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Sputtering on cold starts


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Lately my 00 Boxster has had a hard time holding idle on cold starts. It'll dip really low in the RPM range (500 or so) and run really rough until the engine warms up. Once the engine is warm it runs great. No check engine light or warning lights of any kind. I'm not sure where to start looking for the root of this one? Anyone have any ideas for me?

Thanks in advance!

Xaev

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A few of the obvious things first. Have you tried any of the following:

1. A bottle fo fuel injector cleaner.

2. A better tank of gas

3. OBD2 device to see if there are any codes

Then the less obvious that may have been overlooked:

1. Fuel filter

2. Air filter

3. Spark Plugs

4. Overall general tune-up

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Without you posting any fault codes, it'll have to be a 'finger in the air' guess, so I'd try suggesting an Engine Coolant Temperature sensor, as you can get implausible values from this without the CEL illuminating.

The ECT sensor can fail making the engine think its hot all the time, so does not enrich the mixture on cold start, hence it will start lumpy and smooth when warm. Any OBDII scanner will show you the fault code or if no code, will show the ECT as hot when you've left it overnight. Or it could be something entirely different..(!)

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Taking inspiration from LVDell's post, the trottle body sometimes gets a ring of muck build up around where the butterfly sits, but this does not figure if it idles OK when warm. Worth looking at though.

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Hey guys thanks for the ideas.

It's not the gas I'd think - it's been a few tanks worth and still having the issue. I only ever put Chevron Premium in the car.

I had thought about plugs and filters but figured it would run bad all the time if that were the case. As is, when its warm it purrs.

I'll borrow an odb2 scanner - i was under the impression that if there is no check engine lights there wont be any fault codes to read. Hopefully this'll point me the right direction! The engine coolant temp sensor sounds like it could be the culprit - but wouldnt that be the same sensor that drives the coolant temp gauge? The gauge works great, or seems to.

thanks again!

Xaev

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The Engine Coolant Temperature sensor is the same physical device that provides temp info to the ECU and to the instrument cluster. However when you get to it, you'll see its a four terminal device - there's two separate NTC resistors inside that are electrically isolated from each other. One can go bad and the other work perfectly.

If you borrow an OBDII scanner, when the engine is cold, check for your fault codes but also check the value the ECU reports for the engine temperature. It's a wide range sensor, so should read ambient temperature no matter how warm or cold.

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Jonathan,

2.5l Boxster (M26.20) 996.606.405.01

2.7l Boxster (M26.22) 996.606.410.00

3.2l Boxster S (M26.21) 996.606.410.00

Ask for a sealing washer too. ECT accessable at the front of the engine, remove the cover behind the seats and you see it screwed into the coolant guide assembly - to the right of the crankshaft pully as you look at it.

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Hey guys thanks for the ideas.

It's not the gas I'd think - it's been a few tanks worth and still having the issue. I only ever put Chevron Premium in the car.

I had thought about plugs and filters but figured it would run bad all the time if that were the case. As is, when its warm it purrs.

I'll borrow an odb2 scanner - i was under the impression that if there is no check engine lights there wont be any fault codes to read. Hopefully this'll point me the right direction! The engine coolant temp sensor sounds like it could be the culprit - but wouldnt that be the same sensor that drives the coolant temp gauge? The gauge works great, or seems to.

thanks again!

Xaev

In some cases the engine light will not illuminate due to a bad bulb. Rare, but it does happen.

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Have you replaced the air filter with a K&N or other oiled filter?

This is exactly the same symptom my car had when my MAF was dirty. Started as rough idle when cold and eventually ended up where the car would not run at all. If the filer is over oiled, oil droplets can accumulate on the MAF and cause incorrect readings. I carefully cleaned mine with electrical cleaner and have not had any trouble since.

Just another data point for you to consider.

Graeme

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