Chrysler 300M Enthusiasts Club
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Repair Questions and Answers.

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 #379418  by AndyPre
 June 19th, 2019, 5:45 am
Hi all!

I have had my 300M for just about a year now and it drove without any issues. Two weeks ago, I went on a 2000 km trip home to Germany, and somewhere along the way, the master cylinder and brake booster went out. It started with a loud hissing coming from the footwell and having virtually no stopping power. I went on to my parent's house driving along really carefully and looking way far ahead to avoid braking. I had to drop someone off in Belgrade. The freeway from the south comes over a ridge with a fairly steep downhill slope for several miles. As I exited near the city center, the car was even harder to brake. I then noticed (after putting it in neutral), that at "idle" the RPM's were in the range of 2500-3000! I dropped my passengers off and got back on the highway as quickly as possible. I did not want to leave the transmission in gear, so I could brake, but leaving it in neutral caused it to rev really high. It was 90+ degrees outside, and for the first time ever, the temp gauge went to 3/4, when it is normally between 3/8 and 1/2. I had read somewhere that this may be a sign of head gasket failure... so I got scared. I got back on the highway, cranked up the heat (to reduce coolant temp), and drove on. Everything return to normal, except the braking issue due to bad master cylinder and booster. I eventually got to my parents, ordered the parts from RockAuto (and they got to Germany 2 days later - awesome), and had their mechanic install them, replace the brake fluid, while he was in there as well. Brakes were back to normal, and I left the next day (2000 km back home). All was well until I stopped at the pass over the Carpathian mountains to take care of nature for a minute. On the way down, I used the autostick to select lower gears for engine braking (this is the only time I ever use this feature). It went downhill for about 20 minutes, and I got stuck behind a slower vehicle. I noticed the brakes not being as good as before, as if I were pushing the accelerator at the same time. I put the transmission in neutral, and lo and behold, the revs were up in the 2500-3000 range again. I drove on, only stopped once for fuel, and after a while, everything was normal again. With the exception of the engine temp: I had to wait at the border to Bulgaria for 40 minutes in 95 degree heat. At first, the temp gauge stayed at 1/2 for a really long time (I had A/C on, etc.), but then it slowly crept up to 5/8 and then even 3/4. I turned the engine off and only turned it back on again to move forward a few feet every few minutes. After I got through the border control, I got on the highway, and everything went back to normal.

So... what would cause the rpm's revving that high? It only seems to occur after a longer downhill drive. If the car had an accelerator cable like my old Citroen, I'd say it's sticky but not stuck. But it's electronic... or could it be a vacuum problem? I searched the forum for this issue but did not find anything like my situation.

Should I be concerned about the coolant temp getting that high? I checked the coolant after it cooled down, and it was unchanged. There also seemed to be no water in the engine oil (just from checking the dipstick and filler cap.)

Any advice and / or help in figuring out what's happening would be greatly appreciated.
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 #379422  by FIREM
 June 19th, 2019, 7:31 am
Obviously check the mechanical things like gas pedal, floor mat, cable to TPS and Cruise Control cable for binding or causing throttle to stay open.
After that a vacuum leak will cause high rpm. Booster failure could have been a contributing factor. Check the hose, and check valve for leaks.
Fix that first, then look into the temperature issue. Lean mixture from vacuum leak can elevate temperature. Check fan operation as the temp is ok moving but climbs when stopped.
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 #379429  by AndyPre
 June 19th, 2019, 9:54 am
The gas pedal and floor mat are all fine (checked those on the road when it first happened). The other ones I will look into. The fan kicks in once it gets to the 1/2 mark. I will check the radiator as well, though. Maybe it's gotten too dirty or something. Thanks for the tips on what to check! I will wait for things to cool down first, though. Summer is here... 8)