New user here with a newbie question. I had my car tuned up a month ago. The very next day, on cold start, the engine starts stuttering and the rpm got real low to the point where it almost shuts off. On more than one occasion it did shut off. If I press the gas peddle, it revs properly. I thought I’d give it some time to see if it got better, but it hasn’t. Yesterday, in the middle of the day (car had been parked for 8 hours), on cold start it did it again (93 degrees outside, so it’s not cold weather dependent). It stuttered when I came to the stoplight, and so I put it in park to hit the gas peddle and rev the engine to get it warmer. When I put it back into drive, it stuttered and shut off at the light. It never stuttered prior to the tuneup. My mechanic seems to have no idea why. Any thoughts?
The problem might be with a dirty/defective Idle Air Control valve.
The IAC valve allows the engine to idle anytime the gas pedal isn’t depressed.
You can check for this by using your left foot for applying the brake pedal while at the same time using your right foot to slightly apply the gas pedal when stopping. If doing this prevents the engine from stuttering and stalling focus on the IAC valve.
Thanks for the insight, Tester. Any idea why this might manifest as an issue immediately after a tuneup, though?
Perhaps, but it seems too much of a coincidence to me. Could it have anything to do with faulty or loose replacement spark plugs? Anyone else with any other ideas?
If there were a problem with the spark plugs it would cause a misfire, which would cause the Check Engine light to come on.
I’d do what @Tester recommends. He has a 99.99% success rate on here.
I agree with @Tester
Anyways . . . do you hear any hissing at idle with the hood open?
See if any vacuum lines or hoses are obviously broken, or disconnected
I would also clean the throttle body. Carb cleaner works great for this
This actually might have something to do with the tune up. On many transverse V6 engines, you have to remove the plenum to get to the rear plugs. If the plenum gasket wasn’t replaced, or if something was left off or disconnected . . .
By the way . . . is the check engine light on?
I just picked up the car from the mechanic. They had it for the day to see if they could replicate the problem, but were unable to. @db4690, they went ahead and cleaned the throttle body for me (incidentally, now it’s really hard for me to press down on the accelerator – which is annoying; I’ll be taking it back to them to fix that on Friday).
I will try @Tester 's suggestion tomorrow morning when the engine is cooler. I will also try listening for hissing with the hood open if it starts stuttering tomorrow. The check engine light is not on, though. If the plenum gasket wasn’t replaced will the check engine light necessarily be on?
The computer can compensate for vacuum leaks up to a certain point before it throws a fault code
“Tune-up” is a vague outdated term. Exactly what maintenance was done at first?
@lion9car the tuneup only consisted of replacing all the spark plugs.
I didn’t hear any hissing, @db4690. The brake + gas pedal combo does prevent it from idling low, @Tester.
My mechanic observed the problem I’m having and said he was convinced it was the IAC valve, he said I could buy one on my own and bring it in, he’d install it for free for all my troubles (though I think he offered mostly because I was visibly still not convinced the tuneup had nothing to do with it). Any suggestions on where to buy the part? @Tester, is that Rockauto link one I can purchase off of safely?
I purchase from RockAuto all the time.
Sometimes they’re the only source for some auto parts.
I’m going against the grain this time . . .
I advise you to buy locally, in case there is a problem with the part . . . or in case the wrong part shows up
It will be easier to deal with a real live counter person, versus a faceless entity on the other end of the line