Note: This only applies to the 1987 300D/TD, not all 124's in general. Some of the items may apply to other 124's, for example the A/C information. *** Last updated Jan-2012 *** ===================================== 1- The cooling systems are notoriously problematic. If the car hasn't had a recently replaced radiator and/or fan clutch, watch the temp gauge closely. It should almost never exceed 100C except in extreme conditions (blazing heat, A/C on, climbing a mountain.) A new radiator is about $350, or you can send yours out for a rebuild at Reseda Radiator for ~$125 + shipping. I haven't seen great results with rebuilds... and since the old radiators don't have metal reinforced necks, AND the plastic can get brittle, I'd lean towards a new Behr (not Nissens) unit if possible. "Normal" operating temps are between 80C-100C. Don't freak out if the car runs at 90-95C all the time, that's fine, as long as you can't make it go past 100-105C under high loads. Mercedes says temps above 110C are excessive, and I have the TSB to prove it. ;) Damage should not occur until temps pass 120C... but I'd have the heater on 'max' long before that point. 2- Fan clutch: This is probably about worn out after 15+ years. If the fan has metal blades, it's the old style clutch, and may need replacement. The new style clutch requires a new plastic fan (from the OM606 motor.) Total cost ~$250 for both parts. If the old clutch has a good bearing, it can be re-filled with silicone fluid. If the bearing is bad (fan blade moves front to back more than a few mm), you must replace the clutch. Don't be too quick to condemn the clutch as not working - it requires hot air to engage! That means the radiator must not have a cold spot in front of the clutch, and must not be plugged with bugs & dirt, because if it is, even a NEW clutch won't engage. To test the clutch (after making sure the radiator is clean & hot), get the engine as close to 100C as possible, then kill the engine while watching the fan. It should stop immediately, or within 1-2 revolutions. If it spins 5-10 times, it was not engaged. Compare this to how it acts a warm or cool (40-80C) engine. And yes, the fan does do a lot at freeway speeds, the ram air effect at 70mph IS NOT enough to cool the engine with a bad fan clutch. 3- Electric cooling fan: There is a 3-prong electrical switch at the water outlet, near the upper radiator hose, on the front of cylinder head. The 2-prong connector triggers the electric fan on high speed when the temp exceeds 105C. To test the fan, short the two wires together (with the engine on), and the fan should run on high. This switch is often defective and should be replaced on principle. The original switch is blue color (and rated 105/128C), but I recommend replacing it with the gray-colored switch (rated 105/120C) from the W126, which costs half as much (order part number 006-545-61-24, about $30). To test the low speed fan, find the green or red pressure switch at the AC receiver/drier (behind the left headlight), and short the wires together - the fan should turn on low speed. (If you ever open up the AC system, replace the green switch with a new red one.) 4- Serpentine belt tensioner: The pivot bearing tends to fail and lean at an angle. Look at the tensioner pulleys near the water pump, if they don't look perfectly straight, you likely need a tensioner. ~$100 in parts, not a bad job if you have the tools to R&R the fan clutch quickly. Also the belt shock tends to have it's bushings fail and it causes a rattling noise. Press on the shock body with your fingers with the engine running to see if it quiets down. It will be obvious if it does. Get a new OE dealer "Genuine Mercedes" shock if it's bad, the aftermarket ones seem to have shorter life spans - the OE part is ~$60, the aftermarket shock is ~$30. The OE shock has an improved bushing design, as noted in the factory OM603 engine manual. 5- Oil leaks: Hopefully you don't have many. Cost and location vary widely. Depending on the severity you may choose to ignore them. The oil level senders may seep oil through the body. This is ~$50 and simple to replace at oil change time. The tranny cooler hoses at the front of the engine are almost always leaking, just replace them. The injection pump often leaks oil from all over, you can pull the pump and re-seal it for about $50 in gaskets, and 4-6 hours DIY labor (not a fun job but it's nice to have a dry pump!). The turbo also can leak oil externally, but that's expensive to repair, so try to ignore that leak if possible. A leaky front crank seal will cost $200+ to have a shop fix it (this requires several special tools for DIY). Fortunately, the 603 engine does not suffer from the severe oil leak problem at the turbo oil drain, like the older 617 engine did. 6- A/C system: This can be expensive to fix. The manifold hose (the main hose assembly) can weep oil *through* the barrier, making the outside of the rubber hose wet and oily. Yes, it's actually refrigerant oil! This is the big hose near the ABS pump. If it's oily, you will need to replace it eventually. I just did mine, it's a massive unpleasant job. The hose is ~$400 new OE, ~$100 new OEM, or have your old one rebuilt for about $100. Stick with R-12 if the system hasn't already been converted - please! - and don't ever convert an R-12 system to R-134a, or any other refrigerant that requires PAG/POE oils. You'll be sorry in the long run. If the car has already been converted, well, cross your fingers and hope it was done properly. If the evaporator is leaking, it's a massive job (15-20 hours) to replace as the dash must come out - see next item (#7, below). Try a chemical sealant (i.e., Cryoseal) before replacing the evaporator, this may save you $$$$. 