I would definitely ditch the ARV, it will reduce boost (and MPG) at part-throttle. You just want to make sure you're around 12psi of boost, and not more than 14psi. If you turn up the full load, I'd really recommend installing an EGT gauge, but if you don't... just avoid long periods of heavy throttle (i.e., climbing the Grapevine at 75mph).

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If I only had the money, I'd look into doing the same mods to my 1987 300TDT.

How does the cooling system handle the additional horsepower? I get the impression that the OM603 cooling system in the W124 is marginal, so if the motor were run on a hot day, up a long grade with A/C on it would heat up excessively. Dave, do you have plans to upgrade the cooling system?

I'm still battling an overheating issue on my 300TDT and was wondering if anyone knows of an upgrade radiator for the OM603 in a W124 chassis. I was thinking of looking into a custom aluminum radiator that would have a thicker core, but I don't know if there is any space.

I'm also curious as to what the reduction in fuel economy is with the engine mods for more horsepower.

-Steve

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1987 300TDT smoke silver w/ burgundy leather interior
2000 VW Passat wagon indigo blue w/ beige leather interior
1985 Mustang SVO
1970 Chevrolet K10 fleetside, shortbed

Farting with both at the same time, among other projects. Too many vehicles.

I figure my SDL is safely maxed out where it sits and I'm more than pleased with its performance. I've had the full load up for a long while with no ill effects. I checked under the bumper and there is more than sufficient room for that Z32 style IC, just curious of the pressure drop with it. Looks like it could be easily added without deviating TOO far from stock.

Freeway cruise economy has remained around the same as with city. Takes less pedal to get going. If you drive like a maniac, your economy will decrease.

FWIW - the cooling system is fine in my SDL. More car to lug around too. New rad, 606 fan, 606 clutch, Xerex coolant, red fan switch. Not sure if you can compare the 126 to the 124 as the parts/avail space are greater

If you're overheating, it's probably time to replace cooling parts. I went from around 88-92°C to 81-83° when I tossed in my new rad last summer.

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'85 300SD 245k
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'90 300SEL 326k

Six others from BMW, GM, and Ford.

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The good news is, the full-load mod is basically free (although I'd recommend spending the $100 for a good EGT gauge.)

The cooling system, if in good condition, should be fine... you rarely use the additional power for extended periods (and shouldn't!), so the cooling load is not much above stock. I plan to replace my radiator before too long, but there's not much else we can do to improve cooling, unfortunately. If your radiator is more than ~5 years old, and you have a good t-stat & fan clutch, I bet a radiator would make a big difference.

I fought an overheating problem when I first bought the car, and replaced everything one item at a time.... EXCEPT the radiator, because it was only 4 years old, so it couldn't possibly be bad. Guess what? It was. With a fresh new radiator, I couldn't make the car go above 100°C no matter what I tried. That was 8 years ago, and temps have been creeping up the past few years, so I expect I may need to go through the same drill again.

Fuel economy should be unchanged, as long as you keep your foot out of the pedal. Otherwise it will drop, because you're using that extra power, and there is a penalty for additional power production.

Have you removed the aux fan and cleaned behind it and the condenser? I pulled mine last summer and it had 20 years of accumulated crap behind it.

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1987 300TD 309, xxx 2.8.2014 10,000 mile OCI


Be careful of the toes you step on today, as they may be connected to the ass you have to kiss tomorrow. anonymous

“Be who you are and say what you feel, because those who mind don’t matter, and those who matter won’t mind.” Dr. Seuss

The ARV is the vac line going to the front of the turbo right? Its going to fall off.

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Bingo. If it falls off, don't forget to block off the source, at the fall-off point... on my car, that was just above the vac pump (see photo below).

