Erratic Revs on Start Up

metricspaces

metricspaces

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Hi, I've an S3 8L and I'm seeing something strange - has anyone seen this before when starting up http://www.youtube.com/watch?v=5H1axtcj5MM ?

When I start the engine from cold it jumps up to 1300 revs...then slowly works it's way up and down until it gets to about 900 revs. Almost as if it is doing a throttle body realignment.

 
Last edited by a moderator:
S3 8L - your rev counter has an analoge clock - mine don't have that ? it's an led one.
 
does it take slightly longer to start and seems very slightly down on power too?

had one in at work few months ago and the crank sensor had gone down? was a similar complaint by the customer as what you describe. and weirdly enough, it didnt put the management light on despite the cranksensor fault being in the memory

does it cut out if you unplug the cam sensor? it should make no difference plugged in or not on idle.
when the cranksensor goes down, the engine management uses the cam signal to run the engine in a limp mode.
 
Pretty sure mine jumps up to a similar level when cold but drops down to 720/760rpm according to vagcom and the dial when warm. I understand it to be the automatic choke, but if it stays at 900rpm that's a bit higher than mine. At a guess would say maf, coolant sensor, crank sensor anything to do with engine speed, air flow and temp that's off fooling the ecu into giving it more revs.
I'm sure one of the more knowledgeable guys will be along shortly to correct me or give you the right answer.
 
l have the same problem and my temp gauge takes along time to kick in, l put a new temp sensor in yesterday and l still l have the problem. This is odd.
 
Also check for vac leaks under the inlet manifold.
 
Thats exactly what mine does! I have changed the maf and coolant temp sensor, still there. I dont have any tempreture issues though, mine reads ok. must be the crank sensor?
 
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murran said:
had one in at work few months ago and the crank sensor had gone down? was a similar complaint by the customer as what you describe. and weirdly enough, it didnt put the management light on despite the cranksensor fault being in the memory
Click to expand...

By the "fault being in the memory", do you mean it showed up when you did a diagnostic scan?

I don't have anything shown up when I scan the engine module.

murran said:
does it cut out if you unplug the cam sensor? it should make no difference plugged in or not on idle.
when the cranksensor goes down, the engine management uses the cam signal to run the engine in a limp mode.
Click to expand...

Ive never tried unplugging the cam sensor...is there any online pictures or diagrams that might show me where it is, I could try it :)
 
mine does it, but it dies down pretty quickly, i thought i was simply a fast idle to help the car warm up / start up.
 
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drfeelgood said:
Yer i thought it was just something the car did when cold.
Click to expand...

I don't think so. My car never used to do this. Plus it doesn't happen consistently.

If it was something "normal", then it would happen consistently on a cold start.

For example, I've seen this on a cold start on days when it's 20+ degrees outside, and I haven't seen it on other days when it's a lot colder outside.
 
happens to mine consistently on a cold start, or any time over 7hrs, can anyone point to what the possible cause of this is?
 
metricspaces said:
I don't think so. My car never used to do this. Plus it doesn't happen consistently.

If it was something "normal", then it would happen consistently on a cold start.

For example, I've seen this on a cold start on days when it's 20+ degrees outside, and I haven't seen it on other days when it's a lot colder outside.
Click to expand...

Outside air temp does have an affect on this. But if the engine is cold it is cold regardless of the air temp. If the coolant was warm then this wouldn't really happen. Happens on my car every time I start it regardless of temperature.
 
Same problem here, i presumed it was the maf... I am ordering a new one but sounds like it hasn't fixed your issues!
Mine when cold turns over for ages and sort of chokes into life with revs around 1300 too after a minute or so dropping back to around 800! Seems quite common!
 
Mines recently started doing this, although i've seen no other side effects. Would be interested to know the reason.
 
Well mine does suffer from some of these issues, i know for a fact i have 3 vac leaks (2 are due to split rubber hoses in the pcv system and the other is my rocker cover gasket that is leaking) and i need a new maf, will be interesting to see if these 4 things when fixed will cure my problem.
 
I will take it into Audi and see what they say, However wont be for a while
 
You know i have a feeling a pipe is slightly spilt and causeing it to over rev untill it heats up and closes the slight spilt
 
i have noticed mine has been doing this recently from cold. I have replaced the coolant temp sensor and the vac pipe arrangement under the inlet mani. I did not replace them to cure this particular fault though, i thought it was reasonable for the revs to do this from a cold start. I havent got vagcom so i think il keep an eye on it. Interesting to know the outcome of this thread tho....
 