7- The center vent vacuum actuator is almost always broken (ruptured diaphragm.) Put the A/C on max and see if you get a constant stream of air out the center vents over 10-15 minutes operation. If it EVER stops, the actuator is bad. There's a diverter flap (not the recirc flap) that allows air out the center vents, but this flap cycles during normal operation- hence the intermittent airflow. When the center vent flap is open, you can look into the center vent itself with a flashlight and just see the aluminum evaporator down below. If you can't, the door is shut - because the flap isn't open. To replace this $20 part, you must R&R the dash - a 5-10 hour job. NOT fun. I've done this on many of the 124's owned by my family, and most early 124's need this done. If you pull the dash, replace EVERY vacuum pod, not just the center vent pod! You will not want to pull the dash again to replace the next failed one a year later, believe me. To test all of them, pull the glovebox out, and use a MityVac to apply vacuum to all 7 of the vacuum lines at the manifold. All should hold ~25" vacuum. If not - if they leak down, or don't hold vacuum at all - that pod is bad. DO NOT bother with the rebuild kits sold by Performance Analysis... that kit only works properly on the center vent pod, and on one of the two chambers inside the dual-chamber pods. Just order new pods, it's NOT worth pinching pennies here, given the significant labor time involved in R&R. 8- See if the ELR (idle speed control) is working. There's a round, black trim plug near the fuse box, numbered 1-7. With the engine idling, pull it out, turn it to a different number, and push it back in. The idle speed should change. 1 is slowest, 7 is fastest, it changes approx 20+ RPM per number. Normal setting is #4. If it's not working, it's probably just be a bad connection, usually at the ELR actuator (round red thingy at the top/rear of the injection pump). A bad OVP relay, or engine speed sensor, may also be the cause. 9- The outside temp sensor can be a problem. If it's always WAY off, you can replace the sender ($40?) but even after I did this, it always reads way too high in the summer. It's more accurate in winter. Poor design. If it really bothers you, it's possible to retrofit the newer version, which "holds" the last temp reading at low vehicle speeds, to prevent it from climbing when stopped in summer traffic. If the display is "blacked out" you must replace it - about $300 for a new one (!), or a good used one will be $50-75 from salvage yards. Early 124's have the sensor in the left corner of the bumper impact strip, late 124's relocated it behind the front license plate. I moved mine to the front bumper to get more airflow, similar to the later design. 10- Check the sunroof, power windows, cruise, locks, seats, etc. Often it's just a switch (cheap) but the sunroof can cost a fortune ($1000+) to repair. DIY repair is possible but do plenty of research before attempting this. The early 124's (1986/87) had weaker sunroof lifting arms, late 87 and newer have an improved mechanism that is far less likely to break. Use ONLY the proper Mercedes sunroof lube (Gleitpaste). There are a dozen or so felt pads that the mechanism slides on, these wear out over time. If a working sunroof is carefully disassembled using the factory manual, cleaned, and re-assembled with Gleitpaste and new pads... it usually will work like new again for $50-$75 in parts and 4-6 hours of your time. 11- Performance: The 1987 300D is NOT slow. Check it with a stopwatch from 0-60. Factory spec is a little under 11 seconds. Faster than 12-13 is probably acceptable if it drives OK otherwise. Much slower than that and you have a problem. The wagon is one second slower than the sedan. Could be as simple as a plugged boost line to the ALDA (free fix), or a bad turbo (~$500), or an injection pump (~$750), or both. An engine with bad compression (shot engine) will likely have other symptoms like hard starting, lots of smoke, and high oil consumption. If the car is very sluggish off the line (off idle), the IP is low on fuel delivery at low RPM. This can be compensated for by adjusting the ALDA (remove it to add shims, OR turn the setscrew, re-install). If the car launches hard but runs out of breath past ~3000rpm, either the turbo isn't working (stuck wastegate? damaged impellers?), or there is an exhaust restriction. This assumes all filters are clean, brakes are not dragging, etc. At high elevations, say over ~3000ft above sea level, the car may be sluggish off idle... this is normal and you can't really do much about it. Adjust the ALDA richer until you don't feel a power gain, then turn it back leaner slightly, that's usually about perfect. If you can feel the turbo "kick in" at 2000-2500rpm or so, that is another indication the IP is too lean, and you need to richen the ALDA. There should not be any "kick", the engine should have strong power off idle, and build power smoothly to the redine (5150rpm). 