63mm TB

[Schrade]

Originally Posted by LGAFF

I think you meant open, not closed

I believe they mean the plate is cut to for a setting of 10 degrees from vertical when closed

Yeah - same thing here; we're measuring the angle from different planes.

Now - any way of isolating vacuum on the secondaries butterflies to check seal integrity? Like I was sayin' before, mine looked good, but I got a 50 - 75 RPM swing, until I put in that restrictor gasket. THEN it was ROCK STEADY 675 in DataMaster.

So, how to figure out if vacuum is pulling on the butterflies open AGAINST the return spring, or if they were holding closed, and just a tiny leak is in it being out-of-round??? I think I remember trying to push on the TB 2' cam to check this, and it didn't help steady the rev's............... ?

Isn't someone here who is re-working TB's putting in a 'helper' return spring? Marc perhaps???

[Hog]

I wouldnt be worried about them holding vacuum too much(incorrect, t-body blade sealing is VERY imp[ortant in the LT5), many single blade t-bodies that actually use an IAC motor have a hold drilled in them to help the engine idle.

I would be interested in leaving a modified t-body in the freezer for an hour or 2, then checking for binding.

Vacuum is pulling equally on either end of the blade if the pivot point is central to the blade.

[LGAFF]

I could not find the moly dry lube locally, so I tried a dry graphite lube by PB Blaster which has the same properties as other products that call themselves "DAG"

The stuff locks onto the metal, cannot scratch it off with you nail, good to 850 degrees.....warmed the TB up to promote fast drying and...

awesome seal

[LGAFF]

Drove 35 miles today, idle is just as before....so no issues. Initially it took some time for the DAG replacement I used to loosen up.....it was sticking at 2K when I lifted off the throttle....it would then drop to 850-900 at a complete stop. After a few miles no more issues....

[Hog]

DAG?

[LGAFF]

Dag is a general purpose material that is graphite based and is applied like paint. It's used in various industrial applications that need properties like a thick material buildup, low friction and high temperature range. DAG is a trademark of the Verick, Acheson Colloids company.

It seals the gap between the TB and the plates

[Schrade]

That 2' seal HAS TO be perfect BEFORE any tuning is done (or as perfect as can be).

Once I removed the restrictor gasket, I got a swing in idle of about 50 RPM. Not so good...

I read in an old HOTB article (2006?) that Marc addresses this very thing, because you can't tune UNTIL it's right..................

[Hog]

Thanks, for the DAG info.
I would have thought that the IAC motor could have taken care of slight leakage. Most cars have an rpm swing at idle as the sensors give info back to the ECM in a negative feedback fashion.
Perhaps this is because of the Speed Density system and/or the slower talkback rate of the OBD1 system?

[XfireZ51]

The IAC is a controlled leak. If it's completely closed, that leaves whatever air is coming thru the TB and possible other leaks. That's why you can suspect a vacuum leak of some sort if the IAC steps are at 0.
If you've disassembled the TB, you need to re-do the Min Air Adjustment.


"Isn't someone here who is re-working TB's putting in a 'helper' return spring? Marc perhaps???"

Marc does have a helper spring. I'm using it. TB return spring may get a bit weak after 20+ years.

[Hog]

Makes sense. In the early days of OBD2 controllers guys that were installing lumpy cams were drilling extra holes in in t-body plates to allow for improved idle characteristics.

Ability to recalibrate ECM/PCM's eliminated such a need.

It appears that t-body sealing is very important on the LT5, thanks for the feedback.

I looked up the IAC motor 17112350 for the 1990-95 LT5., $27 from Ecklers or $83 for a GM part from Rock Auto.


Does this motor act on the smallest of the 3 primary throttles in the t-body?