Some fuel trim readings from both of my '96 Mark VIIIs

[sprocket]

I was hoping to change injectors on the DA today but they did not make it in the mail. The car has been very hard to start and the idle is fairly rough. Today I took some live data readings.

First: No stored codes.

It was a difficult start and there was plenty of smoke out the exhaust when it finally started. I can smell the fuel.

The STFT after a short warm-up then showed high on both banks, as high as 42% on bank 2 and 25% on bank 1. Eventually bank 2 went down to 3.1%. Bank 1 never got below 18% and was often in the low 20's.

LTFT on both banks was never anything but zero, which I find quite strange. The idle was rough.

Now onto the other '96 for comparison:

Stored codes P0156, P0161 and a code related to the deleted EGR that I can't remember. P01401 I think.

STFT on both banks always under 10% and dropped to .7% over time.
LTFT however was around 10% on bank 1 and 13-14% on bank 2. I think this suggests a vacuum leak but I can't find one and the car idles very smooth most of the time. I believe it does develop an occassional hiccup over time if left to idle for a long time.

Any suggestions for what I should be looking for?

Thanks!

 

[slowmkviii]

Did you also do tests at 1500 and 2500rpm
If the ltft is zero that could mean that at least to me that the ecu could be bad
try swapping it with the other car im not saying it is but its a free thing to do.
Like you said for car #2 you could have a vacuum leak
But i would be looking for possible intake leak too.

 

[slowmkviii]

Just found this could help you maybe
http://obdcon.sourceforge.net/2010/06/interpreting-generic-obd-ii-scan-data/

 

[sprocket]

Yeah, I just was looking at that page a little while ago. I will check again at 1500 and 2500.

It is about 20 degrees out here in MN and I only had the second car running for maybe 10 minutes at idle when it had those high LTFT numbers. Maybe they were high from the cold outside temp and short run time? They carry over though, don't they?

 

[sprocket]

The STFT gets better in the DA at highway speeds, almost into an acceptable range. I have injectors coming tomorrow and will change them on Tuesday. There coud be a vacuum leak I'm not seeing too.

I'm never getting an LTFT reading. I'm going to take a wild guess here and say that the owner prior to the last owner maybe had the ability to do some tuning. I'm thinking the LTFT might be turned off in the computer to avoid a check engine light. I didn't try swapping the computers. I think there might be 3 different ones for '96, so I don't know if they are a match or not.