i have a 97 4.0 auto 4x4 and then ever its cold out side like under 40 degrees, after it get up to temp it will start trying to die and stoplights and when in park for a minuet or so the rpms will start bouncing by about 100 rpm. Then sometims while coasting or going down a hill with my foot off the gas the rpms will bounce from 1100 to 1500, and worst sometimes while coming to a stop around 15 mph the rpms will bounce again causing the truck to buck unless i give ita little gas while braking. it throw the both banks of o2 sensors are lean and the evap system is not working right, and ive changed the evap purge silonid and the sensor for it. i have no idea ive done plugs wires pvc valve iac looked at all the visible vacuum lines. but it runs fine in the warmer weather. has any one ever had this problem because im running out of ideas and i think that shops are a joke because they'll fix it but screw up ten other things in the process so you have to bring it back to them. any help would be greatly appreciated as this is my work truck.

 

it sounds like the idle air controller is jammed open from carbon build-up

unbolt it from the intake and give the throttle body housing a thorough cleaning

 

It does sound like the IAC, however from some previous experience with mine, it did not throw any codes. It only receives signals from the PCM and is not in a closed loop control. You are receiving lean DTC's I would say you have a leak after your mass air flow sensor creating a lean mixture that the PCM cannot pick up on. You could try spraying carb cleaner around areas on the intake assembly and see if the engine responds or not. That would help pin point a leak. Since you already know that your EVAP system is not working right, unhook and plug any vacuum lines that are hooked up at your engine to eliminate the possibility of a leak in your EVAP system. You could also try with your truck running, unplug the MAF sensor and see how the engine responds. When you unplug it the truck will go into open loop and should even out.

 

Sounds very similar to my nieces VW Cabrio. I had a hell of a time figuring it out.

On that car if the coolant gets even slightly low the coolant tempature sensor winds up not touching the coolant and the car runs horribly.

This was magnified 100x in cold weather.

Just some food for thought.

p.s. The VW Cabrio did not send a SES or CEL for the problem.

 

when the engine is normal operating temperature , dis-connect the EGR valve vacuum hose and see if the idle rpm`s change ( idles normally or idle becomes worse )

it could possible be the EGR vacuum actuator solenoid ( it would jam in the VACUUM PULL position )

meaning the EGR valve is drawing in exhaust gases into the intake manifold at all rpm`s ( not supposed to )

or the EGR valve is jammed in the open position because of excess carbon buildup

i would unbolt the EGR valve , and submerse it in a SEAFOAM bath overnight to disolve all the carbon buildup inside

i dianosed that exact problem many years ago ( licensed mechanic`s were stumped ) go figure

 

basically over time crystalized deposits would form on the vacuum actuator solenoid brass base

( they activate by coolant pressure NOT coolant temperature )

when you are driving and the air flow through the radiator cools the engine coolant , the coolant

pressure drops , the vacuum actuator solenoid opens allowing vacuum flow to the EGR valve , thus

EGR valve opens allowing exhaust gases to be drawn back into the intake manifold

it definately sounds to me that the EGR valve is jamming open because of the engine heat

as you said , the engine runs fine when COLD , but runs like crap when it is warm