2002 Pontiac Montana Running Rough at Idle

I have a 2002 Pontiac Montana with 156000 miles. Serviced regularly. I have replaced the spark plugs and spark plug wires. No loss of coolant and oil is not milky. The van runs rough when at idle. I suspect it is missing at speed just not able to feel until at idle. What else could be causing the rough idle? Thanks

Is the van hard to start when cold ?
Does it run rough at idle when both cold and hot ?
I take it no "Check Engine" light ?

Could be a small vacuum leak, have you checked for weak spark at the plugs ?
Have you tried cleaning the Idle Air Control valve ?
Have you tried dis-connecting the MAF sensor to see if the idle improves ?

Just some thoughts.

I have a 2002 Pontiac Montana with 156000 miles. Serviced regularly. I have replaced the spark plugs and spark plug wires. No loss of coolant and oil is not milky. The van runs rough when at idle. I suspect it is missing at speed just not able to feel until at idle. What else could be causing the rough idle? Thanks

If it is missing the results should show up as a 'check engine' indicator on the dash. A miss should translate to raw fuel entering the exhaust which should trip the O2 sensor to indicate a rich fuel condition, I would think.

BUT, it could also be a faulty or plugged fuel injector, in which case there is no raw fuel, just a piston going up and down for the ride. Someone on this forum suggested using a stethoscope to listen to the injector(s) to see if they are pulsing on and off. You could try this too.

I've used a yard stick or suitable wooden stick as a scope to listen to engine noises before. You place it on the spot you want to listen to and put your ear against it.

If the injectors are working, then likely one is plugged. You can try a reasonably strong dose of injector cleaner to see if it frees the solenoid / pintel in the injector as well. A good drive on the highway would help too.

It could also be a dirty intake butterfly where the gas pedal linkage attaches. You can clean the inside of the opening and the butterfly on both sides using throttle body cleaner and a rag. Don't go crazy with the cleaner, but spray some on and wipe it and the carbon off with a clean rag. go inside the intake too as far as your fingers can reach. There is a small air bleed in the opening that if clogged, will cause the engine to run rough on idle, but not off idle.

The van starts very easily.

Seems to run a little more smoothly at idle when cold.

No Service Engine Soon light.

I did clean the Idle Air Control Valve several months ago. This is a fairly cheap replacement. Do you think it would be worth trying to replace this?

With regard to the Mass Airflow Sensor, do you mean just simply unhooking the wire going to it, or actually removing it?

I have thought about getting a stethoscope to see if I can hear for a vacuum leak and to check the injectors.

I honestly do not believe it is an injector issue because it is not missing that bad. It is just running more rough recently. Air filter is good and clean. Ran half bottle of Seafoam through the gas tank, other half through the PCV valve. Did not run any Seafoam through the crankcase.

If you have access to a manifold vacuum gage, you could check that to see what the vacuum is. Should be 19 - 21 in. HG @ idle. As far as the MAF, just disconnecting the connector, with ignition off, then start the vehicle see if there is a difference.

Since it seems to be better when cold rather than warm, it would make me think a possible vacuum leak.

Great, vacuum leaks are always fun to find, lol.

Thanks

I will go buy a vacuum tester this week. Do you know where the vacuum line that I need to use is located?

If there is a vacuum tree, one fitting with multiple vacuum lines attached to it, see if there is a stem that is capped. Barring that any port on the intake manifold with a vacuum line on it, however insure that the line you remove is not essential to idle operation.

Unfortunately I do not have a vacuum diagram to consult inorder to be more specific.

CHeck EGR valve...disconnect it for a test....

If there is a vacuum tree, one fitting with multiple vacuum lines attached to it, see if there is a stem that is capped. Barring that any port on the intake manifold with a vacuum line on it, however insure that the line you remove is not essential to idle operation.

Unfortunately I do not have a vacuum diagram to consult inorder to be more specific.

There is a vacuum port on the front side (facing you) of the intake near the throttle plate, just to your left of it...it is capped and could be used to attach a vacuum gauge.

You have to take the plastic cover off the intake the covers all the uglies up in order to see it.