Poor start when warm. UPDATE

W

w1ndsurfnut

Member
 Cognac
Haven't had to post for a problem for a long while. Car starts fine from cold. But when it is warm the start is terrible. Doesn't fire on the button if you know what I mean. Generally it helps if I press the gas paddle a little. Then it fires. Runs a little lumpy for a short period. Then it runs Ok. Any ideas. I'm thinking plugs maybe. I'm not throwing any codes. Thanks in advance.
Just to be clear. This is not a faulty battery or starter motor. When it cranks but no start its turning over at normal speed. Its just not firing properly.
 
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Weirdly my previous Z4 would always fire up straight away when cold, but turned over very slowly when warm.

It turned out the battery was on it's last legs, so it might be worth checking your battery health.
 
I had the exact same symptoms you describe. Mine is a 2004 3.0, M54 engine, had about high 50 K miles at the time. I replaced all the injectors, didn’t help. What it ended up being was the ambient air temperature sensor. Part number 65-81-6-936-953.

I replaced the injectors because I thought some were leaking and it was, in a sense, “flooded” and had to crank more to start. And, just like yours it idled rough for a few seconds before it settled down. Part was around $40 USD then. I checked the net and they are about $50-ish now.
 
Could be charcoal canister + evap/purge solenoid connected to intake. My car (N52/3.0si) does a rough idle / start on a 'warm start' which I think is because the charcoal canister might be saturated with vapors.

You can check this PDF which gives some insight to when the car uses the evap solenoid to allow vapors the charcoal canister has sequestered to flow into the intake: https://www.princeton.edu/~sdepasqu/bmw/pdfs/Leak.Diag.Sys.pdf
 
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