Tootie Pang

iTrader: (2)

After three new BBK TBs (two were slightly defective) and a rebuilt factory TB, I have without doubt, determined that engine vacuum is keeping the plates slightly open. See videos. This a brand new TB that mechanically is just fine.
Not sure what to do about this other than drill some holes in the outer edge of the side of the butterfly plates that the vacuum is pulling against the return spring. Ugh.

ZZ42Fast

iTrader: (2)

I'm interested in this too as I have the same problem.

Tootie Pang

iTrader: (2)

I don't know why this is a problem now and not when these cars were new. Weird.

The only thing I can think of is that in the years since, some brands of TP Sensor return spring have gotten weaker and they are needed to contribute to the return force along with the main return spring on the other end. Perhaps some of the aftermarket sensor manufacturers thought they only need their sensor to follow the throttle position, not help it close. MMmmmmm. Suspicious.

ZZ42Fast

iTrader: (2)

I've tried a few TB's too like a Holley and 3 stock.I have 2 x return springs which helps a bit

Tootie Pang

iTrader: (2)

I'm sure if I shave enough plate off of the correct side, I can fix it. I just don't know if the IAC would be able to compensate enough for it.

NoEmissions84TA

iTrader: (1)

When I first read this, I thought Tootie was , but after seeing that others have the same problem, I have to follow this one.

IIRC, the throttle plates on carburetors purposely have the plates offset on the shafts so that the engine vacuum pulling against them helps to keep the plates closed.

Tootie Pang

iTrader: (2)

Hey, just because I'm a little loco, doesn't mean it isn't happening! Lol. I calculated that at 20" hg (10 psi), the weight on each plate at idle (closed) is almost 32 lbs. If those plates are off a little, it could make an inch pound on the throttle shaft trying to keep it open.

LiquidBlue

Thats pretty crazy. My 1st suspect was the spring. unless something is not adjust right.

Vader

Another vote for the throttle plates being installed with the long side at the tip-in end, otherwise known as "backward" from what it should be. It's no great surprise with the aftermarket TBs, but shouldn't be a problem in the Rochester TB unless they have been disassembled.

ZZ42Fast

iTrader: (2)

Not sure you are right Vader.

ULTM8Z

I think the problem with BBK is there is too much play in the center support for the throttle shaft. Allows the throttle shaft to flex under pressure from engine vacuum.

Vader

I'm not sure that I am right, either. If I had the TB in my hands, I could probably identify the problem in minutes. We don't even have a photo.

Tootie Pang

iTrader: (2)

Here's some of it. They also had a problem with smooth operation. I had sent them this video. The plates can be seen clearly. I do not believe they are off-center and they appear to be centric to the shaft (but that's just eyeballing it). I was thinking of drilling a small hole in the lower half of each plate near the bottom. I figure the IAC should be able to compensate for the idle by just running more closed and then the difference in surface area between the upper, full plate and the slightly less area of the drilled out lower plates will create the proper balance under vacuum and relieve the torque that is keeping the TB from closing..

NoEmissions84TA

iTrader: (1)

C'mon Tootie, let's do some math.
You calculated 32 pounds of force on the throttle plates.
So if you drilled 2 holes, each relieving 1 pound each, then you are still at 30 pounds of force.
And then the IAC probably could not compensate.

ZZ42Fast

iTrader: (2)

Those 2 lbs could be all that is needed though?

Tootie Pang

iTrader: (2)

Remember if exactly half the plate surface area is above the shaft, and half below, the vacuum pulling on the entire plate should balance to zero. If one area was larger than the other, that larger area would create more force on it's side of the shaft, creating a rotational force on the shaft.

NoEmissions84TA

iTrader: (1)

Exactly, which is why the plate should be offset to the "holding it closed" side.
But without actually holding your TB in my hand, I can only offer this info.

Tootie Pang

iTrader: (2)

yes but I should've explained better. One hole on each plate, near the outer edge of the side that is turning the throttle shaft in the open direction. Slightly less area on that side of the shaft.

Here's pic. The holes I am going to make are shown with the red arrows. The vacuum and rotational force on the shaft is shown with the dashed blue and green arrows. Notice the blue force is created by the lower plate area and should be balanced by the upper area's force. By removing some of the area on the blue side, the blue rotational force is less and the throttle body return spring can now close the throttle.

