914 high oil temperature
Bruce Anderson
National Technical Chairman
Porsche Panorama, 1990
914 high oil temperature: My 1974 91.E seems to be running too hot. Even with outside temperatures in the 80s, the needle on the oil temperature gauge gets up to about 90 percent of the way to the red zone. Does this mean it is too hot? Could I substitute a calibrated 911 gauge for the 914 console gauge to determine what the temperature really is? If the car is running too high an oil temperature, does it mean there is something wrong with the oil cooler or the air cooling flap adjustment? If there isn't, what can be done to reduce the oil temperature?
Alan Caldwell responds:
This question was the most frequent 914/4 inquiry received during the warm weather months. Causes for higher-than-normal oil temperature can include too low an oil level, blockage of the cooling blower intake or internal passages, improper ignition timing, too lean fuel mixture or a spark plug heat range warmer than specified.
The USA 914 factory installed oil temperature gauges never included any numbers on them and the owner's manual advises that the needle will normally vary up and down within the white zone depending upon ambient temperature and engine load, but the owner is not required to take any special action unless the needle goes into the red zone.
Most Porsche owners don't trust gauges without numbers and the 914/4 situation was made worse by the first console oil temperature gauges (914.641.118.20) in the 1973 models showing very little needle movement regardless of conditions. The white zone on these early gauges was about an inch wide and the red zone about three-fourths of an inch. The sender unit with these gauges had a temperature value of 200°C (392°F) stamped on the headend. the normal operating range of 160 F to 210°F occurred over a small portion of the white z6ne (Figure 1)
In 1974, a new gauge (914.641.118.30) was introduced which had a much larger white zone and much greater needle travel than the earlier design. The sender unit that came with the newer gauge remained the same (part no. 039.919.268)
The first step in oil temperature peace-of-mind is to find out just how high your oil temperature really is.
The quickest and least expensive way to find where you are is to determine at least one pertinent calibration point on your existing gauge equivalent to 212°F.
The most convenient time to do this is during an oil change when all the oil is out of the engine. This can be done by removing the gauge from the car and also removing the sending unit at the left forward side at the bottom of the crankcase. (Keep track of the way the wires are connected to it).
Connect a positive (+) wire from the battery to the gauge (+) terminal and then run a connecting wire from the gauge to the center electrode on the sender. A ground wire from the sender base to the gauge ground (-) and also to the battery ground completes the circuit. The sender can then be immersed in boiling water and the needle on the gauge should stabilize at the point on the scale equivalent to 212°F.
This procedure is only approximate since increases and variations in line voltage with the engine running may cause slightly higher needle positions. However, the results should be close enough to tell you if you have a problem.
Even the single 212°F point obtained with boiling water is a significant position to know on your temperature scale, since it is toward the upper end of the normal operation zone of 160°F to 220°F.
If you want to get really scientific and have an oven or candy thermometer, you may be able to also check a wider range of temperatures up to the red zone by using cooking oil instead of water.
This latter type of calibration was used to arrive at the temperature scales shown in Figure 1. Since we found three different 914/4 sender units stamped with 120°C, 150°C and 200°C, the primary combinations of gauge and sender were also calibrated.
On my 1973 914/2.0, my calibrated gauge has read as high as 200°F on a warm day under normal driving conditions and up to 2501F for sustained track driving. When coming off the track, the temperature quickly falls to the 200 °F region. If your car has the 2000C sender and the late style gauge, it is possible that your oil temperature at 90 percent of the way to the red is about 250°F and is probably too high for continuous running at that point.
Although a calibrated 911 gauge and sender would work in the 914, it would not be particularly convenient in size unless you wish to mount a 911 combination oil temperature and oil pressure instrument in the dash and put a separate fuel gauge in the console.
((LarryM NOTES: this is the way my Euro-spec 914/6 M471 GT is setup; the fuel gage that works is an early VDO VW bug gauge))
Alternatives include a 914/6 combination instrument or the similar 914/2.0 combination instrument that included an oil temperature gauge and was used on the two-liter cars that did not have the appearance group and console.
The 914/6 combination instrument (914.641.101.10) is shown in Figure 2. The oil temperature gauge portion of the instrument has five factory calibration marks. The marks are along the upper edge of the interior face and are not very easy to see without looking at a very small angle or removing the temperature gauge portion of the instrument by taking out the four screws in the back. The calibration points include 50 °C (1220F), 68 °C (154°F), 109.5°C (229°F) when the needle is vertical, 129 °C (2641F) at the beginning of the red-shaded zone and 144°C (291°F) at the beginning of the solid red zone. There is also a red oil temperature warning light at the high end of the scale that was planned for use with the sportomatic cars, but not connected to anything on the manual transmission cars.
The 914/6 oil temperature gauge uses a different sender than the 914/4, part number 901.641.632.00, which is a 150 °C unit. A calibration check of this gauge similar to that used for Figure 1 is indicated in Figure 2. The gauge readings were found to be within five to ten degrees of the factory calibration marks from 160°F upward.
The four-cylinder, two-liter cars without console used two different combination instruments with oil temperature gauges. The original gauge of this type (914.641.101.30) was used until the end of the 1973 model year and pictures of it show a wide red zone similar to the 1973 console gauge. In 1974 and later cars, an updated gauge, 914.641.101.50, was used with a smaller red zone. I was able to borrow one of the 1974 and later gauges and found, like the 914/6 and 911 gauges, it had factory calibration marks on the edge of the gauge face indicating that the bottom of the white scale was 30°C (86°F), the beginning of the red zone was 156.7°C (314°F), and the middle of the red zone was 170°C (338°F).
