2001 Buick Lesabre has a weird vibration

customcruiser91

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Man, this is a long story and I'll do my best to cut to the chase.

In June of this year, we purchased a 2001 Lesabre from a friend. It had 75k miles on it and other than a few minor dings on the rear bumper, is in excellent condition. During the test drive, we had five adults in the car and it drove like a champ, so we decided to buy it.

About 6 weeks ago, it started to idle roughly and act strange (more on this later) under load, so I ran a can of Techron through it, with no change in the idle. Then, since it had the original spark plugs and wires on it, I changed both of those items, per advice from a friend and there was still no change. Here's what it is doing: from a standing start, under moderate acceleration, with a full load, there is a "shudder" in first gear, then it stops and is replaced by a steady vibration that I can feel in my seat through the rest of the gear changes. The gear changes are smooth, but it does seem to go into overdrive a little too quickily. The vibration will continue until a float or coast condition is encountered and it that instant and during the duration of the float/coast condition, the vibration disappears and the car is as smooth as silk. There is absolutely no pulsing of the brake pedal and the vibration is the worst under the greatest load, i.e. when all five of us are in the car. If I'm by myself, there is no shuddering at take-off and vibration is lessened, but is still there.

Being desperate and broke, I finally saved up enough money to have a recommended mechanic hook the car up to his diagnostic computer and he said absolute no error codes came up at all. I've never seen a "Service Engine" light come on at all. The tires are practically brand new. When the mechanic said nothing abnormal appeared on his scan, I decided to replace the throttle position sensor, based upon an article I'd read in an obscure corner of the internet that mentioned how a faulty TPS can mimic a torque converter clutch solenoid issue. The shuddering did diminish a bit, but the vibration is still there. I cannot afford to keep replacing parts and I cannot afford to buy a decent multimeter with a wire-piercing probe in an attempt to diagnose another faulty sensor, EGR valve or the like. The transmission fluid looks good and doesn't smell burnt. They guy I bought it from said he had both front CV joints/axles replaced when he received the car from his mom.

Would anyone happen to have had a similar situation they were able to resolve? I'm not sure in which direction to head next and would appreciate any wisdom or insight and would be happy to provide more information if required.

(It's late and I'm going to apologize for any typos I may have missed.)
 
Sorry that you're having problems with your 'new to you' Buick. These are great cars when they're running right and they usually do but sometimes older cars will have problems.

I wonder if the fuel pump might be failing - not allowing it to keep up with the acceleration demand. Once you level off it catches up and the engine smooths out.

You could measure the fuel pressure and see how it looks - should be around 50 psi.

I also remember some of the other guys saying that if one of the ignition coils are bad or the ignition control module is bad it can stumble under load.

The ignition coils should be easy to check with a HEI spark tester. The ICM will be harder without a good multimeter.
 
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