Epiphany... torque converter and stopped car

[classact2575]

I was doing a bunch of research here yesterday on Subaru torque converters and problems with locking up. (Mine is extremely finicky, and doesn't engage when it typically should, thus resulting in me getting shoddy gas mileage.)

 

Anyway, someone had posted this "How stuff works" article on torque converters:

http://auto.howstuffworks.com/torque-converter.htm

 

This part of the article (pg 2) is what gave me an "hmmm... I wonder?" moment yesterday:

A torque converter is a type of fluid coupling, which allows the engine to spin somewhat independently of the transmission. If the engine is turning slowly, such as when the car is idling at a stoplight, the amount of torque passed through the torque converter is very small, so keeping the car still requires only a light pressure on the brake pedal.

 

Common knowledge for 99% of you, but me being not so mechanically inclined, this was new information. Ever since I bought this car used (about 3 years ago), it's done this funny thing while sitting idle at a stoplight or whatever. I'll be sitting there, foot on the brake, and the car will kind of make this shuddering noise -- almost like something's pulling on the car. I usually just go up to Neutral, and the noise/vibration goes away. The engine's going about 1000 RPM when stopped at a light, in D, but when I shift it into neutral to make the pulling go away, it goes UP to around 1100-1200 RPM.

 

This only happens about 60-70% of the time, doesn't matter if the car is warmed up or cold.

 

Sooo, if the torque converter is involved in the car sitting idly waiting for the light to change, and I frequently have the above problem (combined with the lockup not engaging soon enough)... does this indicate that there's something wrong w/my TC?

 

Like I said, I'm a complete noob and want to learn more about my car -- since I want it to last FOREVER! Any advice or guidance appreciated... thanks!

[idosubaru]

this could still be an engine issue. do you have have a check engine light? when is the last time the plugs/wire were changed? does it have subaru or aftermarket wires and does it have NGK or other non-OEM plugs? and is this in your 93 impreza in the signature - so it would be an EJ18 engine right?

[uniberp]

The engine's going about 1000 RPM when stopped at a light, in D, but when I shift it into neutral to make the pulling go away, it goes UP to around 1100-1200 RPM.

 

This only happens about 60-70% of the time, doesn't matter if the car is warmed up or cold.

 

Your car should be idling quite a bit lower, at about 700 rpm. The shuddering is caused by the engine straining against the tranny, when not returning to a correct lower idle speed.

 

It is not your transmission causing it.

 

It may be caused by a vacuum leak on the intake somewhere. Check\tighten the connections between the MAF and the throttle body. The lower part that connects to the throttle body is especially susceptible to crimping.

 

Something is wrong in an idle circuit somewhere. It can be several things. Search IAC Idle Air Control here.

[x_25]

My car does this also, it always vibrates a little at lights but sometimes it is realy bad. The only difference is that mine does it at 700rpm. I beleve that is is caused by a loose coupleing in the drivetrain somewhere that causes it to bounce back and forth.

[uniberp]

My car does this also, it always vibrates a little at lights but sometimes it is realy bad. The only difference is that mine does it at 700rpm. I beleve that is is caused by a loose coupleing in the drivetrain somewhere that causes it to bounce back and forth.

Look under your hood when it is running and make sure the front crankshaft pulley is not wobbling. It should be obvious if it is. This can cause vibration and requires varying degrees of repair.

 

But think. The drivetrain, aside from the engine and front end of torque convertor is not operating while sitting still. And rarely does a TC have problems. There are many thread about fixing that problem. It's nto common, but it happens.

 

You may have a simple problem fixed by a tank of premium or changing the spark plugs.

[Hocrest]

A mis-adjusted or bad TPS can affect TC lock up.

[classact2575]

Oops - sorry, my bad, I meant about 600 RPM not 1000... not paying attention!

 

I'll check out what you guys suggested and let you know! I'm hoping the TC lockup problem is something relatively simple to fix, although the subie dealer did absolutely crap when I asked them to look into it... "You should do a transmission flush!" was their recommendation.

[subaruplatt]

A new feature in the 08 impreza is to shift the car into neutral automatically after the brake has been depressed for a brief period of time while the shifter remains in drive.

 

This is a known subaru trait and is useful in an all wheel drive car that you don't want to roll backwards on icy uphill parking ramps etc. So putting your car into neutral at stop lights is normal, just don't forget.

 

It would also appear that the generalized article you read on torque converters doesn't exactly apply to Subaru.

[Reveeen]

classact2575:

In "modern" torque converters there are two functions:

1) a fluid coupling (picture a fan plugged in and running, pushing air, in front of another fan, not plugged in, but turning because of the air pushed into it from the running fan).

2) a small clutch pack that provides the "lock-up" function (a very small clutch pack, usually 1 disk, the size of a clutch you would find in around a 350 Honda motorcycle, that has 5 disks, by the way).

 

You are confusing the two separate functions.

 

The lock-up function is electrically controlled by the transmission computer, when a set of conditions are met, that triggers a solenoid that allows fluid pressure to apply the lock-up clutch in the torque converter. If this lock-up function is triggered while sitting and idling in D your car will stall (providing that the lock-up clutch is intact and working).

 

If your car engages lock-up at all we have to assume the clutch in the torque converter is working, it is the control circuit that is faulty (be it electrical, or improper fluid pressure applying the clutch).

If your car never locks-up then you have to look at the above, with the possible bonus of a faulty torque converter.

It is quite normal for a 200-400 rpm difference between D and N, stopped, running, foot on the brake (the fluid coupling).

[cookie]

A lot of Subaru automatics do that, I've heard a bunch of explanations and fixes that even involved an incorrect axle. Do a search here and you may see several ideas. I might just put it down to a Subaru kink myself.