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This is a complicated one and i don't know who to turn to. I drive a low mileage (approx 50k) 1991 Mercedes 300 ce 24. Anyway, a few months ago the head gasket failed and i took it to a garage for repair. The car was repaired given the all clear and then a few days later it started overheating again, i took it back to the garage and they said it was normal for it to overheat (over 100c). I carried on driving and it went to 110C and i knew something was wrong so took it back and they told me that the viscous fan was too weak and i paid for a new one to be fitted. A few days later it started overheating again and so i tried burping the engine myself after reading something on google, still overheating i managed to keep it cool by blasting my fan heaters in the hope that this would keep me going until i got it back to the garage.

 

A few days later, the car started to shudder as it hit around 60/70 with cylinder misfires and then soon after i realised that smoke was now coming from the back seats. The front controls soon went (rev counter, temp guage, oil guage etc). AA towed the car to the garage and after the garage looked at the car they said the electrical wiring needed changing which i got them to do and then they said i needed a new catalytic convertor which after they started work has ended up being the whole exhaust system. The car is still misfiring and shuddering. The garage then said that they they were gonna get the car tested and have come back to me saying that the ECU sensor readings are wrong and require further analysis. I'm at the end of my tether!! I drove this car for 18000miles without a single probem! The amount of money i've spent so far could have bought me another car. Was it the garages fault that the electrical wiring and exhaust system needed replacing? Would oveheating have caused this? Your independant advise would be appreciated...

 

1) Went in for a head gaskget

2) bought a viscous fan

3) electrical wiring had to replaced

4) catatylic convertor replaced

5) mid and rear box replaced (not told about this until they had changed the cat

 

Your advice and comments would be appreciated. This has been ongoing since May!!

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Viscous fans usually seize so overcool, so they were wrong there. Cat won't affect the running but the lambda sensor fitted near it will, so they were wrong there.

 

You say 'smoke from the rear seat', so where exactly, I can't see this has anything to do with underbonnet problems, so might be a coincidence.

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thanks for your reply.

 

could you elaborate on:

Viscous fans usually seize so overcool, so they were wrong there. Cat won't affect the running but the lambda sensor fitted near it will, so they were wrong there.

 

smoke was really coming from under the car - i belive the exhuast system was burning through the car seat and burnt the wires? When showing me, the garage pulled up the back seat behind passenger and showed me a black box with a load of wires that the electrician changed which sorted out the dials on the front.

 

do you think the exhaust system was changed and not really needed to be changed? how can I prove this?

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A viscous fan works by utilising a viscous clutch ie, there is silicon fluid between plates and, as opposed to engine oil, the hotter it gets, the thicker the fluid gets so the plate driven by the engine starts to drag the plate attached to the fan faster and faster. It is only designed to reach a certain speed so if you engine revs reach say 5.000 the fan will not go above 2,000 as further speed is not required to enhance the cooling.

When these fans wear, the clutch plates tends to bind together until there is no slip whatsoever and the fan will reach the same speed as the engine revs hence theoretically 'overcool' by sucking more air.

A seized viscous coupled fan at these sort of revs is dangerous and will shatter as they are only made of a polymer plastic and not designed for high revs. If it shatters, it can take out the radiator, oil cooler, damage the undertray and battery and damage the bonnet so basically recking the front of the engine bay so should never be run while seized.

 

They give increased miles per gallon and reduced noise. When the engine is started from cold, they usually spin at very low revs.

 

Exhaust burning through the car seat. Very very unlikely as the heat transfer from the exhaust to the car interior is very limited and if there should be a point where the exhaust is close to the body, a heat shield will be fitted on the top of the exhaust.

 

Was the cat changed unecessarily, probably yes. I would never change a cat unless it had rusted through, not even if the emission failed at an MoT, there are other things that work in conjunction with a cat and these should be checked first before forking out for the cost of a new cat.

Edited by Conniff
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Viscous fans usually seize so overcool, so they were wrong there.

QUOTE]

 

Sorry Conniff, have to disagree there. The explanation further on is correct however as you correctly point out but bilt in is a failsafe so this shouldn't happen. If it does happen that the clutch so to speak binds or seizes this is very apparent as sounds like a jet engine and the consequential drag pulls down fuel consumption like no tomorrow so to speak. If it had failed in this mode it would have been blatantly apparent to not only the owner but the garage itself.

 

I think out of the countless cases of viscous couplings I have changed and come across I can only recall two that have locked and one of those had a self tapper through it!!!

 

The cat will affect running especially if it has collapsed but will not cause an overheating problem. As you know and point out, the lambda sensor is purely a measuring device so is nebulous to actual mechanical issues.

 

This is a very odd one indeed.

 

Perhaps the op could give a few more details as I'm not aware that mercs put engine and transmission control units under the rear seat. Body control etc yes.

 

Smoke could be new parts of exhaust paint burnig off.

 

All seems a bit odd to me at the moment me thinks.

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I'm afraid I does happen helios, I had a Rover SD1 V8 in with a huge bulge in the bonnet, no underbelly, battery and rad holed and not much fan left, the coupling was seized solid.

 

I think the smoking started before the exhaust was changed.

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True Connif it does happen and the other instance I came across just happened to be an XJ12 where it locked and went through the radiator. Perfect hole though!! Interestingly it must have been of the same era as your SD1.

