Very Rough Idle - Pls Help !

[GarethM]

On my way to work this morning my 1998 W208 2.3K Auto started to idle very roughly whilst queuing in traffic. The revs dropped to just over 500 rpm and the engine behaved as if it was about to stall ( lots of car shake ). Switching to neutral seemed to alleviate the problem by allowing the revs to climb to about 800 rpm.
However as my jouney continued the normal running performace deteriorated and it felt like the car was missing, running on 3 cyclinders or something.

Initial thoughts were MAF ?
Followed by a foul plug/lead ?

Has any one got any ideas what could be causeing the problem. Merc want £130 an hour around here - so thats currently a last resort only.

Thanks in advance.

Kind Regards

Gareth

 

[Benzowner]

I would go for the easiest route first and check the plugs.

 

[GarethM]

Are the plugs under the red engine cover ? Or is the injection "bar" mounted over the top of them ? ( not had to work on this car much yet so sorry for the dumb question )

 

[GarethM]

Ok ive just taken it for a quick run and all "appears" fine now. In my eyes that probably lowers the likley hood of it being plugs.......

 

[Brian WH]

Ok ive just taken it for a quick run and all "appears" fine now. In my eyes that probably lowers the likley hood of it being plugs.......

In this weather it was more likely damp on the plug leads, leading to misfire. Spray with antidamp or WD40 and this should help.

 

[Parrot of Doom]

Have you recently filled up? Maybe you had some dodgy fuel, it happened to me last year at a services. I just kept topping up as I went along.

 

[Glenn Smith]

I would go for a faulty coil or lead, these often breakdown when they get warm.

 

[Will]

When were the plugs last changed? They're cheap to replace and only take about 10 minutes to fit - just under the red cover as you suggest

MAF sensor is around £300 at the dealers, one from an independant Bosch dealer should be around £100 - cheaper than paying £130+ just for the labour if that's the likely cause.

Me - I'd change plugs first, check leads and then see how it goes. If MAF is suspect, at ~ £100 I'd be tempted to replace it anyway and see how it goes.

Will

 

[GarethM]

Thanks for all your replys - VERY much appreciated.

Ive had a quick look at the plugs and they seem fine. Last full service was about 22K ago according to the log book ( 2 minors since then ). Ive got 1/4 of a tank of fuel left so unlikley to be fuel. Heres the weird thing....

Drive home from work ... all fine ( thinking phew ).. then pull into my road and it starting doing it again, struggling to tick over. So i stop, pop the bonnet, check out the coils ( visually ) then go for another quick drive. All fine all the way up to red line ( to put max pressure on coils and leads ), then again it starts, applying pressure to the accelerator makes it feel like its missing, and struggling to idle. Ignition off - then restart straight away and all is fine !!

Missfireing says spark related to me and not MAS/MAF. However would the coil pack ( looks like a transformer to me ! ) "reset" with and ignition off ?

The fact it resets with ign off/on makes me think it must be a sensor ?

Any more thoughts ?

( Oh still cant find the MAF/MAS !! Any directions appreciated ) also can you simply measure the resistance of the coils to check them ?

Thanks again ...

G

 

[Will]

The plugs are something like 2 or 3 pounds each, I'd be tempted to just change them TBH - even if they look ok, there's only four of them (not like some unlucky souls on here with 16 awkward ones on a modern V8 )

Follow the tubing from the inlet back to the intercooler, MAF sensor should be on there with a lead attached to it. If it was consistantly playing up you could disconnect it temporarily to aid diagnosis

HTH,

Will

 

[Wil]

I would highly suspect the leads or coil pack, the reason I'm saying that 'cos I had exactly the same problem when I bought my car, rough idle as if it was missing a cylinder but when driving under load was fine. It threw up a ECU light now and then so I used my own OBD2 diagnostic lead/software to read the f/c and it was showing miss-fire on cylinder 6 detected. I then went for the cheapest option by swapping the plug around from a good cylinder, fault did not move, so I then swapped the lead around and the fault moved hence replaced that lead and now happy as larry

 

[GarethM]

Ok - Car behaved itself on the morning commute. But as it could be any or all three of, Coil(s), leads or plugs, i rang MB to book in for a diagnostics. I was then told that if the engine management light wasnt on then they would probably not be able to tell me what the fault was ! ( at least they told me i guess ).

Will - Cheers for the MAS directions ( totally the other side of the engine to where i expected it to be ! ( ie near the air box ect ! )

Wil - How much / What OBD2 box did you use ?

Ill go with what you guys say and if the problem repeats itself ( which i believe it will ), change the plugs, then leads, then coils ...... I notice on europarts a lead set is only £17 !! however the site also lists separatley a variety of plug caps ! i would have thought they were included in a lead set ? ( GSF want £45 for a lead set ) - will ring MB for a price at lunch.

Cheers for all the help...

G

 

[faraway_flossie]

We had exactly the same problem with our V280, and had to replace the coil pack and leads - coil pack was £175 and leads £170. Luckily labour only came to £140 for 3.5 hours from local garage (not MB specialists!).

 

[Glenn Smith]

Ok - Car behaved itself on the morning commute. But as it could be any or all three of, Coil(s), leads or plugs, i rang MB to book in for a diagnostics. I was then told that if the engine management light wasnt on then they would probably not be able to tell me what the fault was ! ( at least they told me i guess ).

G

So, just out of interest, how would they go about curing the fault, where are all the highly trained technicians that MB say they have got? If i went to a dealer and they responded like that i'd be staight on thge phone to DCUK.

 

[Wil]

Wil - How much / What OBD2 box did you use ?G

I bought the PC based Auto Enginuity package
http://www.gendan.co.uk/product_AUTOPCU.html

At £199 it is not cheap but very affordable, especially at what dealers charge for a quick plug-in diagnostic sessions. OK you won't get full functionality (ie. airbags, ABS etc) but you do get all the standard OBD2 functionality diagnostics such as f/c reading/reset, live sensor monitoring etc. So far it has got me out of trouble for both my cars. The only snag is that your car needs to be OBD2 compliant (Most vehicles from 2001 onwards are) and you have a diagnostic socket under your dash area which looks like a scart socket on your tv. If you have a 38pin Merc socket you have no chance and will require a dedicated Merc diagnostic package.

 

[GazCaff]

Do the petrol models have an idle speed control valve? I'd be inclined to investigate that.

 

[Harley Man]

Gareth, where in surrey are you? If you are near Cranleigh give me a ring as I have a Snap On scanner with Merc specific software and can read the codes for you.

David
07719 454814

 

[Harley Man]

put the wrong phone number on the last post, it should be 07719 454814!

Dave (dopey!)

 

[GarethM]

Thanks Dave - Very generous offer.

Ive managed to tame the problem, only occurs over 3K rpm/ heavy acceleration, and then stays until i turn the ingnition off. As there is no engine management light on im lead to believe there will be no codes to read ? This still fits with leads from past experience and 8 yrs old, 91K then the life of the old ones seems about right too ......

Ive bitten the bullet and ordered, Plugs, Leads ( with caps ect ), and 2 new Coils. Im picking them up tommorow - £220 inc vat all from Merc - so if it fixes it then its not toooo bad i guess.

If it doesnt fix the problem then i would be very greatful if you could scan for any codes ( I always thought it stored a number of codes - ie Parking sensors ect, without the the engine light coming on ! )

Im over near Camberley BTW ( Just the other side of Guildford to yourself. )

Ill report back if it fixes it.....

Cheers

G

 

[Wil]

Hope you get it resolved.
I'm just around the corner from you in Sandhurst