Pontiac Grandam 2000 sometimes almost stalls during starting after parked for longer time, runs fine otherwise

Grandam GT2 sometimes starts as if not all cylinders are firing.
When it happens I push gas a little and while coughing it starts. Otherwise it would not.
It only happens after car was parked overnight or for the entire day. It never happens when car was driven recently.
Car runs perfectly.
Spark plugs, ignition cables, battery, fuel pump & filter, fuel regulator are reasonably new (few years old).
Replaced Idle Air Control Valve, cleaned Intake Manifold sensor wires - no effect.
No error codes stored, and because it does not happen every time - it is hard to measure anything when it happens.
Please point me in the right direction. Thanks in advance.

Guess #1 is that the fuel pump’s check valve is leaking and making the car hard to start. Try cycling the key 3-5 times without cranking the engine. Just ON-OFF-ON. You are doing that to cycle the fuel pump and build pressure. Then crank it and see if it starts right up. If it does it might be time to replace the pump.

Leaking fuel injectors is my guess #2 A little injector cleaner in the tank may clear that up. You test for it the same way and you get pretty much the same symptoms. Try a tankful of cleaner first.


I know you said that the fuel pump was replaced recently, but just for yucks and giggles have a fuel pump residual pressure test performed.

The fuel pressure should slowly bleed down over a certain period of time.

If the fuel pressure drops to zero PSI over a couple of minutes or instantly, that indicates a bad fuel pump.


Thank you Mustangman and Tester!
Because it typically happens in the morning but not every time, I will try cycling the key every day for several weeks in order to see the effect. It will take me probably as long to get a hold of the fuel pressure gauge to test the residual pressure. Will report back when I have conclusive results.

Another idea, the computer may not be injecting extra gasoline like it should on cold starts. First step is to verify the coolant temp sensor (the one used by the computer) is working.

I removed the connector from the temperature sensor and compared the resistance when cold to the identical looking sensor I have from another GM vehicle and resistance matched. Not sure if this was conclusive enough test.

That’s a pretty good test. I think you’ve ruled the coolant temp sensor out. Still a slight possibility the engine computer isn’t reading the sensor accurately. Wiring or connection problem, etc. A scan tool with coolant sensor function could tell you that without much trouble. If the computer simply weren’t connected to the sensor for some reason, that would almost certainly flag a diagnostic code, so that can be ruled out too.

If I had that problem I’d do a visual check for spark on each cylinder when the symptom is occurring, before going down the coolant sensor path or the fuel path, since that’s usually a pretty easy test. Your goal at this point is to narrow it down between a fuel, spark, or other problem.

One more idea, when my Ford truck develops a similar symptom, idles poorly on cold starts, it is usually that the vacuum system has sprung a leak somewhere.

Another clue, not sure what to make of it. When I feel it stalling and help it to start by pressing the gas pedal engine revs up but car shakes as if engine is overcoming some resistance or running not on all 6 cylinders. So pushing gas does not immediately makes it run smooth it still struggles through initial engine revolutions.

Absent serious internal engine problems, misfires are usually caused by a spark or air/fuel mixture problem. It’s easier to check for spark, so suggest you start there. If that’s ok, do a thorough check for vacuum leaks.

Thanks George_San_Jose1,
I did inspect all my vacuum lines, even put additional compressing o-ring on the fuel regulator boot and PCV valve boot.
I am thinking that vacuum leak and/or ignition problem would also manifest itself on the idle at the traffic light, but idle and overall ride is very normal.
The consensus seem to be that fuel system is a culprit, either injector(s) or the fuel pump.