OK I just read through basically this whole thread, lots of info, but not specific areas where to look when buying. I have seen some M104 and M119 based cars up close now and want to add some to do with oil leak details, what to test and look for regarding the transmission, some cooling system checks and a few other things.
At the age these cars have reached if a car has not had fastidious maintenance there is likely to be oil leaking from more than one place. Under the car is likely to be covered in oily grime making oil leaks very hard to locate. IMHO in most cases sellers want to get rid of these cars due to one or more problems that need fixing, so you better be good at finding those problems!
So here goes, some pointers for inspecting the condition of a M104 or M119 based W140:
Both engines: check coolant when the car is cold, plan ahead requesting the car not be started before you arrive, hopefully the coolant is clean with no scum or anything under the filler cap or in the expansion tank. The color should be that of MB specified coolant not green glycol crap or old coolant faded to opaque dirty water.
While the car is still cold remove the aux belt and feel for looseness, rotate by hand and listen for noises on all the pulleys (PS pump, alternator, water pump, tensioners, everything) and tensioners. At this age there is bound to be a loose one or two or the water pump that feels rough when turning, even if it doesn't leak, bad bearing etc etc.
Test the AC and heater at full blast. AC should freeze your balls. Heater should feel like hell after not very long if the engine is at operating temp. Test operation of both sides at different temps to check duo valve operation. Leave the AC on full for a while to heat everything up. The engine fan has a viscous drive and this should engage before the AUX electric fans do, if not the viscous drive needs replacing. Make sure the AUX fans engage if the engine temp climbs above 110 or so. The temp gauge can move about from a faulty gauge in the cluster showing excessive engine temp.
After you have driven the car with the engine at operating temp check for leaks with engine off as the coolant system will be under pressure.
S280 & S320 M104 engine oil leaks to look for:
Engine cam cover gasket at the rear, oil can seep down then wick around to the exhaust side due to the angle of the engine. This might look like a head gasket leak but it is not.
Rear main seal is a common leak with a completely revised rear main seal available from Mercedes to fix. Front main seal does not have this issue but is much easier to check and fix if required.
Front "U" seal between upper and lower section of the cam chain cover, if engine has been ran with excessive oil the engine is more prone to leaks. Mercedes revised down the recommended oil level.
Right hand drive cars have an oil cooler located under the oil filter. The perimeter seal can perish and oil then leaks down the side of the engine above the starter motor.
(Side note: Interestingly the M104 series engines use a cast iron block and have piston squirters unlike the M119 and M120 that use an aluminum block with no piston squirters.)
S420 & S500 M119 engine oil leaks etc:
Oil leaks are not as prominent on the M119. Check the front main seal and the cam covers, you will need to remove the air intake assembly to have a good proper look.
Have a look at youtube videos showing the dreaded cam oiler ticking noise to familiarize yourself with this sound. This can sound quite bad in person but is far from an impossible problem to fix. If you come across a seller that has not disclosed this problem at least you will know what it is if you hear it, it's not the end of the world.
POWER STEERING pump seal at the front manifests as a slow leak, requiring removal and rebuilding. Steering box can leak quite a lot of oil requiring seals to be replaced, access to which is tight.
AUTOMATIC GEARBOX 722.3 and 733.4 mostly on pre 1997 models are hydraulic controlled with mechanical shift linkage mechanism with further vacuum control. Before starting check the oil, it should be low on the dipstick and a healthy red-orange colour. The level comes up with the transmission at operating temp. If the oil is in the darker burnt to greenish black colour it shows abuse of the gearbox or it is out of adjustment, and possibly very old oil and has absorbed moisture from many many years.
When engine is still cold the transmission will delay shifting to 3rd gear, this might seem abnormal but it is normal operation. Check for smooth shifts both up and down, test for healthy kickdown, test manual shifting at sensible speeds. If something doesn't feel right it could be a multitude of causes, ranging from a simple adjustment or vacuum leak up to possibly detrimental wear or failure of an internal component. If the shifter mechanism feels a bit loose it is just old tired shifter bushes, a very cheap fix but it could show the car has not been serviced by a mechanic with a brain.
At full throttle from a stop the car should change through the gears near redline, gear changes should still be smooth but no flailing, 0-100kph time should be close to advertised.
Test reverse, it should not take excessive time to engage and should drive the car with confidence just as the forward gears do. If you need to rev the car for it to move, if it feels weak and/or strange noises in reverse gear are symptoms of the common reverse lip seal and/or spring retainer failure.
