Maybe the leak wasn't the headers...

A few of you read and responded to my post last week regarding a pesky exhaust leak I was chasing after installing new headers. The short of that story was that I removed the new headers twice and replaced the gaskets, installed TimeSerts in some questionable stud bores, and re-torqued the exhaust system to spec. After performing this ritual three times (counting the initial installation of the headers), I became concerned that the problem might be a pulled head stud. Some of you pondered that possibility also.

After the last round of header installation last Saturday (and still hearing the hiss/pop upon starting the engine), I acquired a valve adjustment kit and removed the valve covers. Because of the anticipation of a failing head stud, the first thing I did was to check the torque on all the head studs…all being absolutely OK. I then adjusted the valves to the 0.1mm spec with the prescribed adjusting tool with the engine stone cold. Generally, most valves seemed close to spec. Two seemed quite tight and I adjusted them to the proper clearance. I buttoned up the engine, started it to top up the oil level and of course the popping was evident…I really had done nothing to stop it that morning.

That day turned out to be a clear and mild day, so I took my wife for her first ride in the 911 since the restoration (actually since we’ve owned it). We ran the car for an hour or so part of the time on open road at good highway speed. Definitely, the engine came up to full operating temp. maybe for the first time since before the previous owner parked the car a few years ago. I noted that the spitting/popping exhaust note had completely disappeared by the time we turned into our neighborhood after the ride. Upon starting the car yesterday morning in a cold garage, I noted immediately there was no hiss/popping. Drove it all day and it never made any noise except for the flat six staccato.

The big question…could it have been a carbon deposit on one of the cylinders that had too tight of a valve clearance and carbon built up on the valve seat? When that valve was set properly, could that have allowed the wearing away of the deposit and allowed the valve to seat? Any thoughts? Warren, you must have some words for this, I hope.

You stated that prior to your problem disappearing, you adjusted your valves and two were quite tight.

It is possible that carbon was your problem and small pieces of carbon got between the valves and the valve seats. This is why it is recommended not to remove your spark plugs prior to a valve adjustment. Small pieces of carbon could be dislodged and find their way onto the valves and cause the adjustment to feel tight. Then you adjust the valve to what you think is the proper clearance, the carbon blows out when you start the engine, and viola...your valves are now to loose.

You can prove this theory by going back and adjusting your valves again and see if some are now loose.

I would also recommend a good treatment of Techron or Swepco 503 to work on any additional carbon that may be lingering around the valves and injectors.

Hope your problem is cured for good.

Enjoy, Bruce

I doubt the carbon theory, at least as it applies to the valve seat. Carbon buildup would have caused the clearance to increase, and carbon removal would then cause a 'too tight' valve condition.

My experience with pooly adjusted valves is that they cause some surging at very low rpms (idling through parking lots), but do not impact performance enough to cause stumbling.

My vote is for contaminants in the fuel system or a FI component that is not working properly. This, because it also does not sound like an ignition problem. Dirty injectors is a prime suspect as is fuel pressure. Jump the fuel pump, displace the air flow sensor plate and observe the injectors spraying into six glass jars. The spray patterns should be a nice, even cone shape.

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'83 SC

Superman is dead-on on this one. The carbon between the valve and seat would pull the valve further away from the rocker, increasing clearance. Then if you adjust the valve, it will be too tight when the carbon eventually blows away. My guess would be the fuel system not being too happy about sitting for so long. Or moisture in the distributor cap. There are many things possible in a vehicle that has sat for awhile. Old gas etc.

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Tyson Schmidt
72 911 Cabriolet

Sorry for the confusion guys. Had a left brain/right brain disconnect, or possibly to many malted beverages the night before.

Regards, Bruce