How is a vacuum leak in a 1994LaSabre discovered and repaired?

My engine is shaking when driving between 45 and 60 miles per hour.
It does it sometimes.....and then it topped doiung that yesterday and this morning. Now it does it again this evening. It is not subtle shaking when it happens....it is serious shaking.
So far I have had these things told to me.
#1 Opinion:
One independant shop mechanic (NOT a Buick shop ) said I need a new transmission because of this shaking. He also reported he noticed low pressure (I think the word was pressure) on the #5 cyclinder when he was trying to find out why the car engine shakes. he replaced the spark plugs.
#2 Opinion:
A man on this website suggests that the shaking could be caused by a
vacuum leak.
#3 Opinion:
The local Buick dealer service dept gave it a computer diagnosis and looked at it...They get a 02 sensor code...and say the shaking is caused because there is a problem in the fuel injector wiring. They say a signal is not going between the 5th cyclinder fuel injector and the Power Control Module. The Buick dealer says they will have to spend an hour or two locating the wire...and maybe replace the Pwr Cntrl Module
#4
A relative of mine, retired from GM as a Service Manager for a whole section of the US....drove the car and says it is not a transmission problem. He called a service manager in milwaukee for input. They told me to First replace the 02 sensor. If that does not solve it....I should have the fuel injectors cleaned at a GM shop...because they only do the job right.
?????
What direction should I be looking in? i know nothing about cars....and can't afford experimentation. I surely am glad I didn't let the independant shop replace the transmission , or the Buick dealer replace the power module for $525.00.....on a hunch.

Vaccuum leak.. Spray around the intake manafold with WD-40. A leak will suck in the WD-4o and burn it some improving the idle. But your symptoms infer its not a vaccum leak if it only happens at high speed.

Seems something with your ignition is bad.. Ignition Module, Coil pack, Sensors, Ignition wires, Or even a plug.

Good Luck

I have a similar problem, my car vibrates around 50-60 when in low gear, if put into 3rd, it kinda goes away

I have replaced, plugs, coil packs, wires, everything

someone told me that it could be the shift module, the flying saucer looking part on the front of the tranny that has a vacuum line going to it, that's what i think it is, but i havent replaced it yet

or it could be the tranny selonoids

im not sure, its really ****ing me off, i'm taking this car off the road until i can figure it out

its definately not the O2, thats not going to change anything cause the car would run like **** all the time with a bad fuel mixture

I changed my Mass Airflow Sensor and man that made a huge difference in overall performance, but the engine is still shaking at those speeds, it has to be that the transmission is putting too much of a load on the engine in 4th gear, thats my thought

BUT HOW THE **** DO WE FIX THIS?

[Edited by tman because a member complained of profanity.]

To the first poster: two separate mechanics found a problem with cyclinder #5, which speaks volumes as to your problem. If it is a wiring issue, either one: figure out which wire it is from your PCM (subscribe to Alldata's 94 Lesabre page and you can find a detailed schematic of your PCM's wires), and figure out a way to replace it yourself (actually test it first using an ohmmeter and a spare long wire--run the long wire from the wire you are testing connected to the number 5 injector to the interior of the car where it will connect to one of the terminals of your ohmmeter, while the other terminal of your ohmmeter is connected to the spot on the PCM terminal that corresponds with that wire that you are checking--if resistance = 0 then your wire is good, if resistance = infinity, the wire has an open. Also check the wire's continuity to ground.) Or two, pay the dealer to diagnose your problem and fix it yourself (if it is the PCM). Or three, replace the PCM even if yours if fine simply because the cost to do so may be less than paying a GM mechanic to tell you what is wrong.

Warning: getting a wire run between the under dash area and the engine compartment is not easy.

To the third poster: have a mechanic check your motor mounts, tire balancing, alignment, etc. After you know you have the smoothest running car possible, you will better know how much shaking is going on with the engine. As to causes, again have someone do a compression test, an injector balance test, a fuel pressure test, and have a GM dealership check your tranny and steering for possible problems.

Just some ideas, probably worth the paper they are printed on.