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Functional Question: What makes the engine idle high when cold?

C

ck42

CEG'er
Joined
Jun 4, 2005
Messages
99
When you first start up a cold motor, what exactly is the puter taking into consideration that causes it to do this? (ECT, etc?)

....and, when the engine idles high and then comes back down to normal, is this just an automated 'timer' that is being run or is the computer actually getting feedback that causes it to slowly drop the idle down?

Chris
 
when you start your car it will be in "open" loop operation where the ecm will ignore the o2 sensor and go by a set program for fuel and spark. then when it warms up the car will go into "closed" loop operation. I think also it will have to see a 40 degree difference in the cts , and then a final tempature of over 160 to go into closed loop.
 
OK....I understand the 'choke' operation being an enriched fuel mixture. Is it as simple as that? The ECM says 'It's cold, make it richer'. But, the RPMs fall pretty quickly...even when it's pretty cold out. I can't see any significant engine warming occurring in the ~15-20secs that the high idle occurs.

Interesting info regarding WHEN the system goes into closed loop mode. So, an ECT of 160F and a 40F delta between the ???? (the ECT and the ambient temp sensor in the air filter?)

So, hypothetically, if one of these 2 conditions wasn't true due to a malfunctioning sensor or whatever, then the system would stay in OPEN loop mode (and continue to run rich?)

Chris
 
ck42 said:
OK....I understand the 'choke' operation being an enriched fuel mixture. Is it as simple as that? The ECM says 'It's cold, make it richer'. But, the RPMs fall pretty quickly...even when it's pretty cold out. I can't see any significant engine warming occurring in the ~15-20secs that the high idle occurs.

Interesting info regarding WHEN the system goes into closed loop mode. So, an ECT of 160F and a 40F delta between the ???? (the ECT and the ambient temp sensor in the air filter?)

So, hypothetically, if one of these 2 conditions wasn't true due to a malfunctioning sensor or whatever, then the system would stay in OPEN loop mode (and continue to run rich?)

Chris
Click to expand...

A bad thermostat that encourages the engine to run too cool will keep it in "cold engine" mode longer and waste gas. On cold days, it may never come into closed loop.

I don't know, but I would think the cold engine high idle is controlled by more than just time. I think it is temp controlled but that it comes down it steps.

The warm engine "throttle hang" or "dashpot" mode I believe is mostly a timed event to help keep enough oxygen available for a complete burn to lower emissions.
 
You are right, when the engine is cold, it runs in open loop and uses default (and overly enriched) fuel values.

However, when the engine is first run, the Idle Air Control is opened wider for the first 30 seconds or so, depending on the temperature, to increase the idle. This is done to prevent the car from stalling in very cold conditions, and to warm up the engine more quickly. When the idle drops down to the normal 700-1000 rpms, this does not mean you're in closed loop already, the IAC has simply closed a bit.
 
OK...getting somewhere.

So, ECM reads engine as cold (some varying degree of coldness) and then enriches fuel and tells the IAC to open up some varying amount to gets the RPMs up while it's in the 'warm-up' phase.
 
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