Cold low RPM hesitation
Dec 28, 2008 | 06:22 PM
#1
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From: Kelowna, BC
Cold low RPM hesitation
My '90 V6 has a bit of a low rpm hesitation when it's cold for the first few mintues after start up. It's mostly noticeable off idle, when leaving a stop, etc. Once she's warm it disappears. It's more of an annoyance than a real issue. Cold start valve/injector?
Thanks guys,
Brent.
Thanks guys,
Brent.
Dec 28, 2008 | 07:14 PM
#3
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From: Temecula Valley, CA
The cold start injector is only supposed to work during cranking to start. After the engine starts the ECU is able to calculate a fuel mixture based on intake air temperature, throttle position, coolant temp and air flow. To help the idle out when cold, there's a valve under the throttle body that opens up and allows air to bypass the throttle plate thus raising the engine idle a bit.
Do you know what your cold and warm idle rpms are? Cold could be as high as 1600 and warm should be 800, and it should definitely idle higher cold than warm.
Other things could be: plugs- if the gap is not correct, the rich mixture when cold could foul one or more plugs; coolant temp sensor- if it's malfunctioning the ECU may think the engine is warmer or colder than it really is and inject the wrong amount of fuel; throttle position sensor- if that's maladjusted or failing, the ECU may think the throttle is at a different position than it is and inject the incorrect amount of fuel; and as always, vacuum leaks could affect engine operation. I would probably shy away from the air flow meter, but the air temp sensor in the meter could be sending an incorrect reading and mucking with the air-fuel ratio. There's also the possibility that the EGR valve is not operating properly or you have one or more partially clogged injector(s) that is/are not able to supply enough fuel, but since it runs fine when warm I would probably not concern myself with that until it's the last resort.
It's likely running fine when warm because the O2 sensor has warmed up and gives some feedback to the ECU allowing it to fine tune the mixture.
Do you know what your cold and warm idle rpms are? Cold could be as high as 1600 and warm should be 800, and it should definitely idle higher cold than warm.
Other things could be: plugs- if the gap is not correct, the rich mixture when cold could foul one or more plugs; coolant temp sensor- if it's malfunctioning the ECU may think the engine is warmer or colder than it really is and inject the wrong amount of fuel; throttle position sensor- if that's maladjusted or failing, the ECU may think the throttle is at a different position than it is and inject the incorrect amount of fuel; and as always, vacuum leaks could affect engine operation. I would probably shy away from the air flow meter, but the air temp sensor in the meter could be sending an incorrect reading and mucking with the air-fuel ratio. There's also the possibility that the EGR valve is not operating properly or you have one or more partially clogged injector(s) that is/are not able to supply enough fuel, but since it runs fine when warm I would probably not concern myself with that until it's the last resort.
It's likely running fine when warm because the O2 sensor has warmed up and gives some feedback to the ECU allowing it to fine tune the mixture.
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