MB misfire at WoT

AdrianH · 13-02-23, 11:06 AM

Last month, after a couple of months not being driven I took the MB out for a spin and found that when I opened it right up it stumbled and misfired pretty badly – pops and bangs etc. Hmmm, I thought, seems like the spark is breaking down. Luckily I happened to have a new coil on the shelf. (BTW this doesn't fit exactly like the old one –*I'll do another thread about that when I get five minutes.) So anyway, new coil fitted eventually, took for a test drive and all seemed well, no more misfire. Also, the car running noticeably more smoothly at all settings –*the old coil was 35 years old so I guess it had seen better days.

However, I took the car out yesterday and the misfire is back. Not as bad as before I swapped the coil, more of a stumble, but still annoying and unwelcome. At any setting other that WoT the engine is sweet as a nut. I'll start by replacing the plugs (although the ones in there only have a few thousand miles on them, they were swapped about six years ago). After that, anyone have any ideas where I should be looking?

BackintheFold · 13-02-23, 11:36 AM

Try the blink test after a misfiring drive-don't switch the car off as the codes are lost every time.

84CoupeGT · 13-02-23, 08:29 PM

Have a look at the last page or two of my Coupe GT thread. Mine started out with occasional stumbles when flooring it and got progressively worse until it cut out completely and wouldn't restart. I was convinced it was fuel at first but realised no spark and think I've now tracked it down to a faulty rotor arm losing spark through to the distributor shaft. Unfortunately I've not been able to get to the car since I ordered a new one to confirm it yet, but it's definitely worth a look... mine has burn marks just below the rev limiter so it looks as though that's the problem. The thread will give you other ideas of where to look also...

http://www.classic-audi.co.uk/forum/...=39834&page=11

AdrianH · 05-07-23, 04:40 PM

Just to round this one out, I bought the car out after a couple of months, hoping the WOT stumble would have miraculously cured itself. Of course it had not. So anyway back to basics with a visual check and I observed that I'd not re-fitted the earthing strap that runs from the fuel frequency valve (which is bolted onto the coil bracket) to the back of the cylinder head. 60 seconds with a 10mm socket and full performance restored!

So the initial problem probably was the coil, and the secondary problem was not earthing fuel frequency valve.

Motto –*check the basics.

BackintheFold · 05-07-23, 09:55 PM

My standard response to any remotely electrical issue-it’s the earth.
If it’s not the earth it was the earth and now it’s damaged something else.
But it’s always the earth.

EDIT-Interesting, though. Is the FFV totally isolated from earth? Where does the cable attached to it go? Clearly it needs a signal (not 12v!) to power it, and that was working on yours, but there are two wires plugged in-where does the second one go?

Presumably, that is the earth, and also the cylinder head is earthed in more than one position (3 if memory serves-a big one for engine block to chassis (for starter etc), small ones at the front of the head and the one you note at the back).

AdrianH · 06-07-23, 12:21 PM

Now I'm thinking about it I'm not 100% sure that component is the fuel frequency valve. Little hat-shaped thing mounted towards the rear of the engine bay, on a rubber bush. Two wires going in, plus the earth strap.

vagdream · 06-07-23, 05:34 PM

Has it also got a couple of vacuum pipes connected to it? In which case it is the N75 wastegate frequency valve

13-02-23, 11:36 AM	#2
BackintheFold Senior Member Classic Audi Club Member Join Date: Feb 2019 Location: Near an MB ur Quattro Posts: 1,174	Try the blink test after a misfiring drive-don't switch the car off as the codes are lost every time. __________________ 2006 A3 (daily drive) 1987 MB ur

05-07-23, 09:55 PM	#5
BackintheFold Senior Member Classic Audi Club Member Join Date: Feb 2019 Location: Near an MB ur Quattro Posts: 1,174	My standard response to any remotely electrical issue-it’s the earth. If it’s not the earth it was the earth and now it’s damaged something else. But it’s always the earth. EDIT-Interesting, though. Is the FFV totally isolated from earth? Where does the cable attached to it go? Clearly it needs a signal (not 12v!) to power it, and that was working on yours, but there are two wires plugged in-where does the second one go? Presumably, that is the earth, and also the cylinder head is earthed in more than one position (3 if memory serves-a big one for engine block to chassis (for starter etc), small ones at the front of the head and the one you note at the back). __________________ 2006 A3 (daily drive) 1987 MB ur Last edited by BackintheFold; 06-07-23 at 08:42 AM.

13-02-23, 08:29 PM	#3
84CoupeGT Grown up member Join Date: Nov 2020 Posts: 309	Have a look at the last page or two of my Coupe GT thread. Mine started out with occasional stumbles when flooring it and got progressively worse until it cut out completely and wouldn't restart. I was convinced it was fuel at first but realised no spark and think I've now tracked it down to a faulty rotor arm losing spark through to the distributor shaft. Unfortunately I've not been able to get to the car since I ordered a new one to confirm it yet, but it's definitely worth a look... mine has burn marks just below the rev limiter so it looks as though that's the problem. The thread will give you other ideas of where to look also... http://www.classic-audi.co.uk/forum/...=39834&page=11

05-07-23, 04:40 PM	#4
AdrianH Grown up member Join Date: Jan 2014 Posts: 218	Just to round this one out, I bought the car out after a couple of months, hoping the WOT stumble would have miraculously cured itself. Of course it had not. So anyway back to basics with a visual check and I observed that I'd not re-fitted the earthing strap that runs from the fuel frequency valve (which is bolted onto the coil bracket) to the back of the cylinder head. 60 seconds with a 10mm socket and full performance restored! So the initial problem probably was the coil, and the secondary problem was not earthing fuel frequency valve. Motto –*check the basics.

06-07-23, 12:21 PM	#6
AdrianH Grown up member Join Date: Jan 2014 Posts: 218	Now I'm thinking about it I'm not 100% sure that component is the fuel frequency valve. Little hat-shaped thing mounted towards the rear of the engine bay, on a rubber bush. Two wires going in, plus the earth strap.

06-07-23, 05:34 PM	#7
vagdream Grown up member Join Date: Jul 2015 Location: Llandudno Junction Posts: 165	Has it also got a couple of vacuum pipes connected to it? In which case it is the N75 wastegate frequency valve