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I have a 2004 Grand Prix GTP that blew the top heater hose fitting on the intake manifold which caused the car to go into Overheated Engine Protection Operating Mode. I fixed the hose fitting and now the car is very hard to start once warm and I keep getting the ODBII code P0300-Random Misfire. Could the overheating of the car have caused this? I know when the car goes into Overheated Engine Protection Operating Mode that it alternates firing groups of cylinders.
Any and all help is greatly appreciated.


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I changed the plugs and wires and that seems to have fixed the problem.


   
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The overheating and resulting engine protection mode, probably just magnified an existing problem. You were probably due for plugs and wires soon anyway, or had one about to go that was taken out by the situation that happened. Have seen that many times.


   
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Hi, new to this forum. Great site! I have scanned through the posts and haven't found one that is like my problem. This post may be lengthy as I will outline what has been done and then the current problem. This GTP now has about 170xxx miles on it. Runs really good. A few months ago we had a problem with the thermostat getting a bit erratic but never really got hot. We did a thorough flush on the radiator and engine as it was showing signs of the common dex cool deterioration. I am sure this was not the first change, do to age of the car and past experiences with dex cool. We replaced the thermostat and filled up the cooling system with compatible 50/50 mix . The car did well, running a solid temp, until a few days ago. I then started heating a bit at highway speeds ( 60-70mph) but cooled down immediately at slower speeds and did well in city traffic. Removal of the radiator cap showed some accumulation of sludge again so we bought a new radiator and, from a gm dealer, a new thermostat. Drained and thourghly flushed the engine again. Installed new radiator and thermostat, refilled with 50/50 mix - burped the system 3 times to make sure we had all the air out. We then let the car run with A/C on high until it reached normal temperature (small fluctuation of gauge indicated thermostat opened) and then let it run for about 30 minutes. Gauge was rock solid. Laser scanner showed 180 degrees on front head and rear head. Then we took it for a road test; all fine until we ran it for a ways at highway speeds where up on it proceeded to climb in temperature. Slowed down and gauge returned to normal. When we got home we put the laser temp sensor on it again and found the rear head running at 220 and the front head at 160.
Question is what would make the rear head run that much hotter? Also why would highway speed affect it when it does fine sitting still with the A/C on?


   
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use an exhaust gas analyzer and see if you have hydrocarbons coming out of the radiator.

also coolant leaks from the intake is common. So is coolant leaks around the EGR port area. Pressure test the cooling systen and see if it holds pressure or if any lweaks show up. Check for coolant in the oil, oil in the coolant, coolant in the trans,

make sure the fans are working normally. maybe drive with a scanner on and see what the temperature is doing. then check that against the handheld infrared thermometer.

the temperature on the front head may be lower because it is closer to the fan. (if the fans come on) Engine temp should be close to 210F so 220F on the rear head seems ok. 160F seems low but if the fan has been running the external temp will be lower from the fans blowing on the front. maybe...


   
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Thanks for response, carriedi
The fans are running, there are no leaks in the cooling system and it pressures up just fine. It makes sense that the rear head would read a higher temp than the front, I am just surprised that it is that hot (I've not had occasion to check another since all 6 of my other vehicles are rear wheel drive).
So, if 220 is ok on the rear head that only leaves the problem of it running hotter at highway speeds. At sustained speeds over 60 it will continue to climb but will promptly cool back to normal under 50 or even sitting in traffic. (Have not allowed it to get beyond 3/4 scale on the gauge; it normally runs at about 1/2 scale.) I just now had a thought; could it be the cats are getting clogged?


   
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usually to have an overheat at highway speeds but not at lower speeds is because there is not enough coolant flow through the radiator to cool the engine down. since you have already replaced the radiator and thermostat those should be good. (but I have had new parts that were bad) so then we are looking at not enough circulation or not enough coolant to circulate. That's why when you slow down the temperature goes down. The engine building up heat is dissipated by the circulation of the coolant.

when you have the car sit and are checking the temperature, what temp does the low speed coolant fan come on? and then what temperature does it turn back off?

Is the heater putting out a lot of heat when you put it on full hot? (Should be 140F or more with the engine at normal temp) If it's 140f or more then the flow through it would be okay.

Driving with a scanner hooked up to your car would give you a digital reading of the temperature you could compare to your gauge.


   
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The coolant fan was not coming on until after the gauge reached more than 3/4 scale and then shutting off before it got back to normal so we wired the fans to run all the time. With the fans running it has always held a steady half scale until this highway speed heating started a couple of weeks ago. At first it was minimal then gradually got worse.
The heater is working very good; plenty hot. That was the first thing we checked when considering a possible circulation problem.
The new radiator is an exact replacement and, when flushing the engine (with the thermostat out) we got nothing but good clear flow this time. There was no gunk on the thermostat we replaced nor in the outlet. The only sign of any gunk left was in the radiator. I have had problems with aftermarket thermostats so I only use OEM stats now. Have not had a failure on a half dozen, or so, OEM stats (to date anyway). Oh, we did replace the upper and lower hoses while we were at it.
Will see if I can borrow a scanner in the next few days so we can check actual driving temperature.
Thanks again for your comments and ideas.


   
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