Shuttering when increasing speed

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Old 04-13-08, 08:07 PM
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Question Shuttering when increasing speed

Hello,

Thank you for looking at this!

1994 Buick LeSabre V6 3.8 liters

Shutters on acceleration or on an incline, but runs great when asked to accelerate quickly, as when passing.

When increasing speed the car starts shuttering. A type of shaking caused by the engine suddenly not running smoothly. This occurs most noticeably at about 50 to 70 miles per hour. ( I don't hit speeds higher than that). But I have noticed the effect at lower speeds also.

The car shutters the most when you ask for a small increase in speed or when you try to maintain speed while going up an incline. It occurs when you give it just enough gas to keep up the current speed, especially if the road is level or inclined.

Now, a most interesting thing is, if you give it enough gas, like if you’re going to pass someone, it drops a gear or changes the sound to that effect, and sounds great as it quickly accelerates.
If you ask for stronger acceleration it works and sounds and feels fine. But ask for slight acceleration, and you feel and hear the stammering.

The car idles just a little rough in park, but sounds pretty smooth in neutral. It drives pretty nice other than this.

What could this be, please.
 
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Old 04-13-08, 08:20 PM
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that sounds very much like a secondary ignition system break down. This would include spark plugs, wires and ign coils. how long has it been since it has been serviced? Is it possibly due for plugs, filters etc? plugs and wires are BOTH maintenence items...coils should be replaced when they fail. If it's overdue for plugs, start with those. you can try a little water mist from a pump bottle around the ignition...works very well at night and may show "leaking" plug wires. Look for blue arcing when trying that test. coils can be tested with an ohm meter...or even better, current ramped with a current probe and labscope.
 
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Old 04-13-08, 08:32 PM
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It runs great when asked to accelerate quickly.

It runs great when asked to accelerate quickly.

Could that still be bad wires or coils?


I've owned it about a year now and no service in that time.

Thank You
 
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Old 04-13-08, 08:37 PM
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I agree with Carguy....

But , just for giggles, Humor me...... Next time you drive the car, Dont use the overdrive position on the gear selector, Use Plain "D" or "3".. If it still responds poorly, It is in the ignition. If It straightens out without overdrive engaged, suspect TCC, or internal transmission wear.
 
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Old 04-13-08, 08:42 PM
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sorry unc...but i respectfully disagree. you can't reproduce the kind of load you get in OD with the convertor applied and secondary ign breakdown is almost always load related.

OP...when you accelerate quickly and the engine RPM is higher, load is actually reduced. lugging at moderate speed in OD with the convertor applied is a stress test for ignition components. I'm not saying that it can't be a convertor problem or an EGR problem or the like...what I am saying is that maintenance items need to be taken care of first.
 
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Old 04-13-08, 08:45 PM
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I am assuming it is fuel injected, have you cleaned the injectors and do you run good quality fuel, some times you can get carbon build up on valves which can cause drivabilty issues. You might try some BG44K in the tank. I used to use gm top end cleaner but the last time I tried to purchase it only came in a spray can and wasn't as effective.
O'tools comentary on Murphy's Law (Murphy was an optimist)
 
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Old 04-13-08, 08:50 PM
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sorry unc...but i respectfully disagree. you can't reproduce the kind of load you get in OD with the convertor applied and secondary ign breakdown is almost always load related.
Agreed. Usually I would test this with just enough pressure on the brake pedal to activate the stop lamps, and only momentarily, as to "Pinpoint" a "Cause and effect".

Unfortunately, everytime I recommend this to someone, they come into the shop two days later , complaining of a Brake pulsation, or a "Howling " converter.
 
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