s4 misfire

[therealnovaboy]

my s4 has a slight miss when on idle. it also seems to do it when driving slightly hard at about 3k rpm. my thots are its a coil pack on the way out but was wondering if anyone had any input.

it doesnt always do it but when it does it is a regular miss and recon its only one cylinder. altho now and again it seems like it is more than one. i think it does it mostley when the car has been for a run, stoped for a period then started which leads me to the idea that the heat from the engine is soaking up to the coil pac and breaking down the coil. i cannot be sure.i dont know if the mis at 3k is related but had the idea that because it is around boost that it was a sticky re-circ valve causing a low flow/air stall across the MAF. i dont know if re-circ valves are prone to failure but this a symilar problem/cause my brother had with is 300zx. what is the best way to find out what cylinder has the misfire. i thought about getting a vag-com but would this tell me if a certain cylinder has a misfire. also do you know if i could measure the resistance of the coil packs to see if there is a differance between them thus telling me if one is breaking down.

thanks in advance, any input will be appreciated.

 

[Blue_Thunder]

I reckon your MAF is shagged. Try unplugging it and see if the car runs any smoother.

 

[therealnovaboy]

I reckon your MAF is shagged. Try unplugging it and see if the car runs any smoother.

would this cause a miss at idle, at 3k or both. a MAF fault would show up with an engine diagnostic wouldnt it?

 

[Blue_Thunder]

Oh, and yes you can check for misfires by Cylinder using Vagcom.

The DV's are very prone to failure. The sensible option is to go for a pair of the S3/TT standard valve (part number ends 710N). Think you can get a pair for around £40.

 

[Blue_Thunder]

A faulty MAF wouldn't show up on a diagnostic tool because they rarely fail completely, they just misread so your fuelling goes to sh1t.

A faulty MAF can cause a lot of issues. Certainly a lumpy idle and a flat feeling/hesitation when coming on boost are common symptoms.

Get it checked before it knackers your EGT sensors!

 

[therealnovaboy]

what s the prone fault with the dump valves?

 

[Blue_Thunder]

Torn diaphragms. These wouldn't cause a noticeable problem at idle though.
Other common faults to look for are torn TBB, torn F-hose, leaks from plastic intercooler caps, various other split hoses, etc.

I must be in a good mood... usually I just shout at people and tell them to use the search button.

Oh what the hell... USE THE SEARCH BUTTON!!!!

 

[therealnovaboy]

ill try unplugging the MAF. see if it helps. i take it the missfiering should stop on idle and shouldnt drive with it disconnnected? will it store as a fault if i disconnect it? i tried search but kept getting info on s3's, i dont care much for s3's. thanks for your help tho

 

[aragorn]

The reason you find all the S3 stuff is that "S4" is too short a search term. So it only searches on misfire, which is waay too generic as you found.

Not really an easy solution though.

 

[Blue_Thunder]

Or better still, go to Advanced Search, then select only the B5 forum. :icon_thumright:

You will be ok to drive the car with the MAF disconnected for a short while, just don't go hooning it around too much as this could also cause problems with the EGTs.

Disconnecting the MAF will log a DTC (fault) which will show up on a diagnostic tool, but you don't need to delete the code once you reconnected it for the car to work properly again.

Just unplug the MAF, see how it idles, then take it for a short drive and see if you are still getting the hesitation at 3K. It may not be the MAF, but it's a fairly good place to start looking.

You should also look to getting it pressure tested as soon as you can to rule out any leaks. There are loads of B5 S/RS owners in your part of the world, i'm sure you can find someone to help you out with a pressure test.

 

[S4twiggy]

my MAFwas ******** last week and i was having the same problems mate, if you do unplug the MAF only dive it under 2500rpm and only drive for a short time or you will nacker the EGT sensors. get vagcom on it mate as this will pick up your codes so you can diagnose the problem, had my maf changed last week and the car is perfect now, surprising how bad it makes the car feel just from the ******** maf.

 

[andybnwc]

3k sounds like maf to me too , dvs and boost leaks will usually throw it in to limp mode.

 

[gaz71]

3k sounds like maf to me too , dvs and boost leaks will usually throw it in to limp mode.

Don't want to sound stupid but what exactly is limp mode

 

[therealnovaboy]

think the MAF sounds like it could be at fault. i will get a vag-com and run a diagnostic then carry out a leak test. i was wondering if i needed the obdII vag com plug because its a 1998 model. are the ones on ebay any good and what one is it i need i thought it was obdII but they dont seem to list it for the s4.

limp mode is a safty mode the map goes into when it detects a fault that has any effect on emmisions or damaging something .limp mode is a map with pre determined values and usualy reduced boost. its so you can limp back to the garage.

 

[aragorn]

There are various "limp modes" across different cars, some will limit RPM's to a much lower level, some will reduce boost, some will let the car do nothing but limp to the hard shoulder.

The boost-cut type is probably the most common on turbocharged audis, but the S4 ECU is pretty sophisticated, and has a whole range of things it can do if its unhappy, including cutting boost and stopping you getting full throttle, all while still driving nice and civilised so you'd never know the difference (bar it being slower obviously)

No idea why UK cars dont have a "check engine" light like the US cars do. It would be quite valuable!

 

[gaz71]

I'm wondering if that is what is up with my car. Drives ok but just doesn't seem to have much kick from the turbo.