ElectronJockey
New Member
- Posts
- 7
- Location
- Uckfield
I am new to posting on this forum but have spent many hours reading other people’s threads and gaining knowledge from it. My problem is with a 1997 P38 with a GEMS engine that runs like a bag of nuts and bolts. I am an electronics engineer and have test equipment and a P Scan Diagnostic tool, so I am able to approach the problems in a logical way (when the whisky clears from my brain). The output of the P Scan shows:-
Live update interval 602 ms
Engine speed 633 rpm
Fuel System 1 Status Closed Loop
Fuel System 1 Status Closed Loop
Calculated Load 34%
Engine Temperature 99 deg C
Short Term Fuel Trim 1 -16%
Long Term Fuel Trim 1 11%
Short Term Fuel Trim 2 -17%
Long Term Fuel Trim 2 11%
Error Status 0 MIL OFF
Vehicle Speed 0 km/hr
Ignition Timing Advance 24 deg
Air Intake Temperature 27 deg
Air Flow Rate 0.04 g/s
Throttle Position 11%
Commanded Air Status 4
Oxygen Sensor Location 17
02 Sensor Output Bank 1 0.645
02 Sensor Output Bank 2 0.805
The engine ECU/ECM has been recently changed (I have tried three different ones) and has done about 50 miles so the long term fuel trims are probably reflective of the previous engine that it was used on. The air temperature and O2 sensors were changed last year. My main approach is to get the Air Flow Rate right (should be about 5.5 g/s) before looking at anything else. Much is written about the MAF sensor on this forum but the MAF does not appear to be the problem here. I have now accumulated 4 MAF sensors and the one currently on the car gives an output of 1.35 Volts. Rave states on page 351 that the output should be 0 - 5 Volts (1.4 V at idle) so there is nothing wrong with that MAF. Injecting a regulated voltage into the ECU/ECM and reading the P Scan gives:-
0 Volts dc = 0 g/s
1 Volts dc = 0.03 g/s
2 Volts dc = 0.16 g/s
3 Volts dc = 0.58 g/s
4 Volts dc = 1.38 g/s
5 Volts dc = 2.19 g/s
So, the full output of 5 Volts from the MAF could not reach the expected 5.5 g/s at idle and therefore eliminates the MAF as the problem. Checking the wiring harness from the MAF to the ECU/ECM shows that it is continuous (less than 2 Ohms) and there is no leakage. Continuity checks were made with an old ECU/ECM that was opened so that measurements could be taken at the PCB and they were again less than 2 Ohms showing that the plugs were mating properly.
It appears that something other than the MAF can affect the reading of Air Flow and it is probably related to the adaptive compensation.
My question is this. What other factors/inputs can affect the Air Flow Reading from the ECU/ECM? I know that other things are suspicious but I want to get the air flow right first.
Live update interval 602 ms
Engine speed 633 rpm
Fuel System 1 Status Closed Loop
Fuel System 1 Status Closed Loop
Calculated Load 34%
Engine Temperature 99 deg C
Short Term Fuel Trim 1 -16%
Long Term Fuel Trim 1 11%
Short Term Fuel Trim 2 -17%
Long Term Fuel Trim 2 11%
Error Status 0 MIL OFF
Vehicle Speed 0 km/hr
Ignition Timing Advance 24 deg
Air Intake Temperature 27 deg
Air Flow Rate 0.04 g/s
Throttle Position 11%
Commanded Air Status 4
Oxygen Sensor Location 17
02 Sensor Output Bank 1 0.645
02 Sensor Output Bank 2 0.805
The engine ECU/ECM has been recently changed (I have tried three different ones) and has done about 50 miles so the long term fuel trims are probably reflective of the previous engine that it was used on. The air temperature and O2 sensors were changed last year. My main approach is to get the Air Flow Rate right (should be about 5.5 g/s) before looking at anything else. Much is written about the MAF sensor on this forum but the MAF does not appear to be the problem here. I have now accumulated 4 MAF sensors and the one currently on the car gives an output of 1.35 Volts. Rave states on page 351 that the output should be 0 - 5 Volts (1.4 V at idle) so there is nothing wrong with that MAF. Injecting a regulated voltage into the ECU/ECM and reading the P Scan gives:-
0 Volts dc = 0 g/s
1 Volts dc = 0.03 g/s
2 Volts dc = 0.16 g/s
3 Volts dc = 0.58 g/s
4 Volts dc = 1.38 g/s
5 Volts dc = 2.19 g/s
So, the full output of 5 Volts from the MAF could not reach the expected 5.5 g/s at idle and therefore eliminates the MAF as the problem. Checking the wiring harness from the MAF to the ECU/ECM shows that it is continuous (less than 2 Ohms) and there is no leakage. Continuity checks were made with an old ECU/ECM that was opened so that measurements could be taken at the PCB and they were again less than 2 Ohms showing that the plugs were mating properly.
It appears that something other than the MAF can affect the reading of Air Flow and it is probably related to the adaptive compensation.
My question is this. What other factors/inputs can affect the Air Flow Reading from the ECU/ECM? I know that other things are suspicious but I want to get the air flow right first.
.