I don't want to hijack this interesting thread on the power upgrades for the 603, but I have done pretty much a long list of items to try and get the cooling system under control. (I have also posted many times on the topic and searched extensively.) I first got my 1987 300TDT in May of 2004 and since then I have replaced: the radiator, the thermostat, the water pump, the fan and clutch assy., and the auxilary tank. I have also changed out the coolant from that green stuff to G05. When I first got the car, I also had the coolant tested for hydrocarbons since I figured the overheating might be due to a bad head gasket and blowby. The mechanic said there was no evidence of blowby. Another mechanic claimed that the missing encapsulation panel under the engine was probably causing the engine to overheat, but I'm not sure I buy that one. I'd put the encapsulation panel on, but I don't have one and they cost $500 plus my engine has an oil leak from around a head bolt. The engine does not exhibit any signs of a cracked head (hard hose after cooling overnight, etc.). The temp sits at between 80 and 90 on the gauge. (It used to sit closer to 80 until I replaced the thermostat.) My 300TDT only overheats going over the Cuesta grade, which is a long hill of about 5% grade that is over 3 miles long. The temperature will get up to over 110 degrees C on the gauge on a day that the ambient temperature is 70 degrees F. (The A/C is off, as are all other electrical accessories.) The car also seems warm up quickly (in my opinion). The temp. will get up to 85 degrees C within a half mile to mile of driving on a regular 70 degree F day. The only thing I can think of that might be causing the overheating is a cracked head or bad head gasket but my 300TDT doesn't exhibit the classic signs. I do have a "14" head. I also have not checked for debris behind the aux fan. I would give just about anything to be able to resolve this overheating issue. Sorry for hijacking this thread......anyhow back to 603 power mods.

I'm assuming EGT means Exhaust Gas Temperature. Dave, great write up on adjusting the injection pump full load screw. If I do the mod, I think I'd definitely opt for the intercooler, although from looking at the install procedure it is quite involved. I wonder if an air to water intercooler would be smaller. So where is the EGT gauge mounted in the interior?

-Steve

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1987 300TDT smoke silver w/ burgundy leather interior
2000 VW Passat wagon indigo blue w/ beige leather interior
1985 Mustang SVO
1970 Chevrolet K10 fleetside, shortbed

Steve, have you started a thread on your cooling problems? If not, you should do that, and post a link here... that way we don't get mixed threads. But based on your description, I'm thinking either airflow (stuff in the condenser, behind aux fan, in rad fins, etc)... or internal corrosion (citric acid flush)... or a flow problem (corroded housing behind the impeller, plugged passages in block/head/gasket, etc).

You are correct about EGT. An intercooler install is difficult on the 124 due to lack of space. It's much easier on a 123 or 126 because there's a lot more room for plumbing, etc. Here's photos of my gauge cluster:

What color is that called? I like it a lot. My cars paint was kind of rough when I bought it and I don't like the gold color it is so I was thinking of switching to a dark gray or blue.

So have you ever pulled the aux fan and cleaned behind it and cleaned the condenser?(See post #20) Also does you aux fan come on when you go over the Cuesta grade? If not you can change the aux fan switch/sensor. I have the 100/110 in my car; although some (Dave) think that is too low. Have you tested your aux fan? A new radiator isn't going to help if it doesn't have air flowing through it.

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1987 300TD 309, xxx 2.8.2014 10,000 mile OCI


Be careful of the toes you step on today, as they may be connected to the ass you have to kiss tomorrow. anonymous

“Be who you are and say what you feel, because those who mind don’t matter, and those who matter won’t mind.” Dr. Seuss

I would kill for my engine to be that clean.

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1969 280SE
2023 Ram 1500
2007 Tiara 3200

Well when you change the head, scrub every little piece like Dave did. That sure is a clean engine.

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1987 300TD 309, xxx 2.8.2014 10,000 mile OCI


Be careful of the toes you step on today, as they may be connected to the ass you have to kiss tomorrow. anonymous

“Be who you are and say what you feel, because those who mind don’t matter, and those who matter won’t mind.” Dr. Seuss

I'd have to replace the head, winter salt has corroded the aluminum.

I'm hoping to never have to replace the head.

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2016 Corvette Stingray 2LT
1969 280SE
2023 Ram 1500
2007 Tiara 3200

My car is Midnight Blue, paint code 904. It's non-metallic paint.

Side note about the cooling system... the electric fan does not do much while the car is moving at freeway speeds. It's designed to help at idle and/or low speeds. If the car is getting too hot at 70mph climbing a grade, turning the electric fan on earlier won't help much. Definitely check to make sure the fins are clean and airflow is not obstructed...


Don't worry, it no longer looks that clean. Bio300TDT is correct, that photo was taken shortly after the new head was installed back in 2002. While everything was apart, I did scrub the engine compartment. But unfortunately it doesn't stay looking like that if you actually drive the car. I do try to rinse it off about once per year, mostly to remove dust & dirt... I've managed to eliminate 95% of the fluid leaks. Here's another photo from 2002...

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