br1anstorm
Active Member
I have just been on a 3 week trip in France in my 1990 W126 560SEC, and noticed a curious problem which oddly, seems to have almost disappeared since I got back. I've scanned the forum for clues, but don't quite know where to start with troubleshooting.
The symptoms were:
a) a slight 'stuttering' or hesitation when cruising steadily on very light throttle at around 3000 revs/130kph on the autoroutes. Sometimes this was quite obvious - almost like driveline shunt - and at other times barely noticeable;
b)with cruise control set at 110 or 130 kph, a sort of on-off 'surging', again like driveline shunt, as if the cruise control couldn't quite get the fuel/air setting right and was constantly but slightly jerkily correcting itself;
c) on upchanges through the auto transmission on light (gradual) acceleration, a similar sort of hesitation at each change point, as if the 'box couldn't quite decide whether to shift to the next gear or not. On heavier-footed acceleration, no such problem, just normal smooth upshifts. Made no difference whether transmission was set on 'S' or 'E'.
Now here's the puzzling aspects. First, these problems only seemed to show up once the car had been running for a while. For the first 10-15 mins after start-up, all seemed normal. Is this a clue? Second, the problems only seemed to be evident while we were in France (!). Back in UK, before and after the trip, symptoms (a) and (b), at least, have barely been noticeable.
I'm not sure if the three symptoms all have the same cause, or if they are unrelated. My initial instincts were to suspect either dirty fuel (causing a misfire or affecting the injectors) or that the engine didn't like 'normal' French (95 octane) supermarket petrol. I tipped in a dose of injector cleaner, and I took to filling up with 'super' (98 octane). Sometimes I got the impression that this seemed to improve matters, but not always...
I was sufficiently bothered to get a French MB dealer to do some brief basic diagnostics: in the time they had, they only checked the ignition system, and said all was fine (so I assume no misfiring - plugs, dist cap and rotor OK, and timing all right?).
Now I'm back in UK, and in the last week or two I've used the car much less - and no long motorway trips. I have continued to fill with 'super' (Shell rather than supermarkets) though I had previously always used 'normal' petrol. On the trip up from Dover I had a separate and totally weird one-off problem with the climate control blowing hot and the recirc switch flashing (on which I've posted a separate query); but this seemed to reset itself the next day, and surely can't be related, unless there really are electrical gremlins at work under the bonnet!
I'd still like to pin down what is - or was - wrong, but need advice on what to check and where to look. Do the symptoms suggest some sort of electrical/ignition issue, or is a fuel or injection problem? Or something else entirely? It's tempting to blame dodgy French petrol, but over the course of three weeks and 6000km I filled up in places all over the country, so that seems unlikely ...
Any thoughts would be welcome.
br1anstorm
The symptoms were:
a) a slight 'stuttering' or hesitation when cruising steadily on very light throttle at around 3000 revs/130kph on the autoroutes. Sometimes this was quite obvious - almost like driveline shunt - and at other times barely noticeable;
b)with cruise control set at 110 or 130 kph, a sort of on-off 'surging', again like driveline shunt, as if the cruise control couldn't quite get the fuel/air setting right and was constantly but slightly jerkily correcting itself;
c) on upchanges through the auto transmission on light (gradual) acceleration, a similar sort of hesitation at each change point, as if the 'box couldn't quite decide whether to shift to the next gear or not. On heavier-footed acceleration, no such problem, just normal smooth upshifts. Made no difference whether transmission was set on 'S' or 'E'.
Now here's the puzzling aspects. First, these problems only seemed to show up once the car had been running for a while. For the first 10-15 mins after start-up, all seemed normal. Is this a clue? Second, the problems only seemed to be evident while we were in France (!). Back in UK, before and after the trip, symptoms (a) and (b), at least, have barely been noticeable.
I'm not sure if the three symptoms all have the same cause, or if they are unrelated. My initial instincts were to suspect either dirty fuel (causing a misfire or affecting the injectors) or that the engine didn't like 'normal' French (95 octane) supermarket petrol. I tipped in a dose of injector cleaner, and I took to filling up with 'super' (98 octane). Sometimes I got the impression that this seemed to improve matters, but not always...
I was sufficiently bothered to get a French MB dealer to do some brief basic diagnostics: in the time they had, they only checked the ignition system, and said all was fine (so I assume no misfiring - plugs, dist cap and rotor OK, and timing all right?).
Now I'm back in UK, and in the last week or two I've used the car much less - and no long motorway trips. I have continued to fill with 'super' (Shell rather than supermarkets) though I had previously always used 'normal' petrol. On the trip up from Dover I had a separate and totally weird one-off problem with the climate control blowing hot and the recirc switch flashing (on which I've posted a separate query); but this seemed to reset itself the next day, and surely can't be related, unless there really are electrical gremlins at work under the bonnet!
I'd still like to pin down what is - or was - wrong, but need advice on what to check and where to look. Do the symptoms suggest some sort of electrical/ignition issue, or is a fuel or injection problem? Or something else entirely? It's tempting to blame dodgy French petrol, but over the course of three weeks and 6000km I filled up in places all over the country, so that seems unlikely ...
Any thoughts would be welcome.
br1anstorm
