Car jerk (yank, twitch, hitch, jerk) during constant speed

Note: English is not my first language.

I have one 1998 Chevy Prizm (the same as Toyota corolla),
automatic transmission, only 77,000 miles.

A month ago, I had someone changed the front brake pads for me.
Since then, the car experienced weird problems.

Symptom:
=============================================
When driving at about 35-40 miles or about 50 miles in constant speed,
(in other speeds, it happened relatively fewer)

I could feel sometimes the car was jerked by something backwards in a
very short moment,
then pushed forward immediately for a very shotment.

Or it can be described as someone suddenly applied brake then
immediately accelerate the car.

Someone explained this is because "transmission slip", the gear
switched to a lower gear for a short moment and jumped back to
the higher gear. That's why the driver felt something dragged car and
then pushed the car.

The other mechanic explained as engine (or one of cylinder ?) died
for a short moment due to spark plug or spark plug wire.

So for, I changed spark plug and transmission fluid (and gasket)
but the symptom still there.

Before I would spend more time and money on the car, I would like to
know what may cause the problem, what is more accurate or
professional word to describe the problem and symptom.

===============
Transmission slip
Engine stall
Engine stumble
===============

Regards,

-Jeffrey

"Gazebo" <bigtang@gmail.com> wrote in message
news:1165169157.044362.242820@16g2000cwy.googlegroups.com...[color=blue]
>
> Note: English is not my first language.
>
> I have one 1998 Chevy Prizm (the same as Toyota corolla),
> automatic transmission, only 77,000 miles.
>
> A month ago, I had someone changed the front brake pads for me.
> Since then, the car experienced weird problems.
>
> Symptom:
> =============================================
> When driving at about 35-40 miles or about 50 miles in constant speed,
> (in other speeds, it happened relatively fewer)
>
> I could feel sometimes the car was jerked by something backwards in a
> very short moment,
> then pushed forward immediately for a very shotment.
>
> Or it can be described as someone suddenly applied brake then
> immediately accelerate the car.
>
> Someone explained this is because "transmission slip", the gear
> switched to a lower gear for a short moment and jumped back to
> the higher gear. That's why the driver felt something dragged car and
> then pushed the car.
>
> The other mechanic explained as engine (or one of cylinder ?) died
> for a short moment due to spark plug or spark plug wire.
>
> So for, I changed spark plug and transmission fluid (and gasket)
> but the symptom still there.
>
> Before I would spend more time and money on the car, I would like to
> know what may cause the problem, what is more accurate or
> professional word to describe the problem and symptom.
>
> ===============
> Transmission slip
> Engine stall
> Engine stumble
> ===============
>
> Regards,
>
> -Jeffrey
>[/color]
It is difficult to diagnose a problem without experiencing it first hand.
The symptoms you are describing generally are caused by transmission slip,
an engine misfire, or a fuel problem. "Engine stall" is when the engine
dies altogether and has to be re-started. The symptom may be described as a
transmission slip, engine stumble, or engine surge.

Try driving with the overdrive off to see if the problem goes away or occurs
at a different speed. If the problem goes away, then the transmission may
have a problem like a slipping internal clutch.

When you replaced the spark plugs, did you use genuine Toyota spark plugs?
What was the condition of the spark plugs you removed? Were the tips black
or wet with gas?

Check for a vacuum leak with a vacuum gauge. You should have 14 to 16
inches of mercury at idle. Check the black plastic intake tube between the
air filter and throttle body for cracks or holes.

Another possible cause is a problem with the EGR valve or EGR control
circuit. A vacuum leak could also cause EGR problems.

I think your best bet would be to take a road test with a competent
technician so that he or she can experience the problem and provide an
informed diagnosis.
--

Ray O
(correct punctuation to reply)