:1. As the car warmed up the difference between the commanded and actual AFR lessened. However as soon as the ECT hit 70c the AFR went lean at 16.x:1 at idle MAP. At driving RPM/MAP the AFR was 14.7:1. After about 5 min of driving the AFR at idle went back to the commanded 14.63:1

There seems to be evidence that there is another table or parameter that is affecting the AFR at idle (on the 98s) for a period of time just after startup. Every restart is going lean if the car has been sitting for 30min or more. If I just turn the car off and restart it within a few min the lean condition is not there.

Other possibilities:
I thought about how I had lowered the IAC effective area table (drilled TB) and thought that maybe there was additional air coming in the IAC/TB hole that the PCM was not aware of and therefore not compensating for. As an experiment I changed the table from 4 positions down from stock to 3 positions down from stock and flashed the PCM. On the startup the AFR went lean at 16 – 17:1. My IAC correction is only off by -.71.

Maybe an idle airflow table being off might cause my lean problem and the lean time on restarts is the time it takes the PCM to learn out the bad values. I might need to put the IAC effective area table back to stock or lower it 6 positions from stock and see what happens.

As I thought more about it I remembered that I had cut my VE while cranking table by 40%. I loaded the stock table back in and flashed the PCM. When I started the car the idle AFR went lean to 17-18:1 so this did not solve the problem

Any thoughts or suggestions?


Thanks

John

[WicketMike]

i was having the same exact problem last week on my 98'.

I posted some threads about it. i didnt get any help.

Once the car reaches 170 or so. it goes back down to 14.63 but at the lower temps its really lean 18-19.1 afr.
I tried driving it before it reached the correct temp and it stays lean even while driving, so i had to stop and let it idle some more to reach the higher temp.

[ringram]

I think you dudes will have to pump up your ve tables.
While in open loop (cold ect) fueling will be mainly determined by a correct ve. If you are running lean you need to bump up the ve in the area of rpm vs map. With the maf plugged in its correcting the bad ve for you.

Have you dialed in your VE tables using a wideband or ltft's? if not try 10% in the idle area and see what happens.

If it works post off your order for an LC-1 or equivalent wideband, it will dial in your ve fast and easy. ltft tuning is tricky as it averages the whole ve table adjustments over a limited number of cells. 9 or so from memory..?

[WicketMike]

if we bump up the idle area it will be 10% to rich after the car warms up.
its only during the warm up period. Once it hits the desired temp. 170-178?? then its fine. This is with the maf unplugged

[ringram]

Ok, but dont forget once its warm you go into closed loop and the o2 sensors will make it run at 14.63:1 whatever happens anyway, whether its lean or rich they will compensate.

Can you paste your ltft tables here?

[jnorris]

Ok, but dont forget once its warm you go into closed loop and the o2 sensors will make it run at 14.63:1 whatever happens anyway, whether its lean or rich they will compensate.

Can you paste your ltft tables here?

We are running in open loop speed density mode and once the car has been running for 5 -10 min the commanded AFR matches the WB AFR.

John

[ringram]

Oh ok, sorry I thought you were in closed loop. Should have checked first.

[jnorris]

Oh ok, sorry I thought you were in closed loop. Should have checked first.

No problem. I appreciate the feedback.

John

[jfpilla]

Let my try this. I see the same thing when in SD. It looks as if Charge temp. blending{B4901} and PE mod. based on coolant temp.{B3617} are the cause. Unless it's coincidence. When I apply the factors from both tables, to Commanded, the value I get is pretty close to actual logged. I also noticed that as MAFg/s go up the actual AFR richens. Since MAFg/s affects the blending factor that also makes sense. I'm curious too. :???:

[Black02SS]

How can B3617 be the cause? If the car is at idle I don't see how a PE table can effect it. I too have the same problem and I notice a trend here... All three of us have 30# SVO's... Coincidence..