Rumbly idle, but giving it gas smooths it out


#1

Hi guys, this is something my car has been doing for a while but it seems like it’s gotten gradually worse, or at least I’ve been noticing it more. The car’s a 1996 Mercury Mystique 2.0L 4-cyl, 74000 miles. The issue arises when I am idling in drive (like at a red light). There is a distinct rumbling that you can feel through the seat and the steering wheel, and see if you look at the wheel or dashboard. It’s not visible from outside the car, though.

The vibration is worst when the car is idling low – typically around 500 rpm. When I’m driving, the vibration is not noticeable. Also, when idling, the rpms “pulse” upwards to about 700 rpm, and the vibration goes away. It will be rough for about 5 seconds, then bump up to 700 rpm for a second or so and smooth out. If the car is in neutral or park, the vibration goes away.

So I suppose there’s two issues: 1) the low idle that “pulses” upwards, and 2) the vibration itself. Is the vibration just an effect of idling too low, and raising the idle might fix the problem? What would cause a low idle? Or, if the idle does not seem too low, what would cause vibration that is dependent on RPMs?

The scheduled maintenance is up to date, but the car is due for 75K maintenance next weekend. The PCV system was cleaned and valve replaced, air filter replaced, and spark plugs and wires replaced 12000 miles (~1 yr) ago. The idle air control valve was replaced at around 55000 miles (~2 yrs ago), I believe (the previous owner, a friend of mine, had had trouble with the car stalling at idle, and replacing the IACV seemed to fix it).

Thanks in advance for your advice. You can tell how good your advice has been in the past that I haven’t posted on the message board in six months!

Sam


#2

The 500 RPM figure is too low, and the engine is lugging. Even 700 RPM is too low, as I think this engine has an idle spec of 800-900 RPM. This means the IAC (idle air controller) is having trouble maintaining idle. It vould be getting lazy or it may be dirty. You have very low miles for an 18 year old car, so a dirty IAC and passages would be my first guess.


#3

Thanks – what’s the best way to clean it? Just take it off and hose it down with carb cleaner? Thinking back, I was mistaken about when the IAC was replaced – it was about four years ago. Do you think it could have developed a fault in that time, or is it more likely just dirty?

If cleaning the IAC valve doesn’t work, where would be the next place to look?


#4

I agree.

There’s something wrong in the idle air control circuit.

From what you describe, it sounds like the Idle Air Control valve is hunting for the proper position. And that’s what’s causing the idle speed to change.

You can sometimes check for this on a Ford product by rapping on the IAC valve with the handle of a screwdriver while the engine is idling. If the idle speed changes there’s a problem with the IAC valve.

And because it was replaced once, doesn’t mean it can’t fail again.

Tester


#5

"what’s the best way to clean it? Just take it off and hose it down with carb cleaner? " Yes, but I would use throttle body cleaner and you may need a soft rag or brush like a toothbrush to break up heavy deposits.


#6

Cool, thanks again for the great advice as always. I’ll do some cleaning and screwdriver-tapping next weekend when I get under the hood.


#7

And if by some chance cleaning or replacing the IAC valve does not cure the problem you might consider the possibility of a vacuum leak. Even a small leak can affect the idle.

The easiest way to check for a vacuum leak is with a vacuum gauge. Those gauges are cheap, easy to use, and will reveal in seconds if there is a leak anywhere or not along with info about various other engine ailments.

Point being is that it’s possible to have a vacuum leak inside the dashboard for instance and while a vacuum gauge will not show you where the leak is at it will tell you whether one exists or not. By pinching off various lines and watching the gauge the cause can be located.


#8

I don’t see mentioned that you have cleaned the throttle body. If it so dirty that the IACV cannot compensate then engine will idle badly. If the idle smooths out when you raise the idle speed up to normal than problem likely IACV or dirty TB.

In addition have them clean the MAF sensor during the scheduled maintenance. A dirty MAF can contribute to low idle condition.


#9

If the replacement IACV was aftermarket 4 years might be all it was good for.
Hopefully cleaning it (and the throttle body too) will cure the problem.
If you have to replace it again I’d get the part from the dealer.


#10

I did clean the throttle body and MAF sensor about a year ago or so, but it won’t hurt to give them a once over. It’ll be cool to see how much gunk has accumulated in that time (hopefully very little).


#11

You said “when idling, the rpms “pulse” upwards to about 700 rpm, and the vibration goes away. It will be rough for about 5 seconds, then bump up to 700 rpm for a second or so and smooth out. If the car is in neutral or park, the vibration goes away.”

Is your A/C or defrost on when this happens? If so turn it off and see if RPM pulses and vibration goes away.