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  • 2003 300m Special runs hot after thermostat replacement

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Repair Questions and Answers.

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 #395111  by skyhawk61
2003 Special was running cool (1st mark on gauge)on chilly mornings; thermostat was stuck open so it was replaced by reputable Mopar shop (lots of experience with 300m's) with OEM Mopar thermostat. Engine now runs warm, with measured temps at top hose 230 degrees and little or no circulation noted in coolant tank. Pulled the water pump to check if impeller was loose on shaft. Checked OK, so another Mopar thermostat was installed, along with a new radiator. No change. Temp needle runs at first mark above half-way during normal driving, but it you turn the heater on full blast it cools down to just above the mark below half-way at idle; goes above half-way going down road. Engine coolant was bled correctly and there appears to be no leaks anywhere. Shop now looking at head gaskets; they suggested the open thermostat was masking this problem which became apparent when we fixed the thermostat.
The factory service manual alludes to an internal coolant bypass passage that allows pressure and flow to equalize somewhere near the thermostat housing. Is this correct, and could it have become clogged? (278,000 miles) If so, how can this clog be cleared? Thanks for your help. It's a really nice car otherwise. DSC
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 #395113  by M-Pressive
It was bled with the nose up in the air?

Head gaskets rarely go on these cars.

I would attempt to bleed it again with the nose on ramps.
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 #395115  by FIREM
Long read but full of great information. After verifying fan and all other issues such as bleeding you may need to do the thermostat relocation procedure.
Much less invasive and expensive that pulling the heads. While the head gaskets are rarely fail or leak there is a passage the appears to get blocked over time.
It has been about 4 years and 30k since I did the relocation with no reoccurrence of any cooling system issues.
 #395118  by skyhawk61
Thanks for your suggestions. Both fans are working normally, and we did the "nose in the air" bleeding several times with no effect. I'm not happy with having to pull the heads, but if it comes to that, where exactly in the block is this "bypass" that's referred to in the FSM? The Mopar guys at the shop told me they've not dealt with a "clogged bypass" in these engines before now. (Maybe most of their customers don't keep their LH cars running for twenty years like I do.)
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 #395119  by FIREM
Right Bank (drivers side) has a passage that leads down to the thermostat area ( described in lengthy linked thread)
Trust me you do NOT want to pull the heads on 270k engine. You are going to see things you do not want to see. By the time you add all the “while we have it apart “ stuff discovered to the head gasket job parts and labor co$t will easily exceed your love it number for the car.
Thermostat relocation $200-$250 job will put miles of smiles on the car.
 #395121  by skyhawk61
Thanks, FIREM, for all the great information. The Mopar shop has the front end off the engine already (for water pump and belt inspection/replacement, so I've already reached the "well, we've already gone this far, might as well go ahead and do the rest of it" point. I've added your well-researched write-up on the problem to my files for future reference. I see you've retired to Myrtle Beach, about a 5 hour drive from my location in Hickory NC. We missed Mopars at the Beach last time, but we make most of them. Thanks again.
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 #395122  by FIREM
Let us know how it works out, hope it goes well. The official “Mopars at the Beach” folks have retired to Florida and taken the show to Daytona. Another group “ is taking the event over in October.
 #395849  by skyhawk61
OK. I ended up replacing both head gaskets ($). There actually was a small compression leak on the #1 cylinder, and the bypass port was clogged as well. We put everything back together and the car seems to be running at its normal temperature. You gotta love old cars! Really.