For idle, as long as the two holes do not flow more air than the IAC normally allows for idle, the ECM should be able to run the IAC more closed to keep the idle balanced. It's a brand new throttle body but I don't care. I gotta see if this works.

NoEmissions84TA

iTrader: (1)

I understand your thinking completely. On paper, the theory is plausible. In the real world, I just think it won't work and cause you more problems.
But there is only one way to find out - drill them holes and test it.
I would be looking for a stronger return spring, or some sort of additional spring.

GeneralDisorder

iTrader: (1)

Let's back up a second here. What are your IAC steps at idle under these conditions?

What's you idle vacuum, and what's the timing set at? And what cam do you have in here?

GD

QwkTrip

iTrader: (22)

Maybe just wind the return spring one more loop.

Tootie Pang

iTrader: (2)

The cam specs are as follows:

270/276 @ 0.004
214/220 @ .050
.452/.465" LIFT
112 LSA, 108 ICL

Timing is stock.

Last time I checked. vacuum is 18-20"

Not sure about the IAC, but when I take the housing off it's open more than a 1/4" off the seat.

I've messed with the return springs. It can help. I may do that, but I don't want the pedal too heavy either.

ULTM8Z

You're getting 18-20" vacuum with that cam? At what RPM?

Tootie Pang

iTrader: (2)

About 600 rpm.

Let me recheck everything today. It's been several months and I don't keep good records. I remember checking it to see if I was getting a crazy amount, but it came in at a good number.

aliceempire

iTrader: (14)

Has any efi GM throttle body needed slots or holes? I think I'd just be looking at clocking the spring one turn tighter. If it still doesn't work then I'd be looking at further correcting the throttle shaft. The shaft is probably binding from vacuum and possibly heat, causing too much friction for the spring to overcome with it's current setting.
And my first question is a genuine question, not making a jerk comment.

GeneralDisorder

iTrader: (1)

You need to check that the IAC isn't open too far. If it's open too far then the throttle has to be closed too much - which may be the source of the problem. Opening the throttle stop, allowing the IAC to close, and re-calibrating the TPS to the new closed position may be all that's required.

GD

ULTM8Z

Yeah, I'd be surprised if you were getting that much at 600 rpm. I'm getting 16-17" at 800 rpm in neutral, and my cam has 2 deg less duration @.050.

I think 18-20 is closer to a stock cam.

Tootie Pang

iTrader: (2)

I agree. Has to be something. Looking at GD's comment below. The TB does it cold too.

ULTM8Z

What I'd also do is with the TB out of the car, push on the center area of the throttle shaft (from the outside of the TB) and then operate the throttle linkage. See if you get smooth return back to closed.

When I tried a BBK 58mm one time, I was getting enough throttle shaft deflection under vacuum to cause the blades to bind in the bores. As I said in an earlier post, there was too much play in the center support area.

With the engine off, my off-idle tip in worked perfectly. With the engine on, the blades would stick in the bores due to poor machining tolerances). Opening the blades would eliminate the issue, but I couldn't open the blades any more and maintain my idle speed, so I was in a no-win situation. Had to return it to the seller.

Any rate, I know your symptom is different, but could be related to the same root cause.

Tootie Pang

iTrader: (2)

Now I hadn't thought of that. And the less closed the plates are, the less rotational force which may allow the return spring to take care of the rest. I will try that first.

Tootie Pang

iTrader: (2)

20" vacuum on the button.

GD may have it. I opened the idle screw position on the throttle until the IAC seemed to go no lower than 5 counts or so (after repeated starts). At some point on the way to 5 IAC counts, the throttle no longer hung coming closed. Cracking the plates must have backed off the rotational force on the shaft letting the return spring close it up. Car behaves nicely- for tonight. Will drive it around a bit more tomorrow. I am cautiously optimistic.

ZZ42Fast

iTrader: (2)

Awesome, fingers crossed. The number of times I've done similar tweaks only to find I celebrated too early though...

Let me know- GD could be crowned a hero

Bill Chase

If you look at the clock spring that the old accel/lpe throttle body used, it's the same as stock. There are actually two springs in the stock setup, and they are wound opposed to one another with some small slits in the plastic that keeps both springs tight, and the inner can come loose.

I just took apart my lpe 58mm and painted it, I ran into this problem and it took me a few minutes with small needle nose pliers to correctly locate both spring tabs. I would check there first. On stock throttle body you'll have to grind off the peened over shaft that holds the cantilever in place to remove the spring. But once you get it apart the problem will be apparent to you.

