L Jetronic Tuning -- Surging Idle and High RPM Lean Out
Posted: Thu May 11, 2023 11:34 pm
I am hoping to get some L Jetronic insight and also provide some information that may help others tuning theirs. Two issues for discussion:
1). In Closed Loop Mode (Oxygen Sensor connected) as my engine warms from Cold Start idle speed grows from 550 RPM to 800 RPM as the sensor becomes active. With slight throttle above idle, the engine RPM oscillates up and down a bit. This goes away with increased pedal.
2). I am concerned that my engine may be leaning out excessively at high RPM and I am looking for insight on proper tuning for high RPM as well as idle.
As background I have a stock 1978 USA 528i L Jetronic system running a 3.5 liter BMW M30 with larger 320i intake runners, Metric Mechanic 275 degree sport cam, ported head, b35 exhaust manifold and oversize exhaust. Engine was rated at 260 HP when rebuilt 25 years ago and has 125K miles on the rebuild. L Jetronic is stock except for an adjustable fuel regulator (currently set a 2.5 bar without vacuum which is factory spec for 528i and 530i).
Surging idle is usually caused by a vacuum leak, which I do not have. My system pulls up to 20-22 IN HG. If I add air and reduce vacuum to 14-15 IN HG the oscillation at light throttle turns into an oscillation at idle (Closed Loop Mode only). If I disconnect the O2 sensor to move to Open Loop Mode, the idle is stable (about 425 RPM), and it does not increase to 800 as engine warms. RPM oscillation at light throttle (or idle) is not experienced.
Anticipating your suggestions: Aux Air Valve works properly (I have swapped this with another BMW unit, exchanged with a new Benz unit and also ensured there is proper function hot and cold); Coolant Temperature Sensor has been exchanged with new; Thermo Time Switch works properly and Cold Start Injection has good spray and shuts off properly; fuel pressure is correct; Throttle Body is properly adjusted as are Throttle Switches and Butterfly; AFM seems fine and is at factory spec (hasn't been adjusted); AC Bypass Valve works properly. Originally I thought the Oxygen Sensor might be a fault so it was replaced it with a new one. None of these seem to effect the Closed Loop problem I am experiencing.
I have previously experienced some stumble with a cold engine which was usually fixed with idle adjustment or a new Oxygen Sensor. Also I have wondered if my idle might be too rich (spouse complains of gasoline smell). It was only when I replaced old rubber hoses, rebuilt my brake booster, AC system and power steering all as part of an engine compartment restoration, that I encountered this idle speed instability in earnest. So I bought a Heathkit Exhaust Analyzer to help with the diagnosis and tuning.
I believe spec on a 528i is 0.2 - 0.8 %CO at idle speed (Haynes) while a 530i (Open Loop Mode - no oxygen sensor) is 1.5 - 3.0% CO. In Closed Loop Mode my system is rich versus 528i at 1.9 % CO at cold start (13.9 Air Fuel Mixture) at 550 RPM. I then warms to 3.0% CO (13.4 AFM) at 800 RPM. In Open Loop Mode my idle is steady at 425 RPM 2.2% CO and 13.7 AFM with mixture levels nicely in the middle of the 530i spec range.
If I adjust the Idle Valve on the Throttle Body to raise Cold Start idle speed I move the oscillations into idle from light throttle and the idle RPM then surges to cutoff at about 1400 RPM then drops abruptly oscillating (Closed Loop Mode only).
The Bosch Injection book recommends adjustment of the AFM bypass air valve to deal with surging idle. The procedure recommends adjusting from a lean position to richer until surging stops. Also I could: move to a 3 or 4 wire Oxygen Sensor to minimize the time at low Cold Start idle speed; or eliminate the Oxygen Sensor (Open Loop Mode only).
Separately, I had heard that older big six BMW L Jetronic engines tend to lean out a high RPM. At 3450 RPM, no load and in Closed Loop Mode I register 0.0 %CO and 14.8 AFM. At 3000 RPM and no load Open Loop, I register 0.14% CO and 14.6 AFM. So interestingly, Open Loop Mode has a both a stable idle and light throttle and seems to have slightly less lean out at high RPM.
If I only used Open Loop Mode I could use the Throttle Body Idle Adjustment to raise the Idle Speed to 800-850 RPM spec and perhaps also adjust fuel pressure up slightly to potentially reduce lean out at high RPM. Open Loop Mode seems a bit more powerful.
But I have to believe getting the Closed Loop Mode with the Oxygen Sensor to work properly would be the best solution with benefits in mileage and clean air (the car has no Cat) if I can figure out how to make this work.
I have attached graphs of AFM and % CO as RPM increases in both Open and Closed Loop Mode with no load. And also a graph of how RPM increases from Cold Start to Operating Temperature at Idle in Closed Loop Mode with Vacuum and AFM measurements.
