Maleficio

Debris moving through the system certainly sounds plausible for the fluctuating pressure. I experienced this once in my 93 Trooper right after I had the engine flushed with solvent called Mileguard. Pressure would never rise above 30 psi. Then I heard a loud groaning sound, CEL came on, I shut it off, fired it back up, suddenly oil pressure shot up to 55 psi, and I've not had proper pressure problems since.

Glenn Evans

For those who haven't read 918-S's previous thread on this, his lifters were noisy at idle when he had the fluctuating pressure on the highway. This indicates that the low oil pressure was real and that the problem is not with the sender or its connections.

Those oil changes are very frequent, so the sludge isn't the product of some longer term reaction with the oil.

Glenn Evans

The pressure relief valve opens at 8 bar. This pressure will be reached at relatively low rpm when the oil is cold and, if I correctly recall a chart of tests Devek did some years ago, will open at high rpm even when the oil is at normal operating temperature (this will depend on the oil viscosity). The pressure relief valve is between the pump and filter, so gets unfiltered oil. If a piece of grit got caught between the piston and its seat, the pump would not be able to maintain pressure at lower rpm. The grit almost certainly would be dislodged the next time the pressure was high enough to open the valve, and probably would have fallen out when you changed the spring.

The problem with this theory is that there should be grit in the filter if it can get picked up by the pump. Maybe the grit pickup was a one off?

Bill Ball

You say you see pressure of 150 PSI on start up. That shouldn't happen. The relief valve is immediately after the pump and should limit pressure to 80-100 PSI. However, you describe moving the pressure sender from its stock location to the outlet for the eliminated cooler. That shouldn't change the reading. So, that could indicate the relief valve is sticking at times, first closed, then open. Could you or someone annotate/revise the oil path diagram to show your arrangement in terms of pressure sender, etc.?

Sludge might indicate blowby from unsealed rings, overheating of oil or I don't know what. As venerated as the Castrol GTX name is, I read tests done a long time ago by an independent motorcycle magazine that showed loss of viscosity in a few 100 miles. Modern synthetics are so superior in terms of heat stability, freedom from varnishing and sludging, etc. that I switched all my vehicles to M1 a long time ago. When you disassemble 928 motors you can tell instantly which ones ran synthetic vs. dino oil.

Mrmerlin

I would second Bills suggestion to change oil brands ,
I have experienced viscosity breakdown a few times using GTX and i now use Valvoline dino or Mobil 1

918-S

Thanks for the comments guys, Here's some fuel for the oil break down theory. Remember, this engine is midship in a 914. It is likely seeing engine bay temps higher than an engine with a cooling fan and frontal air flow. I am pumping coolant from the engine to the front of the car and back. Because of this I watch my gauges like a hawk. (which is why I spotted the oil pressure fluctuation) I have a front and rear temp gauge. one connected to the stock block location and one on the return side hose up front. I have seen temps in the range of 220-260 at times in heavy traffic at a crawl. So cooking oil is not out of the question. Normal temp with T stat working is rear 185, front 180-182 on the return. If I had more room I would plum in an oil cooler. In fact I think I should anyway if it turns out this is the issue.

Glenn Evans

The pressure relief valve spring compresses 5.9mm under a 2000 gram load, giving a spring rate of 4 N/mm or 23 lb/in. The thickness of the washer which fits under the relief valve cap is 1.5mm. By fitting two washers the relief valve should crack at 33 kPa or 5 psi below its rated 8 bar (116psi) ie 111 psi.

If your gauge can peg at 150 psi, the spring must be preloaded at least half an inch by something, allowing for the extra washer. The relief valve piston is 22mm long. My guess is that yours is reversed ie instead of the spring fitting into the piston (about 20mm), it is bearing directly on what should be the crown of the piston.

Glenn Evans

I think that you have answered your own question. I think that you have overheated your oil and burnt the polymers which act as viscosity improvers, destroying their ability to increase the base oil's viscosity when hot ie you effectively are running a 20W (thin) oil. Possibly, the crumbly black grit is those burnt polymers.

