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Need help with some transmission symptoms...


DV8
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1965 Riviera 425

 

If there's one black hole in my overall knowledge of auto mechanics it's in transmissions so I'm reaching out as none of the symptoms I'm having seem to match any of the diagnosis in the shop manual exactly.

 

Symptoms - Car stalls on warm up only when in Drive or Reverse.  The best way I can put it is there seems to be an unusual amount of drag on the engine as soon as it's put into gear.

 

It's not a matter of idle or fast idle adjustment.  The engine runs smooth as ever even on start up in Park or Neutral.  Increasing the idle or fast idle does just that and it still stalls as soon as I put it into gear.  Once the car is warm the symptoms go away completely for the rest of the day until the next morning.  It will go if I give it enough gas.  My work around has been to just shift it into Neutral at the first couple of red lights until things have warmed up.  I regularly change the oil and have recently changed the transmission fluid/filter/gasket.  All fluid levels are where they should be.  No slips and shifts into 2nd and 3rd when it should.

 

Any help or advice would be much appreciated.

 

Cheers,

-DAVID

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I’m certainly no expert either but as a keen amateur I’d start by checking the vacuum modulator and it’s connecting hose on passenger side of trans..if not working or vacuum is being lost at hose then might explain at least some of your symptoms ….the vacuum from engine is used to help change gear via modulator…and it’s as cheap place to start 😀

 

cheers

kev

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I had a similar problem some years back. 

Then for some reason I took out and inspected the idle needle screws.

One was fine, the other looked like it did 3 rounds with Mike T.

Replaced it, did a tune and it was fixed.

Tom K

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Sounds good. I'll start with checking the vacuum and the idle mixture screws which sound simple and inexpensive enough if anything needs to be replaced.

 

I could use a little more information on the torque converter slipping though. Is there some kind of test I could run on that?

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1 hour ago, RivNut said:

A 1965 ST400 does not have a lockup feature.  

Well that ran in a ditch!  Just funnin'.  By design, torque converters do slip, it is a fluid coupling, as opposed to engaged clutch.

I guess converter slip means different things to different people. At idle, a converter has little transferred torque VS under load and higher rpm when it becomes more efficient at transferring torque (power).   John

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That sounds like it could be the issue. I need more 'slip' at idle. So how or what do I do? Again, I know very little about transmissions.

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My “extensive” knowledge of transmissions - which is nil - would be to replace the converter, if that turns out to be the problem.  My guess comes from when I had an 83 Riviera and the the solenoid for the lockup on the TH325 transmission was bad. Once locked up, it would not release causing the transmission to stall the engine when I came to a stop. Similar to stopping a car with a manual transmission without disengaging the clutch.

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3 hours ago, RivNut said:

My “extensive” knowledge of transmissions - which is nil - would be to replace the converter, if that turns out to be the problem.  My guess comes from when I had an 83 Riviera and the the solenoid for the lockup on the TH325 transmission was bad. Once locked up, it would not release causing the transmission to stall the engine when I came to a stop. Similar to stopping a car with a manual transmission without disengaging the clutch.

Ed,  you just said it has no lockup feature.  The converter is not the problem,  PROBLEM IS ENGINE IDLE TUNE.   John

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It idles perfectly in Park and Neutral. It's only when I put it into Drive or Reverse that I get issues. Anyone would be hard pressed to get it to idle any smoother in Neutral than it does. If I set the idle speed or the fast idle any higher it just idles higher and still stalls when put into gear. This all leads me to the conclusion that it is NOT an issue of adjusting the idle. If there is an adjustment that only affects the idle RPM's when in Drive or Reverse please enlighten me.

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On 6/15/2021 at 5:17 PM, moran75 said:

I’m certainly no expert either but as a keen amateur I’d start by checking the vacuum modulator and it’s connecting hose on passenger side of trans..if not working or vacuum is being lost at hose then might explain at least some of your symptoms ….the vacuum from engine is used to help change gear via modulator…and it’s as cheap place to start 😀

 

cheers

kev

 

DV8, don't ignore moran75's input.

 

What you-re experiencing is very likely due to a vacuum leak. Check all the vacuum hoses, the transmission vacuum modulator hose and the modulator. Inspect the vacuum line going from the modulator to the intake manifold; if you find transmission fluid in that line, your modulator is leaking and needs to be replaced.

 

A/C controls, power brake booster, vacuum operated headlight doors or any vacuum accessory could be leaking enough to cause this.

 

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I had a 1964 Cad 60S with TH400 (first year) and over the years it (1) had a slightly rough, then (2) hard-to-maintain, low idle, especially when cold--and later THEN (3) it would not upshift out of first unless I ran it up to 28 or so, shifted to neutral, then back into Drive.  The cause was a three-inch piece of vacuum hose forming a 90* angle which connects the steel vacuum tubing to the vac modulator.  The hose was apparently cracked, then split.  10 cents worth of vacuum hose fixed ALL the problems, and I drove the car another 40K miles before selling it.

 

I'm lazy in my old age and like to try the easy solutions first.  So I suggest you crawl under the car and replace that damned little piece of hose ANYWAY, because if it hasn't failed now it certainly will in the next few years.  I sincerely hope that something that simple could be the solution to your problem.

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  • 5 weeks later...

Thanks for everyone's comments and sorry for the lack of update. I replaced the vacuum modulator which seemed to help especially with kick down. My next step and hopefully the other half of the issue is I'll be doing a complete ignition tune up including replacing plugs, wires, points, rotor, cap, adjust the dwell, timing and idle and choke and do an oil change. I realized it's been too long and too many miles since I did most of that. I'll keep you posted.

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