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Discussion Starter #1
My 89 Graduate is now running well after a valve job, resulting in 150ish lbs compression across the board rather than 0 in #1 before due to a burned exhaust valve.

My question: Would running with #1 not really combusting at all, due to low compression, for some time, have dumped a lot of unburned gas into the O2 sensor and cat, and maybe killed both? I'm in Calif, so I've got to pass a smog test to be able to register this thing. I'd don't want to replace more than I need to, but wonder what's common after this kind of failure. I can get a cat done locally for $120, but if I don't need to spend it, I'd rather not. Thanks for any advice.

Andrew
 

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1966-2013
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An over-rich mixture certainly can kill a cat in a very short time, (I managed to burn a brand new one down in about 15 blocks once due to a fat idle mix), though I prefer the club or laundry sack method.

You'll likely have to remove what you've got for peek to be sure.

The honeycomb will be very obviously damaged/melted if that's the case.

As to the O2 sensor, that can be iffy, but if you're going to replace the cat, might as well go the extra bit and get that delt with too IMO.
 

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Discussion Starter #3
Well I passed smog yesterday, so running for some period with zero compression on one cylinder didn't seem to hurt the cat or the O2 sensor too much. I had to buy new mid and rear exhaust pieces, and weld up a couple holes in the cat, but got OK numbers:

15 mph: HC 93/124 CO .17/.78 NO 236/815
25 mph: HC 72/98 CO .17/.66 NO 151/754

First number is measured, second is max allowed. So I've got a fair amount of unburned stuff happening, I'd guess (high HC). I had new spark plugs, and compression is about 150 across the board. Poor oil rings allowing oil to be burned? Cat going south? Whatever, I'm safe for two years.

Thanks for everyone's advice on this.

Andrew
 

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Looks like the only place that's even remotely close is HC.

In these next two years, you might want to check the valve seals.

They'll let some oil directly into the cylinders too, especially when you downshift to engine brake with your foot off the throttle. (engine vac will suck oil right past them if they're not in great shape)

You had to weld up some holes in the cat, and it still passed? Remarkable.
 

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Discussion Starter #5
Has maybe 30 miles on a Norman Racing valve job, with exhaust valves, seals, and all guides.

I did a pre-test before the official in-the-computer test, and the HC numbers were about 15-20% better. Maybe it cooled down some during the intervening 15 minutes? I got it good and hot before the test.

Yes, I was surprised about the cat too. I was prepared to have to get a new one.

Andrew
 

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Dunno about 15 minutes cooling the cat, though I suppose it's not unreasonable.

Then again, if it sat, then got put it right straight on the machine for the test it's likey things were a bit off due to the enrichment at startup and the O2 sensor not quite being fully back up to temperature. (and a cooler cat LOL)

That's what annoys me about some of the test places here: they let the car sit all day, start it up and within 3 minutes have it on the machine.

Of course it's going to fail. It's not even up to 'end of the driveway' temperature yet.........
 

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Based on your numbers, Andrew, you did some damage to the catalyst, not bad enough to melt it, but bad enough to lower some of the conversion efficiency.

If you were running really rich, the CO would be high, too.

As for the cat cooling down- for sure that does happen. I've seen it a lot here at work cooling donw ON THE EMISSIONS CYCLE, let alone just letting it idle. It can be a real PITA if you don't work with it.

The other thing is to put some good miles on it- which will help break in the valve seats and probably stablize the combustion deposits.

But it could be a lot worse- we just finished some cars where the builders did screw up the wiring, and we were left with a large marble of solid alumina (the substrate). It does not take long to put enough energy into the exaust for it to 1) glow red hot in the cat then 2) melt the bricks at ~2300F.

Eric
 

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Discussion Starter #9
Thanks Eric. I assume it's not running very rich now due to the consistent .17 CO number, but was doing so when running for an unknown number of miles with #1 producing no compression at all. I put about 20 miles on it that way, and the previous owners drove it daily as the #1 exhaust valve was burning away, producing progressively less compression. They only stopped driving it when their mechanic said, pre-inspection, that it was running too poorly to pass smog. But I don't know how many miles that was. Probably thousands.

Andrew
 
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