Oil Pressure Monitoring and Warning System - how does it work?

[zcacogp]

Chaps,

I changed the cylinder head on S2 a while back. The replacement was fully rebuilt and it all works very nicely as a result; a chunk quieter, I'm sure there are a few more horses in the mid-range and I'm very happy.

However the oil pressure warning light has taken to coming on very slightly since the work. It happens when the engine is hot and the car is in traffic and idling (i.e. lowest pressure conditions), and when the light comes on it does so very gently - only a slight glow. The slightest tickle of the throttle is enough to make it go out, and the pressure gauge doesn't drop below about 1bar. (For reference, the manual says "An occasional brief flickering of the oil pressure warning light at idle speed and normal engine temperature is no cause for concern if the light goes out upon acceleration.")

I'm not worried about engine damage from low oil pressure but am curious as to why this has started happening, particularly as I don't think the oil pressure has actually dropped since the work; I clearly recall looking at the oil pressure gauge one very hot day in traffic a couple of years ago and seeing it at 0.5bar, and the light wasn't on. Now, with slightly higher pressure, the light is coming on. Hence my question; how does the oil pressure monitoring and warning system work? Are the gauge and light driven from the same sensor or from different sensors? I know that the fat cylindrical Oil Pressure Sensor that plugs in by the power steering pump has two terminals on it, one for the gauge and one for the light, but are there any other oil pressure sensors on the car that could have failed? Or any other explanation for this slightly odd behaviour?

Thanks,


Oli.

 

[scam75]

Hi Oli

I don't know how the system works, or indeed have an S2 with the lower power engine fitted (!), I would however say your oil pressure is a tad on the low side and that maybe a thicker oil would help assuming your bottom end is in decent order. The lowest I ever see is 2.5 bar and that's when it's been ragged on a very warm day and has returned to idle. During normal driving I usually see 3 to 3.5 bar at warm idle in warm conditions, higher when cooler. I use a good 10/50 fully synth which I find works well.

However this is assuming you are seeing correct readings, for verification of you should connect an independent oil pressure gauge, which would be my first port of call.

Stuart

 

[robwright]

+1 for that. Normal pressure should be 2.5-3 bar when warm.

 

[edh]

Does sound a bit low...
I only know of one sender - don't think there's anything else.
What oil?

 

[zcacogp]

Chaps,

Thanks for the answers. It does occur to me that I could look at the wiring diagrams to find whether there are any other oil pressure sensors. I know that there were two on the Mk1 Golf so would be surprised if there was only one on the 944.

Talking of pressure, an external oil pressure gauge is a very good idea, thanks. However the 16v cars will always have lower pressure than the 8v cars; the 8v Golf GTi's always had an idle oil pressure a good 1-1.5bar higher than the 16v cars as they are only feeding half as many valve devices with oil, and the hydraulic tappets use a fair bit of pressure. I'm hence not that worried but will invest in a pressure gauge - thanks for the suggestion.

For reference, it's a 10w40 semi-synth oil, changed regularly. The bottom end of the engine has nigh on 200,000 miles on it but there was no perceptible bore wear (i.e. no lip at the top of the bores) when the head came off. The head is, as stated, newly-rebuilt. The concern was more that the behaviour of the light, although not of the gauge, seems to have changed.

Thanks for the help.


Oli.

 

[edh]

The bottom end of the engine has nigh on 200,000 miles on it

Possibly your answer.... but I wouldn't worry, as you live in a place where it's almost impossible to drive above 40mph, let alone use high revs /full throttle []

My experience is that the 16v 944's have at least as good oil pressure as the 8v ones, but it's probably all about the state of individual engines.
Sandwich plate under the oil filter is your answer - but you need one with a good gasket (not an ebay cheapie). FWIW, I saw up to 8 bar cold on a 16v engine with 10w60, so I changed to 10w40, which dropped to 6 ish. Seems to hold pressure fine for track use.

 

[edh]

double post []

 

[zcacogp]

Thanks Ed. When you say "Sandwich Plate under the oil filter" what are you referring to?