My car is an e9 but I am posting here in appreciation of Peter Florance who's work among others introduced me to the L Jet system which has served me so well over these years.
1). In Closed Loop Mode (Oxygen Sensor connected) as my engine warms from Cold Start idle speed grows from 550 RPM to 800 RPM as the sensor becomes active. With slight throttle above idle, the engine RPM oscillates up and down a bit. This goes away with increased pedal.
2). I am concerned that my engine may be leaning out excessively at high RPM and I am looking for insight on proper tuning for high RPM as well as idle.
As background I have a stock 1978 USA 528i L Jetronic system running a 3.5 liter BMW M30 with larger 320i intake runners, Metric Mechanic 275 degree sport cam, ported head, b35 exhaust manifold and oversize exhaust. Engine was rated at 260 HP when rebuilt 25 years ago and has 125K miles on the rebuild. L Jetronic is stock except for an adjustable fuel regulator (currently set a 2.5 bar without vacuum which is factory spec for 528i and 530i).
Surging idle is usually caused by a vacuum leak, which I do not have. My system pulls up to 20-22 IN HG. If I add air and reduce vacuum to 14-15 IN HG the oscillation at light throttle turns into an oscillation at idle (Closed Loop Mode only). If I disconnect the O2 sensor to move to Open Loop Mode, the idle is stable (about 425 RPM), and it does not increase to 800 as engine warms. RPM oscillation at light throttle (or idle) is not experienced.
Anticipating your suggestions: Aux Air Valve works properly (I have swapped this with another BMW unit, exchanged with a new Benz unit and also ensured there is proper function hot and cold); Coolant Temperature Sensor has been exchanged with new; Thermo Time Switch works properly and Cold Start Injection has good spray and shuts off properly; fuel pressure is correct; Throttle Body is properly adjusted as are Throttle Switches and Butterfly; AFM seems fine and is at factory spec (hasn't been adjusted); AC Bypass Valve works properly. Originally I thought the Oxygen Sensor might be a fault so it was replaced it with a new one. None of these seem to effect the Closed Loop problem I am experiencing.
I have previously experienced some stumble with a cold engine which was usually fixed with idle adjustment or a new Oxygen Sensor. Also I have wondered if my idle might be too rich (spouse complains of gasoline smell). It was only when I replaced old rubber hoses, rebuilt my brake booster, AC system and power steering all as part of an engine compartment restoration, that I encountered this idle speed instability in earnest. So I bought a Heathkit Exhaust Analyzer to help with the diagnosis and tuning.
I believe spec on a 528i is 0.2 - 0.8 %CO at idle speed (Haynes) while a 530i (Open Loop Mode - no oxygen sensor) is 1.5 - 3.0% CO. In Closed Loop Mode my system is rich versus 528i at 1.9 % CO at cold start (13.9 Air Fuel Mixture) at 550 RPM. I then warms to 3.0% CO (13.4 AFM) at 800 RPM. In Open Loop Mode my idle is steady at 425 RPM 2.2% CO and 13.7 AFM with mixture levels nicely in the middle of the 530i spec range.
If I adjust the Idle Valve on the Throttle Body to raise Cold Start idle speed I move the oscillations into idle from light throttle and the idle RPM then surges to cutoff at about 1400 RPM then drops abruptly oscillating (Closed Loop Mode only).
The Bosch Injection book recommends adjustment of the AFM bypass air valve to deal with surging idle. The procedure recommends adjusting from a lean position to richer until surging stops. Also I could: move to a 3 or 4 wire Oxygen Sensor to minimize the time at low Cold Start idle speed; or eliminate the Oxygen Sensor (Open Loop Mode only).
Separately, I had heard that older big six BMW L Jetronic engines tend to lean out a high RPM. At 3450 RPM, no load and in Closed Loop Mode I register 0.0 %CO and 14.8 AFM. At 3000 RPM and no load Open Loop, I register 0.14% CO and 14.6 AFM. So interestingly, Open Loop Mode has a both a stable idle and light throttle and seems to have slightly less lean out at high RPM.
If I only used Open Loop Mode I could use the Throttle Body Idle Adjustment to raise the Idle Speed to 800-850 RPM spec and perhaps also adjust fuel pressure up slightly to potentially reduce lean out at high RPM. Open Loop Mode seems a bit more powerful.
But I have to believe getting the Closed Loop Mode with the Oxygen Sensor to work properly would be the best solution with benefits in mileage and clean air (the car has no Cat) if I can figure out how to make this work.
I have attached graphs of AFM and % CO as RPM increases in both Open and Closed Loop Mode with no load. And also a graph of how RPM increases from Cold Start to Operating Temperature at Idle in Closed Loop Mode with Vacuum and AFM measurements.
My car is an e9 but I am posting here in appreciation of Peter Florance who's work among others introduced me to the L Jet system which has served me so well over these years.