W124 300TE rough idle/misfire help please

[Leylandp76]

I recently bought an E reg 300TE (12 valve model) with 164K miles. The car has no service history as it was a recent dealer trade-in. Mechanically it seems OK and although not 100%, not bad for its age cosmetically.

I've probably done 2000 miles in it, mostly motorway. Last night I was stopped in heavy traffic for a while (not for the first time) and the idle started to get quite rough. When I moved away there was a bad misfire, but under hard accelleration the misfire went away.

The misfire returned again after another slow section, to such an extent that I could barely accelerate out of an uphill junction. Once again, it cleared with a bit of a rev, but returned (not quite as badly) quite soon after.

I stopped the car, left the motor running, and checked under the bonnet. I checked the radiator cap first - no excess pressure there, or steam/heat etc. I checked the oilcap and there is a slight vacuum as you would expect. Apart from the miss, nothing obviously wrong. No rattling, knocking etc. After I switched off the motor I also checked the oil level and it is fine.

The motor did not get hot at all, and has shown no tendency to get hot since I have had it. I am careful about checking oil and water levels as well.

Any ideas? I'm worried it's a headgasket on the way out, but is this symptomatic of problems others have had? Any help appreciated...

thanks

Alistair

 

[sunil sood]

could be fuel related or ignition related

check the following-spark plugs,distributor cap and rotor.
spark plug leads.

fuel filter-when was it last changed.

 

[Leylandp76]

thanks for the suggestion. I'll check those - as I said, no history but it looks like a car that has been serviced, but how often or how well I don't know.

Do you know if heads/head gaskets are a problem on these cars?

 

[Spike]

I must say the symptoms do sound remarkably like the plug leads are shot.

 

[Mike Buley]

I have a 300te 12v H reg with 180k.

Exactly the same problem. Rectified completely by new leads, dizzy cap and rotor arm.

hope that helps,
Mike :lol:

 

[kiwi_bloke]

newbies advice hehe

try playing with the timeing hehe....

josh

 

[Rog_E]

Saved me asking the same question!

My problems seem similiar:

Runs smooth, misfires (complete loss of power for a split second) then runs fine.

On two occaisions the loss of power has resulted in me parking unexpectedley at the side of the road (Mercs don't break down do they!?) and the engine is running on two or three cylinders and no throttle response.

I changed the fuel relay, no joy, so I'll go for the HT stuff now and report back!

 

[Leylandp76]

Thanks all, I guess I started thinking bad news too soon. I'll order all the bits from GSF and do it this weekend... BTW I haven't even SEEN the distributor cap on it, how hard is it going to be to change? Who knows, maybe I'll get a smooth idle after all this. :wink:

 

[TimN]

Have a look under the bonnet in the dark. Damaged leads and distributor cap will often reveal themselves as blue tracking alone the outside surface.

 

[dstrong]

Ah someone else with the same problem!

I have a 300TE 24 and have just received new leads/cap/rotor from GSF and have myself a little job to do!

My car has 150,000 on it and full MB service but the leads are totally shot!
I have had the car for 4 weeks now and just had a look over it when a misfire started, the lead from the coil to the dizzy has worn through to the braid in more than one place, the dizzy cap is well worn and the rotor arm is rusty! All this on a car that had a MB service 1000 miles ago! Are they meant to check the leads etc. or do they wait for a fault? Interestingly the leads are of 3 different types so they have not been replaced as a set.

Let's hope that next week we have happy tales to tell!

Cheers

Darryl

 

[Nik_Codling]

changing dizzy cap

Hi Alistair,

Regarding your earlier post concerning location and changing of the dizzy cap, I did the dizzy cap and rotor arm on my 300CE just before Christmas. It's slightly fiddly when you haven't done it before, but then that's often the way.

It's situated on the end of the cylinder head, by the fan.

You will notice that the leads are retained in a black plastic "sleeve" which runs along the top of the head. This can be opened by first removing the black plastic cowl on the end of the cylinder head (it unclips easily) and then unclipping the sleeve.

Word of note, it might be worth marking the leads with the relevant sockets on the dizzy cap before you remove them, or drawing a little diagram so you don't have the game of guessing where to re-fit the new leads...

Once the leads are out of the way you can see that the dizzy cap is held on with three socket-head (allen) screws. I recall it's something like a 5mm allen bit, or thereabouts. The top two are fairly easy, but the bottom one is a bit awkward to undo, as clearance isn't great.

I used a ratchet with an allen bit adapter, to give me reasonable leverage. I had to sit the ratchet inbetween two of the fan blades, and this just about gave me enough clearance to undo the screw. You'll see once you try it, but it is possible!

Once off, the rotor arm is accesible. Held on with two (or is it three?) allen screws. Quite small heads, so be careful when you undo them. One of mine almost rounded off on me, i was lucky...

My old dizzy cap was completely shot, the contacts were heavily pitted and blackened, i'm amazed it ran at all!

In the best traditions of Haynes "re-assembly is the reverse of removal". The only awkward thing is getting the new leads to sit in the plastic sleeve.

Hope that helps.

BTW If you are in West Sussex I'd be happy to lend you a hand.

Nik

 

[dstrong]

I replaced leads/cap/rotor at the weekend and what a difference!!! Misfire gone and it now pulls away from rest really smoothly. A job worth doing.

 

[Rog_E]

Replaced my leads, cap and rotor arm.

Much smoother running, especially noticeable when in gear and the brake depressed putting a load on the engine.

So far the misfire hasn't re-occurred so fingers crossed.

Putting the new leads back in the plastic conduit on top of the cylinder head was fun (not)! :?

 

[GaryC]

What ignition module is it? is it same as the E300? the M103 engine? if is the M103 engine, same as mind, it should have a vacum hose connected to the ignition module, check the hose see any leak there... air leak there will cause the ignition timing go wrong.

hope this can help...

 

[Leylandp76]

Fixed - thanks everyone... here's what it was:

Well I bought new cap, rotor arm and spark plugs. Decided to leave the leads for the moment. Managed to find the cap and took it off just using a normal allen key (heyyy, I used to work on Minis and Jags, so this was easy!).

The cap and rotor weren't too bad, so I was feeling a bit unhappy that there was nothing obviously wrong, but I replaced them anyway, they were showing signs of burning out. i also figured there could be a hairline crack.

But then I took the plugs out. The plugs were utterly fouled up. So thick with coke that the plug gap had almost disappeared, I have never seen worse. At this stage I felt really quite sick, because I thought this is a pretty surefire indicator that my motor is burning a lot of oil. Except it doesn't smoke, and now i think of it, the oil level hasn't really fallen much since I bought it. So, I looked at the plugs a bit closer, and then looked at the replacement plugs I got from GSF.com and i could see the difference. The correct plugs are Bosch H8DC, but the car had been fitted with Bosch H5DC. Basically these are a significantly cooler heatrange plug, I think used for high compression applications such as turbo motors, and in my poor old normally aspirated 300 they just fouled up. With new plugs, cap and leads she purrs, runs very smooth and has a load more power.

thanks everyone for the tips.