91 Buick Lesabre

Will Do. Could it be also a bad modulator? Maybe this will help, that tells exactly how it began.

This transmission has been a problem for years. Over the last 5 years or more it has gotten driven very little. It used to not like to shift at times until it reached 50 or so. After a short time it would start working better. Also a sudden down shifting or something could piss it off to not like to shift normal again until after some time went by. The time it went out was when I was having problems with it not getting enough gas and stalling and having to wait for a few mins for it to run again. I was getting pissed and after doing this once I dropped it into gear with the car reeving up more than usual and at the moment it never did shift again. What would most likely would that be a cause of?

I am not a mechanic, so more making guesses. I had a modulator leaking fluid into the vacuum line, when the line swelled and fell off it would not shift. With your description of problems, I suppose you could try any of the additives you listed. If I were to do that I would drop the pan and use fresh transmission fluid.

the modulator has great vacuum pressure going into it. The modulator itself is not tight and is able to turn easily. I would think it should be tight, does that matter any?

I really think it needs to go to a good trans shop for a diagnosis rather than guessing. The only modulator issue I had caused a hard shift not no shift. No shifting can be anything from lack of proper pressure to sticking or non-operational solenoids, or even shot 2nd and 3rd clutches, without checking the service manual. Even the electrical solenoids though require pulling the side cover which requires some disassembly of the drive mechanism. On one car I got off lucky for about $300 to just replace the solenoid pack. On another it cost me over $500 to replace the harness inside. There is really not much for a DIYer anymore to do when it comes to transmissions.

If the transmission has a failed modulator it’s likely that the transmission is now low on fluid. Engine vacuum operates that valve and when it fails it allows fluid to be sucked into the intake manifold.


Its not low.

The car is not worth more than 500 and that would be only if I parted it out selling the motor the gas tank I just put in and the hood which is that cool open backwards style. And that would be pushing it at 500.

I work seasonal, and for the past ten plus years I have been taking this to work the first day and the guys at work are always in shook that the thing is still operating. So just trying to get another year out of her.

Well you’ve got a car that won’t shift that you don’t want to spend any money on and isn’t worth anything. If you had a service manual, you might be able to check the continuity of the solenoids but that’s about it. So throw in some of that transmission cure-all and hope for the best. I doubt its the modulator but not expensive. Also check for vacuum leaks, but otherwise you aren’t going to get very far in just low gear. Sooner or later there comes a point. Its just a year early.


It is not running as good as I thought. It does run good for about 5 mins then begins to run worse and worse. The first time I did not run it as long to notice this. I put a fuel pressure tester on it. At the engine it reads around 32-34 when running. Book says it should be at least 40. After a few mins the pressure will begin to drop till it gets to 10 and stalls. If I remove the vacuum line on the side of the fuel regulator it will actually jump up to the lower 40s for a few mins and begin the slow decline. Does this mean its just a bad fuel regulator? Why would the pressure go up when disconnected?

I’m not a mechanic so don’t rely on me but the vacuum must be pulling on the diaphragm in the regulator to reduce the pressure available. So if the hose is removed it will not regulate the pressure but allow the full on pressure. But I may be wrong. It would tend to lead me to believe that it might be the regulator. Wouldn’t be the first time I changed a regulator and made no difference. One thing you can do is check for gas in the regulator and if it is wet then it needs to be replaced anyway. But gee why bother if the trans isn’t working? At any rate if you do change the regulator be very very careful when taking it out. Even after I was sure I had taken all the fuel pressure out of the line, I still got a whoose of fuel pulling it off. Didn’t hit me but getting it in the face would not be fun.

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True is like fighting a battle that is near the end. But it was always a great running engine. And if I could get it running good enough to take down the road perhaps that Lucas could do its thing and get it shifting. I did talk to a trany expert shop here and he said it sounds like more of a likely internal trany problem vs the solenoid or modulator.

I confirmed the books way of testing is full of crap. It says hope up the fuel pressure tester to the spot on the engine and turn key on. It reads 0, Did this on my perfectly working 98 Le sabre and it read 0. Even when running it took a good amount of seconds for the gauge to go up to the lower 40s. When I disconnected the regulator on there it went up to 50

I installed a new fuel pres regulator. Did nothing. Same thing. What causes pressure to slowly drop? Happened with the old pump and filter and regulator, happens with the new pump and filter and regulator. Could it be a line partly blocked? But if I put the tester after the filter where these is nothing but a few inches of metal line then a hose up to the filter that would rule that out if it drops there also. I believe it did drop when I put it there first but I will test that again.


I dropped the tank to try a different pump. After I dropped it I checked the voltage at the fuel sender plug. It read 8.25 volts. A friend said to switch the relay. So whatever I did, same reading. So I am suppose to have around 12 volts correct? So bad wire bad ground bad connection is likely the problem correct? The wires going along the passenger side of the car do not look bad, it is rusty by the door but the wires were well wrapped. In the trunk where the wires from the sender go in that part is very rusty and there seems to be some kind of ground at that spot that I think you call a quick connect terminal connect ground that is extremely rusty.