inokiyo

Hi Guys, hi John,

I posted a thread here in regards to my S4.
https://rennlist.com/forums/showthre...t=running+rich
I got a rebuilt MAF and the car is running worse now! I am fully confident with the new MAF, so would this mean the LH computer? I also installed a new temp 2 sensor. Although i did not think this was the problem, I wanted to change it anyways to clear all doubt.
Before the new MAF, the S4 would idle fine when cold, then after about 15 minutes either driving or let idling, the idle would rough up (going up and down) and idle rich. It would have the above symptoms with the following already done;
New battery
Knock sensor
Thermostat housing
Cooling hose
Breather hose
Idle control valve
vaccum lines
fuel pressure regulator (one)
O2 Sensor
I installed the new MAF, with the battery disconnected, confirmed it was installed right, and did not disturb any lines in the process.
Now, about 10 seconds after fire up, it runs richer, and the rough idle is more pronounced.
I tried switching back to the old MAF (battery disconnected) and also the old temp 2 sensor, thinking if I drop the original parts back in, it would go back to its previous state. It runs the same as with the new MAF installed! So now the old running condition is history.
Any thoughts? Should I replace the LH myself and pray, or bite the bullet and take it to a repair shop? Do you think the LH is highly probable to cause this?
Any, I mean any advice would help greatly!
Thanks always!!
Ken (inokiyo)

Rich9928p

Ken,

You've done all the right things. The LH is your only logical next step. They are known to fail in this manner.

John Speake

Hello Ken,
I assume you ran the car long enough for the LH ECU to adapt after the battery ?

You need to drive the car for a few minutes.

Did this car ever run right for you, or did you buy it as it was described in your original thread ?

Mrmerlin

Hi once the computer has been disconnected from the battery it must reinitialize itself, so a few miles with some moderate romps on the go pedal and all should be well

Normy

Do new MAF's come pre-set to the correct output voltage?

I don't know if your MAF has an adjustment pot- it should....but maybe you might consider leaning it out. It's possible everything is in order, it just needs an adjustment. Use a 3 mm allen wrench, and turn it counter-clockwise 1/2 turn at a time and watch your idle improve.

N!

AO

I don't think the S4's have this Normy...

MBMB

Ken,

Where are you? The next logical step is to swap computers with a functioning car and see if the problem moves with the computer.

I had a very similar problem; after eliminating all possible under-hood causes I replaced the LH computer with a rebuilt one from Rich and the problem disappeared.

inokiyo

Hi guys,
Thanks for your support and advice!
After installing the rebuilt MAF, I did have the motor idle for a few minutes, assuming that this would adjust the LH. I will try to drive it a few miles to see if this improves anything. Will keep you updated!
Thanks again!!
Ken

Gretch

Check your WOT switch signal.

Rich9928p

To go into more detail regarding Gretch's suggestion (good suggestion Gretch), here are the instructions. The reason for this suggestion is that if the LH "thinks" the throttle is wide open (as the switch tells it) the algorithm goes to "open loop" calculation and ignores the input from the 02 sensor and runs from the internal fuel load map (plus other inputs such as engine temp) and enrichens the fuel mixture.

This is how to check the idle and full load switch, you need an Ohm gauge. I've attached the LH connector diagram so you know where to look for the signals.
Use LH connector position 5 for the Ohm meter ground connection.

1. The throttle idle switch (+ lead goes to LH connector position 2)
With no pressure on the throttle pedal, there should be close to zero ohms (closed circuit, just wire resistance) at connector position 2. Gently press on the throttle, the reading should quickly go to infinite (open circuit)

2. Full load throttle switch
With no pressure on the throttle pedal, there should be infinite ohms (open circuit) at LH connector position 3. Press on the throttle, the Ohm reading should quickly go from infinite to close to zero as the throttle is opened more than 2/3 of the travel - the reading should stay near zero until you release the pedal and the throttle plate closes at the 2/3 or more mark.

Rich9928p

Normy,

All S4 and newer 928s (1987 - 1995) that come with cats and 02 sensors do not have a CO adjustment at the MAF. The base control for these is managed by the LH.

The 1984 [Euro] - 1986 928s do require base CO adjustment at the MAF. For S4s and other 928s sold without cats and O2 sensors, the base CO adjustment is made via a potentiometer that is situated on the LH mounting plate in the kick panel inside the car (quite different from the earlier 928s that have the adjustment done at the MAF). Look at the blue arrow in the attached document. Any adjustment done at the MAF is ignored on 1987 and newer cars.

For you owners of S4 and newer 928s with cats and O2 sensors, that extra wire/connector that is dangling around next to the LH is the connector for the CO adjustment pot - this remains unconnected in your 928.

Hope this info helps all who read this thread.

John Speake

Hi Rich
That CO pot position was used for only the very first S4s - all the later cars have the same adjuster but it is mounted above the fuse/relay board about 1/2 way along.

I appreciate that all US market cars never had the pot as they were cat/O2 equipped. In the UK cats were an option only for many years - I think the legistlation made them compulsory only after '92, although Porche supplied the UK market with cat car from about MY90.

Regards

inokiyo

Just got back from driving the S4 for 20 minutes. it stalled at almost every traffic light, stop sign, and if it didn't, would take forever to bring the rev up with the pedal depressed. Barely managed to get it into my garage. Half of the *** is black with soot now.
Time for a beer, but will look into checking the wot switch tomorrow.
Will keep you updated!
Thanks!!
Ken

jon928se

If the O2 sensor isn't working (on a car with cats) doesn't the LH default to an overly rich default mapping because it can't see the correct coding plug for a non cat car ? IIRC the pot that controls idle mixture for a non cat car (with no o2 sensor) gets richer as the resistance increases. Thus if resistance is infinte (idle pot missing completely) the mixture at idle would go as rich as the lH lets it?

Gretch

Yes, without a signal from the O2 sensor, the fuel mixture goes to FULL rich.............