Burned valve on Saab 900

Harold_89_911 · 11-19-2005, 08:58 AM

I lost compression in my # 3 cylinder on my way to work. Pulled the head last Wednesday and found the problem. My big question is what could have caused this, so I can fix the real problem instead of just the consequence. The car is a 91 Saab 900 non-turbo with 180k miles. The ignition/fuel system is the Bosch Jetronic, which is very similar to the Motronic on my p-car. The engine ran for maybe a total of 15 minutes after I lost compression but I don't think that would have created the difference in deposits shown on the pics. Any expert combustion chamber deposit detectives here?

island911 · 11-19-2005, 10:07 AM

No expert here, but I will say that I don't think you can tell much by the combustion chamber deposits . .. due to the burt valve, it's been burning funny (low compression) for a while.

Valves burn, more often than not, when the valve isn't returning to it's seat.

I would guess the hydralic follower (aka 'lifter') or the spring are problematic. Else, it just hapeen to get a chunk of carbon build-up (or other) stuck on the valve seat.

beepbeep · 11-19-2005, 11:05 AM

clogged injector, resulting in lean burn thus high EGT's which burned one of exhaust valves.

Just swap the injector. Find a similar car on junkyard and swap both exhaust valves and injector.

RoninLB · 11-19-2005, 08:13 PM

not my words.

"The key elements in good valve cooling are:
Good valve face to valve seat contact (it needs to be nearly perfect); and
Good valve stem to valve guide fit; and
Cool cylinder head temperatures.

Flat statement: I believe that virtually all valve problems originate with the factory or the overhaul shop.

The hole through the cylinder that takes the valve guide must be true, straight and centered. The valve guide must be true and straight. Finally, the valve rim must match precisely the face of the valve seat, and both mating surfaces must be wide enough to provide enough surface area to conduct the heat.

This calls for some very fine machine work, and sadly, the factories haven't done it very well. The main hole will always be microscopically off-center, and it will never be perfectly straight. Close doesn't count here. The guide must also be machined or honed to very tight tolerances, and it must be straight and true.

But that's only part of the story. Once the valve is installed nice and true, there remains the task of making the correct metal-to-metal contact between the rim of the valve face and the valve seat (which is itself yet another insert that needs to be placed with great care).

The usual way of doing this is to put a fine grinding compound on the surfaces, stick the valve in, and spin it, or rotate it back and forth, so that the two surfaces grind away at each other, hitting the high spots on both surfaces, eventually leaving a perfect match. This is called "lapping," and it's an evil chore.

Ideally, that area of contact has to be some minimum width, and it must be equal all the way around, or the valve head will be cooled unevenly. Uneven cooling will cause hot spots and cold spots, and the valve head will actually warp a bit. When that happens the contact is not even all the way around, and a microscopic gap opens. At first, the pressure of the combustion event is probably enough to smash the valve head closed and correct a small warp, but eventually, the gap will allow a tiny amount of the combustion gases to leak past. Once this begins, it's only a matter of time. This is where you begin to see a loss in compression, sometimes very rapidly!

Remember, that combustion event can be upwards of 3,000-4,000ºF and that heat blowing through a small crack will cause an intense hot spot on the rim of the valve. With the rest of the valve getting cooling, and this little arc not getting any."

Joeaksa · 11-19-2005, 08:36 PM

I would lean towards beepbeep's comments about a injector. Would pull all of them and get them cleaned. Many diesel shops do this for a reasonable price.

Also ask them to check the flow pattern on the bad cylinder first and report back with their findings. If its clogged even partially it could have done this.

JoeA

Vipergrün · 11-19-2005, 09:53 PM

Hey Harold, what's up?!

Harold_89_911 · 11-19-2005, 10:29 PM

Hi Brad:

Funy you should ask. Just took my 5-year old daughter to the new pool in East Ventura and drove close by your old house and was wondering how you were doing. Independently, my wife also asked me what happened to you. She did not even know that you left the area.

We are doing pretty good, still busy with the new house. As you can see, I got myself a nice little project here. This is the most involved project I have ever done on any car so I could have used your help here. I did get to use the special socket that you made for me to turn the engine. Thanks again for that! I'm taking my time to work on it since I have to wait until the kids are in bed and I can't go to bed too late since I have to go to work too. I had to kick the p-car out on the driveway since there is only space for one car with all the junk that we still have in the garage. The p-car is doing good and will probably made this years turkey-day run. I will take some pics for you.

Hope you are good and maybe see you next time we are in Colorado. Take care, Harold