Quaz

I have seen quite a few posts recently about preventively replacing injectors in hopes to head off possible cylinder wall scoring. Now I am no longer a mechanic by profession, but I used to be and I have built several engines over the last 20 years. None of those engines were Porsche flat sixes, but one of them was a 928 S4 32v V8. So I have seen what a damaged engine looks like and I have seen what causes failures. Which leads me to this discussion and I just want to talk about injectors. Specifically the value of replacing them at a certain age. In all my years working on cars I have never seen "leaky" injectors that wash out a cylinder. I have seen temp sensors cause a car to go rich and wash down cylinders. I have seen fuel pumps and regulators go crazy and increase pressure and wash down cylinders, but I have never seen a leaky injector wash down a cylinder to the point where it causes wall scoring. Not that it can't happen and I am some super expert on it, but I personally not seen that kind of failure. So some of my observations about injectors.

• Injector failure is random, so if we were seeing injectors fail they would cause random cylinders to fail. Not the same 2 cylinders on every car. The data would be all over the place.
• When you have injectors reconditioned they are pressure tested to hold a specific PSI. If they fail that test they are junk. If an injector was to leak down fuel over time it would only be bleeding off excess pressure at a very slow rate. Probably at a rate in which the fuel would evaporate before it pools. Assuming there even is enough to pool. So again not enough to wash out a cylinder.
• If gas was leaking into the cylinder in enough volume to "wash" the cylinder out there would be gas in the oil. That gas in the oil would be very very easy to smell and would cause the oil lever to increase over time. In a 9+ quart system it would take a fair amount of gas to dilute the oil to the point it was no longer effective.
• Cylinder scoring from my experience is caused by ring problems, detonation or expansion issues of the metals. (mostly from the rings or detonation). As soon as the engine cranks oil is splashed back up to the bottom of the cylinder and lubricated, it would take a lot of starts to score a cylinder from washed out liner. Not saying it can't happen, just saying it is unlikely. So lubrication issues not related to oil starvation typically don't cause scoring.

Those are my thoughts and I am trying to think through this injector replacement thing logically before everyone puts injectors on their 100K tune up list. Recently I had a 1997 BMW M3 with 250,000+ miles on the original Bosch injectors. Car ran perfect, had great compression and when the head was off there was zero scoring. I have had several VW turbo engines go past 200K miles and never an injector problem. If injectors failed around 100K miles we would see this on other cars because they all use the same injectors from the same supplier. Discuss away and maybe some logic will come from this thread. :-)

dporto

^^ Good points/questions - maybe Jake Raby will chime in as he seems to be a big proponent of this injector theory. *Even though the injectors may be the same, it's possible that the idiosyncrasies of the M96 are causing them to fail earlier than with other platforms.

TexSquirrel

Jake never said it was THE cause, he said it was ONE of the causes.
There are other reasons why it affects one bank over the other.

Quaz

As I was thinking about this more why have we not seen this on the Mezger engines? I realize this new block setup is very unique and was a first for Porsche, but why are the 2.5/2.7s in the Boxster not affected as much and it carried over to the 987 Caymans/Boxsters and 997s even as different parts and components of the injection systems changed over. Mostly this is just rambling at this point, but I am not seeing a need to replace injectors at some interval unless I know there is actually a problem with them. For instance if there is a specific failure which is what we suspect to be contributed to by a leaky injector I would like to see what parameters of that injector fell out of spec. Meaning you can put the injector on a bench and measure spray, response, resistance, pressure and leak down. It is a pretty definitive way of proving this theory.

Flat6 Innovations

Different animals.. The Mezger, along with all aircooled engines use Nikisil plated cylinders. Nikisil will streak and consume oil when too much fuel is present, but they will not score. Often once the over- fueling issue is corrected, the issues go away. The only thing that will touch the surface of a Nikisil bore is a diamond, or a tungsten- carbide cutter. remember, Nikisil is a composition of Nickel, Silicon, and Carbide.

That said, I have collected enough conclusive data to support this that I don't need to substantiate my feelings. I know what keeps my engines alive, and that's what I do. Its not like I sell fuel injectors, or any other part.. I have no benefit from sharing this with you..

In fact, the best thing I could do is not share anything, let the effects occur, and watch my line att he door for engines get longer and longer....all while no one has figured out the things that create the equation of failure. Do as you wish :-)

On a bench you don't have a real world scenario. You don't have warm fuel, you don't have engine vibration, you don't have a hot engine that stays hot, and even gets a little hotter just after being shut down. Further, you don't have all these elements working in unison with the engine in a car, using unknown fuel. You don't have all of this happening, then requiring the injectors to hold fuel pressure without leaking down. You can't test this scenario in a lab.

TexSquirrel

The Mezger engines are not the same as the M96 and M97, so they really don't compare.

