Engine sputtering - O2 Trim Low

[RSTurboFWD90]

My '90 Turbo sputters at idle and cruise (closed loop) but runs fine at WOT. I have logged the O2 sensor and O2 Trim and found that the trim stays low indicating the ECU thinks it's rich most of the time (or so I have read). You can smell it is rich and my last emissions test passed but was higher in HC and CO than prior tests. Here are graphs of the logger data.

IDLE:

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O2 is very random with a range of 0.06 to 0.94, O2 Trim has a range of 35 to 60%

2400 RPM:

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A similar looking result. O2 Trim range is 0 to 50%.

Can anyone offer any insight to these results. I suspect a sluggish O2 sensor. No CEL for this problem.

 

[andymoraitis]

Plain and simple the failing O2's dumping extra fuel and even if it hasn't tripped a CEL it's time for a replacement.

You can find OEM replacements for dirt cheap here:

http://www.sparkplugs.com

Enjoy,

Andy

 

[RSTurboFWD90]

Thanks for the source. It is a good price. The best I could get from a dealer is 50% higher.

I know the trim should average 100% but should the O2 signal be more regular and square?

 

[RSTurboFWD90]

A new O2 sensor didn't fix it. I did find a leaking capacitor in the ECU.

 

[RSTurboFWD90]

I had the ECU repaired and that fixed the problem.

 

[andymoraitis]

I forgot you have a first gen and are subject to leaking caps.

Glad you fixed it anyway and at least you now have an O2 that will handle leaded if you need to run race fuel.