94 Buick LeSabre problem

My 94 LeSabre sometimes vibrates or shakes when going on the road. It does it mostly when I am cruising at about 50 mph. It's something like an hesitation but somewhat rhythmic. If I lift the foot from the gas pedal or
press it until the car shifts down that vibration or hesitation stops. It also does it at lower speeds. It's not the CV joints or the computer. I took it to a transmission place and they said it must be the engine, maybe a bad spark plug, wire, etc. I replaced these and also switched the Ignition Units and Coils with the ones from another Buick I have and it didn't fix the problem. When I read the codes one tells me that cylinder # 5 and #6 misfire, #5 almost all the time. But when I check the spark at the spark plugs it is strong and doesn't appear to miss and the engine doesn't shake or hesitate. When cold it does it more and I can notice a vibration at idle as if one spark plug wasn't working well, but not as if a wire was disconnected. Compression is almost even on all cylinders except number 1 that is lower about 12%. However, spark plug number 4 looked dirty as if it wasn't burning well, but not that bad. Otherwise the car run OK and starts at the first time. Vacuum and gas pressure are normal. Any idea what may be causing this?
Thank you
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Check the throttle position sensor. And... clean the throttle body. I'd clean the throttle body first since that requires not parts purchase. The TPS on that motor is a bit of a pain to replace and to be done properly it has to be calibrated when you replace it, but it's a fairly common failure point. It's only a pain to replace because of where it's located. Requires you to "get down under that part, and in between these two and behind that one over there...".
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I replaced the throttle position sensor and adjusted it recently and cleaned the throtle body. Could it be because of carbon build up in the cylinders?

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cleaned
I doubt that. These things burn really cleanly today. Just look how nice your plugs look, even after 100,000 miles on them.
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Try gently depressing the brake pedal with your left foot while cruising steadily at 50 mph. Keep the right foot on the accelerator so as to maintain steady 50 mph. Does this scenario stop that vibration/hesitation? Yes means the lockup torque convertor is at its critical parameters for trying to decide whether to lock up or unlock. Often the simplest solution here is to drive either a little slower or faster. No means this is not the culprit & diagnostician needs to look further. s
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Thank you for the tip. I did try it and it stopped. Just a little pressing of the brake pedal did it. Is there any adjustment or part replacement that I can do?

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Don't think 94 has an adjustment, but if it does, it'll be on a cable from tranny to throttle linkage. Will have a flat metallic disc inside a plastic retainer. (Has a tiny hole in the center of metal disc, for your recognition). Press disc inward enough to allow you to slide the cable further thru its retainer--lengthening its distance from tranny or shortening it, just a tad, will alter the go-into-lockup speed. Try lowering speed 1st.; may hafta use trial-and-error--no problem. HTH, s
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I don't see any cable that can be adjusted and neither comes from transmission. Just by touching the break pedal, even before the brake light come on, the vibration/hesitation stops. I wonder if disconnecting some switch at the brake pedal would do anything useful?

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Hi, Alpha one, what is happening as you gently depress(or harder) the brake is the lockup torque convertor is being disconnected--ie, car is not utilizing the efficiency, gas mileage, lower rpm's, and cooling it receives when IN lockup mode. It's vaguely similar to car not shifting into the last gear, & would feel that way to a non-mechanic. On older cars, we could cut/disconnect a wire to render lockup inoperative. I've personally not known a tranny to burn up from any lack of cooling, but I've heard it will run hotter. They did get worse gas mileage, but lockup shudder/binding when approaching a stop would be elimnated. Sorry, but someone else would have to tell you any repercussions or method of permanently disengaging the '94. I think the method would consist of finding proper wire that completes lockup circuit (somewhere near tranny) when computer calls for it & cutting it. Were I to do it, I would leave it where I could easily reconnect it if other malfunctions should appear--maybe even splice in a toggle switch so I could engage at smooth rpm's & dis-engage at critical/shuddering rpm-range. Still think accelerating thru critical rpm-range more quickly could pretty much 'hide' your problem, at least from you who happens to be the one bothered at the moment. Good luck & HTH. s
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One more thought: talk to a tranny shop or 2 & ask. There may be a lockup-control solenoid not properly or fully engaging which could be replaced by merely pulling pan. If so, this may be a correct, permanent, relatively inexpensive fix. Nothing beats doing it right! Luck to you, s.
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Thank you. I am going to ask first.

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