Coolant Flush

[dablk1]

This is for a 1991 300D 2.5. I've read through a lot of posts but some of the info is conflicting and I want to make sure I do this correctly. First...is the block drain plug the large bolt above the thermostat housing? Secondly...where am I suppose to insert the hose while I'm flushing the system? I plan to use a mild flush chemical and change the thermostat since I've started having high operating temperatures.

[2.5Turbo]

The block drain is near the block heater under the exhaust manifold and behind the turbo. It's not just a bolt, it's sort of a banjo bolt type plug that when loosened allows the coolant to flow through the center. You can attach a smallish diameter hose to the plug to direct the coolant into a bucket, but it isn't totally necessary unless you want to keep your tie rod dry. Make sure you also drain the radiator by either pulling the lower hose or opening the drain valve. When I flushed mine I disconnected the lower radiator hose at the radiator and put the hose in the expansion tank and set the water flow so it wouldn't run dry but also wasn't overflowing. Make sure you run the engine a while with the heater on MAX to flush out the heating circuit as well. Make sure you flush with water numerous times with drives in between to get the car up to temp in order to get all of the chemical out once you're done.

[dablk1]

I don't have a block heater...is the banjo bolt a 17mm or close to it with something smaller inside of it? I also want to make sure that I understand you. You removed your lower radiator hose to flush the system? Was this after you drained the block and radiator(closed both plugs) and then put the hose in the expansion tank? If this is the case, while you were flushing it...the only opening was the expansion tank, which your hose was in and the lower radiator hole? Is this correct?

[HuskyMan]

not sure on your car, but on mine it is 17mm wrench. place the car on ramps that are rated for the weight of the car, be sure and put it into park and block the rear tires. then, while lying on your back shine a line up onto the right side of the engine block. your deep socket 17mm should slide right on. merely loosen the drain plug on the radiator and let it drain while the engine block drains.

2. tighten both drain plugs.

3. install a backflush kit into the heater hose coming from the firewall going into the engine block. attach a garden hose to the backflush "T" connector. I use an oil cap I bought from the car parts store to block the top radiator inlet. loosen the top radiator hose and let it hang over the top of the radiator. run the engine with the heater on high while doing this procedure. turn the garden hose on full blast. run until the water coming out of the top radiator hose runs completely CLEAR.

3. disconnect garden hose and place cap on "T" connector. reconnect the top radiator hose to the radiator. purchase some Shout ultra gel advanced from either Lowes or Target stores. mix half bottle in two gallons of warm water and pour into radiator overflow expansion tank. then fill with water from a garden hose. leave the cap OFF while flushing.

4. run engine approximately 20 minutes with heater on high.

5. next, I did the mercedes citric flush and had very good results. it mixes with two gallons of warm water , fill just like above into the radiator expansion tank. black crud was coming out of the radiator drain plug for the next two days!

6. place your garden hose onto the "T" connector for the prestone back flush kit. loose the top radiator hose again and place a stopper (I used an oil filler cap from an auto parts store) into the top radiator inlet. start the engine and turn the heater on high. this is the final flush so don't be afraid to let the engine run for 20 to 30 minutes. note that MORE black crud will come out of the top radiator hose! I plan on doing this procedure every two years for the next six years. my cooling system was just too down and out nasty to allow it get into that shape again.

6. this is an excellent time to replace old worn out hoses. squeeze the hoses to see if they sound brittle. if so, time to replace them. I replaced my radiator with a new updated nissens unit which is killer!

7. replace the thermostat with a mercedes OEM thermostat. use your finger to apply permatex sealant along the edge of the thermostat housing.

8. important! ONLY use antifreeze which is SPECIFICALLY rated for diesel engine use. two I am aware of are Cummins and Amsoil. there may be others
but I like the Amsoil because of the 750,000 mile over
the road diesel use rating. fill 50/50 with distilled water and you are good to go!

diesel engines run HOT and a special diesel rated antifreeze is required to keep the cylinders cool. don't use a diesel rated antifreeze and the engine will begin to degrade.

[dablk1]

Is the backflush kit an absolute necessity?