12 - Smoke should be minimal. Smoke at hot idle is not a good sign. Constant smoke all the time is also not good. Smoke only at WOT means the car might need to be driven hard ("Italian Tuneup") to clean out the carbon buildup, especially if it has been driven sedately for the prior months or years. After a few full throttle runs on a GOOD engine, smoke should be almost non-existent in sunlight. Ignore smoke at night in headlights, you will always see that, even with a perfect engine. Black smoke is usually excess fuel, blue is oil, and white can be coolant (or unburned fuel, in some cases.) A slight puff of black smoke when leaving a stop may be normal, and/or indicate a slightly rich ALDA adjustment. 13- Oil consumption should be very low. Mine uses about a pint per 4000-5000 miles, with 250k on the clock and Delvac-1 5W-40 diesel synthetic. A quart per 4000-5000 isn't bad. When you start using a quart in 3000, or 2000, or worse... something is NOT right. Always use xW-40 or xW-50 viscosity oil that is diesel rated (CG/CH/CI/etc). Mobil-1 5W-40 'Turbo Diesel Truck Formula' was my personal favorite; but not any more since Mobil cheaped out and switched to a Group III base stock. Instead, I would recommend Amsoil "AME" 15W-40 Heavy-Duty Diesel oil, or Red Line diesel oils (either 5W-40 or 15W-40). These are true Group IV/V synthetics and will handle extended drain intervals. 14- Trap oxidizer: The cars came from the factory with a catalyst called a 'trap oxidizer'. It's a huge round metal can above the turbo, about 10 inches in diameter. This has been replaced under factory recall with a straight pipe that looks like a shiny foil-wrapped iron log, about 4 inches in diameter. If the car still has the trap, go visit your dealer and Mercedes will replace it for free, and you'll get a whole new exhaust system too! The new exhaust includes a small oxidation catalyst which will be downstream of the turbo, about under the passenger seat. You will see a noticeable increase in power and MPG with the trap removed. 15- Emissions junk: With the trap removed, the Air Recirculation Valve (ARV) on the turbo is not needed. Find the small vacuum line to the turbo (not the large rubber wasgate line!) and plug it with a BB. Do the same with the EGR valve. This can increase part-throttle power, as well as MPG, and also may correct a cruise surging problem. If you want a cleaner engine compartment, all the solenoids under the air cleaner can be removed, along with the three vacuum lines that go across the top of the radiator. Don't forget to plug the unused ports on the 5-way splitter above the vacuum pump, or replace it with a 3-way splitter. The large black line is a vent to the passenger compartment. 16 - Cylinder head: The original heads were flawed. The casting was too thin in some areas and they will crack under high heat loads. (The trap oxidizer is thought to exacerbate this issue.) Look behind the #2 injector for the casting number, in the format 603-016-xx-01. If "xx" is number 14, the head is original. If it is a number higher than 14, the head has been replaced. Numbers 17, 18, 20, and 22 are the new, stronger heads. The early symptom of a cracked head is high pressure in the cooling system when the engine is cold. In the morning, with a cold engine, squeeze the upper radiator hose. It should have low, or zero, pressure - perhaps even slight vacuum. You can squeeze the hose together easily. If the hose has HIGH pressure, so much that you can't pinch the sides together with your fingers, that's bad - it could be an early sign of a cracked head. A soft hose only means the head is good IF the cooling system can retain pressure. If there is a leak, a soft hose means nothing. No oil or coolant will have mixed, you cannot pronounce the head "good" based on clean oil & coolant. As the condition gets worse, you may start "burning" coolant, and at that point the car should NOT be driven until it is repaired - otherwise the block can get damaged from steam/corrosion, and require a replacement engine (used = $2000+, new = $7000+). If you replace the head, never install another #14 head, always get a #17 or newer head. A used #17 (from a 1990-95 3.5L engine) usually costs $800-$1200, plus another $500 in assorted parts & tools, and 15-25 hours of DIY labor with the factory shop manual in hand. A new #22 head is $1800-$2200 and does NOT include valves, springs, lifters, or camshaft. Shop labor will usually be 10-15 hours at $60-$100/hour. A car with the updated head is, therefore, worth more than one with an original #14 head. 17- Oil in the coolant (or vice versa) usually means a failed head gasket, and not *necessarily* a cracked head. But if the head is a #14, don't be surprised if it's cracked when you take the engine