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Re: ari: need help with low rpm cyl. miss...
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Posted by Ari Novis (more from Ari Novis) on Thu, 8 Apr 2004 07:15:57 Share Post by Email
In Reply to: ari: need help with low rpm cyl. miss..., shawn, Wed, 7 Apr 2004 08:16:29
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The other folks have provided a lot of good information.

Unless you've got a leak in the wastegate actuator, I'd leave it alone. It only comes into play under boost, which you don't get at idle. I would simply suck on the hose to the actuator - if you can't pull air through without a major headache, the diaphram is OK. Same with the BPC - it just routes around the turbo.

Make sure you don't have a vacuum leak at the charcoal canister - pull the right front turnsignal assembly and peek in. Sometimes the hose comes off the canister, and you won't see it.

The temp sensor changes fuel flow depending on engine temp. It provides more fuel at cold temps, because when cold, you get fuel condensation on the intake manifold and cylinder walls. This is fuel unavailable for burning, so you need to richen the mixture.

I would try and find a way to run the compression test once with the engine cold, and then with the engine warm. I'll bet that the compression in the 'bad' cylinder is higher warm.

In order to get an air-fuel mixture to burn, you need a minimum amount of compression. It's possible that at low RPM, you just don't have enough compression in that cylinder to reliably combust. If the loss of compression is due to wide clearances, those will close up as the engine warms. Also, at higher RPM, you actually get higher compression than at idle - first, the column of air coming in is being accelerated by the pumping action of the engine - this provides a 'ram air' effect. Second, there is some pressurization from the turbo, even at 1200. This may be just enough to let the mixture burn.

I would try a compression test cold, both 'wet' and 'dry'. The dry test is done first - just measure compression with a cold engine. Note how fast the needle gets to the final number - does it jump right up, or does it 'pump' up? Then do a 'wet' test - drop in a teaspoon or so of oil, turn the engine over a half-dozen times, and re-do the compression test. If the compression is higher, or jumps up much faster, then chances are the problem is in the cylinder walls/rings - possibly a cracked ring. The oil acts as a seal between the piston and walls.

If the compression is higher when warm, I'd suspect a very minor head gasket leak. You could have a void right near cylinder #1, and when things heat up, things expand and it seals. Maybe re-torquing the head bolts would be a good idea.

Just to make sure #1 is the issue, let the engine misfire at idle and shut it down. Quickly pull the spark plugs, starting with #1. Is the plug wet with fuel? Are any others?

Other possibilities are a bad/sticky fuel injector. If the injector is sticking open, it'll let more fuel in. At idle, it can be so rich as to not fire. Why it clears up when warm, well, I'm just guessing here.



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