Daveinrenton

I replaced the 48 mm TB with a BBK 52mm on my '87 L-98 Vette. I immediately had problems with the throttle being held open once running. When the engine is not running, the spring tension on the TB is very strong and the TB slams shut like a nervous clam. But when the engine is running, it is as though a vacuum in the intake manifold is "pulling" the TB open and the engine idles at several thousand alarming RPMs. I mounted strong throttle springs to pull the TB closed and for a good while I got normal performance and smooth throttle. About six months later, the pulling started again and I have to kick down the engine to give all those springs a running start and the TB closes completely. This is insane for the TB to behave this way especially with the extra external spring tension helping the TB spring. How has this been solved by anyone and how can I get to other manufacturers of TBs that will work without modification in my Vette? Thanks for your comments. Dave

Silver96ce

Before you do anything else check the operation of the throttle body with the throttle cable disconnected. I believe I read either in threads on here or somewhere else about problems with the throttle bodies made by BBK and the blades sticking. If all is working properly then take a look at this thread -

http://forums.corvetteforum.com/c4-t...le-issues.html

which discusses the throttle body cable being too short and causing the blades to stay open.

Daveinrenton

When the engine is off, everything is fine, no sticking and lots of spring tension. When the engine is on, the vacuum thing starts. Both the throttle cables for the pedal and cruise control are loose when this phenomenon is happening. If the cable was too short, it would be exerting pressure on the TB but there is zero tension. I had heard sometime in the past that the butterflies may be mounted and proportioned around the BBK TB axel in such a way that the pull of the manifold vacuum exerts pressure on the too-large surface causing this effect.

I have not tried simply removing the cables and starting it to see if my earlier assessment on cable tension was right so I'll do that exercise. My thought is that if the cable was too tight, I would not be able to kick it down to normal idle and I can.

I appreciate your response.

GRIS

Had a similar problem when i bought my '96. It had a 52mm bbk that would not throttle down. Nearly got me in trouble before i learned to kick it down. Replaced it w/ 48mm bbk which is ok so far. Wish i had know this was a bbk problem; would sure have gone to something else.

Cruizin

Thinking out of the box - try unplugging your throttle position Sensor (TPS) lead from the unit on the passengers side of the throttle body shaft and give both half's a good blow out preferably with dry compressed air.

Moisture in this plug can cause all sorts of speed variations as the ECM receives incorrect voltage signals. I've seen this happen every now and again after boil overs and after sitting in very damp weather.

C409

..... First , verify that there are no vacuum leaks ... Second , set the minimum idle as described in the factory service manual ... the ONLY reason that I could think of that would cause external return springs to work at all would be that the shaft and or throttle body itself are very worn where the throttle blade shaft passes thru the throttle body ... was the 52mm TB new or used ? .... the engine will not develop enough vacuum to open the blades ...........

383vett

Vacuum pulls the blades closed, not open. You must have a problem with linkage geometry or your cruise control is acting up.

Daveinrenton

Thanks for input everyone. I will check all suggestions. On the vacuum pulling closed instead of shut, that depends on which part of the butterfly is bigger and that was something I read in a thread long ago on this same subject. The BBKs have a bad design on the butterfly causing the issue possibly. And BBK does not respond to consumer inquiries. I got nothing out of them. The car is in the garage for the winter so I will be able to fuss with it to try to get my smooth throttle back and get rid of those springs that are absolutely needed to have the TB work at all correctly. P.S. this was a brand new unit and replaced once when I had the initial issue. Same thing with two different new units one out of the box.

bjankuski

This is a common problem with throttle bodies. I have noticed that as the throttle body shaft bore wears the throttle blades will hang open at idle when the vacuum level is high. The load from the vacuum puts enough force on the throttle blades that the shaft binds in the worn housing. There a few ways to address this, spray some lubricant at each shaft bore when the problem occurs, install bushings in the worn housing to correct the problem, or live with it.

I have installed bushing on mine and others I have used the spray trick, usually the spray trick only lasts a few months.

leesvet

Its impossible.
Engine vacuum cannot pull throttle blades/butterfly open. The vacuum of SPACE wouldn't do it either.
Think this thru...the shaft is center of the blade, equal-distant from one edge to the other. Vacuum is applied against the whole blade. Not just the bottom half that swings inward...besides, if you got the calculator out and did a quick calculation of vac in hg X the surface area in sq inches of the blades, and compared that against the spring tension that has to be fairly high, it would be something like 100:1. That's IF the vac was on only HALF the blade...but its on the entire blade surface so that negates any force the vac would have on the blade.

