TH400 Modulator Valve

[DoubleDingo]

Okay all you guru's in GM-Square-Body-Land, my new 1981 truck with 69,000 miles on it runs great, however, it shifts quickly and softly which is just not cool. And it doesn't downshift when going up hills or when taking corners at slower speeds, so the fuel economy is being affected because the engine is not at good torque points.

I checked the vacuum line and it was all mushy at the trans. So I put a new piece on there, but not until I drove it without the vacuum hooked up. It wouldn't shift which is what I expected, and I didn't want to rev it up too high through the neighborhood. After hooking up the vacuum line I decided to start manually shifting at the desired speeds and downshifting the same way. Forgot how hard the TH400 shifts at higher rpm's.

Anyhow, Saturday I was leaving a bbq, started up the truck and saw white smoke coming out of the tailpipe. It didn't keep smoking, but I suspect there is ATF being sucked up through the vacuum line. I checked the fluid level and all is well there, except it needs to be changed. Sunday when I started it up it did the same thing.

I was planning to switch the modulator valve anyways, and want the adjustable kind. Which one do you guys recommend? And does ATF come pouring out when the old modulator valve is removed? Also, do you know of any other issues that may cause white smoke to come out the tailpipe? It's not blue smoke and doesn't smell like motor oil.

Thanks.

Shane

 

[Swims350]

white is usually antifreeze or something along those lines I think.

easy way is just to pull the line and check for fluid in it.

 

[89Suburban]

Yikes, lol.

 

[crazy4offroad]

Check the spark plugs for water, if you find one or two you know about where your problem is.

 

[DoubleDingo]

Curoius, would it pure like a kitten if water was being introduced into a combustion chamber or two? And are you guys thinking it may be a head gasket? No water in the oil.

 

[HotRodPC]

I usually use those smaller bell type Red Stripe gold smaller adjustable modulators. Yes, it does sound as if you need an adjustable one. You're sure you don't have the adjustable type? Doesn't matter, they are like $10 and take 2 minutes to swap out and makes no mess when you pull them off. Nike and Just Do It !!! Then let us know the results. Remember on the adjustables, turn the screw IN to make the shift point delayed and/or a bit firmer.

What type do you have now?

 

[89Suburban]

Curoius, would it pure like a kitten if water was being introduced into a combustion chamber or two? And are you guys thinking it may be a head gasket? No water in the oil.

If you were burning trans fluid out the exhaust, you could not mistake the smell of it. So take a good whiff of it next time, and pull the vac line and see if there is residue in it. I am surprised HR did not chime in on whether or not that was even possible for trans fluid to leak into that vac line.

HR????

 

[HotRodPC]

If the intake was getting just a sip of it, it would do just as he described. I have a feeling he might a have a canister type modulator and why I asked. It's hard to get the ATF to suck out of those type, so yes, he could get a sip and small plume of white smoke and be gone.

 

[DoubleDingo]

I usually use those smaller bell type Red Stripe gold smaller adjustable modulators. Yes, it does sound as if you need an adjustable one. You're sure you don't have the adjustable type? Doesn't matter, they are like $10 and take 2 minutes to swap out and makes no mess when you pull them off. Nike and Just Do It !!! Then let us know the results. Remember on the adjustables, turn the screw IN to make the shift point delayed and/or a bit firmer.

What type do you have now?

Payday is Friday, so I will ask for the small one with the red stripe. The one on there now is not adjustable. I stuck the little screwdriver in there and it didn't catch on any groove. I will let you know the results. I know it will be much better than it is now. I had a '56 Chevy Sedan in high school with a 350/TH400 setup and that sucker shifted nice!!

 

[DoubleDingo]

If you were burning trans fluid out the exhaust, you could not mistake the smell of it. So take a good whiff of it next time, and pull the vac line and see if there is residue in it. I am surprised HR did not chime in on whether or not that was even possible for trans fluid to leak into that vac line.

HR????

It just smells like it's running rich. I didn't get a chance to smell the smoke Saturday or Sunday, the breeze blew it away too quickly. I need to find my carb adjuster tool, can't find it anywhere. But I know how to get it to turn up, go buy a new one...lol...

 

[DoubleDingo]

If the intake was getting just a sip of it, it would do just as he described. I have a feeling he might a have a canister type modulator and why I asked. It's hard to get the ATF to suck out of those type, so yes, he could get a sip and small plume of white smoke and be gone.

Yep, I believe it's the canister type. And the old piece of hose that was on it was all mushy like it was soaked in oil. But it wasn't smoking until Saturday. Must be the manual shifting loosening things up a bit.

 

[crazy4offroad]

Water vapor dissipates quickly, if it is atf smoke it will linger a while.

 

[DoubleDingo]

I also spoke to a local guru about the puff of smoke upon start-up and he said it's the nature of the beast for this year truck. It only happens when it has sat for a while and only lasts a couple seconds. It happens because the valve seals are o-rings. The way to fix it is to machine the head to accept the umbrella type valve seals, or just accept it leave it be unless it is burning oil. It's not burning oil so I shall leave it be. If it starts burning oil, Mr. Vortec shall come to save the day!!

Bought the modulator valve at lunch. Red Stripe! Truck is sitting in the driveway so I can switch the modulator valve after work and deal with a hot transmission. Also got some in-line fuel filters and an in-carburetor fuel filter.

I shall post how it shifts after switch-a-roo.

 

[HotRodPC]

Good News. So you see the tiny screw inside the vac hose inlet? That's the screw you'll turn inward to make your shift later and should also make it a bit firmer. Just from experience, I'd suggest just turning it in 2 full turns before you install it and that should put you right where you need to be. Keep in mind, light throttle take offs will still be early and light, that is normal characteristic of Th400 wo a shift kit. But half throttle and full throttle you should be about right but you can adjust futher to your liking.

 

[lusktommy]

I also spoke to a local guru about the puff of smoke upon start-up and he said it's the nature of the beast for this year truck. It only happens when it has sat for a while and only lasts a couple seconds. It happens because the valve seals are o-rings. The way to fix it is to machine the head to accept the umbrella type valve seals, or just accept it leave it be unless it is burning oil.

If it was valve seals bad and they are leaking a little the smoke would be a faint blue smoke not white.....and valve seals dont just instantly start leaking unless its been sitting for awhile and allow the seals to dry and crack then when you start it up and drive it around and start it up again the next day you'll see the faint blue smoke again its not white.