Porsche Engine Woes

When I bought my 1977 Porsche 911S in 2006, I paid what I felt was a very good price. I knew there would be maintenance and repair problems to solve in a 29 year-old car, but I estimated that even if it amounted to another ten thousand dollars, I still had gotten a good deal. If only it had been that little!

 
Although the original engine in 1977 was a magnesium block 2.7 litre, a previous owner had swapped that for a 1979 aluminum block 3.0 litre engine. The car had 259,000 km on it when I bought it, but the engine had been rebuilt more recently than that.

 
The first major job was to rebuild the transmission, since it was well worn and grinding when shifted from second to third without double-clutching. In the spring of 2007 I had this done, after having considerable difficulty with it at my first track days in May 2007. Suddenly it became very drivable, but I began to get concerned about the engine.

 
Over the balance of the summer, the engine began to use more oil and its temperature would frequently reach 120 C. Clearly something had to be done to bring these parameters back to normal. By mid-October, the car had become impossible to start and compression and leak-down tests were well below specification. Between November 2 and March 29, 2008, the engine was rebuilt to a higher performance spec. We increased the bore to 98 mm, for a displacement of 3.2 litres and reground the cam shafts to a more aggressive profile. The power of the engine had increased to 220-230 bhp and it should have run a lot better.

 
However, throughout 2008 there were several track days when oil consumption and/or temperature were higher than normal. We addressed these issues by improving air flow to the oil cooler. In addition, numerous repairs and modification were required to the brakes, ignition, wheel bearings, shift linkage, CV joints, etc. The list of things to check and tools to take to the track kept growing. Nonetheless, it was a successful track season and I looked forward eagerly to 2009.

 
Over the winter of 2008-2009 I installed brake cooling ducts, rebuilt all the brake cylinders, installed delrin sway bar bushings, added tow rings for the trailer I planned to buy, and adjusted the valves. The 2009 track season got off to a good start, with a trip to Virginia International Raceway. While there, I was black-flagged for blowing oil smoke in Turn 1, but couldn’t find anything wrong. Later, at Le Circuit Mont Tremblant in June and July, people were telling me that I was trailing a mist of blue oil smoke. I attributed it to the fact that the engine builder had intentionally left off the seals on the exhaust valves, as was his habit.

 
However, in July at Calabogie Motorsports Park, with no warning the car used a litre of oil in one 30-minute session and the temperature threatened to exceed 120 C. Oil was leaking out of the tank and the engine, so I loaded the car on the trailer and took it to the engine builder a few days later. He confirmed a problem by doing compression and leak down tests and immediately set about removing and dismantling the engine. We found that all piston rings had broken! The only conventional explanation was detonation, but damage to the piston tops was very minor and only to one. We had to assume some inexplicable incompatibility between the pistons, rings and cylinder barrels, absent any other explanations. To his credit, the mechanic rebuilt the engine in about four weeks – some kind of record.

 
While the engine was out, I had the oil coolers flushed and cleaned, to ensure that maximum cooling would be available. I got the car back together near the end of August and followed the break-in regimen to the letter, checking the oil after every 300-400 km. The valves were readjusted at 1800 km and the fuel injection mixture calibrated. I had some problems with stalling and failure to restart, which we traced to a failed MSD ignition box. I borrowed a replacement and sent mine to MSD for reconditioning. During the break-in routine – which took me only 18 days – the odometer clicked over to 300,000 km. At 3300 km I had reached the point when I could use a redline of 6000 rpm, so I took it to Calabogie for a lapping day. The engine felt a lot stronger than the previous one and compression was clearly higher – I could feel it on deceleration far more than with the previous engine.

 
For the first 3 stints I didn’t go above 5000 rpm, as I worked on tire pressures and getting comfortable with new rear tires. In the fourth stint I took it to 6000 on one of the two long straights and noticed that I was only going an indicated 170 km/h, which is slower than before. Then I saw that the temperature was at 120 and the oil level indicator showed zero. I pitted and found I’d used a quart – in less than 40 km. When I added a quart, oil smoke was puffing out of the filler pipe. I then learned that I was blowing blue smoke at every corner, on acceleration. After getting home I checked the compression and on my gauge all cylinders were 140 psi (instead of 175-185), except #6 which was 65 psi.

 
I contacted the mechanic and discussed the second engine failure with him. After hearing the entire description of the break-in procedure, the first three conservative stints at the track, and the subsequent engine failure, he’s at a bit of a loss. He’s beginning to suspect a failure of something in the oiling system, i.e. pump or scavenging bits, that would cause a build-up of oil in the (normally dry) sump and lead to overheating and excess pressure.