I did just google "944 S2 Oil Pressure" and came across a post of mine from 10 years ago on Pistonheads! Here:

http://www.pistonheads.com/gassing/topic.asp?h=0&f=48&t=318536

As a point of reference it's interesting, and should probably add that oil pressure of less than 2bar is only when the engine is very hot in a traffic jam. Normal idle (i.e. engine at normal temperature, idling while stationary, fans not on) is significantly much higher.

(Does it sound like I am trying to talk myself out of believing that I have a sha99ed bottom end? [] )


Oli.

 

[edh]

I mean that's the easiest way to get another oil pressure sender in there..

 

[zcacogp]

Ed,

Thanks again. I've tightened up the big bolt on the end of the crank pulley this morning, although I don't think it moved as it didn't seem loose. Torqued it to 180Nm but I don't think it was the problem.

I also took the Oil Pressure Relief Valve out for a squiz. The plunger moves smoothly but the O-ring on the end was very flat so I replaced it, and there was no seal behind the nut head which may explain the slight oil leak I've had since I bought the car! It turns out that an aluminium sump plug washer is exactly the right size for it so I put one on and will see how we get on. I'm not thinking that I have solved the problem but I do seem to have found a couple of oddities so am glad to have corrected them.

We'll see what happens now!


Oli.

 

[Tommys86]

Hi Oli,

Agree with the above both comments, track and road car S2s once hot - see around 2.5 - 3bar (well certainly the road car does, to be honest I rarely look at any dials at the track )

You mentioned the OPRV check, has your oil cooler seals been done before? And was the correct alignment tool used when fitting the OPRV?

Tom

 

[PSH]

Hi Oli

Just to add to the valid comments given so far....according to the 944 handbook oil pressure can drop as low 0.5 bar, having said that i would be very concerned if mine did. Now my engine is very fresh, however, when very hot the pressure can still drop to just over 2 bar on idle, mostly at 3. The most important thing to check is that even when very hot the pressure goes up to between 4.5-5bar in response to opening the throttle. It may be a sender issue or just a partial blockage somewhere, I noticed a 1 bar increase after an oil flush on mine. The pressure should always read 4.5-5 bar when cold.

Pete

 

[zcacogp]

Tom, Pete, Thanks.

Tom, thanks. That's good real world experience of S2 oil pressure there! What weight of oil do you use in your two cars? No, oil cooler seals haven't been done as far as I know. And no, I didn't use an alignment tool when refitting the OPRV; I'm told it's not necessary unless you have had the whole oil cooler apart and, having removed and replaced it, it seems to work fine. (Also, having done the job, I'm not sure what the alignment tool would do as the OPRV is essentially a long thing that screws into a hole. I would expect the position within the hole to be set by the position of the tread that it goes into as threads are self-centring, although perhaps that's why it is necessary when the whole thing has been apart.)

Pete, thanks again. My engine seems to tick all of those boxes; oil pressure at cold idle is 5 bar and, when hot, rises when the revs rise and will easily get past 4.5bar when given a few revs, even when it's properly hot. It's interesting that you gained pressure after a flush - maybe I should try that. As I said before, I don't think I'm worried that my engine has low pressure as the pressure hasn't changed of late (and I don't think it's changed much since I bought the car 11 years ago - I recall seeing 1/2 bar on very hot idle on a very hot day at least 8 years ago and being worried, but reading the owners manual allayed my fears.) My concern is more that the behaviour of the warning light is a little peculiar and does seem to have changed, and I was wondering why that is.

For reference, I've now driven the car a little bit today and it never dropped below 2 bar. I didn't get it very hot and I didn't drive it very hard. I'll see what happens when I do both of those things.

Thanks again for your help.


Oli.

 

[Tommys86]

Correct you're fine to remove and replace the OPRV provided you don't move the case, so the threads are on the oil cooler adapter case (best description) not the block so if the case moves you can get misalignment - which I'm guessing it leads to it not seat properly > I have no idea if it equals to a minor or a major pressure issue (which you're not seeing), plus if it's grossly out I suspect you can't fit the OPRV anyhow?

As you know I ended up doing mine twice, as I found that bulletin regarding port depth in the block for the connecting pipe and gluing a specific washer in (absolute pig of a job to measure due to access). I also couldn't remove the pipes to the cooler, again access and cutting down spanner to fit being the challenge along with the force required, but as mentioned on here previously it is possible to remove the lot with some encouragement and a few swear words.

