Idle bounce while coasting after valve cover gasket replacement

I’ve got a 2004 Mazda 6 with the 3.0L V6 & a 5-speed, with about 128k miles.

The car runs fine, but was leaking oil onto the exhaust manifold and so I recently had the valve cover gaskets replaced. the mechanic could not get replacement gaskets from Mazda, and so changed the valve covers themselves also, and consequently needed to replace a couple of hoses that connect to the valve covers.

Since the repair, the car drives fine - acceleration seems normal, I don’t notice any performance issues.

The CEL is NOT lit.


  1. When the car is in neutral, but rolling, the idle consistently bounces back and forth precisely between 1250 & 1500 rpm. This occurs always, and happens whether the gearshift is in neutral or the clutch is depressed. With the radio off I can hear the engine surging when this is going on. As soon as the car comes to a complete stop, the idle immediately drops to the a rock-steady position just below 1000 rpm, as it always has.

  2. I assume related to the above, but since the repair my gas mileage has dropped from its usual 18-19mpg to 15.7-15.9mpg as recorded at two fill-ups, and my current tankful is looking to return equally low mileage.

I brought the car back to the shop and the mechanic took the car out for a drive with me, with his computer hooked up while we drove. He found nothing wrong, nothing out of whack, and saw no evidence of any air leaks.


  1. Seems strange to me that the valve cover gasket couldn’t be obtained, and new valve covers were needed. I want to believe the garage is being straight with me. Is this really plausible?
  2. What could cause the idle to behave this way? I understand what a valve cover does (in my earlier years I changed the cylinder head on my old Saturn) and can’t figure out how or why changing the valve covers would have an effect like this. That leads me to conclude this is more likely caused by the new, slightly different hoses attached to the valve cover, but I don’t know exactly what those hoses do or why they connect to the valve covers, so I’m stuck here.

Thanks for any info.

Something is wrong with their story. Couldn’t get new gaskets??? How would changing the valve covers help? The mating surface on the engine is still the same, so even if you replace the valve covers with different ones, you still need the same gaskets.

The hoses on the valve covers are for the PCV system. I don’t know what your shop did, but you need a new mechanic.

There’s an air leak somewhere, introducing air after the MAF sensor, throwing its reading off.

Valve cover gaskets

NAPA has them $40.
Advanced $49.
Autozone $45.

Time for a mechanic that knows what he’s doing.

I think while leaning in and doing the job, he accidently pushed off a vacuum hose or there is a rip in the intake boot.


I think that this is a case of a mechanic who is both a crook and somebody with little skill.
Replacing both valve covers instead of simply installing a couple of very easily-obtained gaskets?

The OP should RUN from this charlatan’s shop, and just pay a few bucks for a competent mechanic to locate the vacuum leak caused by the so-called mechanic’s poor skills.

My vote is with the others. Gaskets can be had all over the place.

Just curious, but how much did he hit you for on the valve covers and if he claims that he can’t get gaskets for them then what did he do for gaskets on the replacement covers?

Offhand, I might guess at a vacuum leak and possibly related to the PCV valve/grommett/hoses/whatever.
Based on the alleged lack of gasket availability the entire thing is suspect.

Much as I don’t like it, I have to agree with all of you above in that I was screwed. I’m angry about it, but don’t have the luxury of indulging that, and still need to fix my car. So moving forward…

What can I do to fix this?

  • Yesterday I removed the MAF, completely doused the Intake Air sensor and the wires inside the plastic tunnel with MAF sensor cleaner spray, and let it air-dry (3x), and then reinstalled the MAF. Nothing changed.

I’ve been driving the car this way for a month now. I’d expect the Check Engine Light would have lit by now if there was a vacuum leak. The CEL hasn’t come on - can I rule out a vacuum leak?

I don’t understand how the involved systems actually function, that would cause the engine to very consistently rev up & down like it is when I’m coasting in neutral, but immediateley return to a proper, steady idle the moment the car comes to a full stop. What can cause this?

Thanks for any info.

I just re-read this thread more carefully, and I do see a couple of suggestions above regarding air leaks, thanks for the suggestions.

OK4450, Circuitsmith, is there a reasonable systematic method that I could use to locate the air leak, without expensive, specialized tools?

Use an unit propane torch and flow the propane around suspected leaks such as the intake manifold, or both ends and along any hoses attached to the manifold or plenum. If the engine speeds up and smoothes out, you have found your leak.

Oldtimer meant “unlit” propane torch rather than “unit” propane torch. The idea is that the vacuum leak will draw the volatile propane in and cause the engine to rev. It does work.

Anyway, a can of starter fluid works too, and is cheaper than a torch if you don’t already have one.