1990 Carrera 2, 135,000km, manual.

With engine cold, the idle drops very low then goes back up, then drops back down again (some sensor is obviously detecting that the idle is too low and telling the fuel injection to bring it back up again). When warm the idle stabilises. Oil pressure is around 3.5-4 at cold idle, going to under 5 with increasing revs. As the oil temperature goes up to the 1st white mark (one quarter of the way up the scale), the oil pressure at idle (800 revs approx) suddenly increases from over 1 to around 3. Even just a few more revs (1500) is enough to send the oil pressure rocketing to 5. Very quickly the gauge is pegged at 5+ and can't go any further up. When I drop the revs, the oil pressure gauge needle drops like a stone (much quicker than usual) down to under 1, and then moves back up to 3. If I drive at higher speeds (100km/h) the pressure is 5+ and it seems like the oil temperature goes further and further down until it is not far above where it starts from cold, as though the engine is being overcooled.

Ambient temperature is about 18 to 20°C (having just dropped a couple of days ago from 25+), oil level is about 1/3 down as shown on dipstick when hot with engine running and on oil level gauge.

I do a 40km daily commute with the car. This started the day before yesterday (more or less coinciding with the drop in ambient temperature, but that may not be linked), and continued yesterday. Today I didn't dare take the car to work, so I'm using the bike instead, as I'm afraid of blowing a gasket if the pressure is too high.

Any advice gratefully received, as my mechanic is still on holiday (the whole of France comes to a grinding halt from the beginning of July until early September).

 

First of all, the pressure cannot get too high. The pressure should be pretty much pegged when using the throttle in almost all situations. The oil pressure should be stable and quite high when idling cold (like 3.5+) but when warm the oil pressure should be around 1.5+ when idling. The fact that yours fluctuates is a bit curious - one question I have is that does the oil pressure fluctuate because your revs are also fluctuating?

The fuel injection has nothing to do with fuel pressure.

 

First of all, thanks for your very rapid reply!

I was under the impression that the pressure could build up to such a point that seals could blow, and that there was a pressure relief valve which opened to guard against this.

To answer your question, yes, the oil pressure fluctuates with the revs, but what has changed is that it did this in a linear fashion before whereas now i just need to increase revs a bit and the pressure goes off the top of the scale. Also, the warm oil pressure used to be around 1.5 at idle and now it's at 3.

(I wasn't talking about the fuel pressure in relation to the fuel injection, I just meant that when the idle gets too low and the engine is going to stall, the DME obviously kicks in and tells the fuel injection system to increase revs to avoid this.)

 

Are you sure your oil pressure sensor is OK? Mine used to be a bit unreliable and I had to replace it - I could sometimes get oil pressure of 1.5 before I even started the car.

 

The short answer to that is no, i'm not sure. If this sensor were faulty, could it influence the engine cooling system resulting in the oil temperature being too low? It does seem like the changes in oil pressure are being detected, but perhaps amplified. As I mentioned before, my mechanic is out of town, so no hope of checking this or indeed connecting another oil pressure gauge as a reality check. I really want to get an opinion on whether it's safe to continue driving the car while I wait to get it checked. I don't want to go and break something expensive.

 

I hadn't noticed the comment on your oil temp.

Did all of this start happening together? I don't think there is any chance at all that your oil temp gauge is giving you the true temperature when it drops to the "almost cold" level. Even a water cooled car wouldn't manage that, and no amount of air flow is going to reduce the temperature in an oil / air cooled car that much.

To me, it sounds like you have some other problem - maybe an earth or something - which is causing your oil temp and pressure readings to fluctuate.

 

if your temp gauge is showing a reasonable temp being maintained and you have no warning lights then personally i would drive my car like that, as I am sure it is just the oil pressure gauge that has a fault somewhere. Of course, the choice is yours though...

 

Yes, this all started happening together. If the car sits at idle then the oil temperature comes up to 8pm-9pm as it always has, but oil pressure is an indicated 3 and a small blip of the throttle sends it right up (so the oil temperature gauge appears to be working correctly, at least at idle). If the temperature goes above that then the oil cooler fan switches to high as it should (so the valve to the oil cooler appears to be opening when it should, so temperature sensor on oil cooler would appear to be ok). But as soon as I go for a few kilometres at around 100km/h with no stopping the temperature just goes down and down until it reaches just above zero.

