W202 ?? C280 Straight Six Misfire

[w124coupe]

Some people have stripped and repaired the loom but you will need a fair amount of time on your hands to do it properly (if it needs it).

The issue is that the shorts cause damage to the ECU (in particular the output stages driving the coils) if not fixed.

You may well be fine but for this age M104 its a sensible first check.

Hopefully the previous owner was scared by a loony quote from a Merc main dealer and it will turn out to be plugs/boots/leads or something nice and easy.

[w124coupe]

Originally Posted by Phantom Mark

I have the plastic top cover off but I popped the air pipe back on as I noticed two round holes which appeared to need sealing, didn't wanna risk oil or debris getting in or out

Gonna run the car in a min and see if I can see any shorts/sparking going on in the dark, will take a look at the loom tomorrow if nothing obvious shows tonight.

Is there a cheap way to fix the loom ?? starting to wonder if I made a mistake with this car, which is a real shame as on the surface it appears to be a really nice motor for the money I paid for it.

Check back in later with results, thanks again guys.

The two holes are part of the crankcase breathing system and sometimes are a source of air leaks (unmetered air after the MAF).

[Phantom Mark]

Ok, last update for the day after battling with the MB Security system lol.......

As I said earlier, think I have found a consistant link now.

Car runs smooth as silk from cold for a couple of mins, then drops into the missing problem, so as I see it, could be a plug or lead breaking down or maybe something to with the fuel injection system ?? Based on how I know a carb car works it's like when the choke is on too long etc, may explain the back farting through the airbox if it's choked up ?? Could this be related to my suspected thermostat problem ?? Ie: Fuel injections version of "choke" not turning off and fouling a plug etc ?? or of course it could simply be a plug fouling up as it gets warm because its knackered, however I do know for a fact according to service history the plugs were changed very very recently, in the last 10k in fact ????

Sorry for long winded stuff, I know as much detail as possible is valuable normally right ??

Cheers

[Phantom Mark]

Ps.

The fact that the car will run smooth on all cyls is a positive sign no ?? Even if it is only for a couple of mins from cold ??

[grober]

Make sure you have the correct heat range plugs fitted. Even the plug manufacturer lists can be reproduced wrongly! The recommended plugs were listed on the front slam panel originally but the label might be gone. These engines don't like the multi electrode design but prefer the simple single electrode type. I prefer Bosch or NGK=BCP6E/BCP6ES You may have an inlet manifold/associated pipework air leak = the car runs fine on a rich mixture but as it leans off with increased engine temp the misfire starts.

[Phantom Mark]

You may have an inlet manifold/associated pipework air leak = the car runs fine on a rich mixture but as it leans off with increased engine temp the misfire starts.

Sounds like a good candidate, reckon I will change the plugs so I know they are done, according to history the current plugs have done 10k over the last 2 years.

Any common spots on the inlet side where splits / leaks occur ?? Would some carb cleaner sprayed around that area be suitable for finding any leaks ??

[w124coupe]

Originally Posted by Phantom Mark

Sounds like a good candidate, reckon I will change the plugs so I know they are done, according to history the current plugs have done 10k over the last 2 years.

Any common spots on the inlet side where splits / leaks occur ?? Would some carb cleaner sprayed around that area be suitable for finding any leaks ??

Oh yes....(and carb cleaner is ideal for checking as it will add fuel through any intake leak and cause a momentory increase in revs before the ECU balances the idle again).

I would change the plugs (and the under-coil boots) anyway - the correct ones are very cheap and may well be fouled by the cool running you also mentioned....

On the cross pipe, check the MAF connection, the two crankcase vent rubbers and the seal where the inlet air temp sensor is inserted (clean up the sensor bulb while you are there - they pick up oil residue and under-read).

Also the rubber where the pipe attaches to the throttle body (while you are there, clean up the throttle butterfly if at all dirty as this often causes unstable idling).

Under the inlet manifold are two rubber pipes that are also part of the crankcase vent system - check these are intact and tightly fitted.

Then move onto the vacuum connections and check them all for air tightness - there are many!

A biggie is the vent valve that allows the engine to burn fumes from the petrol tank - connected to the manifold by a pipe it has "MOT" stamped on it (MOT indicates the side that should be connected to the MOTor). This can stick open and let in lots of unmetered air. Pull the motor-side pipe off and block it and see if the car runs better. Often can be freed if needed by a squirt of WD40 (don't use anything with silicone in it as it will destroy the O2 sensor when burned). This valve pulses about 2 mins after a cold start and can be felt throbbing if held.

This valve is a source of problems because of neglect "upstream". Under the wheelarch is a charcoal canister that's supposed to be changed at 70,000 miles. Often they are ignored and eventually turn to a rusty mess allowing the valve to ingest carbon and rust and so become blocked open and let in air....

Also, re your temp problem (which will also contribute to poor running):

This could be thermostat or could be wiring loom or sensor. You should have a 4-pin sensor on the thermostat housing which the ECU (and A/C) uses plus a 2 or 1 pin one that drives the gauge in the cluster. If you decide to change the thermostat, check the wiring to the 4 pin sensor (again inside the outer insulation) as this is another place for "loom rot"!

If it has been running cool, expect fouled plugs to be an issue!

[Phantom Mark]

Thanks for all that mate, plenty to be getting on with in that list for sure

I think all the above is just about within my DIY knowledge !!!

[w124coupe]

All quite straightforward and better than paying someone else :-)

I would add that its worth checking to see the MAF is clean while you are at it - see it as a "tune up"?

I'm still a bit worried about the "backfiring through the airbox" part - that IS unusual for an injected electronic ignition Merc - hopefully you don't have any burned valves!

Did that happen on acceleration or overrun?

[Stratman]

"Backfiring through the airbox"

Sounds like a sticking inlet valve.

[bennesspipers]

If it is a sticking inlet vave,put a bottle of additive in your tank,fuel system cleaner, readily available from Halfords or similar places.

[Phantom Mark]

Only under acceleration from slower speeds, bangs a lot when sub 30mph, whilst it is still missing it doesn't bang and fart the same once cruising at 50mph and flooring it for example, I did try for a brief moment to try and encourage it to "clear" itself out because I know it has been stood around for a couple of months before I bought it.

When you say sticking inlet valve that worries me, taking the head off and regrinding valves and stuff is well beyond my ability, not hard i'm sure but beyond anything I ever did before !!

Will get down frauds tomorrow and pick up some plugs, "redex" and maybe some wynns additive (same stuff I used to loosen hydraulic lifter on my Beema ??)

Will definately give everything you mentioned a clean once I start stripping it down, thanks a million again guys.

[w124coupe]

Wide open throttle at low revs means any unmetered intake air leak would form a large proportion of the air supply and so lead to a weak mixture even with the ECU at maximum trim.

Higher up the range, the leak would form a smaller proportion of total air and so the mixture would be nearer correct...

[Phantom Mark]

I see a busy day ahead tomorrow

[w124coupe]

Originally Posted by w124coupe

Wide open throttle at low revs means any unmetered intake air leak would form a large proportion of the air supply and so lead to a weak mixture even with the ECU at maximum trim.

Higher up the range, the leak would form a smaller proportion of total air and so the mixture would be nearer correct...

But also, wide open throttle at low revs is the most hostile to a spark so would also highlight a plug/boot/coil/lead issue!