W124 warm engine misfire - diagnosis needed before the grim reaper calls!

[howeshouse]

Okay, it's an E280 estate, 1995, and most of it goes really well. But about a year ago it dropped to 4 cylinders. Swapping out one of the coils seemed to fix it for a while, but ever since then it hasn't really liked working on all 6.

It is always fine from cold, but when it gets warm, and most particularly when in slow traffic, it starts to miss. This is mild at first - difficult to tell whether it is one or two cylinders, but rapidly degenerates until it feels like there are only two working. At this point, the power output is a trifle low, and pressing too hard on the accelerator stalls it. But if you just feather it, it will generally keep going (albeit it at around 10-15mph, so you have to have a robust attitude to the queues behind, especially up hills). Turning the ignition off and then back again sometimes cures it completely, sometimes just for a few minutes, sometimes just a few seconds.

Trouble is that every now and then the misfires stop completely, and suddenly there are all 6 cylinders raring to go and as it is invariably in a low gear, the vehicle leaps forward like the proverbial startled greyhound. Having experienced this once, and nearly taking out a pedestrian, my wife is understandably reluctant to drive the car.

We have been given various diagnoses - the most common one is "new head gasket", but without any supporting evidence that just looks like £1,500 to spend with a significant risk that it won't cure the problem at all. A new ECU has also been mentioned: another £600 with a similar prognosis. The wiring harness was definitely dodgy, so I have changed that, along with all three coils, but I think I may be close to the limit of the number of times that we can spend upwards of £500 "just in case" it will solve the problem.

So, unless all of you experienced people can help me to find the problem, it seems inevitable that it will be just another sad E class estate on ebay "0.99 starting bid, no reserve". And it such a great car when it is going well...

 

[Bolide]

There are lots of threads about this on the two forums. Most say to replace the OVP, crankshaft sensor and temp sensor

A search of the forums should find lots of reading matter

Nick Froome
www.w124.co.uk

 

[Juddian]

Just a suggestion, if you have exhausted normal diagnosis of other sensors etc, why not send the ecu to the chap on the Isle of Wight.

I'm ony thinking ecu because youve replaced loom and coils which were presumably causing missfire.

He'll test it and if needs be repair it, i've used him twice and both times repair has been good (both times on different 124 320's).

I think testing is reasonable, but overhaul will be about £300, not cheap but his work is good, he repaired when i other said the ecu was scrap (and did i really want the 'scrap' ecu returning), and its still running our coupe with no probs.

Be careful with some on the net, they talk a good repair but beware.

 

[Ian B Walker]

Something that costs about £15 might help. Under the coil packs are rubber extension pieces, try changing those. I did on an errant 230k and it cured the problem. Maybe it might help you

 

[kth286]

You should get the codes read first thing anyway, as there might be a fault code that is obvious.

Where you located ?

AS I said in my previous response to your wiring loom question it did not sound like a loom problem at that time.

But at least you have a new loom and coils.

 

[howeshouse]

I thought you were probably right at the time, David, but it just felt like something I could do to break the impasse! I took the car to the local MB to read the codes: "needs a new head gasket". Just doesn't sound like something that mechanical to me: thanks for the suggestions to date - looking at the posts, the OVP relay seems to kill everything rather than just a few cylinders, similarly it seems odd that the temperature sensor would have that effect too, though at least that looks moderately cheap and easy to replace, unlike the crankshaft sensor which I could imagine might just give those symptoms but I don't think I am going to be able to get too with the limited skills and tools at my disposal. I like the sound of the rubber things under the coils - would that be the things that connect the HT to the plugs? I did take those off and clean them up when I swapped out the coils, but they are certainly an easy swap! And I can see that they might drop out one at a time, especially once the engine starts vibrating with the missfires. But would they all come back at once the way that the car does? Not sure, not sure... I am in North West Leeds. Had lots of enthusiam but rather mixed success fom both local independents and main dealers, but am very open to suggestions. Just seen from the sticky thread that there is someone in Skipton that has been recommended (but it is 26 miles uphill to there, so I might need to start on Sunday for a Monday drop off!)

 

[kth286]

I forget now what you have covered in total, but indeed the temperature sensor is a key feedback item in the fueling loop.

