Unstable Idle @ startup 3.8L

[IceHammer]

Ok, this isn't a problem yet, but I'm curious if you guys have any idea whats happening, because I dont.

When I turn on the car it lights up immediately and hovers at the high idle mark ~1000rpm or so for the standard 5 or 10 seconds, then drops to 800rpm or so. Once it drops to near 800, it starts to sound like its got a really radical cam in it, sort of heaving quickly. A little too fast to see it on the tachometer, so I can't give a good rpm range, but it cant be more than +/-75rpm from the desired idle. This lasts for maybe 30 seconds and then it settles out.

I'm thinking some kind of scratchy reading causing the idle air controller to overcompensate back and forth?

Like I said not currently a problem, but it could be eventually. Any thoughts?

[truekoa]

Re: Unstable Idle @ startup 3.8L

so it idles up and down for a bit before evening out? What have you checked?

Could be a few things, TPS, IAC, or MAF sensor, or maybe even the MAF screen to name a few.

Anything happen recently that started this? Any codes?

[darkflare999]

Re: Unstable Idle @ startup 3.8L

i believe the tps could be bad...i had a problem like this couple months back...and it didnt throw a code till a few miles...also check the iac...1 quick and cheap way is to just clean it up and test it....you should do that before looking into the tps

[firebird19_99]

Re: Unstable Idle @ startup 3.8L

most likely sensors something similar happened to me but went away once i changed the plugs out. Though i might have gotten the sensors wet and once they dried out the problem went away. TPS or ETC my guess

[IceHammer]

Re: Unstable Idle @ startup 3.8L

so it idles up and down for a bit before evening out? What have you checked?

Could be a few things, TPS, IAC, or MAF sensor, or maybe even the MAF screen to name a few.

Anything happen recently that started this? Any codes?

Haven't really checked anything yet, like I said its transient so its not really a priority. But I imagine it will get worse eventually.

I'm pretty sure it isnt the TPS, this issue is when no foot is on the pedal, and its the electronic throttle so IAC is more likely on startup.

How do you check the IAC? Pull back the intake and watch it?

No codes, it started happening this spring and hasn't really increased in magnitude since then. It seems to have a shorter duration or not be there at all when starting the car hot if that helps.

[LETZRIDE]

Re: Unstable Idle @ startup 3.8L

do you have a cam? My car did this when I put my cam in...my solution was to bump the idle up at idle

[Blizzard242]

Re: Unstable Idle @ startup 3.8L

My car does this same exact thing too and has for a long time. I can't remember what I've done to try and fix it but I never really got rid of it and it hasn't gotten any worse.

What all have you modded if anything?

[IceHammer]

Re: Unstable Idle @ startup 3.8L

Nope, I haven't put in a cam, although it does sound like an engine with a radical cam when it does what im talking about.

Only modifications to the car are sound system, chassis or cosmetic, nothing at all on the engine so there shouldn't be any influence from those.

Maybe the ECU is just having trouble running in open loop and it settles down when it switches to closed?

[truekoa]

Re: Unstable Idle @ startup 3.8L

I think if it was the ECU, it would idle somewhat high and then settle down to a lower RPM and stay there when it is warmed up. It shouldn't idle up and down randomly until it warms up which is why we think its a sensor.

[ssms5411]

Re: Unstable Idle @ startup 3.8L

I had to bump my idle to 1000 RPM's with my cam and rockers, stock is 750 rpm's. If its warm and doing it look at cleaning the IAC or wait until it throws a code. Could be a injector missing too,