Merc 126 engine cutting out

[Iain B]

I have a 1992 W126 500 SEL that has covered over 222K miles. In the last 6 weeks or so, the engine has been badly misfiring to the point of cutting out. It is fairly random, but usually occurs after the car has been driven for a while - anywhere between 5 and 120 miles. If it cuts out completely, it restarts immediately. Sometimes, I can drive all the way from London to Manchester without it missing a beat; other times I have driven 150+ miles with it misfiring the whole way.

I had noticed one of the 2 fuel pumps getting a little noisy over the last year or so, but with no other symptoms. When it's misfiring, however, it seems to be accompanied by what I can only describe as a squeaky buzz from the rearmost fuel pump.

The symptoms are a bit like fuel pump relay failure, but I changed this to no effect. I also had the fuel filter replaced. My garage (Merc experts) have looked at everything from fuel pressure to ignition, and can find nothing wrong. They didn't seem to be particularly bothered by the noisy pump, and suggested the crank sensor might be at fault. They don't think it's the pumps, as the intermittent nature suggests an electrical problem. I'm not so sure, but I don't want to commit to the expense of replacing 2 pumps and associated corroded piping if it is a simple electrical glitch.

I'm at my wits' end on this one!

 

[grober]

If you are driving 150miles + with an engine misfire ITS TIME TO SPEND SOME MONEY before you damage the engine!

Why not change the crank sensor as suggested? If that doesn't cure it change the noisy fuel pump. If that doesn't cure it have a look at the EZL ignition unit- at the least remove it and put a fresh layer of heatsink compound between it and the wing.

 

[bdabdabeav]

we had a 126 280se in the workshop a while ago. customer complained of juddering, misfiring and cutting out at t junctions. checked car over and found oil pressure gauge not working. so ran a oil pressure test by bypassing the oil pressure switch and found even under load it didnt exceed 0.5barafter much diagnosis found big end bearings and journals have worn so much that required new engine! the noisy pumps are common and its just excessive use of a motor, as even though its lubricated by the fuel all pumps wear out eventually. dont know if this will be any help

 

[Iain B]

Thanks Graeme - my garage also agree that I should spend some money...£1000+ for the pumps and piping (so I'd like to be certain it's that). Never heard of the EZL ignition unit - I'll do some research.

Iain B

 

[Iain B]

Thanks for that. The oil pressure's good and the compressions are all 150psi. There's a little oil usage through the valve seals, but I think the engine is in fairly good shape. I may have to bite the bullet and spend the money on new fuel pumps, but I'm a little concerned that my garage is not convinced that's the problem.

Iain B

 

[219]

A grand for the pumps and piping seems an awful lot - unless this is also inclusive of labour .

I haven't had to buy a pump for my 500SEL , but I did replace the pumps on one of my W124's (300TE) . I priced them up at Merc and at Eurocarparts - I think Merc wanted something like £150ish apiece for them and ECP were more like £80 , still for a 'genuine' Bosch part . In the end I sourced two good used pumps from Boss Parts in Leeds for something like £30 for the two - the unions came undone OK so I didn't change pipes , just the filter at the same time . The problem with one of my pumps was that the electrical conection had corroded away due to exposure to road salts . I know some of the short pipes can be relatively expensive £20,£30,£40 apiece , but £1000 does seem like a lot of money for what isn't really that big a job .

I'd ask for a breakdown of costs , I think .

 

[Adam230K]

If the car isn't worth much you could just drive it til it dies

 

[talbir]

Plug wires /HT leads and distributor would be an ideal place to start. Maybe the ignition coil is breaking up ?

CPS if bad usually causes cut-out rather than misfiring.


talbir

 

[grober]

Cps if bad usually causes cut-out rather than misfiring.


Talbir

good point!

 

[Iain B]

Thanks for that - yes, I did ask for a breakdown, and it was all above board. Two pumps at about £190 each, plus plumbing that has corroded beyond re-use in my 18 year old car. They showed me the price of every part.

 

[Iain B]

They don't die - they just get old and cranky...

 

[Iain B]

Thanks Talbir - the ignition (including the coil) is all good. The CPS is a possibility, but I'm told they're rarely a problem on these cars. In desperation, I took the car in while it was misfiring. They cleaned one plug (no. 7) which was fouled with oil from worn valve stem seals, and it's run perfectly ever since (well, one week...), so I may get the seals done - they said that a misfiring plug could "confuse the EMU". Does that make sense...?