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Question dropped valve seat questions

It seems to me that this is the single most catastrophic event to happen to a Type 4 engine. Why does it happen to Type 4's? What causes it, and what can be done to prevent it?
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Old 12-03-2002, 07:32 PM
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Get a hold of Dave Darling. He is the "expert" on this subject.

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Old 12-03-2002, 08:29 PM
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It is not that common in 914's but it does happen. Why? Djet has hoses and seals that age and leak air. Air not going thru the TB is not accounted for so the A/F mix is too lean. Lean means a hotter burn and hotter head temps in a air cooled engine. Aluminum heads and steel valve seats are dissimilar metals in expansion. The head goes thru alot of too hot of heat cycles and weakens so the seat falls out. This is more typical of buses since the engine pushes around all that weight ! Buses run Ljet which also can go lean due to air leaks.
Common fixes for backyard mechanics were done (adjusting the mps without calibration)other than fixing the air leaks, ie seals, gaskets and hoses.
Geoff
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Old 12-03-2002, 09:18 PM
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Another fix is toi use better valve seats installed with higher than factory spec 'crush'.
It's covered in this thread.

Rebuilding 2.0L heads
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Old 12-04-2002, 07:05 AM
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Actually, Geoff, most vacuum leaks in a D-jet car don't change the mixture. There is no air flow meter (like the 1.8 has); the EFI guesses at the amount of air going into the motor by measuring the air pressure in the manifold. Most vacuum leaks will raise the manifold pressure and be interpreted as more air going into the motor, which is mostly correct. Some leaks do cause mixture problems, notably fuel injector seals and the hose going to the MPS.

Still, the main point of your thesis is correct. Heat is a very major contributor to dropped valve seats. 914 engines can run hot (various reasons, from running lean, to dirt on the fan or cooling fins to missing engine tin and on and on) and that will tend to weaken the material of the heads. Add the differential expansion of aluminum and the "sintered metal" of the stock valve seats, and you can wind up with a problem....

There are other possible contributors. Raby blames hydraulic lifters in part for a lot of dropped seats. (You'll have to ask him why; I don't know how this is supposed to happen.) Over-large valve clearances can result in "pounded" seats, which can weaken. Incorrect seat installation can cause problems. And so on.

It may have happened to me three times so far, but I still don't have any truly good answers about the process....

--DD
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Old 12-04-2002, 08:53 AM
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A quick test I have done is to take off the adjusted MPS for my motor and install a 049 MPS (1.7L). My oil temps go up about 20-25 degrees (240F) at operating temp. Thats how much a mis-adjusted MPS can heat up a engine!

So I have to say that also mis-matched FI parts is another cause of lean mixture problems in 914's. Hey, this part looks good and is close to the part that is maybe causing the FI problem. In it goes, and yes it will run just not the right way. Alot of mechanics in the past (even today)did not understand Djet, so there has been alot of part swapping.
Geoff
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Old 12-04-2002, 09:18 AM
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