You need to change to a full synthetic oil with a high flash point and as narrow a viscosity range as possible; preferably one which states that it contains no viscosity improvers. Full synthetics are better able to withstand high temperatures and have a higher viscosity index than a mineral base stock even before viscosity improvers are added.

You also need to fit an oil cooler! I presume the 260F you mentioned was oil temp. The coolant must be dangerously hot also. Is your radiator getting adequate airflow? Is the sump getting airflow? Don't underestimate the amount of cooling it does. I suspect that it's not getting as much airflow as it would up front of a 928.

dcrasta

You need to get those oil temps down, your are borderline burning the oil at that temp.

Per 'Elephant' racing -http://www.elephantracing.com/techtopic/oiltemperature.htm

Properly maintained 911 engines can last over 300,000 miles when oil temperature is kept in the optimal 180°-210°F range. As oil temperature rises engine life declines rapidly. If the temperature is too high the engine will fail in short order.

As someone has said, Oil temps over about 210-220F lead to breakdown of the viscous additives.

Also, a lot of the cooling of the engine comes from the circulation of the oil. Deleting the Oil Cooler is (In my opinion) a mistake that may end up costing you an engine. You may be able to get away with it on the east coast in the fall and spring, but idling in traffic or extended high speed / High RPM runs will be a disaster.

Get those Oil temps down below 220F please!

918-S

Thanks again Glenn, Ya, I just working through the thought process trying to rule out bearing damage as a cause for the drop in pressure. I think now that I have removed the pan and cleaned it out and the pressure valve and removed the second aluminum crush washer I'll fill the system back up with cheap dino just to see if pressure resumes at a normal level. If I'm back to normal I'll dump the dino and get Synth.

Anyone have a suggestion on synth oil? What are you using? Mobil 1? Amzoil? We have the Amzoil head quarters here in Duluth/Superior WI. Oh ya, and if I'm good on pressure I'm going to figure out how to add a oil cooler back in.

Bill Ball

Oil selection arguments are legendary here. I've used M1 15/50 exclusively with no issues, while beating the living crap out of the motor, and I only do once a year/15K miles changes (as Porsche indicates). Amsoil is very good too. Holds viscosity very well - often increases over time, which isn't all that desirable either, but it's a marginal issue. There are others, but those are two good choices.

918-S

Cross posted from the bird because sometime one guy may post here and another post there.

I added the dino GTX today because it was cheap. If the problem repetes it would do it with any fresh oil. I used a Mahle filter because it was all I could get in town today. Not that Mahle is a bad choice. I just usually use Mann. I fired it up and let it idle at 150 psi. I drove it about 10 miles. There was a fluxuation but only to 60 psi when I lugged the engine once. after warm up the pressure dropped around 90 psi for a short while while I was driving at 55 mph. So far I think it's ok.

I'm planning on draining the GTX I just put in and getting synth. I'm going to start a cooler thread and let this one die for now.

IcemanG17

Glenn
Given your high oil temps in traffic I would seriously consider a full race oil..... Personally I have had great success with Amsoil racing 15-50 (racing has more zinc)... I do not have an oil cooler at all....stock USA 4.7L engine and my oil temps range from 225-240F in the winter to a peak of maybe 270F under full race conditions.... For reference my typical water temps are around the 2nd white line (stock gauge) and 190-200F on a digital gauge....

Currently I have 49 HOURS on track on a stock-ish engine.....just a 3/8th spacer and OB oil pan....always used Amsoil racing....but I do think any quality product will work....

918-S

What 3/8 spacer? tell me more

Glenn Evans

918-S,

You should not be seeing oil pressure over 116 psi (8 bar) in a 928 engine. As I said earlier, 150+ psi is so far over the pressure relief valve's opening pressure that I believe that the relief valve piston must have been reversed on installation, compressing the spring by an additional 20mm (0.8 inch). Either that or the piston is jammed closed.

The piston is steel, so you should be able to extract it with a magnetic pickup tool. I realise that this will be a pain, but the oil pressure is way too high and you should find out why. Just to reassure you that the piston really is magnetic ....