idles ruff & hesitates at low speeds

[cjexotic]

Hi, I am new here so hope I am doing this right. I have a 92 Lumina 3.1, just turned yesterday 253,000 miles. I am having trouble with the idle and it running at the low speeds. If you get it over 65 it runs like a champ. But under that and the lower the worse it jerks and when you come to a stop sign or light it will shut off. It will restart but it idles ruff. This is what I have replaced on it.

New plugs, wires, catalytic convertor, new exhaust, exhaust donuts, radiator, struts, fuel filter, throttle position sensor, IAC. Most of these items were not replaced due to this problem I just wanted you to know what has been replaced.

Now at low speeds I get to a spot that it vibrates like its going over rumble strips then if you continue to press the accelerator (easy) it will get pass that. If you press the accelerator to fast then the tranny drags and then slams into the next gear or passing gear depending on how fast you go. Now it did lack power up hills and smell really bad but that turned out to be the catalytic converter. Once its over 60-65 you would never know anything is wrong with it. The warmer it is the more apt it is to shut off at lights and stop signs but never shuts off. But it has not shut off just going down the road but you could get whip lash if not careful.

Thanks for any help or suggestion you could give. Cj

[jeffcoslacker]

Have you ever cleaned/replaced the EGR valve?

Does it do it even when the motor is cold, or only after it warms up for a few miles?

[jeffcoslacker]

I'm a little concerned you may be feeling the beginnings of a faulty torque convertor lockup also, but usually they will cause the motor to stall hard when stopping, after being warmed up.

But occaisionally one will just get draggy and chatter and cause excessive load on the motor at inappropriate times...

[richtazz]

I agree with you on the TCC solenoid Jeff. It sounds like the torque converter isn't un-locking as speed decreases or when throttle is applied.

[cjexotic]

The stall can happen at anytime. But mostly after its warmed up.
no never replaced or cleaned the EGR valve. Where would that be located?
The TCC solenoid is that diffenent than the torque assembly itself? If so where is it located? Would not any of this set off codes. I have had it to the garage and they are stumped as it is not coding anything wrong.

Cj

[richtazz]

The TCC solenoid is located in the valve body in the side cover of the transmission, and it controls the torque converter's locking and unlocking at highway speed for improved mileage. If it is slow to disengage or stuck, it will cause the car to buck and stall as you slow down (like a stick shift when you don't push the clutch in) or hesitate/stutter/buck when you attempt to accelerate once at speed.

The EGR valve is located next to the throttle body on the intake. If stuck, it will throw off the air/fuel mixture causing erratic RPM fluctuations, stalling, surging, etc..., but I think your issue is more related to the TCC myself.

[cjexotic]

If coil packs are bad what would the car do to let you know this? Would it code?
thanks cj

[richtazz]

In an obd-I diagnostic system, the computer will not set a code for misfires. A bad coil pack can cause bucking, hesitation, etc... due to lack of spark at the cylinder that the coil controls. If you have an old style timing light, you can hook it to the plug wires and point it at the underside of the hood to check spark pattern. It should be a nice, steady flash with no gaps. If you find one that fires intermittently, check the other plug wire on that same coil pack (each coil pack fires two cylinders). If both fire intermittently, the coil is the issue.

[tblake]

yeah, i just found that out.... About GM's "waste spark" method. For those of you that dont know, GM's coil packs like whats used on the lumina's, both plugs connected to the same coil pack get fired at the same time. One spark is used as the power stroke, and the other is just wasted on another cylinders exhaust stroke. May be kind of helpful in diagnosis if a spark plug or plug wire could be to blame. One could just swap the plug wires on the smae coil pack around and in theory the motor should run as it always has.

[tblake]

sorry if that's off topic.

[SpinnerCee]

A failing CATalytic converter will exhibit those same symptoms.

An exhaust backpressure check found this out for me after first taking the car to a tranny shop.

The failed or failing CAT and the conditions it causes (engine runing too rich) may also cause the primary O2 sensor to fail as well, so both should be checked out.

[cjexotic]

ok, i changed the coil pack (the whole unit and the plate), crank sensor, plug wires, plugs, catalytic converter, brakes, brake rotors, it runs quite a bit better.

But I still have the vibration at certain speeds (like between 45-55 mph). Also if not enough speed has been built up it will do it on a hill. Tranny is lagging.
Well the mechanic says don't put anymore money into her as she is dying. Ok but they can not tell me what is causing her to vibrate. Any ideas?

[SpinnerCee]

Still seems like the CAT(s) are the issue.

An exhaust restriction will show itself most when you have the greatest throttle opening -- more specifically when you you surpass the ability for the exhaust to efficiently remove the spent exhaust -- Unfortunately cat's don't clog completely, so the car will probably run fine at mild to part throttle and low RPM and low load, but at high RPM under load the engine is effectively vapor locking and choking on the exhaust.

Manifold vacuum can also indicate a severe exhaust resticion -- Chilton's say a slow drop from normal (17-22) at idle and/or a gradial drop to zero at part throttle (2000-3000 RPM) indicates a blocked exhaust.

It's also important to realize how important vacuum readings (aka. Manifold Absolute Pressure [MAP]) are to the computer and engine performance -- Vacuum effectively indicates engine load, which in turn tells the computer how to adjust ignition timing and injection duration among other things like the tranny shift points, so a poor vacuum signal will cause the computer to compensate poorly for various driving conditions (like climbing a hill).

[cjexotic]

So SpinnerCee are you telling me to change the map sensor? I already put a new cat on it and a new pipe and muffler. Where is this map sensor at? Thanks for your help. The vibration comes and goes and almost (to me when driving) sounds like the torque convertor. But I can not be sure of that. She does have 253,600 miles on her. I just can't afford another car right now and i have put so much in this one.

[cjexotic]

Would this also make the car vibrate under a load like hills and some speeds when exerating. If I back off or put through that vibration its stops when I get passed that point such as 50mph or at 60-65 you would not know anything is wrong with the car? If it was the map sensor would it not code? We are not getting anything when coding it.