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Kevin
Kevin, UK Car Mechanic
Category: UK Car
Satisfied Customers: 1820
Experience:  Diagnostic technician at vw /audi specialist
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We have Audi S5 4.2L V8. I have an missfire at cylinder 1 at

Customer Question

Hi, we have Audi S5 4.2L V8. I have an missfire at cylinder 1 at idle or under 1100RPM. Misfire totally stop when engine is warm.
I went to Audi Dealer and after 4 month send to me invoice for Changing motor 26000 euros Its a joke. For a car without any problem when moving/running. Dealer said to me all tested/checked(injector, chain, plug,...) no problem they again did compression test perfect all 12 bar (cold and warm) I was already tested compression and same result. Finally they told me changing motor because they said cylinder Wall have problem.. Its for me impossible because have good compression..I made Cylinder leak down test on cylinder 1 and result are approx 5% leakage. Seems to be normal with good compression result.I made special test called Dynamic compression test and I found cylinder 1 have 0,6 bar less than other...I have all video of my test if needed.Regards
Submitted: 6 months ago.
Category: UK Car
Customer: replied 6 months ago.
I tried to switch coil and spark plug with another cylinder result say in the cylinder 1. I have OEM Audi diagnostic tool. I can see all real time missfire and all is from cylinder 1.
Expert:  Kevin replied 6 months ago.

Hi im kevin did missfire improve at all when plugs were swapped what did plug removed from cylinder 1 look like ?

Customer: replied 6 months ago.
Hello kevin,Hum no misfire encrease normal with other plugs swapped.The plug cylinder 1 and all other look good no oil or other.
Expert:  Kevin replied 6 months ago.

Ok from what your saying i would have to agree with you .the bores in this engine have a coating on them that is supposed to strengthen them and prevent wear .this coating is common to fail and cause your type of issue meaning engine replacement .but this almost always causes plug fouling due to higher oil consumption .my first recommendation just to rule it out would be to swap and injector from another cylinder to rule this out as you can electronically test them but cannot fully test without removing and sending them to be tested and audi wont have done this

Customer: replied 6 months ago.
Hello, if was coating, why compression test and cylinder test result good ? And why no problem after engine warn and when cold if rpm highter than 1100 rpm no problem too ?The injector 1 switched with cylinder 4 and result is same.
Expert:  Kevin replied 6 months ago.

Ok if youve tested injector 1 ,plug 1 and coil 1 ,compression is good and leakage good id be sending ecu away for testing before going any further .im agreeing with you i dont believe engine is the cause

Expert:  Kevin replied 6 months ago.

Ok i just read your post further .you have a valve sticking in my opinion if it goes when revving up and is only there around idle speeds it has to be a valve

Customer: replied 6 months ago.
When I left my car to audi shop , they already replaced Ecu motor with new one and result is same too ;) Its on my invoice.
Expert:  Kevin replied 6 months ago.

It has to be a valve sticking .this is the only thing that wont show on the tests carried out

Customer: replied 6 months ago.
valve sticking maybe and left when engine is warm ?I don't writed but the car comes to Audi dealer in 2012 for complete removing 1-4 cylinder head for valve job (new valve,...) only camshaft was not changed.
Expert:  Kevin replied 6 months ago.

Im guessing that one of the valve guides is worn on cylinder one causing valve to stick open slightly at idle

Customer: replied 6 months ago.
I made another test I don't know if you know this test. Its dynamic compression test. And I saw on cylinder 1 approx 0.6 bars less than other cylinder.About your valve guides, how I can see, detect and replace?
Expert:  Kevin replied 6 months ago.

The only way is to remove cylinder head .this would confirm an issue of this type .

Customer: replied 6 months ago.
oh ok.. So for you its cylinder related issue?
Customer: replied 6 months ago.
can not see anything or test anything with cylinder valve gasket removed ?
Expert:  Kevin replied 6 months ago.

In effect yes the valves anr run through head .the head will have to be removed to access and then guides checked

Customer: replied 6 months ago.
ok thanks. You know maybe the cost of this job? Its impossible to have participation from Audi if this job already made in 2012..?
Expert:  Kevin replied 6 months ago.

You are probrably around the £1200 mark

Customer: replied 6 months ago.
Its impossible to have participation from Audi if this job already made in 2012..?1200GBP total or its only for remove and reinstall cylinder head ?
Expert:  Kevin replied 6 months ago.

Thats a rough estimate of the total job .until head is removed there is no way to know how many guides are worn and. Need replacing

Customer: replied 6 months ago.
Audi can't participate if this job as already made in 2012.. total invoice was 4900 euros..If not I think my better option is too find used perfect condition cylinder head and make remplacement.
Expert:  Kevin replied 6 months ago.

That is another option

Customer: replied 6 months ago.
ok. I will ask Audi technician. For you its valve related problem like valve guide about my symptom and all test made.
Expert:  Kevin replied 6 months ago.

Correct its the only thing that would fit your sympthoms with the tests you have carried out .please remember to rate my service

Customer: replied 6 months ago.
ok thank a lot. Yes agree with you. The only test showing me problem its dynamic compression test(engine running) with 0,6bar less than all other cylinder. So cylinder breathing problem ? ;)You already make this test? Its a good indication on this engine ?
Expert:  Kevin replied 6 months ago.

Correct. It shows an issue and the fact audi have already carried out valve work would make me more suspicious

Customer: replied 6 months ago.
For you its older valve job related problem ? ---
Expert:  Kevin replied 6 months ago.

Its possible but proving that is going to be a problem

Customer: replied 6 months ago.
I am agree..
Expert:  Kevin replied 6 months ago.

Please remember to rate my service and let me know if you need anything else at all

Customer: replied 6 months ago.
little question, can be camshaft or intake valve carbon build up too ?
Expert:  Kevin replied 6 months ago.

I dont believ so

Customer: replied 6 months ago.
ok, and with compression test and leak down test worn valve guide can't be detected right?
Expert:  Kevin replied 6 months ago.

Not always no

Expert:  Kevin replied 5 months ago.

Please rate my service

Expert:  Kevin replied 5 months ago.

Please rate

Customer: replied 5 months ago.
Ok thanks. Today I checked odis error log. And I can see again misfire cylinder 1 normal but now I have this too :
http://wiki.ross-tech.com/wiki/index.php/16805/P0421/001057Regards
Expert:  Kevin replied 5 months ago.

This code shows the cat is becoming damaged likely be the excess fuel in exhaust from missfire

Customer: replied 5 months ago.
ok this can't be the cause of misfire ?
Expert:  Kevin replied 5 months ago.

No wont be the cause