Previously posted about run issues with 1999 disco I running issues. Mainly poor throttle response and backfires. Kept getting codes for o2 sensor heater circuits malfunction as well as random misfires. After replacing maf, cam pos. sensor, searching for vacuum leaks, checking all wiring and connections, removing ECM and checking for moisture to no avail, I followed advice here and went to basic start of this which was o2 driven codes.

I started by replacing exhaust from manifolds to muffler with a magnaflow oem fit setup with cats. Then came fun part - determining which o2 sensor was actually correct for this car. Of the 12-15 part numbers I obtained from denso, Bosch, ntk, and land rover, I eventually settled on an ntk 25044, which fit the 12 mm bung fitting on magnaflow exhaust and had correct terminal plug for factory harness. Good news - now have good throttle response. However, just got check engine lite after running for 100 miles or so. It says p1187 which I understand is sbefore cat o2 heater circuit fault.

I thought is was wrapped up with this diagnosis and could go on to patching oil seal leaks, but now wonder, why this similar o2 code? Should I clear code and proceed to see if it comes back, or assume bad o2 sensor? Additionally, fuel mileage still seems appalling. I'm guessing 10-12 mpg on vehicle with new plugs, wires, etc, etc. does it take ECM a tank of fuel to set new run parameters based on data from new sensors?. Should I disconnect battery to "reset" ECM after installing new o2 sensors. Any additional help appreciated.

 

If you can borrow a scanner check to see if fueling is going into closed loop. If it stays in open loop all the time, will use more gas. Won't go into closed loop unless ECM likes all the sensor inputs. The O2 fault may be holding it back from closed loop fueling (fuel controlled by computer moment by moment, vs using a "memory fixed value".

 

ok - i checked and it is consistently going to closed loop. no more o2 sensor codes. now I'm getting 1177 and 1179 after about 50 miles of running after i clear check engine lite. as i understand those are fuel trim over limit. the running data typically shows short term fuel trim on both banks as +25% and long term fuel trim at -25%. i have not really run for over 20 miles at a time on highway. could this be causing problems with giving the ecm data to utilize?? also, could i have a maf sensor problem. the values i am seeing are from 0.01 lb/min at idle up to high of about 0.08 lb/min under hard load. from what i remember reading, those values seem off by a decimal point. I'm thinking i saw somewhere they should be in the range from 0.10 to 0.80 or so.

 

It is possible to clean the MAF. There are cleaners made for them. The trick is to do it with engine stone cold, don't turn on the ignition, as sensor heats up a tiny wire to 350 degrees inside. Cold spray makes that snap and then you have a sensor to buy that sells in some places for $1400.