[HuskyMan]

well.....that depends. I'm one that believes in giving the
car the best treatment possible. the back flush kit is
about five bucks at the car parts store. a sharp kitchen knife is used to cut the heater hose coming from the fire wall going into the engine block. insert the "T" fitting and screw on the cap. easy to do. the pay off? when you loosen the top radiator hose and place a stopper into the top radiator inlet, then attach a garden hose to the back flush kit and turn on the water full blast while the engine is running and heater is on, you will be AMAZED at how much nasty crud comes out of your cooling system!

call your neighbors over for a cooling system party, because I can assure you THEY will be amazed at what comes out of your car! combined with the Shout Ultra Gel followed up by a back flush, followed up by the Mercedes Citric flush, followed by yet a final back flush, and your system will be about as clean as it can get.

after I did all of the foregoing, I drove the car for a few days with nothing but water in the cooling system. I then drained the radiator and MORE gunk came out! so.....I placed the garden hose onto the back flush "T" and flushed yet a THIRD time! more crap came out which totally blew me away......all I can say is I am glad I went to this extreme.

AND.....this was after a so-called "Professional" mechanic had installed a new water pump in the car. he used the wrong anti-freeze (for a diesel) to boot. several years later had the car at another mechanic who just happened to notice that the bottom hose clamp for the lower radiator hose was loose and dangling. obviously, the previous mechanic hadn't bothered to tighten the hose clamp. the hose could have easily blown off dumping coolant all over the street. this and other issues caused me to start doing as much DIY work as I possibly can. I can screw the car up for half the money I pay a "professional".

FYI, the flush system I listed came from dieselgiant.com

[dablk1]

Ok...I'm sold on the back flush kit. WHICH ONE IS THE HEATER HOSE????? Is it going into the block right between the 4th and 5th cylinder? I don't see a hose that's coming from the firewall going straight to the engine that doesn't branch off along the way. After it's cut, does it need to be replaced?
By the way...all I picked up was the Prestone Super radiator flush.

[vstech]

prestone kit is ok, but you first need to empty your coolant, then flush well with water, then use the detergent gel from lowes. then flush again, and drain well out the block, then use the prestone kit, and follow those directions to finish cleaning.

[dablk1]

Is this Shout ultra gel that you guys are talking about the actual laudry stain remover?

[vstech]

yup. it removes traces of oil from the coolant passages so the acid can work properly.

[2.5Turbo]

I think the plug was 17mm. I was able to get at it relatively easily from above with my 3/8" ratchet and a 3" extension.

Be careful with those flush kits. The tees have been known to fail and drain out all your coolant.

Keep in mind I had the engine running when I was flushing so the water pump sucked the hose water out of the expansion tank and ran it through the engine/radiator and out the lower hose.

I did a citric flush on mine because the car ran a little warm on long hills and I didn't know how much gunk was in the system. Turned out there wasn't much in there; the citric solution was only a little cloudy when I drained it. A new Behr radiator (old one was original and leaked when system was pressurized), water pump and thermostat along with the flush has brought the system back to near new performance and condition.

[HuskyMan]

thanks for the update on the flush kits. I think I will replace mine with a straight hose and only use the "T" fitting when I do my next flush. as far as which hose to cut, on my car the heater hose runs from the firewall to the engine block between 4th and 5th cylinders. the flush kit includes instructions that help locate the proper hose to cut. also, be sure and squeeze your other hoses to see if they make a "brittle" sound. if they do, time for replacement. also, if replacing the hoses, do not use vaseline to lube the fittings, use KY jelly or silglide instead.

a lot to remember, but if you perform coolant service on the car every 2-3 years it will help to maintain the system for a long life. and, you get the satisfaction of knowing you did the job right.

[dablk1]

Well te flush job was a success. Coolant temps returned to normal. It appeared that more crud came from the exterior of my radiator than anywhere esle. I removed my radiator since I also replaced my pulley and damper. When I took it out I noticed how dirty it was bugs, dirt and such. I sprayed the fins and flushed it with my hose, reinstalled it and then did the complete cooling system flush.

[HuskyMan]

depending upon the age and condition of the radiator, one may consider changing it out at the next scheduled coolant service interval. reason: if the barrier between the radiator and the transmission cooler fails, coolant will enter the transmission which means new transmission time. here's the line up:

Price of new radiator: $200 or so
Price of new transmission: $2000++

for me, it was a no brainer.

[dablk1]

I was way ahead of you on that one. Thanks for the tip htough. It's all about preventive maintenance...