 

The smoke problem does mystify me ( which is expected with smoke I suppose) as even if appeared before the exhaust system had changed does not tie up with normal (thermal incidents).

 

Too vauge at the moment perhaps?

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with respect to the viscous fan. one the mechanics put his finger in the fan which stopped the blades from turning. The car engine was running so i guess that was around 1000rpm?? I was told that if the viscous fan was working properly you wouldn't be able to put your finger in and so therefore this weakness was causing the overheating.

 

As for the smoke. The smoke from the backseat was so bad that i couldn't drive and had to be towed. When i pulled over, smoke was also coming from both sides of the of the car under the seat area The car was misfiring and shuddering when trying to maintain a constant speed. One thing, with the shuddering, when trying to demonstrate this to the garage i found that turning the engine off and on again, it took a bit of time to make it shudder - i don't think it needed cooling down but literally within a minute or two off and on i would have to wait for the misfire noise.

 

When the shuddering noise was pointed out to the garage, the back seats were taken off and we found that black box with the leads had become too hot. the car was lifted up so we could walk underneath it, the garage said that the exhaust system was too hot, to the point that this could not be touched. In hindsight i'm not convinced that it needed changing but not sure where to go from here....

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so basically the next problem that has been identified is the sensors and the fact that the sensor readings are wrong. In particular:

 

I was told the following:

 

ecm/icm further test needed

 

30 catalyst tem sensor wiring probably

 

34 engine coolant temperature sensor

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Thanks for that suf as it confirms my statement on the coupling. If the mechanic was brave enough to stick his hand in there then the coupling is shot and is perhaps might have been the cause of the original overheating. If the fan was locked or working normally and he put his hand in there then he would be pretty "armless by now"

 

It's not unusual to have a coupling failure in conjunction with a head gasket and the garage should have isolated the fan fault first though this is not easy to do at idle depending on speed. I've seen it time and time again. Generally one would go with this type of fan first.

 

The problem I'm trying to get my head around is the smoke as I can't see how it is related. Can you give an indication of the smell? There are very disticnt differences in them.

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as for the smell. it was more like a bonfire smell, - burning smell.

 

btw, when the car was lifted up and we were looking at the exhaust i'm sure the garage said that he would expect to see a heatshield protecting the exhaust from the under carriage and it looked as though a layer of the under carriage had burned away when compared to another part of the under carriage

 

also, do you recommend any websites i can look up parts like viscous fan. i had a quick read of wiki but it was very brief

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As regards looking up the coupling, wiki conniff has explained it well in the operation only emitting that they are designed to fail as yours has done. But as we know there are times when it goes the other way and will lock up but believe me you'd know it. In fact, when locked the noise is incredible!

 

Heat shields from a car of that era would be expected over the silencers/expansion chambers but not necessarily over the pipes.

 

The smell from electrical items burning is quite disticntive as is that of clutches/brakes etc. It's as such that you can tell it on a motorway with lorries etc. As also is rubber.

 

The fact that it smelt like a bonfire does start to point me towards a gearbox failure and you vibration faults along with overheating.

 

When you have been able to enjoy the car so to speak have you noticed any change in the way it shifts gear?

 

The symptoms are starting to tie up now and do link in with overheating of the engine as well believe it or not. Can you confirm if the wiring to the units under the seat showed signs of wiring melting etc and if they do is the melting consistent all around the wire or just underneath?

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before the head gasket failure, the car was extremely smooth. the fact that the shuddering is only when trying to maintain a constant speed as opposed to accelaration /decelaration, how can we determine the gearbox? also the shuddering was experienced even when left in park and i tried to maintain a constant rpm say 2000

 

with respect to the wiring, i'll have to take another look at the car which is still at the garage.

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the car passed it's mot in june, surely if they haven't supplied me with another emissions test and the exhaust or the cat bvisouly hadn't rusted and now we know that it hasn't fixed the shuddering or misfiring then it didn't need replacing? What would you suggest i do to confirm this?

 

A viscous fan works by utilising a viscous clutch ie, there is silicon fluid between plates and, as opposed to engine oil, the hotter it gets, the thicker the fluid gets so the plate driven by the engine starts to drag the plate attached to the fan faster and faster. It is only designed to reach a certain speed so if you engine revs reach say 5.000 the fan will not go above 2,000 as further speed is not required to enhance the cooling.

When these fans wear, the clutch plates tends to bind together until there is no slip whatsoever and the fan will reach the same speed as the engine revs hence theoretically 'overcool' by sucking more air.

A seized viscous coupled fan at these sort of revs is dangerous and will shatter as they are only made of a polymer plastic and not designed for high revs. If it shatters, it can take out the radiator, oil cooler, damage the undertray and battery and damage the bonnet so basically recking the front of the engine bay so should never be run while seized.

 

They give increased miles per gallon and reduced noise. When the engine is started from cold, they usually spin at very low revs.

 

Exhaust burning through the car seat. Very very unlikely as the heat transfer from the exhaust to the car interior is very limited and if there should be a point where the exhaust is close to the body, a heat shield will be fitted on the top of the exhaust.

 

Was the cat changed unecessarily, probably yes. I would never change a cat unless it had rusted through, not even if the emission failed at an MoT, there are other things that work in conjunction with a cat and these should be checked first before forking out for the cost of a new cat.

Edited by suf
typo
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