Hi guys
reaching out for help on my low idle
car starts fine it’s a 1990s4 auto with 70k miles
after around 20mins driving and once warm the idle is very low ie sub 500 in park and will drop as it shud even further in D this gets progressively worse as the car gets up to normal temp to the point it stalls if in gear which looks around 300rpm with a bit of shifting in and out of N you can keep it going but now impossible to drive comfortably after a while

ive tried battery reset and used the udt999 tool both with lights on etc to try idle adaption—
the battery reset makes no difference and the udt999 software says error after 60s ??? I think it says unable to perform can’t see why

Both lh and ezk and maff checked by john


any ideas ?

Cheers
johnny

 

When is the last time you checked your flywheel flexplate end play?

 

Check all the driveshaft bolts and crank endplay as Speedtoys suggested

then when was the last I take refresh
based on your report your ICV is frozen

do a search for ICV operation 87 on mrmerlin

 

The ISV uses a pulse modulated voltage signal to determine how much it opens and closes to thus achieve the set point of 675 rpms for idle. For a normally functioning ISV if the voltage supply is disrupted the default position of the ISV is such that it can maintain a more or less normal idle with the a/c off when fully warmed up. Stick the a/c on and the revs drop.

That you are seeing low revs thus suggests one of two things- either the ISV is jammed in a low airflow position or something else is dragging the engine speed down due to the ISV no being able to overcome the resistance.

At this moment in time I would think the most likely scenario is the former [the ISV jammed in a tight position]. Heaven forbid the latter scenario is in play you could have the deadly TBF syndrome in play however do not jump to that conclusion unless and until the ISV proves OK.

Irrespective of whatever problem is in play, have you done anything to check that the flex plate couplings are holding, that the front plate has not crept and that crank end float is as it should be at around 0.2mm? This is something every S4/GTS owner should know and be aware of.

 

hi
i checked and reset the flexiplate around two years ago when got the car it moved a bit when released but nothing drastic
I’ve pulses the icv with the utd999 dish tool and you can hear it switch ok -surely I wouldn’t have correct revs at cold start if icv was stuck plus goes to around 800 while warming up only gets really bad once fully warm

other bit is the o2 sensor is disconnected and I’m running no cat could these have affected ability to run idle stability thru the diag tool ?

not something I understand but how wud tbf cause low idle after warmup ?


This low rev does get worse the more things are on ie headlights and window heater or a:c shud the isv not be better at compensation
One other thing I’ve noticed battery voltage drop below 12v at the lower rpm this could also confuse if source voltage for isv is low ?

Cheers

 

When was an intake refresh done?

Its most helpful to include the running condition in the first post a disconnected O2 sensor is important to know plus your testing of the ISV
and the results
Has the MAF ever been rebuilt?

Have you had the alternator off for freshening
IE checked the brushes
is the car in a wet environment?

Have a fresh O2 sensor installed use a factory part
adding pictures of the engine and the exhaust headers would add info to the condition of you car

 

Yep maff rebuilt
no full intake refresh as you guys would call it thou receipts show replacement isv in last five years I’ve owned car for three
there was a gt chip installed which hid the low rpm according to John speake we have put correct s4 chip in and now the low idle
the gt chip was causing extremely high idle on cold following cam belt and water pump change 4weeks ago

cheers

 

Johnny,

The info that idle speed is slowing down when fully warmed up is the bit that concerns me as does the fact that you found some pre- load tension earlier and released it and now perhaps think "all is well".

Many examples show slippage of about 2 mm and live to tell the tale. Sad to say some do not. Given we are looking for something that "explains the problem" when hot, the fact that you found the slippage previously should be of concern, especially if you have done nothing to mitigate this engine killer permanently. A sad fact of auto ownership is when one starts the car one day and finds it is seized- often that is the first clue. To be clear, I am not saying this is the issue but it is not hard to visualise why a hot engine might slow things down if the flex plate clamp has slipped ever more than previously and friction generated when hot is heating things up more than usual- it is not something I have noted in the many cases I have followed [with interest] but it is something you need to eliminate as a possibility. As I m concerned the slipping flexplate is a pre-cursor- a warning if you like that all is not well and something "bad" is going to happen if not taken care of promptly.

