W124 - Odd idling - occasionally dying

[dot]

Evening all, in addition to the 123 I’ve recently got hold of a 1991 300TE. It’s been pretty well looked after, but I have taken it on with a couple of issues. I’ve decided to post here instead of the electrical section, forgive me if that‘s incorrect. M103 engine I think.

When I fire her up from either cold or warm i wouldn’t say she struggles to start because she always does, but it’s a slight hesitancy followed by an engine sound that possibly says she ain’t firing on all 6, but as soon as I rev the engine within a second she sounds sweet as.

Whether connected or not, this happens too while out and about. I should say at this point the previous owner has fitted a new accelerator cable and has played about a little with the adjustment of it thinking this is the route of the issue but I disagree.

At indiscriminate times, at standstill, at slowdown but never when driving the tickover revs fluctuate from just under 500rpm to almost 1000. When at the lower end it has cut out 3/4 times, but if I’m at standstill and I stick her in neutral the revs return to normal idle, but could also fluctuate after sitting there a while.

Any ideas anyone?

Thanks

Dot

 

[optimusprime]

Welcome to the forum ..First off ..most problerms with the M103 engine and lots of older type Mercedes is 80% electrical, so if the HT side was not serviced in your ownership then now is the time to do it . Stick to Bosch or Beru parts ,with none resistor spark plugs . Start with battery conections remove and clean them add a dab of grease on the luggs and refit ,dont forget you will need radio code if the battery is disconnected . - Same with earth points,, renmove them and clean the body and eath braid and grease them up and replace.-- Look in the engine area for any small bore , or large vacuum pipe and rubber connectors split or missing because the vacuum on these engines is very important so everything works as it should .Fuses in fuse box will be made of white metal if they have never been replaced if they are change at a later date,, they can be a problem. Also clean out the IACV see > C < in picture . Remove from the car you might find split hoses on this part as well that need replacing .Clean with white spirits ,,,shak it a little to remove the crud .....

 

[dot]

Thanks Optimus that's really good info to start from.

Question though - my locks are working fine, so would that lead me to think the rest of the vacuum system is also ok, or should I begin checking it out? I've got a smoke tester now, and am itching to use it

Strange thing when trundling along, she's lovely, not missing a beat, but I will get on to these asap.

Ta,

Dot

 

[190]

Question though - my locks are working fine, so would that lead me to think the rest of the vacuum system is also ok

If it is the same as my 190e. The locks have a separate vacuum pump under the rear seat.

I had a similar issue with the 190e which used the M102 4 cylinder version of your engine. It turned out to be a fouled plug which I think was the beginnings of a head gasket failure. It might be worth a quick look at the plugs starting with No 6 at the rear of the engine as that is where they tend to go.

 

[John757]

I had exactly the same issue just after I bought my G wagen...
I took it to a specialist.. and he said it was running too lean.. after attaching a sniffer to the exhaust.
he tried to adjust the mixture with the air screw, but it was not making any difference....
then he did something weird.. he sprayed brake cleaner near the large vacuum pipe that goes from the brake booster to the inlet manifold ... and the engine immediately revved higher...
"Vacuum leak" he said.. "the brake cleaner is being sucked in improving the mixture..."
we then found the leak where the vacuum pipe attaches to the brake booster... someone had melted it to try and fix it as it had cracked... new one solved the issue....
" too much air... not enough fuel" caused poor slow running... was ok driving it... but when hot it kept cutting out.. or at low speeds... been great last 8 years...worth checking.

 

[optimusprime]

Check out the HT side first . But use WD40 for that it could set fire if you use brake cleaner thats if you dont know what your doing . And as above ,,vacuum in the engine is another seperate system it uses vacuum right from the inlet manifold . Your economy gauge on dash , where is pointer at tick over in park?.

 

[dot]

Check out the HT side first . But use WD40 for that it could set fire if you use brake cleaner thats if you dont know what your doing . And as above ,,vacuum in the engine is another seperate system it uses vacuum right from the inlet manifold . Your economy gauge on dash , where is pointer at tick over in park?.

Thanks Optimus - I don’t have an Economy gauge - to be fair she ain’t that thirsty. I’ll be looking at this at the weekend. Dot

 

[MrGreedy]

Great advice already here in this thread.

The cutting out during idle for me immediately made me think vacuum, but there's loads of thing mentioned already that are very cheap DIY all worth checking.

I think if I was checking the plugs, going to that effort I'd also replace them all at the same time unless there was recent invoice for the job already being done. Get a quote from MB Edinburgh or Newcastle via their eBay store of MB Grangemouth over the phone. Then you can be sure you have the correct plug and gap, and this was discussed at length on a post on here recently with the conclusion that MB dealers can be pretty competitive, and the three I've suggested are usually some of the cheapest.

 

[optimusprime]

So George you dont have this little helpful device in your instrumernt cluster --econo gayge bottom left

 

[dot]

So George you dont have this little View attachment 129834helpful device in your instrumernt cluster --econo gayge bottom left

Noooo, Niet, nada, rien!

 

[ray_hennig]

Noooo, Niet, nada, rien!

No, neither do I in a 1991 300CE-24.

But I did, years ago, in a 1987 300E.

R

 

[Bellow]

If it was related to plugs and HT chances are it would be cutting out on throttle-opening not idling.
If this car is pre-cat, when it does its idling from 1000 to 500 to zero rpm, you should be able to smell at the tail pipe whether it has gone lean or rich.
Lean is vacuum leak somewhere.
Rich is something causing the mixture to richen or, an ignition induced mis-fire.

I'll wager it is the mixture deviating. Ignition failures are sudden death not gradual.

 

[dot]

**update**

Finally had a day to sort this, or at least try. I purchased leads, plugs, cap and rotor.

Changed cap and rotor first, no difference.

Changed leads, no difference.

Changed plugs, set to 35 thou… bingo!!!

Some oil inside cap and plugs were battered. I’ll monitor this… thanks for advice all… I’ll be back for more though!!

Dot

 

[ray_hennig]

**update**

Finally had a day to sort this, or at least try. I purchased leads, plugs, cap and rotor.

Changed cap and rotor first, no difference.

Changed leads, no difference.

Changed plugs, set to 35 thou… bingo!!!

Some oil inside cap and plugs were battered. I’ll monitor this… thanks for advice all… I’ll be back for more though!!

Dot

Those plugs are a disgrace. Never seem anything like them.

R

 

[optimusprime]

Shout when you have everything you bought up and running .