92 Lesabre hard idle on startup and trouble shifting

[OptimistPrime]

I have a 92 Lesabre with about 58k miles on it. Usually it's a good car, but today it idled very hard when I started it up. When I put the car in gear, it would not shift out of first. However after a minute or so of driving, the idle returned to normal and the shifting started working again. If the car sits at idle for a bit, like at a long stoplight, I can hear the RPMs begin to increase. Not sure where to start looking with this. TPS and MAF are both new parts, and both are AC Delco.

 

[2007LucerneCXL]

Age is the issue here that items can rot out or become brittle, while the milage is extremely low, so it's not driven that much (?). Has the transmission fluid ever been changed? Plugs and wires? Fuel filter? Any vacum hoses? These are just some ideas as maybe putting a can of Seafoam in the tank and some highway driving. Any more info you can share may help others on the forum to offer suggestions.

 

[OptimistPrime]

I bought this car from my grandma's estate. It only had about 25k miles on it then. She had kept good maintenance of it but had never driven it, other to the grocery store and back. It's had a few problems with sensors, but other than that the thing runs pretty well. I've never changed the transmission fluid, but it does have plenty. I guess it's probably about time to change it. No leaks, though.

All the rubber seems to be in good shape. This car was kept in a climate controlled garage at grandma's house. It doesn't have a lot of the rot associated with these cars when they sit up. I just changed the oil and it was clean, no coolant in there. I'm very apprehensive about changing transmission fluid, after flushing my S10 made the solenoids lock up. This old heap probably has a ton of trash in that system from sitting so many years. So I need to change the fluid, but not flush it. Could it have a clogged trans filter? I ran Seafoam in it a couple years back. Never in the oil, just in the fuel system.

I'm driving the car about 60 miles tomorrow, so I'll see how it goes.

 

[2007LucerneCXL]

Great car and it seems to well maintained, but age on some parts can make them act up while appearing to look good. A scan may show what is happening or at least point to the area to look at. Not knowing the history of why the other parts were replaced something could be related to this new one, just another view.

Drive it and see, may need to see if it's just a stumble or a problem.

Here's another idea of what could be happening.

Buick lesabre: rpm..rpms..cruise control..the car will jump or stumble…

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[OptimistPrime]

Thanks for the link. I'm starting to lean towards the MAP. Maybe it's related to the other issue I'm having. There's something else I didn't mention, because it's hard to describe and has been ongoing for years now. The car makes a noise. It's hard to say what it sounds like, exactly. No mechanic that has looked at it has had any clue. It's not valves, or lifters. It sounds almost like playing cards stuck in the spokes of a bicycle. ppffffffffffffffttttt

It always makes this sound at low and mid range throttle. One mechanic said maybe it was because my exhaust manifold gasket is slightly torn, and flaps a bit. But it sounds like the noise is in the intake, not the exhaust. For years I've just driven the car with this noise. It's annoying, but I turn the radio up really loud, and that seems to help. Lately the noise seems even louder. Maybe I'm in the end stages of MAP sensor failure. I've never seen one die, but I wonder if the ECM not getting a proper pressure reading on startup would cause that hard idle. This would make my day if the two problems were related, because the noise has become embarrassingly loud.

I drove the car about 60 miles today. It drove great, despite the really loud ppffffftttt noise. I'm taking it to the same place tomorrow, then I'm going to get a MAP sensor and figure out how that goes in. It's worth a shot to change out, if such a simple thing can save this car and fix a very irritating noise at the same time.

 

[2007LucerneCXL]

We use to use cigars to find exaust leaks and starting fluid to find vacuum leaks, we were brave and stupid souls, but the engines were primitive compared to these lol. I hope you find the solution.

 

[Zeroboostbuick]

The noise you're talking about could be exhaust manifold/down-pipe related.

Hight idle could also be due to a dirty IAC ( intake air control ) valve.

You can drop the transmission pan to drain some of the fluid and replace the trans filter while you at it. Get new trans fluid from gm dealer. For their price you won't get a better fluid. Amsoil or Mobil1 ATF is better but costs much more.
Trans pan gasket is reusable and superb quality.

 

[OptimistPrime]

To update, I started the car today with the hood up, to see if I could notice anything amiss. When the engine started, it idled as rough as a dump truck, and I could hear a very clear hiss coming from the driver side of the motor. When I gave it a bit of throttle, the bad idle eased off, but I still hear the hiss.

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[LARRY70GS]

See if the transmission has a vacuum modulator. A leak there will cause a no shift condition and the vacuum leak will cause a rough idle.

 

[gnhbodyguy]

I would pull the IAC pintle and clean it.

 

[Zeroboostbuick]

LARRY70GS is correct.
There should be a vacuum line going to the transmission. Visible on the driver side of the engine.

 

[OptimistPrime]

That will be the next thing I check. Thanks for your help, guys.

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[OptimistPrime]

I'm having trouble finding this vac line. Is it supposed to be on the top or bottom of the engine?

I also don't see where the IAC is on this motor. On my truck it's right next to the throttle body, but I don't see it here.

 

[LARRY70GS]

Look for the vacuum modulator

 

[LARRY70GS]

gm vacuum modulator pics - Google Search:

 

[OptimistPrime]

After examining the modulator, I noticed its vac lines were all rotted out. I replaced them, but still the problem persists. I can hear a very clear rush of air going through that modulator. It's definitely where the hiss is coming from. So I'm thinking the modulator must be bad. Maybe the baffles, or whatever is in the inside of it that does the modulating, is all blown out. I'm looking for a part store with the item in stock.

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[OptimistPrime]

To update, I found a store and got the replacement part. I'll be putting it on first light. I'm concerned, though, that the modulator isn't bad, because the guy at the store said a bad one would be leaking fluid out the vac line port. Mine doesn't do that.

 

[LARRY70GS]

If the diaphragm leaks, it is possible for the engine to suck in transmission fluid. This can happen very gradually depending on the leak. The presence of transmission fluid on the vacuum side of the modulator is a sure sign it needs replacement, but even if you don't see ATF, it can still be bad. An easy way to test them is to check them with a vacuum pump. The modulator should hold vacuum, and not leak down.

 

[OptimistPrime]

Thanks again for all your help, Larry. I was able to get the modulator changed out. One thing I noticed was the hard idle on cold start now only lasts a few seconds, instead of like 5 minutes. That's progress, and should help with the no shift condition. However, this thing still idles very hard. I can hear a very subtle hiss every time I give the engine light throttle. If I give it hard throttle, I can hear the motor sort of backfire. Sounds like the manifold isn't attached properly. I'm letting it cool down so I can feel around on those pipes for any breaks.

The strangest thing is the self-revving engine. I can hear the motor revving itself up occasionally. But it's not getting any throttle. Not sure why it would do that.

I think I'm at the end of my rope with this one. I might have to take it to a shop, which will be hard because none of the ones nearby are reputable. I don't know what else to do, if that exhaust manifold has come loose. I assume fixing that involves welding.

 

[LARRY70GS]

One thing you can do is put a vacuum gauge on it. The engine, fully warmed up, in Park should make 20" of vacuum. If you have a leak, it will be less. Spray carb spray around the intake manifold and throttle body. When you find the leak, the gauge will rise, and you will hear the engine RPM change. If you can hear it, it's leaking somewhere.