Valve timing and it's effect on compression
 

My 2.7 has been running like a pig since the accident. It's running on 5 cylinders and is now in the capable hands of my mechanic.

The car is overdue for a valve adjustment and I've asked him to tackle that too. It's been 8,500kms since the valves were last adjusted.

Before doing anything else, he's performed a compression test to see why it's running on only five cylinders. He's found that four cylinders to be at 165 psi and two at only 110 psi. He's telling me that since the motor is relatively fresh and new (9,000kms/5,500miles) the discrepancy is probably due to the valves being so far out that it's caused a loss of compression. Apparently, my plugs are pretty bad too, very fouled and dirty on those two cylinders.
I'm freaking out here. Is that much of a compression drop likely to be caused by simply bad valve timing? I'm praying this motor doesn't have to come apart again...

Valve timing....
 

and compression can be directly related. If you do not have sufficient gap between the cam lobe and the valve stem, then it is considered tight. If it is so tight that the valve never gets a chance to fully seat against the head, you will loose compression back through the valve that is not fully seating. If the valve does not seat against the head long enough between cylinder firings, the heat keeps building up in the valve and it can "burn". The time a valve seats against the head, it is loosing heat to the head. The head is a heat sink for the valve. There is a critical amount of time the valve needs to release heat to the head so it will not eventually "burn". The newer valves can handle alot more heat (sodium filled), but you don't want to leave a valve "tight". Unfortunately, you cannot hear a tight valve. The loose valves warn you that it is time to adjust the valves...but if they sound good , you could still have a tight valve and be loosing compression as well. Usually though, a tight valve that isn't seating all the way will be prone to either backfiring through the exaust if it is a tight exhaust valve and backfiring through the intake if it is a tight intake valve.

Ask your mechanic to write down which valves were tight (by how much) and which ones were loose (by how much). Keep this info. for the next adjustment to see if there is a pattern of valves that get tight or loose. Same w/ the condition of the plugs. This long term documentation can be helpful in future diagnosis.

JGLhttp://www.pelicanparts.com/support/smileys/yltype.gif

JGL,

That's a fairly concise explanation, thank you. I have decided to just wait and see how it goes after the valves are done and the compression test is done again.

If it's still bad, then I'll start worrying.

I have been getting quite a bit of popping through the exhaust lately. That's what made me think it was due for a valve-adjustment. Is this what you mean?

"....probably due to the valves being so far out that it's caused a loss of compression. Apparently, my plugs are pretty bad too, very fouled and dirty on those two cylinders. "

Ordinarily, valves that are adjusted too tight would lead to a loss of compression. However, that would not cause the plugs to appear as they do as mechanical compression and the ignition system are fairly independent systems. Let's hope it's nothing serious.

Sherwood Lee
http://members.rennlist.org/911pcars

Adam,

The spark plug fouling is a concern ...

The injectors for those two cylinders should be tested. And, if compression doesn't improve after a valve adjustment, then I am predicting that carbon trapped between valve and seat is the culprit for your compression loss!

If carbon or something besides valve adjustment is the source of the compression loss, then I recommend the following procedure. Since I am sure your mechanic won't suggest it ... have him try pouring some Berryman's B-12 Chemtool (or a 33%/33%/33% mix of Acetone, Toluene, and Xylene) down the injector ports on those two cylinders while cranking the engine over with Fuel Pump Relay and CDI-unit unplugged. Then pour a few more ounces into those injector ports with the engine static, and let it sit overnight. Repeat the cranking/pouring treatment the next morning and repeat compression test. This B-12 'treatment' has worked for me on several engines to remove carbon deposits trapped under valves and return compression numbers to normal on all cylinders ... and a few Pelicanheads have used it with success, too! Lots cheaper and quicker than a teardown, too!

Warren, Would it also be advisable for him to change oil immediately after this procedure? Would the chems wind up in crankcase? TIA. :cool:

I don't think the contamination would be bad enough to need having the oil changed immediately, that is, before all of the tests and diagnostics are finished ... but yes, before resuming normal deiving and use.

I don't think a few hours of running the engine with a light or no load for the purposes of resolving the compression and injector/spark plug issues would be a problem. Like a lot of the old, traditional 'pour through' procedures with solvents and light oils (carb cleaners like B-12 or Techron, oils like Bardahl and Marvel Mystery Oil) for decarbonizing an engine ... it will be a smoky, messy aftermath, for a few minutes until every bit of the solvents and residue are burned out of the heat exchangers and muffler, anyway! After the engine is running right and warmed up, an oil change would be the last logical step in the procedure.

You might want to consider removing the thermal reactors beforehand-if it's possible. I've heard of cats clogging up with these procedures.
A engine performance teacher who seemed pretty experienced suggested getting a spray bottle with half water and half antifreeze (to lessen the heat shock) and misting the intake. He said he used it on a big block with carbs to steam clean the engine. I don't agree with this procedure on a lightweight racing engine though, as I thinks the antifreeze would leave a residue.

Dumb question:

Why would it need a valve adjustment so soon? 8,500kms is only about 5200 miles. Isn't that a little short for them to be out far enough to hurt compression?

Tom

Thank you all for your input! :)

Warren, I'll be printing out this thread and giving it to my mechanic to read. I don't know if Berryman B-12 is available here in Melbourne, but with those ingredients and percentages supplied, I'm sure we could mix some up. I'll give that a try and see how it all goes.

John, the car is Australian (Euro) spec, so there are no thermal reactors or catalytic converter. Just the regular heat exchangers.

Tom, I'm surprised that it needs a valve adjustment too... :rolleyes:

I'll keep everyone posted...

Buddy,
Sounds like pain, lots of pain there. one can hope that it is valv adjustments and not a rebuild. escpecially seeing that motor is so young and strong. it was not even fully run in yet was it?
hope joe can come up with the rabbit out of the hat trick :Dand save u some $$$$$$