Figure 3 shows the 914/2.0 combination instrument and the oil temperature calibration using a 200°C sender and the same method as for the console gauges. The results were very close to the factory calibration marks: the calibration thermometer indicated 308 °F at the beginning of the red zone instead of 314, which seemed within the accuracy of the method and gave some confidence that the rest of the numbers weren't too far off. The overall calibration of the combination instrument was also reasonably close to that of the console instrument with the smaller red zone. In the gauges checked with the stock 200°C sender, the red zone starts at approximately 300°F.
A nice advantage of the dash-mounted combination instruments with their removable oil temperature gauges is that small white transfer numerous can easily be added to the face of the gauge so you don't have to remember the calibration. In addition to the Porsche gauges, VDO offers an aftermarket oil temperature gauge with numbers on it (the range is from 120°F to 300 °F) and the sender specified for it is the 150 ° C unit. The gauge is somewhat smaller in diameter than the stock console unit (approximately two inches compared to 2 3/8), but an adapter is available which allows it to fit.
We also compared the electrical resistance versus temperature curves of the VDO 150°C sender (901.641.632.00) used on the 914/6 with the VDO 150 °C sender used on the aftermarket VDO gauge. Below 180 °F, the 914/6 sender had substantially greater resistance, so it is probably a good idea to stick with the sender recommended for a given gauge.
Current thinking is that continuous operation of air-cooled engines at oil temperatures over 220-230°F is not desirable and will degrade engine longevity. If you find you really are running in the 250°F range, you should first check all the contributing factors.
The 914/4 engine oil level needs to be at or slightly over the full mark under warm running conditions. The viscosity range of the oil should be high enough to cover the maximum expected ambient temperatures. Make sure all cooling passages are open and that the intake to the blower is not blocked. Adding the fully-screened engine deck lid that was used on the 914/6 competition cars will also allow more cooling air into the engine compartment.
The thermostatically-controlled cooling air flaps that are designed to assist in initial engine warm-up might be a contributing factor if not properly adjusted or if the thermostat has failed. These two flaps, one on each side of the engine at the blower housing exit under the engine shroud, are mounted on a spring-loaded rod that runs across the top of the engine and is actuated by a cable that is attached to a bellows-type thermostat on the lower portion of the engine (Figure 4). The flaps control all the cooling air that flows over the cylinders and oil cooler. The system is fail-safe to the extent that the spring forces the flaps to be fully open if the cable should break.
Under normal operation, the bellows contracts when cold and pulls the cable down and closes the internal flaps. As the engine warms up, the bellows expands and allows the spring to open the flaps. The thermostat is designed to open the flaps at 150 to 160 °F. You should make sure that under hot running conditions, there is enough slack in the cable to allow the flaps to be full open. This is easy to check by manually twisting the control rod to determine if the spring is holding the flaps all the way to the open stop. If the bellows fails in a way that prevents it from expanding properly, it is possible that it could prevent the flaps from fully opening and thereby restricting the cooling flow. Oil coolers for the 914 have generally been quite reliable. If there is any problem with a cooler, the most likely possibility is that dirt or debris is lodged on the top of the cooler and is preventing the cooling air from going down through the air passages to pick up the heat.
It is probably also worth checking to make sure that the two air deflectors mounted on each side of the lower engine compartment forward edge are in good condition. These flaps were first added to the 1973 models and, along with the shorter rear valance panel across the rear of the car under the bumper, helped reduce engine temperatures by more efficient extraction and mixing of the hot cooling air from the engine with the air moving under the car. Higher than normal cylinder head temperatures can be caused by use of a nonstandard spark plug that has too hot a heat range or by a fuel pump that is not putting out the specified pressure and causing a leaner mixture.
If all of the above items have been checked and found to be all right and there is still too high a temperature, the ultimate step is the addition of an auxiliary oil cooler system. The basic elements of an auxiliary oil cooling system include a new oil filter base that contains attachments for lines to the cooler, a thermostatic valve that protects the cooler from over pressure during cold operation, the radiator, lines and fittings. All of the parts are available from various aftermarket suppliers.
The primary problems in fitting an auxiliary cooler to the 914 relate to finding a convenient-sized radiator and location for it on a body that doesn't have much extra space. The most desirable location is probably that used on the 914/6 GT where the radiator was mounted in a cutout in the front bumper and lower spoiler with the oil lines running to the rear behind the rocker panels. The problems with this installation are the major changes that have to be made to the body components (bumper, spoiler and possibly the luggage compartment) and the related changes in appearance.
The most common mounting positions used by previous owners have included lower body mounting, rear mounting alongside the transmission or under the trunk and mounting under the engine deck lid screen. None of these positions result in as good a flow of cooling air through the radiator as desired, but most owners still have obtained a substantial reduction in oil temperature.
A recent installation in my local region that seems to work consisted of a reworked 911 engine oil cooler mounted in the right front fender (similar to a 911) with the lines running through the right-hand rocker panel. Holes were drilled in the body shell behind the horn/fog light grills to allow air into the fender area where the cooler was mounted.
Installation of an auxiliary oil cooler is not inexpensive or simple, but may be the only approach to attain moderate oil temperatures in a hot climate.
Andy
Andy
here is the combo gage calibration pic
