600SEC W140 acceleration / heavy load misfire

[growlygrowler]

Folks

I've got a significant misfire on acceleration and the car (a '93 600SEC) doesn't want to go anywhere near the red line. If I ease off the throttle a little, then the engine seems happier. I'm also getting less miles per gallon than I should.

I don't have an exhaust gas analyser though I do have a Chinese copy of Star Diagnostics. It reads both LH systems ok and shows no errors. Live data comes though OK. I do, however, get an 005 error on EZ2 pointing at knock sensors 1 or 2 A29g1 or A29g2 - left hand cylinder bank. Knock monitoring/ adjustment comes up as switched OFF on EZ2 though it's all OK and switched ON on EZ1.

I looked at a few plugs and they're fine if a little widely-gapped. No soot, no wear, no breaking, no damaged electrodes. Leads look very new, I removed the left-hand dizzy cap and it all looks clean over there. No damage to king leads from HT coils. Air filters are a bit mucky but serviceable. Engine loom looks like it's been replaced.

My wife just bought the car (seriously, seriously nice motor) off ebay. It's an ex diplomatic car from London which has a £16000 engine replacement bill from 10 years ago (sh1t, I hope I never get a bill from MB like that) though seems to have done only a couple of hundred miles a year ever since. I don't know if it's been serviced for years, so will do that as a priority.

Any ideas? Anything "normal" for these, or anything obvious I'm missing?

cheers

Dave

 

[jaymanek]

mas air flow sensors?

 

[grober]

Could be as Jay says but I suspect that the EZL ignition unit = EZ2 on the cylinder bank giving problems may have gone faulty. These units are heatsinked on to the wing---over time the heat transfer compound dries out or the the wing surface corrodes allowing the unit to overheat. Its always a good idea on these models to remove the unit and give it and the wing mounting surface a clean up and renew the heatsink compound. As a test at the same time I would swap over the units to feed their opposite distributor. If your fault code read outs swap banks then this would be a good indication of the location of the fault. This refers to a V8 but principles will be the same.K6JRF Auto Page

ps you could try the same swap approach to test the MAS SENSORS

 

[growlygrowler]

ok, thanks for the ideas. I'll give it a go

 

[neilrr]

The part # for the W140 V12 EZL is 013 545 70 32.

I have two available if you do find you need one or both.

 

[jaymanek]

grober where do you get the compound... EZL's are a common failure on the W126's and as I have two it would be a good investment..

Although the nuts are normally seized on resulting in breaking the thread welded to the car...

 

[growlygrowler]

grober where do you get the compound... EZL's are a common failure on the W126's and as I have two it would be a good investment..

Although the nuts are normally seized on resulting in breaking the thread welded to the car...

You can buy the heatsink compound from most electronics distributors. Try farnell.com

 

[growlygrowler]

The part # for the W140 V12 EZL is 013 545 70 32.

I have two available if you do find you need one or both.

OK, thanks. Due to business trips it'll be a couple of weeks before I start tinkering, but will let you know if I need one
cheers
Dave

 

[grober]

Most of the online electric component suppliers will stock suitable heat sink compound. Maplins Maplin Electronics ? Website- 2TB Seagate External Drive RS components Welcome to rswww.com or CPC FARNELL CPC | CPC - Over 100, 000 products from one of the worlds leading distributors of electronic and related products. There are different types-- ceramic,metal-silver/alumium, or carbon based. I like the silver based stuff as it has better thermal transfer properties but its more expensive. Still its cheap compared a new EZL unit if you can get one! You can even pick it up in your nearest PC world.
here's an example. Akasa AK-450-5G silver based thermal compound: Amazon.co.uk: Electronics & Photo

 

[BTB 500]

It's an ex diplomatic car from London which has a £16000 engine replacement bill from 10 years ago

It would only have been 7 years old at the time - wonder what happened?

 

[BlackC55]

What is the value of both air mass sensors at idle?

 

[bh13coupe]

You can buy the heatsink compound from most electronics distributors. Try farnell.com

or try Maplins if u want a walk in shop (not the Hi De Hi TV show)

 

[growlygrowler]

It would only have been 7 years old at the time - wonder what happened?

No idea. All I have is a copy of that bill. The book is all stamped by Mercedes at Woking, but apparently according to the Doris on the phone they "have a new system and can't get at the records on the old system", which sounds like "can't be ****d" to me. I don't suppose I'll ever know

 

[growlygrowler]

What is the value of both air mass sensors at idle?

Can't remember off hand, but I'll check and get back to you in a couple of weeks. I have to go away to work (to make money to spend on all these toys!!!) in Germany but will have a few days at home at the end of the month, so will have a play then. They were registering values if that's a clue.

Would a MAF sensor likely only play ball at low revs / speed / load? I could be utterly wrong, but this still feels like an ignition issue to me, especially given the woeful mpg it's returning. It smells rich even standing. It occasionally coughs at idle and just doesn't seem to have the terrifying turn of speed that a V12 shold have. I did 270 miles on a tank before filling it up with £100-worth tonight! Ouch. Severe pain in the Amex card. That's about 13-14 mpg. It think that's what we'd describe as a drink problem. My 1970 V8 Range Rover with Zenith Stromberg carbs did better than that.

I need to get an exhaust gas analyser and see how much pure hydrocarbon is coming out the tailpipe.

I don't have access to a rolling road, so would have trouble looking at what's going on under load ... unless I can persuade my mother-in-law to do her Top Gear Stunt Man impressions, borrow Chubby Brown's flying helmet and sit on the front bumper with the diagnostic multiplexer and laptop whilst I ton it down the A1. Now there's a thought.....

 

[BTB 500]

The book is all stamped by Mercedes at Woking

Woking Motors M-B were actually in Walton-on-Thames (near Hersham)!

 

[growlygrowler]

Woking Motors M-B were actually in Walton-on-Thames (near Hersham)!

Yes, that's the one. it's all "darn sarf" or "London" to me....

 

[growlygrowler]

Sorted!!!!

Yes, that's the one. it's all "darn sarf" or "London" to me....

Well at LAST I made the time to get this looked into. The problem was that the nearside catalyst had collapsed into a melted lump of goo. Cutting the box open, removing the goo and welding it back up again sorted the issue. Now it goes like sh1t of a shiny shovel and wow does it sound the part. <big evil grin>

Thanks for the help,guys

 

[grober]

Ah well so much for the hi-tech theorising. Wonder if it was an MB original cat or a "cheap and cheerful" replacement? Collapsed catalytic converters seem to becoming more common--or maybe the cars are getting older. Glad you got it sorted.