I use 5W 40 Fully synthetic oil on the road. The race car I think we use 5W 50 fully synthetic "Motorsport" oil - obviously because that gives us those few extra horses?!

I do have in my garage some 10W-40 Shell Helix oil which I was planning to use, thinking it might quieten down the engine as mine does sound louder then the race car... this is all a bit subjective and may not actually be the case as race car has a different exhaust etc.

Engine flush might be worth a go - have you done this since the head change? My friend had a head rebuild (for a completely different car) but was shocked how mucky (machining dust/swarf) his head came "ready for fitment".

Failing that, guessing you might be able to check the sender with a meter at idle? Electronics is not my thing - but guessing someone on here can help.

Tom

 

[zcacogp]

Chaps (and Particularly Tom for his very helpful last post),

Quick update; it's amazing what a difference a new pressure sender can make. The thing now sports somewhere a little over 2.5bar at idle when properly hot (i.e. three-figure speeds for an hour on a motorway). Interestingly it now won't show more than 4.5bar when cold, strongly suggesting a sender quirk. I haven't rested it in the extreme conditions of a 90-minute traffic jam on the Maastrict ring road in 30Deg+ ambient heat which gave rise to the 0.5bar on the old one but will be sure to report back when I do so.

I suspect that what I have learned is something I was told a long time ago - namely that absolute oil pressure isn't that important, it's whether there is any sudden change in pressure. Having read around it seems that engines with genuinely low oil pressure show this in other ways (mainly with noisy tappets for long periods of time) and almost all 'oil pressure problems' are actually problems with the monitoring equipment - i.e. gauge or sender. I also recall the words of an older and wiser friend many years ago when I asked him whether I should install an oil pressure gauge in my old Mk1 Golf GTI. His reply was "No, you'll only spend all your life worrying about what it says".

An aside; if anyone else is looking to replace their oil pressure sender the correct thread size is M18x1.5, 0-5bar measurement with the warning light at 0.25bar. However it's much cheaper to buy one with an M10 or M12 thread and a converter to make it fit the engine. I bought a Bosch sender (branded 'Mercury') of the right spec from a boat chandlers for £10 and a converter for £6. Porsche prices for this part were somewhere over £80.


Oli.

 

[zcacogp]

Chaps (and Particularly Tom for his very helpful last post),

Quick update; it's amazing what a difference a new pressure sender can make. The thing now sports somewhere a little over 2.5bar at idle when properly hot (i.e. three-figure speeds for an hour on a motorway). Interestingly it now won't show more than 4.5bar when cold, strongly suggesting a sender quirk. I haven't tested it in the extreme conditions of a 90-minute traffic jam on the Maastrict ring road in 30Deg+ ambient heat which gave rise to the 0.5bar on the old one but will be sure to report back when I do so. For reference, Hartech say that hot idle on an S2 should be between 2 and 3 bar.

I suspect that what I have learned is something I was told a long time ago - namely that absolute oil pressure isn't that important, it's whether there is any sudden change in pressure. Having read around it seems that engines with genuinely low oil pressure show this in other ways (mainly with noisy tappets for long periods of time) and almost all 'oil pressure problems' are actually problems with the monitoring equipment - i.e. gauge or sender. I also recall the words of an older and wiser friend many years ago when I asked him whether I should install an oil pressure gauge in my old Mk1 Golf GTI. His reply was "No, you'll only spend all your life worrying about what it says".

An aside; if anyone else is looking to replace their oil pressure sender the correct thread size is M18x1.5, 0-5bar measurement with the warning light at 0.25bar. However it's much cheaper to buy one with an M10 or M12 thread and a converter to make it fit the engine. I bought a Bosch sender (branded 'Mercury') of the right spec from a boat chandlers for £10 and a converter for £6. Porsche prices for this part were somewhere over £80.


Oli.

 

[blade7]

You can probably buy 5 litres of "synthetic" oil for £20. I'd use something like Millers CFS 15w50 for a leggy engine. It's got enough ZDDP for 944 engines too.

 

[GlennS]

My experience is that the 16v 944's have at least as good oil pressure as the 8v ones, but it's probably all about the state of individual engines.

Indeed. I seem to remember your oil pressure being so "good" that it blew the oil filter clean off. Twice [8D]