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06-20-2004, 10:32 PM | #1 |
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Burnt exhaust valves?
I had a thread on my car's woes a while back and the shop pulled the heads and discovered burnt exhaust valves on three cylinders. Some had material missing off them.
Previously I ran a compression test w/ #2 low (120ish) and the other three close to 0 (materail missing on the valves would explain that). I did a search on burnt valves and didn't find anything. Anybody see this before? I was on my way to the first HPDE event (driving the speed limit) when it happened, the car had been running fine just prior. Thanks Big Sky
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06-20-2004, 10:48 PM | #2 |
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I don't think I've read any posts on burnt exhaust valves, Mike....at least not without some easily identifiable cause.
Do you think something went through the motor and dinged them before they burned? Did you ever get that fuel analyzed??? It sure seems odd to me that everything was five by, and all of a sudden *POOF*....dead motor. |
06-20-2004, 10:58 PM | #3 |
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I never got the fuel analyzed, but it appeared and smelled (no diesel smell) "normal"
I agree- very odd. It's possible something went through the motor, but wouldn't have a gues to what. The up side is I got a new 04 wagon Mike |
06-20-2004, 11:02 PM | #4 |
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Properly done the wagon is one of teh ultimate sleepers...
Ya gotta make up a little sticker..."Your ass just got handed to you by a wagon " |
06-20-2004, 11:55 PM | #5 |
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From what I have learned in the past burnt valves are caused by the engine running at a very high speed for a very long time. Saw it on TV once from someone running away from the cops, car died from bad valves.
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06-21-2004, 12:15 AM | #6 |
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Do you use an exhuast cutout? My mechanic/shop told me that the lack of backpressure (even in a turbo car) can make exhuast valves get stuck open and burned etc. something like that.
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06-21-2004, 12:17 AM | #7 | |
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Quote:
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06-21-2004, 05:45 AM | #8 | |
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Quote:
a turbo is happy with NOTHING behind it, if you could sit your turbo down on a little couch, get it to open up about its mother and childhood, eventually you would find out that it desires to have nothing behind it, no down pipe, no mid pipe, no axleback, no whistle tips, nothing. a turbo by its very nature is back pressure. its a giant mass of spinning blades that the exhaust has to push through. |
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06-21-2004, 06:04 AM | #9 |
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rep that schnit erik |
06-21-2004, 06:50 AM | #10 |
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A bad exhaust leak at the head can cause a burnt valve.
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06-21-2004, 07:13 AM | #11 |
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Hard to tell with out some investigation.
There are several things that can cause burnt valves. If you over rev the engine and slightly bend the valves so they don't seal completely, or have carbon build up so they don't seal you can burn valves. They will burn due to hot combustion gasses jetting through the opening formed by the poor seal. Warped cylinder heads due to excessive head temps, that cause the valve seats to distort, or bad (worn, oversized) valve guides or low coolent flow (bad water pump) that fail to carry away enough heat from the valves/ cylinder heads, can do the same thing. Running lean or screwed up timing that causes excessive EGT and or opening the exhaust valves a bit too early due to the cam skipping a tooth (bad tension idler) or poor ignition timing so that the fuel air charge is still burning when the valves open is another cause. Running a fuel that burns much slower than the fuel the engine was tuned on would have the same effect. The same thing could be caused by spiking the fuel with too much toluene or xylene and blowing burning fuel out due to the slow evaporation of those fuels in high concentrations. You might want to check your fuel pressure and send the injectors out to be checked to be sure that bad gas didn't plug up injectors or you might have low fuel pressure or excessive fuel pressure that locked up the injectors, and that caused a lean condition. Larry |
06-21-2004, 07:34 AM | #12 |
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I agree with Larry's comments. Burnt valves are a progressive sort of thing, usually caused by conditions that keep the valves from making good contact with the valve seat. Once a "hotspot" develops, it just gets worse, until you start losing pieces of the valve.
Good gas, good tuning (timing and A/F ratio), good cooling system maintenance, and good valve springs are important. Good luck with the new wagon. Is there a separate auto-x class for them, or will you be embarrassing sedan drivers? Last edited by RayO; 06-21-2004 at 07:39 AM. |
06-21-2004, 10:41 AM | #13 |
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Thanks for the repies guys. A couple of things I can rule out out is timing- we checked that thinking that had to be the cause of 0 compression in three cylinders- the cam and crank timing was spot on, belt in perfect condition.
Possible blocked injector(s) is a possibility- I'm switching gas stations w/ my new wagon. Montana doesn't get the best gas anyways- 91, and I think in the winter w/ oxygeniation it's worse. Engine hasn't been overreved, just barely bumped the limiter a little a couple of times. Timing is very good (according to delta dash logs). This does sound like it may have been a progressive deal, but the car did seem to be running strong up to that fateful day. It's interesting that I don't find anything much iin the forums on this- lot's of pistons letting go, ones w/ holes in them etc, but not much on burnt valves. Would some of those fuel treatment products (Redline, Motul, etc have them) be worth getting in the habit of putting in on occasion to prevent carbon build up? Wagon is same class- I'm not too worried- KC won the National Championship in STX last fall w/ his wagon Big Sky |
06-21-2004, 12:03 PM | #14 |
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Anything that keeps carbon buildup out of the equation, especially on a turbocharged engine, has got to be a good idea.
I use the Techron (sp) fuel system cleaner every so often (~5,000 miles), but I've never had the cylinder heads off, so I don't know how well it really works. There are other (more expensive) products and specialized treatments designed to remove buildup at the valves (intake more so than exhaust). Larry's post about the slower-burning fuel was thought-provoking. I imagine that it could (at the extreme) cause a problem, but I would think that we don't have enough overlap in our standard cams to get that much unburned fuel behind the exhaust valve. Oh, wait, that's why we have three cats, isn't it? |
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