Overheating whats the problem and solution
12-04-2005, 07:41 PM
#1
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Overheating whats the problem and solution
After running 3 sessions at VIR DE my 911SC began to overheat. Oil temp ran up to the redline. The facts: Ambient temp about 42 degrees. 79SC stock except for SSI, 140000 miles on engine with a top 20000 miles ago. After 5 laps the temp began to rise. Nothing unusual about the laps. Oil is 1 Qt below full. 10-40W Shell, recently changed. Valves adjusted before headed to the track.
Ideas of others (maybe mine too). Oil cavatition. Was a problem on 78-79 and reportably fixed on the 80. How? Alternator bearing may beginning to fail. No unusual noises but the green alt light flashed once. Also, when pitting the car would not idle. Ran ok at low speed, without bucking but when you depressed the clutch it quit. Alternator belt is new (fairly) and popular opinion is not too tight nor too loose.
Why after a run would this begin to happen? Once cool, starts and operates normally without overheating. Got it home and off the trailer and it runs normally, no unusual noises. Help.
Ideas of others (maybe mine too). Oil cavatition. Was a problem on 78-79 and reportably fixed on the 80. How? Alternator bearing may beginning to fail. No unusual noises but the green alt light flashed once. Also, when pitting the car would not idle. Ran ok at low speed, without bucking but when you depressed the clutch it quit. Alternator belt is new (fairly) and popular opinion is not too tight nor too loose.
Why after a run would this begin to happen? Once cool, starts and operates normally without overheating. Got it home and off the trailer and it runs normally, no unusual noises. Help.
12-04-2005, 08:21 PM
#2
RL Technical Advisor
Hi:
OK,...in no particular order, but you should thoroughly address all these points:
1) Pull the sump plate at the bottom of the case and see if you have the venturi-type screen around the oil pump pickup tube. Further, make certain that the screen and plate are installed correctly to prevent cavitation.
2) Make sure that the alternator belt is tensioned correctly (.5"-.75" deflection at mid-point) and not glazed from slipping.
3) Provided that you've not found any problems yet, pull the fan housing and look to see if you have any mouse nests on top of the engine oil cooler under the shroud.
4) Make sure that the boots on the spark plug connectors are nice and pliable. When they get hard, they cannot seal and cooling air leaks precisely where its most needed; the heads.
5) Make sure that your ignition timing is at 34-35 degrees total at 6K.
6) Make sure that your idle CO is at 2.0-2.5%.
And lastly, replace that stock SC trombone oil cooler with a Carrera one, if you haven't already done so. This is a prerequisite for any SC doing DE events.
OK,...in no particular order, but you should thoroughly address all these points:
1) Pull the sump plate at the bottom of the case and see if you have the venturi-type screen around the oil pump pickup tube. Further, make certain that the screen and plate are installed correctly to prevent cavitation.
2) Make sure that the alternator belt is tensioned correctly (.5"-.75" deflection at mid-point) and not glazed from slipping.
3) Provided that you've not found any problems yet, pull the fan housing and look to see if you have any mouse nests on top of the engine oil cooler under the shroud.
4) Make sure that the boots on the spark plug connectors are nice and pliable. When they get hard, they cannot seal and cooling air leaks precisely where its most needed; the heads.
5) Make sure that your ignition timing is at 34-35 degrees total at 6K.
6) Make sure that your idle CO is at 2.0-2.5%.
And lastly, replace that stock SC trombone oil cooler with a Carrera one, if you haven't already done so. This is a prerequisite for any SC doing DE events.
12-04-2005, 09:07 PM
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One more thing to check - this happened to me when I owned an SC - the oil thermostat in the right rear wheel arch. It is closed under cold conditions, thus helping the engine warm up quickly and then should open when temperatures reach 120F (or something like that). If it fails, then it will not open and allow flow to the front cooler. This is easy to check: bring the engine up to full operating temperature and check if the front cooler is warm. If not, you found the problem.
If you are going to run track events, take Steve's advise and get the Carrera or some other high surface area cooler installed.
It was cold at VIR and my turbo engine temperature was higher sitting at idle than when I was running full 30 minute sessions - all thanks to an auxillary oil cooler.
If you are going to run track events, take Steve's advise and get the Carrera or some other high surface area cooler installed.
It was cold at VIR and my turbo engine temperature was higher sitting at idle than when I was running full 30 minute sessions - all thanks to an auxillary oil cooler.
