Monatna stumble

I have a 2004 Pontiac Montana with 90K. If the overnight temperature has been below 40ish, there is a noticeable hesitation or stumble (almost as if the engine cuts out for a second-but never actually stalls out) on the 2 to 1 downshift when braking to a stop while the vehicle is in open loop. As the engine warms up the stumble becomes less noticeable until the temp gauge reaches the 1/4 mark at which point the stumble is gone. It won’t return until the following morning - the next cold start. Our mechanic has cleaned the Throttle Body and the MAF- no change. Their transmission guy said he did not think it was a tranny issue. It has no other symptoms and seems to shift normally at all other times. Ideas on what to do next??


Here’s an idea . . . you may have flat intake manifold gaskets

When cold, they’re not sealing well, and you have false air. And thus the pcm has to command extra fuel, in an attempt to overcome the leak.

Once everything warms up, the gaskets do a better job of sealing, because they have expanded a bit

Next cold morning, the cycle repeats itself

An evap/smoke machine would quickly confirm or deny this, but everything has to be cold

Another thing you could try is to hook up the scanner to the stone cold engine. Start the engine and while you’re observing short term fuel trims. If I’m right, the fuel trims will be very positive, double digit numbers probably, with the engine still cold and idling. spray some carb cleaner at the intake gaskets. If the fuel trims dramatically change, you may have found your problem

Would the gasket issue only show up in the cold 2-1downshift or would there be other symptoms?


When the engine is cold, is it stumbling at idle, or only when the transmission is downshifting?

Only on the downshift from 2 to 1. Idle is normal, all other shifts are normal.


In that case, disregard what I said

The problem might be with the Idle Air Control valve.

If the transmission is shifting from 2 to 1, the vehicle must be slowing down with your foot of the gas and applying the brakes.

The IAC valve controls the engine idle speed anytime the gas pedal is released.

It could be that IAC valve is hunting for the proper position when the 2 to 1 down-shift occurs and the engine almost stalls out.


Would that cause the problem only in open loop but not in closed loop?

In the open loop the engine is cold.

Temperature can effect IAC valve function.


When I drive it a short distance (enough to have entered closed loop), leave it sit for a few hours in 0 degree weather (enough for the engine temp to have cooled - engine temp gauge starts off below the first hash mark) and then drive again the problem won’t occur. It will only resurface if I leave it sit for the better part of a day or overnight. Presumably long enough to have the vehicle now restart in open loop ?

Has anyone looked at the data stream from the ECU? Specifically the incoming air temp and coolant temp sensors? Is the check engine light on at all?

I am not sure if the ECU data was checked. There is no check engine light on.