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Discussion Starter #1
I have had my 2005 300C for about 3 years (20,000 miles) without any issues. I am a GM service technician so I perform all my own maintenance work at the exact mileage or before. Roughly 2 weeks ago my oil pressure sender failed and caused a check engine light. I was short on cash at the time and just ordered my new oil pressure sender a few days ago. That was my first problem. Yesterday I was coming down my dirt road (about an 1/8th of a mile) and my car began overheating when going over the uneven parts in the road. It has never overheated on the dirt road or when on pavement, it's never even increased past normal operating temperature. The cooling fans were on and my coolant level looks fine, so I don't know what caused it. Today I took it for a drive around my house for about 20 minutes, ran great. Sat in the bank drive through without issue even. Came down my dirt road without issue, until I hit the uneven part of the road. The temperature goes back to normal very quickly once the road flattens out. My scan tool is only for GM (Tech 2) and our multi-make scan tool at work is under lock down because our new service manager is being ridiculous, so I haven't been able to scan it. I've never had a problem like this though, any ideas?

**When I say I'm a GM service technician, I mean services, such as maintenance and brakes. I'm not extremely knowledgeable about diagnosis. Thanks.
 

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That is a likely electrical problem. Some wire or connection is being jostled by vibration. This could take a while...
 

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Discussion Starter #3 (Edited)
Ok, then I should add that one time my driver's head light went out for about 20 minutes and then came back on. I attributed it to a possible loose connection in the bulb itself due to the dirt road, because I checked all connections and wires and everything looked like it should. Maybe the oil sender is related.

*UPDATE: After talking to my wife who drives the car every day, only 1 fob is working and that it occasionally says RKE battery low. I have the car today and remembered that I could check for codes with the key. That's how I found out about the oil sender code. However it will not show my codes now with the key method (check engine light is on).
 

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1) Look for a bad electrical ground in the vicinity of the radiator fan. There are a couple of ground connection points to the body just
behind the headlights.
2a) Get a new key fob battery, CR2032 as I recall.
2b) Some early key fobs had bad solder joints associated with the battery hold-down clip. Check the solder joints when replacing the
battery. You may not have to check as the battery may just fall off of the circuit board!
 

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Discussion Starter #5
1) Look for a bad electrical ground in the vicinity of the radiator fan. There are a couple of ground connection points to the body just
behind the headlights.
2a) Get a new key fob battery, CR2032 as I recall.
2b) Some early key fobs had bad solder joints associated with the battery hold-down clip. Check the solder joints when replacing the
battery. You may not have to check as the battery may just fall off of the circuit board!
One key fob was broken exactly where you stated and I soldered it last night, as well as replacing both batteries. The same key fob works and it says RKE Low at all times.

I will check the grounds today if I can get some good weather, if not then i will tomorrow morning. The fans come on before I get to the uneven part of the road and they stay on till I turn the car off. Hopefully I can look at it today, I hate having a car down, and I hate not knowing if I'm actually overheating or if it's electrical. As soon as I can check those connections (and hopefully get a wiring diagram) I will be back on. Thanks DESIREE
 

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Discussion Starter #7 (Edited)
Thanks for those manuals.

I got a chance to take about a 15 minute look at it today. All the connections around the fans and headlights were tight and all the wires looked fine. Still looks like new actually. I was thinking about when my oil sender went out and it was when I was turning slowly, just like ALL the other problems (except the fobs). I'm going to "jingle" around the wire harnesses and connectors with the headlights on later in the evening to see if I can narrow it down to one harness (pigtail). I just found out I have a fog light out, although it is staying out rather then a one-time intermittent outage like the driver's headlight. I will update as soon as I find out more.
 

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Also check that all of the fan-related fuses and relays in the PDC are secure in their sockets.
 

