hesitation/misfire on light acceleration - was it dirty (French) fuel?

[br1anstorm]

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 ignition/timing settings, and said they were fine.

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

 

[Legoman]

could be that the sparkplugs are in need of replacing , higher octane fuel burns more easily and is less likely to foul the plugs , a good condition set wont have a problem with this but if there getting old then this could be a clue , i suspect a possible weak spark , you need to check your rotor arm , dizzy cap , leads and plugs , the plugs should be your first port of call , check the deposits on them , that will give you a clue most likely.

 

[jibcl500]

OR could it just be french fuel........


jib

Dad always told me never trust a nation that eat pond life!!!!!!!

 

[br1anstorm]

could be that the sparkplugs are in need of replacing , higher octane fuel burns more easily and is less likely to foul the plugs , a good condition set wont have a problem with this but if there getting old then this could be a clue , i suspect a possible weak spark , you need to check your rotor arm , dizzy cap , leads and plugs , the plugs should be your first port of call , check the deposits on them , that will give you a clue most likely.

Thanks Legoman. I confess I don't know how old the plugs are (not replaced in the 3 years I've had the car, but I haven't done that many miles in it). But - though I didn't see exactly what checks the MB dealer in France did, surely the workshop diagnostics would have shown up any fault or feebleness in spark, or tracking etc in distributor cap/rotor?

I will have a look at the plugs. But I'm a bit reluctant to start randomly replacing bits (plug leads, dist cap, whatever) until the source of the problem(s) is clearly pinned down. Doesn't help, however, that the hesitations etc seem to be variable/intermittent. I was hoping that someone might spot a pattern, or common thread, in the car's behaviour that would narrow down the search...

Although the W126 doesn't (as far as I know) have any system for showing fault codes, would a dealer or specialist have 'tuning' equipment to test for these sort of faults?

br1anstorm

 

[television]

Hello brian, you do have a 38 pin socket under the bonnet I believe

 

[Legoman]

the variable hesitation and intermitence of it suggests a sparking problem , 3 years on short mileage trips is likely to kill the plugs or moisture in the dizzy cap , im pretty sure given your symptoms its likely to be something in that area.

 

[br1anstorm]

Hello brian, you do have a 38 pin socket under the bonnet I believe

Apologies if this sounds dim... but I fear I don't know, because I'd be unable to recognise it! What does it look like and roughly where might it be located?

I have diagnostic sockets on a couple of my other (non-MB) cars - one has a simple 8-pin fitting right on top of the block, the other has it cleverly concealed under the ashtray in the centre console. And my Jeep flashes fault codes on a light in the dash. But there's so much under the SEC's bonnet that it's a bit hard for a non-expert like me to tell which connections are which.

If there is a socket into which a dealer can plug a reader, then that would obviously help to track down whatever problem there might be....

Oh and on Legoman's point, although I haven't done a lot of miles in recent years, when I do take the car out it is for decent long trips from time to time, usually when the weather is good. It's not used for commuting or shopping trips: so it really shouldn't be the short trips/fouled plugs syndrome

br1anstorm

 

[br1anstorm]

problem solved...

This is my Magnus Magnusson "Mastermind" moment (you know - 'I've started, so I'll finish').

The problem I described when I started this thread is solved, but I claim no credit. A recent trip south for family reasons gave me the chance to seek the help of Derrick Wells and his people - and they cracked it!

The electro-hydraulic actuator (EHA) valve on the fuel distributor head was faulty. Replaced it with a good one, and the engine now runs and accelerates smooth as silk. No more hesitating, no more jerky upshifts - all is well.

Thanks to those who offered suggestions. I post this just in case it helps anyone else who has similar symptoms and reads this thread. And I have to say that Ray, Andy and all the guys at Derrick Wells were outstanding: they couldn't have been more helpful, and they sorted out some other problems too, at very reasonable cost. I strongly recommend them...

Now on to the next task - that tiny, but serious, little bit of rust under the rear window!

br1anstorm

 

[Alex Crow]

glad we solved your problem! (well, me actually)

 

[br1anstorm]

glad we solved your problem! (well, me actually)

So now I discover you're on this forum! Special thanks to you for nailing the problem. You did a good detective job... and saved me a lot of headache.

br1anstorm

 

[Cole@MBS]

Nice work A/P.

Good to have a thread closed with what the fault was!!!

 

[Alex Crow]

yes indeed. so many of these threads get really close to resolution, then the o/p dissapears never to be heard of again. it makes us wonder if the problem is fixed but the poster is ignorant!! i can vouch for br1anstorm though, a genuinely nice chap, and took the time to come back to tie up the looses ends here. much appreciated.