At cold the sensor does not come into play because the ECU operates on fixed pre programmed pararmeters.

When the engine is up to temp then the sensor is a key item.

It is cheap and easily replaced at the top of the engine. Only use proper Merc and give your VIN.

The crank sensor is highly unlikely in your case as it either works or does not; and the engine dies completely until it has cooled down a bit and the car can be started again, until next time the sensor goes out of spec.

The three coil extenders to the spark plugs are another item as mention previous post. They are just a coil of resistance wire inside the rubber tube and can make and break contact with the top of the spark plug sufficient to go open circuit. They get brittle with age and heat and wire inside loses it's flexibility to ensue it stays in contact with the top of the plug.

If you have access to a clamp on timing light you can clamp each of the three leads going to the plugs in turn and see if the light flashes. At least that will tell you if a coil is breaking down at operating temp. I would do that first. Timing light are always useful even if you have to buy one.

 

[Bolide]

I took the car to the local MB to read the codes: "needs a new head gasket"

The codes on the W124 are obviously more advanced than I thought...

Nick Froome
www.w124.co.uk

 

[howeshouse]

Yes, you get a lot for your £50 (+VAT) in Yorkshire... I'll order up a temperature sensor today and let you know what happens - thanks again for all your suggestions.

 

[howeshouse]

The temperature sensor was only £25 (inc VAT), a stock item with the local dealer and very easy to replace, but unfortunately hasn't solved the problem. Any suggestions as to what to look at next very gratefully received. Those plug connectors under the coils, perhaps? Or back to (a different...) dealer to try and get some sense out of the diagnostic codes? Thanks, Adrian

 

[ant500sl]

A brief description of the engine management may help.

The HFM system that you have has a misfire detection built in so that when a cylinder misfires there is no excess fuel able to damage the catalyst. It does this by shutting off the injectors to 1 or more cylinders. When the car is restarted the fault is cleared and you are back on 6 cylinders again. A fault code reading should point in the right direction, make sure the codes are all cleared first as you may have loads due to all the work you have been doing. Get the fault to occur and then get the codes read again. This will hopefully tell you where the problem lies.

It sounds like you are losing at least two cylinders, as it came on suddenly the first place I would look is at the ignition leads as they will be breaking down around now if they are original. It may be that when you swapped the coils around you actually remade a connection in the leads which is why it solved the problem initially.

As an aside I had exactly the problem on my C36 which shares the same ignition system. Upon examination the leads which were from a main dealer a couple of years ago had been incorrectly crimped and had corroded badly. Remade the cables up for about 20 quid and its now as good as new, although the crimping tool did cost £95!

These cars are so well built that its almost always the simple things that cause the problems.

Good luck.

 

[howeshouse]

Hardly dare to make this posting in case it upsets the delicate balance, but I am cautiously optimistic that the problem may now be sorted: the leads between the coils and the plugs made no difference, but the replacing the other three seems to have done the trick - many thanks for all of your good advice!

 

[Cosworth]

sometime the wiring to the coils can fail and cause a mis fire

 

[dieselman]

You can buy an HT 'flash tester' for about £3 which is great for finding any HT fault.

http://www.mikeogara.net/tools/engine-drivetrain/flash-test-vehicle-ht-ignition-tester.html

 

[Omega]

Hopefully fixed by now, but may be of benefit to others:
I had a similar (???) problem with a 230TE (1983 vintage)
After 6 call outs by the AA (to no avail) they sent out an old man of about 70 late one night.
He said that it was "more than likely the shroud covering the distributer cap" !!! I couldn't believe it,,,, (he added that he had known of this fault for a while [& was the same on a certain Citroen model].
After all the young dudes had failed to fix it, I was a little hesitant.
He asked if I wanted him to take the shroud off (as even if it wasn't the cause, wouldn't be a problem) It was riveted to the cap via the fixing holes,,,, but I said "O.K"
Ten minutes later, the car was as good as new - & never ever bothered me again.
Apparently, condensation can sit in the gap between the shroud & cap, causing shorting etc.
The AA wanted to start charging me after this (after 40 years of no call-outs) so we parted company.

 

[MotardMan]

This thread might give hope to all those E320 owners out there fearing a loom replacement. Well done everyone, excellent tech help, and a resolution (I whispered that bit...!).

Jim.