I'am asking mechanics on that "just answer" , at 22 quid i better get a good answer lol
 
Dani_B19 said:
Well mine does suffer from some of these issues, i know for a fact i have 3 vac leaks (2 are due to split rubber hoses in the pcv system and the other is my rocker cover gasket that is leaking) and i need a new maf, will be interesting to see if these 4 things when fixed will cure my problem.
Click to expand...

Yeah, think I will be looking at all the areas mentioned

1) VAC leak - visual inspection & get it pressure tested
2) Replace temp sensor- only a few pound, might as well
3) Replace thermostat - only a few pound, might as well
4) Replace crank sensor

As for the MAF, I only replaced my MAF a year ago, doubt it's gone again, but might check that too. Anyone know how you'd check whether the MAF is causing this issue? The normal test for the MAF is running up to the redline and recording the airflow with VAGCOM...if this shows correct g/s figures, then you know the MAF isn't causing any performance issues, but could it still be causing this problem at low revs\start up?

It would seem strange if it was the MAF. If you've a dodgy MAF, then you will always have performance problems. Whereas this startup issue is sporadic.
 
Issues such as this are 99% down to vac leaks. Hunting at idle, erratic idle on cold start, they are all typical symptoms of vac leaks. So many have had them on here now...
 
Welly said:
Issues such as this are 99% down to vac leaks. Hunting at idle, erratic idle on cold start, they are all typical symptoms of vac leaks. So many have had them on here now...
Click to expand...

Thanks. What's the best way to pin down where the leak is?
 
My 2002 A3 sort of did thism, fixed when the throttle body was cleaned out of all it's gunk.
 
I have the same issue, it gets worse when the engine is 30 deg +. Checked for leaks, cleaned the TB and no difference. Pretty sure its the crank sensor but at €110 im reluctant to change it. Does it take longer to fire than normal especially when warm? If so prob the crank sensor.
 
Can't you get one from a scrapper to test before buying? That's what I'd be doing.....

At least that way you can tell if your sensor is shot.
 
Mine does this too and since changing the clutch switch did not solve my over revving problem when changing gear on boost, I reckon ive got some spilt pipes some where
 
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Id love to check it Welly but S3's are as rare as three balled pitbulls in Ireland so no chance getting one from a scrapper. Im STILL waiting for my VCDS to arrive to see if it throws up a fault for it. Until then its crank and pray!!
 
talentwalsh said:
Id love to check it Welly but S3's are as rare as three balled pitbulls in Ireland so no chance getting one from a scrapper. Im STILL waiting for my VCDS to arrive to see if it throws up a fault for it. Until then its crank and pray!!
Click to expand...

******.....
 
Could this have anything to do with the PCV valve and the pipe which returns to the intake pipe?
mine does something similar, hav just seen the the return pipe is degraded badly
 
the cam sensor is in a metal cap on the end of the inlet cam on the cambelt end of the engine. theres a 3 wire plug directly on to it.
 
hi mine used to do that so i got fed up with it and tuck it to a higly recomended bosch master tech and it was just the coolant temp sender or loom to it he changed both as they were cheap, my car also turned over for abit before starting and cracked wen warm as if the timing was out but the sender and loom did cure this,
however whilst on his computer he noticed it was running 15% lean but i didnt have the time or money to let him find out why any suggestions?
thanks
 
For the car to rev higher when cold is totally normal.

I had a look at mine on the drive home and it sat at about 1100 as i started it (so about 50rpm lower than yours), although I didnt hang about to see what happened after that.
 
I cleaned the throttle body, was a bit grubby, however problem is still there. its annoying as when cold, the increased revs give the car a slight jerky sensation when driving, also if going up a slight hill, the car continues to accerate even with the foot off the pedal. right werid it is. Its going to audi on wednesday, and im not takeing it back till its sorted! lol
 
This isn't totally normal, as mine didn't do it until the last few weeks (i've had it for 4 years). I've done a vagcom scan and it's coming up as running rich on bank 1, which just happens to be the same bank as the coilpack I replcaed recently. I'm going to replace the coilpack for a newer version (its currently an old one I had lying around) and see if that influences the idling issue (as the idle issue started around the same time as replacing the coil pack).
 
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