There are a dozen or more seals transmission oil can leak from off the top of my head. Rear seal is the easiest to spot, and seals at the front at the input shaft are hardest to fix due to access requiring removal of the gearbox. If there is oil leaking at the front of the gearbox it could be the engine rear main seal, color of the oil should be a tell tale sign.
AUTOMATIC GEARBOX 722.6 is fitted to most or all later model cars. It is roughly a redesigned 722.4 box with an extra ratio added to the rear and a lockup torque converter up front. It is electronically controlled with a manual shifter for some operations. I do not have owner experience with this box but I understand the common problem leak with the main electronic connector at the front, and a rubber boot that can tear with use and age at the rear. Everything else is similar enough to the 722.3/4
Check operation as per the 722.3/722.4 box above, however this transmission does not have a dipstick and was marketed as "sealed for life", so it will be hard to check the oil when looking at the car and there is a greater chance the oil has never been changed since they put it in at the factory.
Re. any of the transmissions, if the car does not have a documented gearbox rebuild factor that into your purchase price if you plan to keep the car for any length of time.
Under the car check the two rubber flexdisks on the tailshaft for cracks. Small cracks are ok but the rubber should be structurally sound. A rumble vibration through the car while driving can be caused by flexjoins in need of replacement. Check boots around sway bar links as the boots on cheap non-genuine parts can crack very easily. Check the suspension bush condition, hopefully you can find a car with fresh bushes!
While driving if the steering wheel shakes it is either from out of balance wheels or out of round wheels/tires. Shaking while braking is warped brake rotors. There can be a slight shake at certain speeds from old suspension bushes but I have not experienced this, correctly balance and round wheels will expose this problem.
Check suspension for clunks and knocks, make sure steering isn't too floaty. Also make sure tires are at proper pressure around 36psi as this can be the difference between thinking the shocks and bushes are gone, to thinking the car rides ok.
Listen to the engine while idling for any noises, make sure the AC compressor and PS pump is very quiet or silent, give the car a bit of a rev but while stationary to listen to any noises but the EMS limits the engine to 4 or 5k rpm.
Final notes: Do not buy a W140/C140 as your only means of transport. Have a 2nd car (another W140 or C140?) as a backup for when it needs some love because she will.
That's all I am prepared to write tonight and I have not touched on the ignition and fuel systems and many other aspects of the car. This is my no means a step by step guide to an exhaustive inspection but if you don't know your way around a W140 the above tips might help a bit. The information is my opinion and I am not a mechanic.
Edit: If anything I typed is an obvious error please notify me and I will correct the post.
At the age these cars have reached if a car has not had fastidious maintenance there is likely to be oil leaking from more than one place. Under the car is likely to be covered in oily grime making oil leaks very hard to locate. IMHO in most cases sellers want to get rid of these cars due to one or more problems that need fixing, so you better be good at finding those problems!
So here goes, some pointers for inspecting the condition of a M104 or M119 based W140:
Both engines: check coolant when the car is cold, plan ahead requesting the car not be started before you arrive, hopefully the coolant is clean with no scum or anything under the filler cap or in the expansion tank. The color should be that of MB specified coolant not green glycol crap or old coolant faded to opaque dirty water.
While the car is still cold remove the aux belt and feel for looseness, rotate by hand and listen for noises on all the pulleys (PS pump, alternator, water pump, tensioners, everything) and tensioners. At this age there is bound to be a loose one or two or the water pump that feels rough when turning, even if it doesn't leak, bad bearing etc etc.
Test the AC and heater at full blast. AC should freeze your balls. Heater should feel like hell after not very long if the engine is at operating temp. Test operation of both sides at different temps to check duo valve operation. Leave the AC on full for a while to heat everything up. The engine fan has a viscous drive and this should engage before the AUX electric fans do, if not the viscous drive needs replacing. Make sure the AUX fans engage if the engine temp climbs above 110 or so. The temp gauge can move about from a faulty gauge in the cluster showing excessive engine temp.
After you have driven the car with the engine at operating temp check for leaks with engine off as the coolant system will be under pressure.
S280 & S320 M104 engine oil leaks to look for:
Engine cam cover gasket at the rear, oil can seep down then wick around to the exhaust side due to the angle of the engine. This might look like a head gasket leak but it is not.
Rear main seal is a common leak with a completely revised rear main seal available from Mercedes to fix. Front main seal does not have this issue but is much easier to check and fix if required.
Front "U" seal between upper and lower section of the cam chain cover, if engine has been ran with excessive oil the engine is more prone to leaks. Mercedes revised down the recommended oil level.
Right hand drive cars have an oil cooler located under the oil filter. The perimeter seal can perish and oil then leaks down the side of the engine above the starter motor.
