Car Shakes idle in Drive and Reverse

DCRUZ126

MEMBER
1995 MAZDA 626
6 CYL
FWD
AUTOMATIC
118,000 MILES

I own a 1995 Mazda 626 ES with a V6 engine. My car shakes when I am in idle in drive or reverse, and when idle in park for awhile. When I shift into drive or reverse, rpm's go really low around 200 or 300 then jump to around 500 and 600. Sometimes the rpms dip so low that the car stalls. When I drive, it vibrates a little to about 15mph, then it runs smooth. I also tested the spark plugs and wires. I had the car running and took all of the spark wires one by one and the engine wanted to die a little each time I pulled one out, which I know is regular when you remove the wires. But when I pulled the spark plug wire from the #2 cylinder, the car wasn't effected and it didn't die down. I just bought the car and a Tune up was done about a month ago. There's spark coming from the #2 spark plug wire so it isn't the wires. Thinking it was a bad plug, I pulled the plug out, and replaced it with a new plug. When I pulled the wire off the #2 cylinder, still no effect on the engine. I also did a compression test on all the cylinders, and the all read around 150-160. I then thought that maybe putting fuel injection cleaner in the car would take care of any clogs that would possibly be in the fuel system. At first it did run a little smoother but then it went back to where it always was with the shaking.

What could this be? Its smooth in park but not in drive or reverse. I thought that it might be a motor or transmission mount, because when I shift to drive or reverse the motor moves forward a little. I was told tho that its normal for the motor to do that. Got any ideas on what it could be?

1 Reply

ERNEST CLARK

MECHANIC
1,730 POSTS

You actually did a pretty good diagnosis. More thorough than most people who aren't technicians.

It definitely sounds as if you have a misfire in that cylinder. (Since you verified spark and compression) Keep in mind, there's two kinds of misfires: Ignition and lean.

What you want to do next is use an automotive stethoscope to listen for the number 2 injector firing. (Which I believe it is doing) Once you confirm it's firing, pull it off and use a container to check it's flow pattern. I believe you'll find it partially clogged.

Also, manufactures advise against using fuel injector cleaner. For one, the store bought kinds isn't strong enough to clean anything. The industrial strength kind used by professionals will actually do too good a job.

What the manufacturers found out: The build up of contaminants actually clogs up tiny holes and leaks in the seals that get worn over time inside the injectors. Once you clean that stuff out, the injector ends up leaking, which causes other engine problems.

After you test the injectors, get back with me if you need more help.