2008 Audi S5 4.2 V8 Misfire

[Rickym1221]

Hello, reaching out to all who can help on this! Let me explain.

S5 4.2 V8 (B8). Car has a misfire on cylinders 5-8. Car idles rough etc.

I have tried many things like spark plus, coil packs, carbon cleaned the valves and intake, swapped sensors from bank 1 to check for faults but nothing.

so… did a compression test and on bank 1 I’m getting all 150-160psi and bank 2 I’m getting 210-225psi.

 

stuck a scope down to see what the pistons are like and wow… covered in carbon! What would be the cause of this?

 

any advice I appreciate 

 

[Magnet]

Welcome Ricky and thanks for joining.

Obviously you have to home in on cause of the low compression on some cylinders - either worn bores/pistons &rings, or valve seating issues. 
What is the history of this vehicle?

How long have you had it; mileage; what do you know of its earlier history; did it have this issue when you bought it; does it suffer from oil consumption etc. etc. 

Some will claim that if you inject a teaspoon full of oil down into the offending cylinder, any temporary increase in compression value indicates worm bores association rather than valve seating. I don’t completely buy into that theory since some of the oil will be sprayed onto the valve seats. 
Way forward as I see it:- check you have valve clearance on the offending cylinders, and the cam lobes are fine. If so, then it’s head/s off at assess and correct the issue. 
Perhaps you could let us know how you get on Ricky.

Kind regards,

Gareth.

p.s. What part of the U.K. are you in? 

[Magnet]

Hello Ricky,

We notice you have returned to the forum since you have had a response. 
Could you please do us a kindness and read the post entitled ‘ Reacting to help and advice given’ under New Members section. 
Thanks. 

[Rickym1221]

Hi magnet thanks for the response! 
 

so from what I’ve read 150+ is good compression? Bank 1 that has no issues is the side that’s giving 150+

bank 2 is showing 200+ 

I’ve not long had the car, what I have found is the misfires are random across bank 2. I’m also getting codes for:

 

p1575: right engine mount

P0660: Intake manifold tuning valve bank 1

i have brand new injectors arriving tomorrow so will be installing them. 
 

thanks

 

ps. From what I’ve researched high compression is caused by high carbon build up. I’ve used a bore scope to look at the pistons on bank 2 compared too bank 1 and bank 2 pistons are covered in carbon

 

 

Edited August 1 by Rickym1221

[Magnet]

Thanks Rickie,

I’m getting 110 on a 92 year old car with a compression ratio of less than 6:1! 

[Rickym1221]

so it turns out I think I did the test wrong. Car was cold, not operating temp as stated on sites that it should be 🤣 apparently you should see around 25+ psi when engine is up to temperature. This true? If so I’ll redo the test with the car at normal operating temp 

Edited August 1 by Rickym1221

[Magnet]

Personally, I’ve never experienced much ‘improvement’ hot/cold on a good engine. Remove all plugs, isolate ignition and fuel feeds, hold accelerator to floor. 
As long as your test is consistent for each cylinder, then any differences in compression should be meaningful. 

[Rickym1221]

Oh wow looks like I did the test completely wrong!  I didn’t remove all plugs I only removed the one I was testing at the time… I didn’t hold the accelerator to the floor. 
 

I will certainly redo the test 

[Magnet]

OK Ricky - glad we’ve been able to be of assistance. 
Could you do us a kindness and:- 

Have a read of the thread ‘Reacting to help and advice given’ under New Members section. (Second time of asking!)
Also give us an answer to where in the U.K. you are. (Again second time of asking!)

Kind regards,

Gareth. 

[Rickym1221]

Sorry I will have a read now, I’m from the Midlands, UK.

 

will post results later today of compression test 

[cliffcoggin]

Ricky.

As Magnet said, the test needs to be done with all plugs out and the throttle wide open. Hot or cold makes no significant difference as long as all tests are done under the same conditions. Crank the engine until the pressure reading is stable, which should only take a few revolutions.

The absolute readings for each cylinder are not as important as differences between cylinders, so if you find one cylinder or one bank is much lower or higher than the others you have a real problem. In your position I would not be spending money on new injectors or any other parts until the head has been taken off for inspection of the valves and cylinder bores. I would find the fundamental problem before spending, possibly wasting, money on unnecessary new parts.

[Rickym1221]

Hi Cliff, thanks for the message. Right ok I’ll have to get it looked at. I was going by many other forums etc I’ve looked at where it states higher compression is caused by carbon build in the cylinders (which I’ve confirmed bank 2 has using a bore scope). 

[Rickym1221]

Not the best pictures… here’s a photo of the borescope. 1 picture is a piston from bank 1, the other a piston from bank 2. Clearly a lot of carbon build up. The carbon is dry on the pistons not wet.

There was mass carbon build up everywhere after removing the intake manifold, not just on the pistons. From what I can see the cylinder walls look clean

injectors are here however so will install them anyway

Edited August 2 by Rickym1221

[cliffcoggin]

17 hours ago, Magnet said:

Thanks Rickie,

I’m getting 110 on a 92 year old car with a compression ratio of less than 6:1! 

Good Lord Gareth. Does it also have a wick carburettor and desmodromic valves?

[cliffcoggin]

39 minutes ago, Rickym1221 said:

From what I can see the cylinder walls look clean

What I meant by inspecting the cylinder walls was looking and feeling for a step near the top of the bores, which would indicate wear of the cylinders and/or piston rings. Ideally one would use a micrometer to measure the diameters of the worn and unworn sections, but feeling for a step is a good indication. Heavy scoring of the bores would also be bad news.

Whatever the cause of the soot accumulation it still has to be dealt with by removing the head in order to clean it. The process was called de-coking back in the day when I regularly got my hands dirty and people knew what coke was.

[Magnet]

Ok Ricky, 

It seems you a must be posting on/looking at other forums, and you’ve still got your knickers in a twist. 
Just treat it one step at a time:- 

Get a proper compression test carried out then report back. 
Don’t bother finding the reason for varying compression pressures until you have established that there are variations. 
Kind regards,

Gareth.

p.s. I didn’t want to spell it out, since I didn’t feel I would need to, but the purpose of directing you to the ‘ Please react to help and advice….’ was that when you had read it, you might see it fit to carryout what it suggests - acknowledge the help you are being given. Instead, you ‘Like’ the tread, but don’t bother to act on it! Good isn’t it?

[Magnet]

You know Cliff, it starts on the first press of the starter button - who claims press button starters are new and ‘posh’? 
It ticks over like the proverbial watch, and does what is asked of it, without any complications of ECUs, remote locking, electric everything, and many other things that you don’t need and hope won’t go wrong! 

[Rickym1221]

Hi Gareth, what is it your after, a like or thank you on the replies you give to me? I come back to the site and acknowledge your replies and are obviously grateful. I’ve clearly not "taken the advice and ran" 

yes… researching other sites that contain information that may not be on this site… that isn’t a bad thing, I’m looking at all possible reasons as to why I’m having these issues but are acting on one at a time. 

[Rickym1221]

UPDATE: doing some measuring in VCDS as I got a code saying low fuel rail pressure. It was reading 1.6bar with just ignition on. Start the car it was at 6 bar!

replaced both HPFP and cleared codes and the car fired right up, no misfire, pulls as it should!!

 

ps: car smoked from bank 2 exhaust quite abit for about 5 mins then cleared 

Edited August 3 by Rickym1221