The M96 & M97 3.6 and 3.8 suffer the most from bore scoring (as compared to the smaller M96s) because they have the most contributing factors.

As I said before, fuel injectors are not THE cause, but one contributing factor.
But it is one that all of the M96/M97 engines, and many other engine designs, have.
Leaking fuel injectors cause issues with LOTS of different engine designs, not just Porsche M96s.
Those issues can be bore-scoring, hydro-lock, failed cats. etc.

Oops, Jake posted while I was typing my post.
Jake has the most experience with these engines, so I don't expect him to explain his findings unless he feels like sharing.
If he says a X causes Y in these engines, then I'll except that as an experienced statement.

Paul Waterloo

Jake: Did you get to putting that video together on how to modify the .2 injectors for the .1 3.4's?

We would love to see it! Thanks!

TexSquirrel

Another good thing you could do for your business is allow quality to reduce (by outsourcing some of the work) so you could increase productivity.
But that's not your business style, thankfully!

Quaz

FIrst thanks Jake for chiming in and thanks for the answer on the Mezgar engine, that makes sense.

As an engineer and working with many different OE manufactures I can assure you there is a way to test this. However feasibility of setting up the lab and cost may be prohibitive, but I highly suspect Bosch, Denso, or another injector OEM has the proper simulations and tests for this exact case. It's not like this is new technology either, injectors have been around in this form since the 80s.

I am not trying to stir the pot and get emotional, instead I really want to understand what is happening and what the proper course of action is. In my garage is both a 3.2 and 3.4 M96 of the correct vintage and mileage for this type of failure, but neither car is at the moment exhibiting any symptoms and I would prefer to keep it that way. Both cars however were southern cars and very well maintained prior to my ownership, but most Porsches are very well maintained. That being said pandora's box has been opened about suspect injector behavior and I would like to understand it better because my personal experience leads me to believe this shouldn't be an issue. If it is an issue there should be a protocol developed to reduce risk. As a business owner and innovator I understand your apprehension in sharing too much information and possible negative effects to your bottom line. On the other hand this is a community which does try to help each other out as best we can without regard to profit. So any additional insight you could share would be greatly appreciated.

Flat6 Innovations

If I didn't want to help, I would have never brought this up.
Yes, you can test this. It is tested in the car, using the vehicle as the lab. When things that are designed in labs don't work, this is how people like me solve problems. Nothing trumps practical application testing, and development.
Someone can learn all they need to about this with their own vehicle that is showing symptoms. I just used a 32.00 bore scope from Amazon to assist me in a couple of the bore scoring videos we shot this week. A fuel pressure gauge, a bore scope, and some common sense and someone can figure this out.

There's a difference in sharing, and arguing. Most people that want to argue have no personal direct experience with the problem, nor the solution. I don't argue with people that haven't experienced it themselves.

Quaz

dporto

"On a bench you don't have a real world scenario. You don't have warm fuel, you don't have engine vibration, you don't have a hot engine that stays hot, and even gets a little hotter just after being shut down. Further, you don't have all these elements working in unison with the engine in a car, using unknown fuel. You don't have all of this happening, then requiring the injectors to hold fuel pressure without leaking down. You can't test this scenario in a lab."

^^This is exactly what I meant when I said this:

"*Even though the injectors may be the same, it's possible that the idiosyncrasies of the M96 are causing them to fail earlier than with other platforms."

Flat6 Innovations

Quaz

Back to my pressure gauge thought, if the system is pressurized it should hold that pressure for a certain amount of time. Does the 996 system have a check valve to prevent back flow to the pump? So if you take a known good set of injectors and get the car hot and then turn off the key the fuel pressure will back down in a specific amount of time. It will be pretty consistent at the same temp. When I designed my own injection for my 2.2 engine back in the day I was having problems when the engine was hot not having enough pressure in the system. What I found was the tank return line was swirling the gas and adding air to the system causing the gas to have bubbles in it when hot. The solution was to put a check valve/restrictor on the return line to slow the rate of return to the tank. (also keeping the tank above the 1/4 level) Later tanks didn't have this problem since the return line was in a different position. I diagnosed this by watching the pressure gauge over several days to see how it acted at different operating temps. Porsche figured this out in 72 and mine was an early 71 LOL Anyway if an injector is bleeding pressure off the system it should be pretty easy to detect by watching a gauge and the behavior from control good to bad. Once diagnosed bad then you just need to figure out which injector, but at that point they all have to come out anyway so you can just replace them all at that point. Am I crazy here? Might be easier than bore scoping a hot engine or trying to get access to the intake runners.....which was my first thought BTW.

It may not be as straightforward as just putting a gauge on the rail, but I think it could be done in this manner. You diagnose boost leaks on a turbo car in this manner and a variation to diagnose vacuum leaks on a 928 S4 in the absence of a smoke tester. Again just thinking out loud here.

Flat6 Innovations