(Side note: Interestingly the M104 series engines use a cast iron block and have piston squirters unlike the M119 and M120 that use an aluminum block with no piston squirters.)
S420 & S500 M119 engine oil leaks etc:
Oil leaks are not as prominent on the M119. Check the front main seal and the cam covers, you will need to remove the air intake assembly to have a good proper look.
Have a look at youtube videos showing the dreaded cam oiler ticking noise to familiarize yourself with this sound. This can sound quite bad in person but is far from an impossible problem to fix. If you come across a seller that has not disclosed this problem at least you will know what it is if you hear it, it's not the end of the world.
POWER STEERING pump seal at the front manifests as a slow leak, requiring removal and rebuilding. Steering box can leak quite a lot of oil requiring seals to be replaced, access to which is tight.
AUTOMATIC GEARBOX 722.3 and 733.4 mostly on pre 1997 models are hydraulic controlled with mechanical shift linkage mechanism with further vacuum control. Before starting check the oil, it should be low on the dipstick and a healthy red-orange colour. The level comes up with the transmission at operating temp. If the oil is in the darker burnt to greenish black colour it shows abuse of the gearbox or it is out of adjustment, and possibly very old oil and has absorbed moisture from many many years.
When engine is still cold the transmission will delay shifting to 3rd gear, this might seem abnormal but it is normal operation. Check for smooth shifts both up and down, test for healthy kickdown, test manual shifting at sensible speeds. If something doesn't feel right it could be a multitude of causes, ranging from a simple adjustment or vacuum leak up to possibly detrimental wear or failure of an internal component. If the shifter mechanism feels a bit loose it is just old tired shifter bushes, a very cheap fix but it could show the car has not been serviced by a mechanic with a brain.
At full throttle from a stop the car should change through the gears near redline, gear changes should still be smooth but no flailing, 0-100kph time should be close to advertised.
Test reverse, it should not take excessive time to engage and should drive the car with confidence just as the forward gears do. If you need to rev the car for it to move, if it feels weak and/or strange noises in reverse gear are symptoms of the common reverse lip seal and/or spring retainer failure.
There are a dozen or more seals transmission oil can leak from off the top of my head. Rear seal is the easiest to spot, and seals at the front at the input shaft are hardest to fix due to access requiring removal of the gearbox. If there is oil leaking at the front of the gearbox it could be the engine rear main seal, color of the oil should be a tell tale sign.
AUTOMATIC GEARBOX 722.6 is fitted to most or all later model cars. It is roughly a redesigned 722.4 box with an extra ratio added to the rear and a lockup torque converter up front. It is electronically controlled with a manual shifter for some operations. I do not have owner experience with this box but I understand the common problem leak with the main electronic connector at the front, and a rubber boot that can tear with use and age at the rear. Everything else is similar enough to the 722.3/4
Check operation as per the 722.3/722.4 box above, however this transmission does not have a dipstick and was marketed as "sealed for life", so it will be hard to check the oil when looking at the car and there is a greater chance the oil has never been changed since they put it in at the factory.
Re. any of the transmissions, if the car does not have a documented gearbox rebuild factor that into your purchase price if you plan to keep the car for any length of time.
Under the car check the two rubber flexdisks on the tailshaft for cracks. Small cracks are ok but the rubber should be structurally sound. A rumble vibration through the car while driving can be caused by flexjoins in need of replacement. Check boots around sway bar links as the boots on cheap non-genuine parts can crack very easily. Check the suspension bush condition, hopefully you can find a car with fresh bushes!
While driving if the steering wheel shakes it is either from out of balance wheels or out of round wheels/tires. Shaking while braking is warped brake rotors. There can be a slight shake at certain speeds from old suspension bushes but I have not experienced this, correctly balance and round wheels will expose this problem.
Check suspension for clunks and knocks, make sure steering isn't too floaty. Also make sure tires are at proper pressure around 36psi as this can be the difference between thinking the shocks and bushes are gone, to thinking the car rides ok.
Listen to the engine while idling for any noises, make sure the AC compressor and PS pump is very quiet or silent, give the car a bit of a rev but while stationary to listen to any noises but the EMS limits the engine to 4 or 5k rpm.
Final notes: Do not buy a W140/C140 as your only means of transport. Have a 2nd car (another W140 or C140?) as a backup for when it needs some love because she will.
That's all I am prepared to write tonight and I have not touched on the ignition and fuel systems and many other aspects of the car. This is my no means a step by step guide to an exhaustive inspection but if you don't know your way around a W140 the above tips might help a bit. The information is my opinion and I am not a mechanic.
Edit: If anything I typed is an obvious error please notify me and I will correct the post.
