Engine "shuddering" when stopped at lights? '90 Celica

Hello, hello.

I noticed while driving today that when I come to a light (and the car
is still in DRIVE), the engine seems to "shudder." Instead of sitting
comfortably at about 800rpm., it will drop down to about 600rpm. and
then back up to 800rpm. It will continue to do this until I apply the
gas and start driving again.

When the vehicle is parked and running, it generally idles comfortably
at 1,000rpm (though it doesn't seem to sit PERFECTLY at 1000, it does
fluctuate SLIGHTLY).

I'm concerned that my gas mileage is decreasing and I'm wondering if
whatever is causing the shudders is the culprit.

In article <1161973180.454376.133190@f16g2000cwb.googlegroups.com>
[email]omnineko@yahoo.com[/email] "J" writes:
[color=blue]
> I noticed while driving today [...][/color]

In the 1970s I had a car (GM/Vauxhall "Chevette" -- European car)
which did much as you describe. Fault proved to be a perforation
in the diaphragm of the carburettor float chamber. How modern is
your car?

Or how about dirty spark plugs? Plugs once were such a source of
fun. (Modern ignition systems seem to have spoiled all of that.)
--
Andrew Stephenson

J wrote:
[color=blue]
> I noticed while driving today that when I come to a light (and the car
> is still in DRIVE), the engine seems to "shudder." Instead of sitting
> comfortably at about 800rpm., it will drop down to about 600rpm. and
> then back up to 800rpm. It will continue to do this until I apply the
> gas and start driving again.
>
> When the vehicle is parked and running, it generally idles comfortably
> at 1,000rpm (though it doesn't seem to sit PERFECTLY at 1000, it does
> fluctuate SLIGHTLY).[/color]

That happened with my Nissan because a bad spark plug wire occasionally
arced to the sharp edge of a piece of metal. There were no visible
cracks in the insulation, and I found the defect through the old method
of running the engine in dim light and spraying water over the ignition
system and looking for sparkles.

Another possibility is the idle air control valve, which routes air
around the throttle plate to regulate the idle speed, or its wiring,
but sometimes it's not the valve itself but a dirty throttle plate and
throttle body. Cleaning it with throttle body spray, rags, and a
toothbrush (not wire brush!) can help, and I was told that the
difference between throttle body and carburetor choke spray is that the
former is made to not damage any anti-stick coating.

"J" <omnineko@yahoo.com> wrote in message
news:1161973180.454376.133190@f16g2000cwb.googlegroups.com...[color=blue]
> Hello, hello.
>
> I noticed while driving today that when I come to a light (and the car
> is still in DRIVE), the engine seems to "shudder." Instead of sitting
> comfortably at about 800rpm., it will drop down to about 600rpm. and
> then back up to 800rpm. It will continue to do this until I apply the
> gas and start driving again.
>
> When the vehicle is parked and running, it generally idles comfortably
> at 1,000rpm (though it doesn't seem to sit PERFECTLY at 1000, it does
> fluctuate SLIGHTLY).
>
> I'm concerned that my gas mileage is decreasing and I'm wondering if
> whatever is causing the shudders is the culprit.
>[/color]

There is a good change that whatever is causing the shudders is the culprit.

If you are wondering what to check, try the following:

- Turn on the AC and see if engine idle speed increases by 100 ~ 200 RPM.
If the AC is already on, try turning it off and see if it clears up.

- Check to make sure the air filter is not clogged and the air filter
housing is tight.

- Check the plastic snorkel between the air filter and throttle body for
cracks and holes.

- Check for a vacuum leak with a vacuum gauge. There should be at least 14
to 16 in Hg at idle.
--

Ray O
(correct punctuation to reply)

J wrote:[color=blue]
> I'm concerned that my gas mileage is decreasing and I'm wondering if
> whatever is causing the shudders is the culprit.[/color]

change the plugs every 30 or 60k. either NGK or ND, only.