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Discussion Starter #9 (Edited)
Last night I "wiggled" every harness, pigtail, and connector I could see up front as well as the body grounds with the headlights on and nothing changed with the lights. I also had my wife turn the wheel while I did this and nothing changed. I turned it off before it reached normal operating temperature since I don't know for sure if it's actually overheating or not. This morning I checked all the fuses and relays after seeing your message and they are all firmly in place. I also got a good light and looked at the wiring more closely and everything looks new. The wiring and connector for the coolant temp sensor looks good as well. The only good news is I did get the code to read. Evidently you have to (if you have an RKE low msg) do the key method and then clear the RKE message, then repeat the key method and it will show the codes as normal. Only problem is they are both related to my oil sender: P1521 and P0520. Nothing about cooling, overheating, or open circuits. Good news is I have all day today to look at it unless something comes up. Thanks for the help so far.

Any chance of the bad oil sender causing the problems? Doesn't seem likely, but it was the first thing to go.
 

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Discussion Starter #10 (Edited)
Didn't get anywhere yssterday. Took out the air box and looked closely at pretty much every connection and wire under the hood and everything looks normal. Any more help would be appreciated. I was considering changing the thermostat since it's so easy and I know it's the factory one, but the fact that it only happens on the way back home on the uneven part of the dirt road makes me think it's not relevant to the problem.
 

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Stop. Before you go any further you must know if the "over heat" problem is real. Get yourself an infrared thermometer. Cheep at Harbor Freight.

We hate to see you going to all this bother looking for the wrong problem.
 

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I presume the overheating problem you mention is brought to your attention by the dashboard temperature gauge. Forget that thing and get into the EVIC and see what the car says the coolant temperature is. Keep that display mode in the EVIC and watch for temperature fluctuations as you do your daily driving. For a stock thermostat equipped engine you could see as high as 225 or so degrees F. If your "overheating" is like that and drops off rather quickly as the fan kicks in, then it's pretty much normal.
 

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Discussion Starter #13 (Edited)
I presume the overheating problem you mention is brought to your attention by the dashboard temperature gauge. Forget that thing and get into the EVIC and see what the car says the coolant temperature is. Keep that display mode in the EVIC and watch for temperature fluctuations as you do your daily driving. For a stock thermostat equipped engine you could see as high as 225 or so degrees F. If your "overheating" is like that and drops off rather quickly as the fan kicks in, then it's pretty much normal.
Ok I will take a look. I wasn't aware that it displayed the temperature digitally, guess I need to drive the car more often. The gauge went to the 3/4 mark. My fans were already on when it began to overheat both times. The temperature falls quickly if you turn the car off and then back on and then will run completely normal until it gets to the bumps and really uneven parts of the road, that's why I have been thinking it's electrical. The gauge moves so slowly though that I can't be sure. I'll drive it today and watch the EVIC when I get to those parts of the road. However, if the gauge was misreading due to an electrical issue, wouldn't the EVIC misread too? If so I could pick up a thermometer like tarheelcracker suggested, but I wouldn't get a chance to do that till later in the week.
 

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Discussion Starter #14
I haven't had a chance to come on in a while and wanted to give an update on this issue. Turns out it was the thermostat. I had to chance my oil sender and I replaced the thermostat while doing it since I had the hose off already. I have test drove the car a handful of times and it runs at normal temperature EVERYWHERE. The secondary fan doesn't even kick on at the uneven part of the dirt road where it use to overheat. I guess the fact that it happened on that part of the road was just a coincidence. I suppose there could have been a connection that I "connected better" since I removed the cooling fans and some other connections, but it seemed like everything was well connected (some of them were a PITA). Thanks for all the help.
 

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I haven't had a chance to come on in a while and wanted to give an update on this issue. Turns out it was the thermostat. I had to chance my oil sender and I replaced the thermostat while doing it since I had the hose off already. I have test drove the car a handful of times and it runs at normal temperature EVERYWHERE. The secondary fan doesn't even kick on at the uneven part of the dirt road where it use to overheat. I guess the fact that it happened on that part of the road was just a coincidence. I suppose there could have been a connection that I "connected better" since I removed the cooling fans and some other connections, but it seemed like everything was well connected (some of them were a PITA). Thanks for all the help.
What was the problem with the t-stat you installed:
A new t-stat that didn't work correctly?
Or did you install it backwards (if that's even possible?)?
Or did you install it with the 'jiggle pin' air bleeder at the bottom??
 
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