Secondary Air Pump

K

kingsgate

Dedicated Member
Joined
Mar 17, 2015
Points
31
Hi me again, I'm still chasing issues on my 2.0 1999 M52TU which involve poor start ups. I'm getting closer to some understanding of the issues but need to pick some brains, that's where you all come in. When my car starts from cold it sometimes starts in what I call 'normal' mode, the air pump runs and the engine is quite quiet and smooth. Most of the time it starts in a slightly 'sick' mode where the engine is noisier, tappet noise and a bit lumpy. The air pump doesn't run during this type of start up and though the revs are higher the cold start is short lived compared to 'normal' mode. I have noticed that when the air pimp is not running that vacuum to the air valve is absent, that is to say the secondary air valve hasn't operated (it's the nearly impossible to find solenoid valve mounted round the back of the intake manifold), there is no vacuum from secondary air valve though it is present at the input to it. I think I have 3 electrical issues here, 1 no air pump, 2 no air valve, 3 no vanos solenoids. Looking at circuit diagrams these 3 can all be traced back to common fuses but so can other things that do work. All 3 items work and check out okay, the only common factor that I can't really check is the ECU where eventually all 3 items are taken to ground to complete their respective circuits. I don't have a diagram of the ECU as I'm sure BMW/Siemans keep that closely guarded and I'm sure it would be quite complex. By the way the ECU is original and having examined it in close detail shows no signs of obviously faulty compnents or dry joints. In fact the ECU looks pretty good for it's age, mileage and the way the car has been neglected by previous owners. My questions therefore are: has anyone else experienced this, if so what was it? Does anyone have a diagram of the ECU, can I have a copy please? I assume the ECU needs an input from a sensor to tell it that the engine is cold so the air pump should be turned on but which sensor does this? Coolant? Oil temp? or intake air temp? Quite why the engine should be noisier in 'sick' mode I can't imagine at present. I do get short bursts of what might be vanos noise that disappear after a few seconds when the engine starts in 'normal' mode. If the engine starts in 'sick' mode I can let it run for a short time and if I switch off and restart the engine it quite often starts up in 'normal' mode, this may indicate either a faulty electrical connection that would need to be common to all 3 or a sensor that is now either registering a slight rise in temperature or a faulty sensor that is now working correctly. I have been able to access oil temp and air intake temp sensors, they read okay in ohms (from what I can tell) but I can't find or see the coolant temp sensor, it seems to be hiding under the intake manifold somewhere. Temp gauge on dash seems to work okay, thermostat has been changed twice. No fault codes show. HELP!!
 
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