On my '98 GEMS-equipped 4.0 Rover V8 Discovery 1 I'm seeing what I think is a excessively negative LTFT. Both banks are pegged at -19.5. STFT appears to act normal. In attempting to diagnose why the engine might be running so rich, I've thoroughly cleaned a cold MAF with MAF cleaner, and checked the MAF g/s. It idles at about 0.10 and rises through to about 0.40 at about 4000 rpm (with no load, just pressing the throttle in Park). If I disconnect the MAF, the engine runs somewhat similar and the MAF g/s reading is "err."

Based on preliminary research -19.5 seems excessive for LTFT, and MAF g/s less than 1 seem insufficient for the MAF. However, I do not have a Discovery 1 benchmark to compare with. Can anyone advise what I should be seeing?

I'm attempting to determine if there's a vacuum leak. There's no indication based on the idle, which is smooth. STFT can show -3 at idle versus 0 at 2500rpm, but other times it will be 0 at idle. The engine was cold, so running a little rich at idle seemed normal. I'll check it when it warms up.

The vehicle runs normally. It idles normal. It drives up to 80mph without any abnormal behavior. It uses too much fuel and doesn't have enough power, but I believe that's normal too.

My questions are what should I see for acceptable range for LTFT, MAF g/s, and if I have a failed MAF, and if there's any other diagnostics I can try. Thanks

 

I've learned from some other threads that the MAF g/s readings around 0.10 to .60 have been experienced by others even after replacing the MAF. It may be that the ECM simply doesn't provide the right data to the ODBII scantool. I don't really understand the reading, but I'm skeptical that it means my MAF has failed. I read some other people testing their MAF based on the voltage reading, but they had the Bosch MAF and I have a Sagem, and don't know what the voltage should be.

After I cleaned the MAF, LTFT rose from -19.5 to -16.4 and then went down again to -17.6

I also noticed STFT seems to vary between bank 1 and 2. Bank 1 is consistently trimmed lower than bank 2. So while bank 1 might be at -1, bank 2 might be at +2.

All four O2 sensors are reading fairly consistent voltages around 1V. The voltage varies when running as expected, but they all seem to be working.

 

Did you ever find out the problem