Truck idles wildly back and forth between 1500 and 500 rpm.

Disconnected the MAP sensor and was able to get it to idle.

Replaced MAP sensor, checked IAC, replaced TPS, replaced vac line to MAP, checked for vac leaks around TB, TB adapter, and intake.

Any ideas?

 

when trying to pass emmisions, when i went too far on the lean side of things, the idle got very eratic.

does this happen in open loop or closed loop, or both. This is very important to know.

you can also manualy set idle, via the throttle blades. Disconenct the IAC, and try to set it to idle at about 450-500 RPM with the IAC disconnected. the reconnect the IAC and see how that goes.

 

I just used the idle air adjustment at the blades actually to fix my erratic idle. After I put the new intake and headers on it needed a higher idle so it's something simple to try.

Also, what is your idle set at it in the prom? How about the stall saver when is that set to kick in, you might be hitting it and then the iac opens to save it and shoots the rpm's up.

 

I've got a similar problem.
This is a new engine with the mods shown in my sig.

At idle the rpms begin to drop until the vacuum drops causing the FP to increase from 11psi to 15 psi. This causes the engine to rev. As the rpms go up, vacuum rises & the FP goes back down. causing the rpms to drop.

If I disconnect the IAC, it idles fine but I can't get it to idle lower than 1000 rpm in park.
I also have a code 42.

Any help would be appreciated.

 

On the code 42, check your grounds.

What vac source do you have hooked to the reg? As rpm's go up, vac should drop, and fp rise.

 

We had T'storms tonite so I didn't work on it.
I'm using the port on the left side. I believe it's the J port. Not sure off the top of my head.
I didn't actually watch a vacuum gage. but as the rpms went up the FP droped then the rpms droped. As the rpms droped the fp went back up, causing the rpms to go back up.
It doesn't last but one or two cycles before it dies unless I get into the throttle.
I'll check the ground tomorow. I did paint the t'stat housing but I am using an unpainted nut to hold down the gound wires.