 

english2, there are several things here that need some correction:

1. The temperature should go up to around the 8 o'clock mark, at which point the oil cooler should open and your oil temp will go down fast to about 7.30 or so if you see what I mean?

2. When your oil temp reaches around the 9o'clock position, the LOW speed oilcooler fan should cycle up and down as necessary (mine ddoes it in aprox 2 sec on, 10 off but I think this varies between car and temp etc).

3. When driving, obviously the temp should go down, but not to zero.

The point of 1 and 2 above is that the low speed fan operating is not a sign that the oil cooler is opening! As stated, the temp should go up and duck down again when the oil cooler opens. Primitively you can test if the oil cooler is opening by driving for at least 20 mins or so and then checking the front wing with your hand to see if it s warm. If it is, then oil is getting to the oil cooler.

Additionally, (and separately in my opinion) I would say that you also have an earthing or sensor problem somewhere...perhaps someone can be more specific as to what to try to isolate these? As I said before, if it wasn't for the temp gauge not working correctly I would definitely drive the car but keep an eye on it. Now, with this information clarified I would not recommend driving the car unless it is to the mechanic.

Good luck!

 

The oil temperature sensor that the CCU uses when deciding to turn on the low speed fan is located on the oil cooler radiator in the front fender. If the temperature rises enough in the radiator then that would mean the mechanical thermostat under the oil tank is opening?

 

Jason - thats a good point, but I am sure I have spoken to people some time ago who said the fans worked even though the oil cooler did not open.....i am happy to be corrected and or educated on this point..!

 

I decided to take the risk and drive the car - I did a 500km round trip from Paris to Northern France this weekend, and the car did not miss a beat. However, at 140km/h, the oil temperature stayed around 7pm (half way between the double line at the bottom of the gauge, and the first graduation). The pressure was pegged over 5 and couldn't go any further, then quite suddenly the gauge went down to 4.5 (as though a valve had opened somewhere), and it stayed there until I reached the autoroute tollbooth where the car idled with the pressure showing 1.5. However as soon as I started off again, the gauge pegged at the top once again and this time didn't go down until I stopped the car. I've checked for oil leaks but can't see any, and the level has not gone down. I'm still mystified.

 

It sounds like your oil pressure is working just fine. As RPMs increase so should your Oil pressure gauge. I will check today while driving, but from memory I would say that just about any time I'm not idling (i.e. whenever I've got my foot on the accelerator and I'm driving) the pressure gauge is pretty much pegged at 5. If you think about what's happening it makes sense. The oil pump is engine driven. This means that it is dependent upon the RPMs of the engine to function. Low RPMs=pump running slower, and pumping a small amount of oil: High RPMs=pump running faster and pushing more oil. As the pump speed/engine RPMs increase the oil pressure will also increase because you have an increased amount of oil moving through tubes with finite area. I think your pressure gauge is fine.
Your temp gauge is interesting however. As DaveK said, it should never return to the cold position, or really below about the white line at about 7 o'clock (Ambient air temperatures may drop it a bit, but it's still an internal combustion engine, combustion=controlled explosion=lots of heat). In my opinion, the oil temp gauge is a much more important gauge than the oil press gauge. If it continues to act strangely I would put the car up until the mechanic returns. However, the oil gauges on a 964 are somewhat difficult to understand at first. Be sure to read the manual for more information about them, and pick up Adrian Streather's book on 964s if you want to understand more about the systems. Hope this helps.

Brian

 

English2:

Your car works like a swiss clock!!! The fact that your oil pressur drops at idle is perfectly normal. The fact that it goes up to 5 and more is perfectly ok and normal. The fact that the oil pressure indicator blips up with the diff rpm's i.e fluctuating idle is perfectly normal! The fact that your oil temp is going to around 9 O'clock is perfectly normal! Enjoy your car and stop worrying!

Don't overfill it with oil. Clean your ISV (with carburator cleaner look at a recent tread like ISV, air-flow meter and idle microswitch clean)

Check if your car has a RS clutch i.e. single mass flywheel (that can cause the rough idle)
if you have a LWF (light wheel Flywheel -> single mass flywheel -> RS flywheel) you should buy a new DME Chip from a reputed Tuner (please no E-Bay crap) an put it in. That should stabilize if not cure your idle problems!

Hope this helps

Thomas

 

english 2, i still think you should get the gauge/sender/sensor looked at. I really don't think there is anything to worry about, i just think somethign to do with the gauge has gone wrong...