apart. The plastic coolant tank may be yellow in color from age. If there are dark, or black, splotches inside... that means it at one point had oil in the coolant. A new tank should be installed ($50), AFTER fixing the head or gasket problem. The new tank will be white/clear and has a silica pack to help control corrosion. Never, ever use "green" or "orange" coolant. The Mercedes low-silicate coolant, which is also sold by Zerex as G-05 Formual, is light yellow in color, almost clear. This is the ONLY anti-freeze to use!!! 18- Check all the usual wear items - brakes, suspension, tires, yadda yadda. The rear torque arms are usually shot. They're about $60 for both and take 2 hours to replace. Most of the other 4 (of the five total) rear links are usually ok, even at high miles, but check the condition of the bushings. The wheel carrier bushing at the outside end of the rear lower control arm is often toast, and it is very hard to see. A special tool is needed to replace these (the aftermarket tool is about $120) and the bushings are $20-$40 each. The front lower control arm bushings are often shot by 200kmi, especially if they are oil soaked. A coil spring compressor is needed to remove the control arm, and the bushings must be oriented a certain way - don't attempt this without the factory manual. 19- If the cruise doesn't work, make sure all the brake light bulbs are good; if so, it probaby needs a new cruise amp (~$200 for a rebuilt unit, $50 for a used one). The cruise actuator on the engine is a very rare failure, the cruise amp is a very common failure. 20- Rough idle is usually engine mounts, they rarely last more than 10 years or 100kmi (usually a lot less). They are about $60/each and aren't too hard to replace. If the rough idle persists, it is probably related to the fuel system - dirty injectors, etc - try a Lubra Moly 'Diesel Purge' treatment first. If that doesn't help have the injectors pop tested at a diesel shop. If they are fine, next item could be injection pump delivery valve seals. 21- Don't forget to check CarFax for possibly odometer rollbacks, I found a nice looking 1987 300D in San Francisco area with "98kmi", that actually had a broken odometer that was rolled back, the car really had well over 200kmi on it. The odometers are another item that fails - make sure it works! They're not hard to remove (and send out to have fixed), but it makes the actual milage on the car hard to determine. Good service records can prove the mileage is accurate. ------------------------------------ Here are some additional comments from the late Marshall Booth (MBCA Diesel Technical Advisor), on the subject of 300TD purchase: The OM60x engines used in the W201/124/'86+ 126s are much quieter then the older OM61x engine. Part of that IS the engine and part is the sound encapsulation used in and under the engine compartment (be SURE the one you buy HAS these panels - some misguided mechanics throw them away and they are expensive to replace)! The hydraulic valve lifters CAN make quite a racket if the car has had a steady diet of conventional oil. Changing to synthetic oil WILL considerably reduce and eventually may even eliminate all but an occasional clack/clatter from these "noisemakers" ;-) The '87 300TD is probably my favorite MB ever! There are LOTS of little things to look at when buying a 13 year old car. Ones that I would look for are: AC system - it must produce good cooling (compressor replacement will usually be well over $1000), the sunroof must work properly in all modes (sunroof repair can be $1000), The tail gate must open, stay up and close flawlessly and the "pull in" motor must engage and pull the gate shut without effort once contact is made, check that the serpentine belt idler pulley (right below the horizontal expanse of fan belt at the front to of the engine) MUST be aligned parallel with the belt - if it's a degree or two cocked, then it MUST be repaired (the costs could be from $100-$1000 depending on what is actually worn or broken), the engine MUST start promptly when dead cold and idle should smooth out within 20-30 seconds! There must be NO evidence of transmission fluid on the grate at the bottom of the bell housing (where the torque convertor lives ;-) as that would be evidence of leaking seals (minimum of $500 to fix) and the transmission should shift very smoothly after the 1st few cold shifts. The power seats should all move effortlessly and all the windows should work at about the same speed. The cruise control should be tested! The radio, antenna and cassette player SHOULD work (though a refurbished radio is available on an exchange basis from Becker for about $130). Wipers (front and rear) should work smoothly. Finally, the car should ride and handle about as well as your 300SD - but just a bit more agile and it should feel more powerful ('cause it is). Let me add that the 124 is notorious for having the wires from the "B" pillar to the rear doors break - seems that the wire was made from stranded wire with too few strands of wire that was too thick to be sufficiently flexible. The 201s and 126s made during the same period did NOT suffer from this failure.