GeneralDisorder

iTrader: (1)

Having the throttle plates open for low IAC counts down around 10 steps is as it should be from the factory. It also prevents the IAC having trouble catching the engine from stalling when the throttle snaps closed. Remember your minimum idle air procedure - the engine should idle with the IAC completely closed and unplugged.

Setting up and tuning my FIRST with a stand-alone..... makes you an expert in what works for TPI idle management in a hurry.

GD

Tootie Pang

iTrader: (2)

I am very familiar with the stock TB dual spring setup. They are not wound opposed to each other. They work together. I have re-indexed the inner spring to mighty amounts of closing force with only some improvement but not a reliable solution.

Tootie Pang

iTrader: (2)

GD got it! Drove it around for an hour. Worked flawlessly. An absolute pleasure to drive. I'll bet that's all my factory TB rebuild needed also (which I threw in the trash) . This is a very confusing thing to sort out.

GeneralDisorder

iTrader: (1)

Yay!

GD

Bill Chase

poor wording on my part, the indexining tangs on the plastic pieces are 180° out from each other. Sorry for the hasty response. I was just saying to look and verify the springs were right. Not sure if the bbk stuff uses the same setup or not. But I do know the accel/lpe piece for 89-92 is the same springs as the stock 89-92 throttle body. Sorry for adding confusion to your thread bro.

Tootie Pang

iTrader: (2)

No worries! I think the BBK has a single large spring. Not sure though, I have not disassembled one.

dan5

Glad you have a solution, but I am surprised it is that sensitive. To pass FMVSS requirements, the throttle body is designed to close under all operating conditions with a broken spring (from high ambient temperature down to -40 degrees). That is why it must have (at least) two springs.

Tootie Pang

iTrader: (2)

That is very interesting. It makes sense. But because the vacuum can create quite a rotational force on the shaft, I imagine engineers have to balance it out so that the driver doesn't have to push down too hard on the throttle overcome that force and ideally is only working against the springs when coming off idle (the maximum rotational force position). In my cases, for some reason the vacuum was creating enough force near idle that the shaft return springs could not overcome it and the throttle kept open a little. UNLESS I closed the plates rapidly before vacuum could establish in which case the throttle plates would hit the idle screw and by the time the vacuum rose, it could not reopen the plates against the return springs. Just weird. Fluid dynamics is a super-computer level area of engineering.

dan5

I'm still a little mystified by your problem. I've just never encountered that. I haven't done throttle design, but was involved in throttle system development and validation. My experience suggests that the system ought to be fairly close to neutral balance without the springs. If anything, there might be a slight offset in the throttle plate mounting to provide some closing effort. Think about the force on each half of the throttle plate trying to turn the shaft. If the shaft is off center very slightly, the two halves of the blade would have unequal force applied to the throttle shaft. There may be some air flow effects, but it isn't a venturi, just a straight bore. I would expect air flow forces to balance.
If you go back to carburetors, particularly old Holley, they would let you remove and replace throttle blades, but they required the plates to be installed in a specific bore in a specific direction. If you looked at a vacuum secondary model, the throttle plate opened with a vacuum diaphragm and was regulated by a very light spring in that chamber. The primary throttle lever would pull it closed if you dropped back toward idle, but if you had the primaries open, the secondaries would self regulate based on speed and load.

GeneralDisorder

iTrader: (1)

What you have to understand is that the factory throttle stop is *NOT* adjustable. GM put a plug over the throttle stop to prevent occurrences just such as this. If everything is in balance as GM originally intended it to be, and the security plug is in place on the throttle stop then no adjustment would be required. Sadly most of these original throttle bodies are clapped out with worn bushings, etc and the aftermarket replacements have no such limitation on the stop adjustment for obvious reasons. Be it poorly designed, or not, the intention on the part of the engineers was that the IAC steps be in the 10-15 range at a hot idle and that the throttle allow enough air for engine idling even when the IAC was fully closed (minimum idle air adjustment procedure). This allows the throttle to be open enough that engine vacuum isn't prone to sucking it open or closed. It also keeps the IAC in a very small range where it can move quickly to the position it is needed, etc.

GD

Vader

Take that thought a step further, and study the Rochester 4MV. It required offset to operate the secondary air talve.