12-06-2005, 03:52 AM
#5
Former Vendor
In addition to the excellent advice above,
7) Check the operation of the on-engine oil thermostat. It regulates oil flow to the on-engine cooler.
You can do this without removal (though the best way is to plop it in boiling water and observe it opening). Get the engine to temp, then very carefully (so as to avoid the heat exchanger) reach under and feel the bottom of the on-engine cooler. It should be too hot to keep your hand on.
Both internal and external tstats start to open in the 180-185 F range.
8) Also make sure the on-engine cooler is clean. Sometimes oil leaks can fill it's fins with oil/dirt that prevents airflow through the cooler. You can check this when you pull the fan, good time to spray some degreaser through it.
9) Is there any reason to suspect the oil temp sending unit and dash unit are mismatched? There are two different setups. This happens way too often when engines are swapped, or the wrong unit is used for replacement, and the result is false readings.
10) Dents in your external cooler lines. These get smashed all the time by misplaced jacks and aggricultural excursions. Significant dents and line closure create back pressure that cause a pressure bypass valve to open in the external tstat, thus short-circuitng the external cooler entirely.
Note the oil lines usually still feel hot to the touch if this problem exists, because the bypass only opens when the pressure is high enough - as it is when RPM is high at a DE. When you get out of the car to feel the lines, you let it idle and the pressure drops - most or all the oil then flows through the cooler.
11) Engine surround sheet metal and rubber gasket in place and sealing properly? Else heated air recirculates bottom to top, elevating engine temps. Not usually a problem when the car is moving fast, but in the interest of completeness....
7) Check the operation of the on-engine oil thermostat. It regulates oil flow to the on-engine cooler.
You can do this without removal (though the best way is to plop it in boiling water and observe it opening). Get the engine to temp, then very carefully (so as to avoid the heat exchanger) reach under and feel the bottom of the on-engine cooler. It should be too hot to keep your hand on.
Both internal and external tstats start to open in the 180-185 F range.
8) Also make sure the on-engine cooler is clean. Sometimes oil leaks can fill it's fins with oil/dirt that prevents airflow through the cooler. You can check this when you pull the fan, good time to spray some degreaser through it.
9) Is there any reason to suspect the oil temp sending unit and dash unit are mismatched? There are two different setups. This happens way too often when engines are swapped, or the wrong unit is used for replacement, and the result is false readings.
10) Dents in your external cooler lines. These get smashed all the time by misplaced jacks and aggricultural excursions. Significant dents and line closure create back pressure that cause a pressure bypass valve to open in the external tstat, thus short-circuitng the external cooler entirely.
Note the oil lines usually still feel hot to the touch if this problem exists, because the bypass only opens when the pressure is high enough - as it is when RPM is high at a DE. When you get out of the car to feel the lines, you let it idle and the pressure drops - most or all the oil then flows through the cooler.
11) Engine surround sheet metal and rubber gasket in place and sealing properly? Else heated air recirculates bottom to top, elevating engine temps. Not usually a problem when the car is moving fast, but in the interest of completeness....
12-07-2005, 07:16 PM
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May have found the problem, but don't completely understand it. It seems that the alternator bearing has begun to fail, mixed with the fact that I powder coated the shroud and fan, thus reducing the fan blade/shroud clearance. Although I had filed the blades to remove the excess paint, it may not have been enough. The shroud now shows signs of the blade rubbing against shroud. As the bearing began to fail it allowed the fan to wobble a little interupting the flow of air, possibly slowing down the fan enough to allow the temp to rise. Once the temperature started upward, and the flow of air hampered, it didn't take long to peak near the very bottom of the first red line. Still, it made no noise, did not do it at idle or without a load.
Go figure. Forrest Gump time! Dumb is as dumb does. Lesson, if you try to make it look good, it will get even by not working anymore, kinda like welfare!
ARF
Go figure. Forrest Gump time! Dumb is as dumb does. Lesson, if you try to make it look good, it will get even by not working anymore, kinda like welfare!
ARF
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12-09-2005, 05:42 PM
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I wonder if the fan belt was slipping, reducing the amount of air going over the engine/engine cooler?
If you are referring to the rear alt bearing, once it gets loose a bit, the fan will hit the shroud.
If you are referring to the rear alt bearing, once it gets loose a bit, the fan will hit the shroud.
12-09-2005, 11:36 PM
#9
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If you haven't already, you may want to read the 911SC FAQ in the engine section regarding some changes made to the early SC engines to prevent overheating under AX and track conditions. Engine section, page 2. Also, there's discussion there and in the performance section on engine cooling.
