W202 ?? C280 Straight Six Misfire

[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]

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]

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!

 

[Phantom Mark]

My fingers and toes are crossed for the plugs sollution !!!

When you mention the "boot" to which do you refer ? the big plastic/rubber grommet bit which plugs into the top of the spark plug ?

 

[momomo]

My fingers and toes are crossed for the plugs sollution !!!

When you mention the "boot" to which do you refer ? the big plastic/rubber grommet bit which plugs into the top of the spark plug ?

Yes those are the ones, they are referred to as boots.

 

[Phantom Mark]

Ace thanks mate, I think I will pop into GSF today and see if I can get a lead set with "boots" as well as the plugs then, no point being half prepared

I noticed in the service history that they have failed to fill in the coolant flush tickets, is this gonna be an easy enough job to do at home ??? I know on many cars this car be a royal PITA because of air locks etc ??

 

[w124coupe]

I would get the boots from Merc - only a few quid and only 3 of them - they are critical on this engine and I'm not sure pattern parts would do the job.

The lead set is expensive and may be OK.

Try plugs and boots first?


Re the coolant, its easy enough to change (leave the heater on full). Make sure you get the right spec antifreeze (or buy from Merc) as the car has a mix of metals and plastics and needs the right (50/50) mix.

Do the thermostat at the same time as changing it loses quite a lot of coolant (or needs it drained first).

There are two drains, one on the bottom of the radiator and one in the engine block on the drivers side low down. You have to drain both if you want to change all the coolant....

 

[Phantom Mark]

Well here is an interesting update.

Bought some plugs today, could not get Bosch so I got NGK BCP6E

All the plugs in general looked quite rich IMHO with a small amount of depositing.

Plugs 2, 4 and six, the "boots" were wet, quite possibly some water ingress from the damp, however all of those plugs were soaked in fuel as well, where 1,3,5 were all dry, the car had not been run at all today before starting work on it.

I cleaned up the boots with a rag before refitting the new plugs, and sprayed a healthy dose of carb/injection cleaner into the throttle body, the flap itself looked pretty clean I must say, I also emptied a full 250ml treatment of STP injection cleaner into the fuel tank (which is roughly quarter full atm)

Refitting and check securty of the cross pipe and maf etc and struck her up, smoothly idled for 5 mins no problem, washed my hands and took her for a spin, no apparant missing for the first few corners, then when I head up the hill out of where I live I got a few backfarts through the air box as before, it simply does not like heavy load at low speeds it seems.

I headed up the local bypass at a steady 60mph and smooth as silk, at the roundabout at the end to come back again it backfarted a little again, again heavy loads slow speed..........on the way back down the bypass I gave it some, used the kickdown and she seems to pull ok all the way to near max RPM, all the way up to a ton briefly.

Not sure how quick this should be, hard to compare when you look at my regular drive !!! felt pretty ok tho compared to my old 2.5 24v 525i (similar power and torque)

So yes, the situation is improved for sure, but not solved........did not replace the leads and boots today as I was not able to get any.

Feeling pretty confident it is nothing serious anyways, because the car is at least showing it is willing to run properly.

Tomorrow I will aim to get boots and leads, however the fact that the car seems to pull well under full load once the revs are up lead me to believe at least it is not those parts, or even the coils ??? I would expect those to break down the more load I put through the engine ?? Seems we have that but only in a low gear at sub 3000rpm sub 30mph ??? A little contradictory ?

Cheers