 

I took the car to the mechanic when he returned from a trip to the Targa Newfoundland. I spent a couple of hours there, doing the compression and leak-down tests and green lighting possible causes of the failure(s). His readings were 160-170 psi and 11-12% leakage in all cylinders, except number 6, which was 70 psi and 82% leakage. And all the leakage was audible through the oil filler neck, meaning rings.

 

We discussed so many factors, I can’t remember them all. Leaky fuel injector, damaged oil pump, damaged oil filter, blockage in external oil lines, pressure level in fuel injection, etc. He can’t figure out where the oil has come from that is on top of the engine, towards the front on the left – the pump is on the other side. And is there a reason, or is it coincidence, that both cylinder failures were in the right bank?

 

A few days later I went back to observe and help with the dismantling of the engine. After mounting the engine to the stand, it took him a little over 2 hours to dismantle it to the point of removing the pistons. It’s a pretty impressive process, involving lots of complexity one doesn’t find in a normal (non-Porsche) engine. The casting of the cam housings is amazingly complex, with built-in bearings for the camshafts and rocker arms, plus oiling passages and spark plug accesses.

 
There were no broken valve springs or damaged rockers, although one rocker had minute scratches in the normal direction of the cam lobe’s movement. The cam lobes were all clear. All the cylinder heads looked normal except for traces of oil that had blown back through the intake. The tops of all pistons were chocolate brown (normal), without any pitting that could be caused by detonation. Every cylinder wall still showed all of the honing cross-hatching. Number 6 had one tiny line running down in the direction of piston travel.

 
Every piston ring was broken. All but two wrist pins were harder than normal to drive out. Some pistons had collapsed on their rings, making the rings difficult to turn or remove. There are surface cracks in several pistons, inside the journals where the wrist pins are located. They run longitudinally through the bore for up to 2 cm. The ends of the rings where the original gap had been were clean and straight, but not particularly bright under magnification, discounting notion of inadequate end gap at the time of assembly.

 
Mahle offers a diagnostic service upon return of their products, which are all engraved with serial numbers and matched at delivery, including pistons, rings, wrist pins and cylinders. The mechanic spoke to their representative at Andial in California, who suggested two possible causes of this type of failure. The fuel injection system might be out of adjustment, worn, or miscalibrated, resulting in an excessively lean mixture at high engine speed. This could result in either excessive heat, leading to failure of the oil and components, or to non-uniform flame spread at the time of combustion, leading to a combustion shock effect that would act like a battering ram on the piston tops. The other potential cause could be a distributor misalignment or maladjustment, also leading to badly timed, non-uniform combustion.

 
We suspect the fuel injection system has either been rebuilt (badly) or abused in some way by a previous mechanic. The solution could be a switch to carburetors, rebuilding the fuel injection or replacing it with a simpler, more modern, computer controlled system like MegaSquirt. The last option may be the most economical and reliable. I have subsequently discussed it with another PCA member who has the system and read an instruction manual for the Bitz Racing CIS to EFI conversion using MegaSquirt components – it may the right solution.

 
The mechanic will ship the Mahle components to them for analysis and any other suggestions of possible causes. We re-examined the JE pistons from the first build and found the same surface cracks in the wrist pin journals. We will consult with JE as well, to get their ideas on the possible causes. While waiting for their response, we’ll separate the block and measure all the components to determine whether the rods, wrist pin bushings or any bearings may have been damaged. Meanwhile, I have removed the oil tank to have it flushed and pressure tested, to find and repair a pesky leak.

 
We will not reassemble the engine until we are very confident that we have identified the cause of these two failures and fixed the problem.

 
More to follow.

 

Update as of 28 Oct 09

Mahle Racing in Mooresville, NC provided verbal feedback today, to be confirmed in writing. Their analysis shows that the pistons had become annealed due to extreme combustion temperatures. The annealing process would have softened the pistons and allowed deformation at the lands and wrist pin journals. They also found signs of detonation at the intake valve area. These signs of damage were uniform across all pistons. The most likely cause is miscalibration or plain dirt in the fuel injection system, allowing the air/fuel mixture to become excessively lean under heavy load, i.e. high rpm’s. All pistons and cylinder barrels are damaged beyond salvage.

The solution will be to install a completely new set of pistons, cylinders, rings and wrist pins, plus a new fuel management system. While we could rebuild the old CIS system or install a new MegaSquirt system, I am inclined to go the route of the tried and true carburetor. A pair of new PMO 3-barrel carb’s, plus the associated linkages, pressure regulator, etc. will be more expensive than the other alternatives, but should result in a simple, robust and easily adjusted configuration. We’ll make a final decision in the weeks to come and plan to have the engine ready by spring.

Meanwhile, I have flushed, pressure tested and repainted the oil tank, cleaned and repainted the engine bay, cleaned and painted the fibreglass engine shroud and powder-coated the exhaust valve covers. This baby is going to look good, as well as perform!

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