Hi all. I'm a newbie having bought a '94 SL280 (R129.058) with the M104.943 engine last Sunday, (my first Merc, I'm usually an Alfa guy, so take it easy on me), and walked headlong into a badly running car.

Symptoms are:
- Stumbles and coughs on a very light throttle after slowing down. (E.g. following slowly in heavy traffic that picks up slightly).
- Yesterday it stumbled and then died on the way to work. I nonchalantly pulled over as if that was what I was doing - turned it off and on again and drove home quick smart!

So far
So after reading multiple threads during the week about badly running M104 engined cars decided I'd better
- replace the suppressor between the coils and the spark plugs, (wouldn't want Arthur Dalton to growl ! ), and
- the plugs themselves, (I know, only F8DC4 plugs. I gapped them at 1mm. They were selling cheap because they're a deleted item so I bought 12!).

While doing that I noticed all the HT leads to the other plugs had white points where it looked like they were shorting so I bought and fitted a set of those too.

Anyway having replaced leads, suppressors and plugs, the car is running better but it still "coughs". Sounds like a pop noise that might be fuel going off on the exhaust? It's not REALLY loud, just quite noticeable...

I can now get it to happen regularly. If I slow down to about 40km/h and then gently depress the accelerator, it'll hesitate and cough and then slowly pick up. If I bury my boot it'll only stumble a little, (hardly noticeable), and then GO like a cut cat!

MAS? Built an LED diag tool.
After reading a bunch of threads about M104 MAS issues, (here and here), I thought that was possibly my problem, but decided I'd better build Arthur's simple diag tool to check the codes. (Found here for other newbies looking. Thanks Arthur! ).

Of course I should have checked my connector type first as those lovely banana plugs I soldered up were never going to fit into the 38pin sockets!

Codes.
These are the first take. I have now cleared them. Should I wait and do it again, (I'm a bit worried about getting stranded now, so I'm back driving the Camry! ).

I got the code explanations from the "Diagnostic Codes_MY 88-98.pdf" doc found here

Pin - Module - codes - my interpretation
8 - BM - 1 - No fault ???
7 - ETA - 2 - Assume my car has ASR - EA/ETA module fault.
Does that mean my EA is ?
4 - HFM - 4, 13 - intake air temp sensor sht circuit, and throttle valve pot actual value too low, (another ETA indicator?).

High pitched sound from under the intake
On top of all this, in the hour I was working on the car getting the codes, (ignition on - engine off), I noticed there was a continuous high pitched sound, (it was after 11 on a cold night!), coming from below the intake manifold. Is there a fuel pump under there, (please say yes although it's a bit high pitched for a fuel pump)? Otherwise it could have been coming from the EA. I've uploaded an MP3 of it, (smartphones are cool!) for your aural pleasure. SL_sound.zip

Where to next?
Any advice? What further diags can I do?
MAS / EA?
Either way my wallet is starting to throb...