question about idle rpm

JeepPilot · **Posted:** Thu Sep 11, 2008 9:39 pm

When I give my KJ a cold start, it sits at about 1000rpm until I get moving and get my oil temp up to normal. With normal oil temp, just slightly below the halfway mark, my engine idles at 600rpm, which is what I've always experienced. Driving around tonight, I glanced down a couple of times to find my engine idle at roughly 550rpm, not sure if I should be concerned about that.

Dunno if I'm just now taking notice to my idle being at 1000rpm on a cold start, or if it was always like that. Given this information, is there anything I should be concerned about, or am I worrying for no reason?? :roll:

tjkj2002 · **Posted:** Thu Sep 11, 2008 11:01 pm

JeepPilot wrote:

When I give my KJ a cold start, it sits at about 1000rpm until I get moving and get my oil temp up to normal. With normal oil temp, just slightly below the halfway mark, my engine idles at 600rpm, which is what I've always experienced. Driving around tonight, I glanced down a couple of times to find my engine idle at roughly 550rpm, not sure if I should be concerned about that.

Dunno if I'm just now taking notice to my idle being at 1000rpm on a cold start, or if it was always like that. Given this information, is there anything I should be concerned about, or am I worrying for no reason?? :roll:

When you start your KJ when the engine is cold it is "open loop",meaning the O2 sensors are not heated to 600 degrees yet to start working so you are running on a preset program,which is a rich condition to prevent engine damage in cold temps.When the O2 sensors start working you go into "closed loop" and the PCM is adjusting your air/fuel ratio alone with timing for the best performance and your idle will drop.Normal idle for a fully warmed up KJ with the 3.7 is 650rpm +/- 25rpm.

JeepPilot · **Posted:** Thu Sep 11, 2008 11:14 pm

Quote:

Normal idle for a fully warmed up KJ with the 3.7 is 650rpm +/- 25rpm.

So should I be concerned if my idle is at 600rpm?? That's what it's been at it's entire life and I haven't experienced any problems in that regard.

tjkj2002 · **Posted:** Thu Sep 11, 2008 11:28 pm

JeepPilot wrote:

Quote:

Normal idle for a fully warmed up KJ with the 3.7 is 650rpm +/- 25rpm.

So should I be concerned if my idle is at 600rpm?? That's what it's been at it's entire life and I haven't experienced any problems in that regard.

Your tach is showing you "relitive" rpm's,like most of the other gauges it is not showing you what the PCM is actually reading.Plus the marks are so small on the tach how can you tell.Hook up a scanner that does live data and it will show you actual rpm's and you will see it's within specs.

JeepPilot · **Posted:** Fri Sep 12, 2008 8:01 am

Ok tjkj, much thanks for the help. I don't imagine I have a problem, but it doesn't hurt to ask the experts for an opinion/advice.

question about idle rpm

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