Strange spark plug variations...

[boiseDSM]

This is addressing a 1990 talon Tsi,

When analysing you sparkplugs, do you often find they have different readings between them? On cylinder #1, the plug had a very dark / thick coating (too rich). cylinder #2 and #4 were as to be expected (just a tad brown), but cylinder #3 was really white (too lean).

I must have taken out the spark plugs more than 20 times over all the years I've had the car and for the most part they have been universally the same color. However, this time they are all over the place. The car is not running too well either. Crusing the car feel fine. WOT and part throttle the car will pull max timing. I originally thought it was phantom knock but after seeing the super white (lean) plug on #3, I am begining to suspect it is real knock which the source of the problem.
Injectors were cleaned and checked about 40,000 miles ago. I don't think it would be a fuel pressure problem because it is usually cylinder #1 (counting from cam gear side) that runs the leanest. Tested for boost leaks at 40 psi and found none. Hold positive pressure for about a minute. What else could make an individual cylinder run super lean?

 

[GTM]

This is addressing a 1990 talon Tsi,

When analysing you sparkplugs, do you often find they have different readings between them? On cylinder #1, the plug had a very dark / thick coating (too rich). cylinder #2 and #4 were as to be expected (just a tad brown), but cylinder #3 was really white (too lean).
...
Injectors were cleaned and checked about 40,000 miles ago. What else could make an individual cylinder run super lean?

This is unusual, however, not meaning to be insulting we don't know if the plugs were torqued to spec or just guessed at since this can result in 1/2 heat range variation. If the plug was too loose it will suck air in and make it lean. Usually I would look for a vacuum line leak or the device controlled by that line which could be influencing an individual cylr, intake manifold gasket leak.

You may be pushing your luck with the 40K since the last time you had the injectors out and checked for delivery and pattern. Did you replace the plugs, did you move them around to different locations, regap? Plug wires starting to fail? If you hear it knocking then back out of it.

You make no mention of a compression check.

There are no absolutes with this type of problem, all we can do is toss out ideas but you will have to do the diagnostic work. Rule out the easy things, then the more difficult and/or then those which cost money.

Cheers,
GTM

 

[boiseDSM]

I'm going to try to swap in another set of 450's to see if the problem goes away (along with a compression check too). I never even thought about the torque specs of the plugs. That does make a lot of sense now that you mention it. They were all tight though.
I can't see it being a vac problem because the car holds boost for a very long time. Also, it seems like that would affect all the cylinders at the same time.

So far here is what I've done...

replaced the following...
O2 sensor
CAS
plug wires
knock sensor
cleaned lifters

Checked cam timing, checked crank timing. checked error codes (none).
I'm not certain that the CAS in it now was out of a 1990. Not certain that it matters because the plug of the CAS is the same.

It's a strange problem. Could the resistor pack be going out and messing with the injectors?

 

[Defiant]

Other than in extreme cases, reading spark plugs on fuel-injected, closed-loop feedback engines is -as you're discovering- an exercise in futility and frustration. The computer is throwing so many different mixtures so many times a minute that the plugs don't have the chance to get an even read.
Truth be told, it was worse with carburetors. You really needed to get the gas cut off when you chopped it, at speed, and it just wasn't that easy to do.

 

[GTM]

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I can't see it being a vac problem because the car holds boost for a very long time. Also, it seems like that would affect all the cylinders at the same time.
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It's a strange problem. Could the resistor pack be going out and messing with the injectors?

Boost leak is static, if your cruise control is leaking and the vacuum source is leaking air so that it washes some of the fuel away from the intake and into other cylrs it could cause this.

Yikes almost forgot EGR can cause this problem depending on it's placement.
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Truth be told, it was worse with carburetors. You really needed to get the gas cut off when you chopped it, at speed, and it just wasn't that easy to do.

That's why they had those electronic solenoids and power valves which helped but were a PIA to diagnose if they were sticking.

Cheers,
GTM