Re: How does your car run when cold? (archive)

I know what you mean about the "snap" - I have been calling it the "switch" effect, like someone flips a switch and car suddenly accelerates. Mine runs pretty crappily (stumbling, inconsistent power, surging) until this switch flips when it runs perfectly, and no, it's not the O2 sensor kicking in because it does it with sensor unplugged. It must be the switch from the enrichment phase to the warm phase, which is based on coolant temp afaik. Mine backfires out the tailpipe when shifting until the switch flips. So unburned fuel must be getting into the exhaust. Strange thing is if you monitor O2 sensor voltage (by preheating O2 sensor for 2 mins before starting car) it reads LEAN until the switch flips, at which time the voltage jumps above stoichiometric.

SO if there is alot of fuel being dumped into the engine during enrichment, but it is not getting burned completely (backfire), I could get a lean reading at O2 sensor, even if there is fuel in the exhaust, because sensor reads CO (burned fuel) and HC (unburned fuel) is not read by sensor. Make any sense?? If this does make sense, why is engine unable to burn enough fuel? Ignition problems? Have new plugs, wires, cap, rotor; all test and visually look fine. Maybe a weak coil?? Car seems to runs fine warm under load tho. Other possibility is maybe bad injector spray pattern, meaning fuel being dumped into cylinder not atomizing sufficeintly to ignite? Possible?? A contradictory factor here is that HC reads VERY low (meaning a good/complete burn) when car is idling/warm (exhaust gas analyzer)...Any comments appreciated!

if the car is cold, you shouldnt' spend too much time betwen 2500-4000rpm, especially not near 4000rpm.

It's been in the mid 30s the past few weeks in seattle and i have no pinging, hestitation, or other issues with my M5. It sits all night uncovered and starts right up, and runs great. It cold-idles at like 11 or 1200 rpm or so, and it stays up there for about a minute of driving. it starts to come down over the course of the next few minutes.

One thing i have noticed is that i'f im holding constant load and rpm, there's definitely a discrete "snap" when the engine just finishes warming up. It's very noticeable. I.e. there's a sudden 100rpm change, even though road and throttle remain constant. It's like the idle control system (do our S38s have a cold injector like the M30 does ?) shuts the last little percentage of cold-start enrichment off but its too large a granularity to not be detectable. I get a kick out of it when it happens :)

Quick survey - does anyone else's car drive poorly when stone cold (in other words, engine at ambient temp, not run for at least 4 hours)- do you experience any hesitation in the 2500-4000 rpm range, or surging, or backfire? As many of you know, mine has these issues but only when absolutely cold. Sheldon's also does it. Drove another M5 today that ALMOST did it.... just wondering if it is a widespread phenomenon in the S38...

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