Help, I have a cold start hesitation that is driving me batty

(this problem is only on a "cold" start)
My 99 Mounty (5L) fires up no problem every morning, put it in drive and give it moderate gas and it stumbles for a few seconds then it is fine until another cold start. I never can reproduce the problem after the initial few seconds.

I have changed the IAC and the TPS. I thought that there may be a build up of carbon on the valve soaking up some fuel at the beginning so I ran some Seafoam a few times via manifold vacuum but didn't help.

I figure that because I am running open loop that the other sensors do not come into play.

Fuel pressure is up to snuff, plugs have been changed 5 months ago..

Any suggestions? I even tried changing the TPS again thinking it is always possible to get a defective new one.

Thanks
Kurt

 

Your assumtion on the other sensors is not quite accurate.
1. HEGO sensors have internal heating elements and take only a few seconds to heat up to the point they will produce sugnals.
2. MAF will also be used to calculate the volume and density of the air moving into the engine.
3. Air charge temp sensor (may be incorporated in MAF) measures the incomming air temp and is used to calculate the density of the air charge.
4. Baro/ MAP sensor if equipped is used to calculate the absolute barometric pressure or (Manifold absolute pressure) and is also used to generate an air charge density calculation.

Of these , I would be looking at the MAF first. Try the sticky link in this forum on cleaning it. It functions by heating a small element inside it. This heating action changes the resistance of the measured signal. As air passes over the element, it is cooled somewhat. The more air passing over it, the greater the cooling action and thus produces a change in the signal voltage that is proportional to airflow. The computer uses the signals from the MAF and TPS to calculate the fuel needs of the engine. The TPS gives a signal that indicates the throttle position. This signal is used by the PCM to determine or anticipate the future fuel needs of the engine, whereas the MAF provides a more delayed signal of the actual engine load conditions.

Dialtone

 

Not sure about OBD-II and the heated O2 sensors. But my OBD-I won't look at the O2 sensors until the coolant temp is 170 degrees F. Why do you suspect open loop? Does the code reader say this or do you just assume since the engine is cold?

So fundamentals say if you don't get air, fuel or spark, you're engine does not run. When you are at idle, IAC gives 95%+ of all air into the intake. The MAF can barely sense this flow so the PCM runs off it's own table where engine coolant and throttle postion have main affect.

But now you hit the gas to start moving, the MAF wakse up and tells the PCM something. Well cold engines like to run rich so it doesn't make sense that the PCM is saying massive amounts of air so therefore give too much fuel to stall. Likely it is saying nothing or just a little air so you get just a little fuel and you are supper lean.

Fundamentals say no gas no run.

Since you are idling OK, you are right? I think the MAF sensor might be the culprit. But if it reads lean just off idle, it will read lean at all RPM's. Are you getting a ping at all under load? With out super fuel you'd really be able to tell.

There is a method to clean these. I think you year is similar to my 1993. Check this web site. http://draco.acs.uci.edu/explorer/

And finally - did you ever disconnect the battery 10 min's to clear the computer's learned correction factors after you changed all the components out? Seems like I'm asking this a lot lately. The computer might still be trying to correct for a bad condition that you have just fixed.

 

I did didconnect the battery to clear the EMC...

Going to try a different gas station....what the heck, anything is possible.