landsharklady

revving very low, any thoughts?
MAF came from John Speakers, who was extremely helpful.
it's now running consistently about 50rpm lower than it should.
no problems with this as yet, just wondering if anyone has any ideas or if this is just one of those 928 things.

BC

You can tell when a car is idling 50rpm lower than it should?

The new MAF will change fueling at that rpm, and richer or leaner will make the idle change up to 100rpm in either direction.

17prospective buyer

Hows it feel now compared to with the old MAF?

69gaugeman

How much is very low?

Mrmerlin

So, you installed the MAF and drove the car to full throttle application?

Try disconnecting the battery for a few mins ,
then drive it to full throttle the computer needs to relearn the new MAF settings

jon928se

As a UK car your '89S4 (put this in your signature by the way so we know) won't have cats or an O2 sensor. Thus it has a a CO pot (a variable resistor) to set the idle mixture. It's located above the right hand side of the central electric panel and is the size of a box of swan vestas matches with a round tube sticking out of one end. Inside the tube is a brass screw to adjust the mixture.

Following a maf replacement you need to re-adjust the CO pot using an exhaust gas sniffer to get the
idle mixture to be around 1%CO. This is of course assuming that it has been adjusted previously to take account of an aged MAF and possible vacuum leaks.

As a baseline with a new MAF the resistance measured across the two connected pins of the CO pot should be around 380 ohms. Beware that there is three pins on the pot - examine the plug and determine which two pins are actually in use, as measuring resistance across the other possible combinations will give readings of approx 1100 ohms (that won't vary when you adjust the screw) and 1100-the reading across the correct pair of pins.

You say the idle speed is now 50rpm too low - what is the current idle speed and is it stable when the engine is warm. IIRC it should be around 675rpm for an auto.

Hilton

As Jon notes, set the CO pot (its above the CE panel) to 382 Ohms as measured across pins 1 and 2, or better yet get the car put on an exhaust gas analyser and set the idle mixture potentiometer to 1% CO. 382 Ohms is the correct value for a new MAF - then as the MAF ages you get the potentiometer adjusted to compensate for the change in MAF output (each year at MOT exhaust test is a good time - just take a screwdriver and do it while the car is hooked up ).

Then once the idle mixture is set correctly, disconnect the battery, wait a minute, and reconnect and start the car. The idle in S4's is computer controlled, so it will run the idle adaption and should settle at 675rpm.

Don't trust the needle - the dash gauge is often 50-100rpm off. Measuring with an inductive timing light or capable automotive multimeter off the signal is the best way to tell for sure.

Here's where the potentiometer is hiding in your car:



If you pull the plug off it and look inside the plug, you'll see which two pins are used and be able to take a measurement without removing it from the bracket.

(note for LHD owners - its in a different location mounted on the same metal plate as the ECU's)

John Speake

As far as can be determined by talking to the owner and the mechanic, this car does indeed have an O2 sensor and hence no CO pot......

Originally the problem reported was that the idle was hunting up and down enough for it to stall sometimes. My thought was that the MAF might be so far out that the LH idle adaptation couldn't cope once the O2 adaptation had hit the end stops. Car mileage is 160k miles, no evidence of MAF ever being changed)
To prove this, I suggested a battery disconnect, then O2 sensor disconnect, then reconnect battery and see how it ran. ... it ran fine, no hunting. Reconnecting the O2 sensor, the car ran OK for a few miles until the adaptation ran out of range... then the idle speed hunting was back.

The old MAF was miles out of spec when I measured it. With new MAF in, car runs much better, idle stable, but reported 450rpm when in gear.....apparently "normal" around 675rpm when in N. Doesn't really stack up....

Without seeing the car it's difficult to diagnose further...

Hilton

Hmm.. I just looked up the car at the DVLA. First registered 01 08 1996.

A quick dig through the old thread for pictures - and this has been covered before. Its apparently from Ireland and was first registered 1/1/89 there.

Randy V

Are you sure the A/C is not inadvertently turned on by having the bottom slider in the far left recirc position?

Bill Ball

Idle stabilizer sticking? George Suennen's car started idling low but Sharktuner showed idle stabilizer signal was maximum. A few WD40 treatments fixed it for now.

LH with defective O2 circuit? I ran into one car that surged and stalled after driving a while. Swapped in a spare LH and the problem disappeared. Installed a rebuilt unit from Louie. All better.

John Speake

Thanks for that info, Hilton - very useful. One of the problems trying to help sort the problems is a somewhat haphasard flow of feedback, sometimes OP, sometimes her mechanic, sometimes her partner..... the car appears to have an O2 sensor but the rest of the exhaust is no stock, probably no cats from what info I have.

John Speake

Hi Bill, thanks for your thoughts. As the idle speed was originally hunting from high to low rpm, it sounds as though the ISV driver stage in the LH and the ISV itself are both OK. When the O2 sensor was disconencted, cancelling the O2 adaptation, the car idled fine as far as can be determined.

landsharklady

Hello

-i've been trying to add a signature with the car's info, but it appears i've not been authorised to do so, very strange.

To clarify:
Car idled very erratically and cut out with old MAF and O2 sensor.
When O2 sensor disconnected/battery disconnected/reconnectedO2 sensor reconnected (we tried all combinations) car ran better for a while, then went back to being very unstable, so MAF sent off.
At present, car is running smoothly but at low rpm with new MAF.

Due to hectic nature of lifestyle (off to be killed in horror movie in Norway on monday) and running band and clothing company, whoever is currently available to deal with issues takes over- which obviously, does lead to a rather confusing stream of info!

But yes, our current situation is car starts, finds it's RPM and stays stable- if low.
If this is just how it's going to be from now on with no further issues, absolutely fine!
The MAF has made a MASSIVE difference to it, it's so much quicker now, it's astonishing, the old one must have really been out of whack!

John Speake

That's fine :-) What are the rpm now at idle when in Park or "N" ?