When the engine is cold the brains adjust the speed such that things run quicker initially and also run richer until temperature stabilises- as I recall the stock maps stop adding additional fuel for cold engines at 82C [i.e. just before the thermostat opens]. .If the ISV were seized in a "fixed position" logic says you would have even bigger problems during cold starts and as I understand that is just not happening. Now you are saying that the ISV is seemingly modulating OK one has to wonder what else could possibly be slowing the motor down short of a blockage in the ISV tract and even that would not explain cold starts being seemingly "norrmal"..

 

Thanks Fred I’ll check the flexi again to be sure it hasn’t moved in the meantime any ideas why Theo’s Utd diag tool can’t run thecidke adaption ? Might be related to some issue why car can’t set it itself etc
plus this loading affect of lights and heater etc shud it affect rpm as much as it does is this another indicator isv isn’t working right ?
thanks !
johnny

 

Johnny,

You should eliminate the possibility of flex plate clamp migration ASAP irrespective of anything else that may be causing your current issue.

The electricals are marginal when everything is switched on and working as it should but I am not aware of anything that would impact the idle speed in such circumstance but that does not mean it is not possible. If the engine harness is in poor condition that can pull the voltage down some just as faulty earth straps can cause too much resistance to current flow. My late S4 had a powerful amplifier grafted into the car and it struggled voltage wise below about 2500 rpms. My GTS chassis has no issues but then it does not have a powerful amplifier draining however many amps irrespective of engine speed..

Faulty O2 sensors can cause issues but whether such can cause your particular symptoms remains to be seen. My 928 runs on open loop so my experience in that aspect is zilch. That you are having trouble running adaption suggests you need to get to the bottom of that.

 

Hi checked the flexiplate it’s sitting very flat no movement since I adjusted it checked with steel rule
also my mistake the o2 sensor is connected

cheers
johnny

 

An aging O2-sensor can cause a weak (lean) mixture at idle, which will cause stalling. I am not familiar with that tester, but it is easy to reset the adaptation-- just disconnect the battery for a moment (engine off, of course). When restarted, the LH will begin a new adaptation for idle and fuel.

Try this: Warm the engine up, until it wants to stall. Then shut it off, disconnect the battery for a few seconds and reconnect, then restart the engine. It is better, or worse?
Another simple check is to disconnect the O2-sensor (3-pin rubber connector in the CE panel area), better or worse?

If the O2-sensor is more than 60K miles old then I would replace it. They are not expensive (compared to rebuilt MAFs) and as they age the mixture does get weaker. After installing a new O2 sensor always disconnect the battery for a moment to reset the adaptation. The LH2.3 uses the O2-sensor to compensate for a weak MAF, but if the O2-sensor itself is weak then the mixture will still be lean. With the O2-sensor is disconnected, the LH defaults to the baseline map values with no correction or adaptation-- which may not be correct, but likely better than an aged, lying O2-sensor.

 

A good result on the flex plate check. I use some white paint to index check for small movements.

If I understood your post correctly you forgot to reconnect the O2 sensor- if so are we to presume the problem has now "disappeared"?

Do bear in mind that the TBF issue may never happen and then again when it does it usually manifests itself as a seized motor with no obvious warning.

 

Fred and others reading this if one of these engines is suffering from flex plate pressure careful listening can provide clues that it’s happening

this can be backed up by hand turning the crank and pulling the dipstick and dripping the oil on a black piece of paper then looking at the oil in the sun
sometimes I can hear the slight screeching sound or the engine laboring at idle or worse a very slow cranking from the starter motor once the engine is warm
Note these are very subtle indicators but most are present when an engine is having this issue

 

• hi alas the issue is still here the 02 sensor was connected all along
• will try it disconnected today and adaption via battery reset as can’t seem to thru dish interface took which is a concern that the ecu is somehow never able to run adaption properly ?

thanks
johnny