Vacuum holding throttle body open - need ideas, shave plates? 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. |
Re: Vacuum throttle body open - need ideas I'm interested in this too as I have the same problem. |
Re: Vacuum throttle body open - need ideas 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. |
Re: Vacuum throttle body open - need ideas I've tried a few TB's too like a Holley and 3 stock.I have 2 x return springs which helps a bit |
Re: Vacuum throttle body open - need ideas 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. |
Re: Vacuum holding throttle body open - need ideas, shave plates? When I first read this, I thought Tootie was :crazy:, 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. |
Re: Vacuum holding throttle body open - need ideas, shave plates? 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. |
Re: Vacuum holding throttle body open - need ideas, shave plates? Thats pretty crazy. My 1st suspect was the spring. unless something is not adjust right. |
Re: Vacuum holding throttle body open - need ideas, shave plates? 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. |
Re: Vacuum holding throttle body open - need ideas, shave plates? Not sure you are right Vader. |
Re: Vacuum holding throttle body open - need ideas, shave plates? 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. |
Re: Vacuum holding throttle body open - need ideas, shave plates?
Originally Posted by ZZ42Fast
(Post 6402103)
Not sure you are right Vader. |
Re: Vacuum holding throttle body open - need ideas, shave plates? 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.. |
Re: Vacuum holding throttle body open - need ideas, shave plates? 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. |
Re: Vacuum holding throttle body open - need ideas, shave plates?
Originally Posted by NoEmissions84TA
(Post 6402265)
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. |
Re: Vacuum holding throttle body open - need ideas, shave plates?
Originally Posted by NoEmissions84TA
(Post 6402265)
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. |
Re: Vacuum holding throttle body open - need ideas, shave plates?
Originally Posted by Tootie Pang
(Post 6402282)
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. But without actually holding your TB in my hand, I can only offer this info. |
Re: Vacuum holding throttle body open - need ideas, shave plates?
Originally Posted by NoEmissions84TA
(Post 6402265)
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. 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. https://cimg1.ibsrv.net/gimg/www.thi...23f988c20b.jpg |
Re: Vacuum holding throttle body open - need ideas, shave plates? 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. |
Re: Vacuum holding throttle body open - need ideas, shave plates? 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 |
Re: Vacuum holding throttle body open - need ideas, shave plates? Maybe just wind the return spring one more loop. |
Re: Vacuum holding throttle body open - need ideas, shave plates? 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. |
Re: Vacuum holding throttle body open - need ideas, shave plates?
Originally Posted by Tootie Pang
(Post 6402735)
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. |
Re: Vacuum holding throttle body open - need ideas, shave plates? 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. |
Re: Vacuum holding throttle body open - need ideas, shave plates? 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. |
Re: Vacuum holding throttle body open - need ideas, shave plates? 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 |
Re: Vacuum holding throttle body open - need ideas, shave plates?
Originally Posted by Tootie Pang
(Post 6402737)
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. I think 18-20 is closer to a stock cam. |
Re: Vacuum holding throttle body open - need ideas, shave plates?
Originally Posted by aliceempire
(Post 6402751)
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. |
Re: Vacuum holding throttle body open - need ideas, shave plates? 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. |
Re: Vacuum holding throttle body open - need ideas, shave plates?
Originally Posted by GeneralDisorder
(Post 6402753)
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 |
Re: Vacuum holding throttle body open - need ideas, shave plates? 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. |
Re: Vacuum holding throttle body open - need ideas, shave plates?
Originally Posted by Tootie Pang
(Post 6402830)
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. Let me know- GD could be crowned a hero |
Re: Vacuum holding throttle body open - need ideas, shave plates? 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. |
Re: Vacuum holding throttle body open - need ideas, shave plates? 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 |
Re: Vacuum holding throttle body open - need ideas, shave plates?
Originally Posted by Bill Chase
(Post 6402845)
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. |
Re: Vacuum holding throttle body open - need ideas, shave plates? 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) :doh:. This is a very confusing thing to sort out. |
Re: Vacuum holding throttle body open - need ideas, shave plates? Yay! GD |
Re: Vacuum holding throttle body open - need ideas, shave plates?
Originally Posted by Tootie Pang
(Post 6402875)
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. |
Re: Vacuum holding throttle body open - need ideas, shave plates? No worries! I think the BBK has a single large spring. Not sure though, I have not disassembled one. |
Re: Vacuum holding throttle body open - need ideas, shave plates? 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. |
Re: Vacuum holding throttle body open - need ideas, shave plates? 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. |
Re: Vacuum holding throttle body open - need ideas, shave plates? 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. |
Re: Vacuum holding throttle body open - need ideas, shave plates?
Originally Posted by dan5
(Post 6403056)
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.
Originally Posted by dan5
(Post 6403164)
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. GD |
Re: Vacuum holding throttle body open - need ideas, shave plates?
Originally Posted by dan5
(Post 6403164)
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. |
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