This is a common cable problem that I have had to figure out with my BBK. Although mine was an Edelbrock BBK, I don't know if that makes a big difference, but the Tb worked perfectly, even though it did want to fast idle after installation. The problem is that the 52 is a deeper body, so the blades/shaft/linkage is slightly further out from the fixed position of the cable mounting point. When closed, the throttle cable is stretched like a piano string.

On the cable mount bracket on the plenum, take one of the two-10mm bolts out
(are there 2 or 3?, I forget) and roll the bracket just about 1/4" to give the cable some slack between the bracket and the TB linkage.

Problem Solved.

If you don't believe this, then take the throttle cable OFF the TB linkage and start up...it'll idle fine.

WW7

My cruise control would cause my engine to go wide open throttle when I first got the car...I disconnected the CC cable and it stopped...WW

Daveinrenton

I get the logic on the vacuum and just because I was able to wiggle the loose throttle cables when this stay-open condition occurred making me believe something else was going on, I have not tested this with the cables removed and I will do it today. I will report findings and, again, really appreciate everyone's sharing their experiences with this not uncommon problem.

bjankuski

Just a little food for thought my previous comments on a worn throttle shaft and worn throttle shaft bore causing binding at high engine vacuum like idle and slowing down from a cruise at closed throttle. When engine vacuum is at 15" of hg (On deacceleration and idle, it may easily exceed 15" of HG) the force on the throttle shaft bore and throttle shaft itself is around 50 lbs. If there is any binding on the throttle shaft that much force could easily hold the throttle slightly open. When you blip the throttle you eliminate engine vacuum and load on the throttle allowing it to snap shut. That is another reason why with the engine off the throttle appears to work correctly.

Daveinrenton

I did the test running the engine without any cables and no change. With the engine not running, the TB opens and shuts as smooth as butter. Start the engine and gently turn the shaft to increase throttle (I did this with no cables attached) and it not only sticks open but there is a grittiness to the action between this stuck-open position and closed throttle. That seems really weird to me. But in the last comment from BJankuski, I did notice that I had to rev the motor and "let go" of the shaft to get it to spring back to closed. If it got in its stuck open position and I tried to slowly twist it back to closed, it seemed a little reluctant to do it. That plays to his theory. I am thinking there is no remedy since the effect is strong enough to counteract the extra pull-back power of two throttle springs I put in place to keep this from happening. It does not happen as badly, but it still happens once the engine is fully warmed up. I kind of wondered why BBK put the extra appendage on the TB for the return springs. There is no such post on the original 4mm TB. The only reason I changed it is that the TB was getting sticky at closed throttle from wear (it is sparkling clean). I would just as soon get this old unit re-built as to goof with the BBK but I have no idea who could do it reliably. Any ideas on that (and others on the current situation if there are any, of course)?

Daveinrenton

I just read more carefully BJankuski's initial concept and have a better understanding of what he meant. I never though of the shaft bushings or whatever they have in there as being the problem especially since this was a brand new unit. How did you retro your TB with super-slippery bushings to keep this sticking from happening? This may be the answer fro my old 48, too, since I thought it was the butterflies sticking on the bore, not bad bushings.

Chuck Tribolet

Not necessarily. The shaft might be just a touch off center. Depending
on which way it's off, the throttle plate would pulled open, or closed.

Curveit

If you've tried everything else, you might try backing off the mounting bolts and re-torqueing them... not quite so tight, and equally. That worked for me, on my stock worn out w/a spring TB. I'm guessing mine was warping the body enough to put it in an extra bind. Good luck with yours...

bjankuski

First take off the air intake leading to the throttle body and spray the three spots where the throttle shaft passes through the throttle body housing with WD-40. Work (Open/close) the throttle body a few times put the air intake back on and start the car and see if the problem is gone. If it is you have confirmed a sticking throttle shaft due to wear of dirt. If you want to fix it, remove the throttle body, and remove the throttle shaft. Determine if it is worn or just dirty, try to polish out the wear or dirt. If it is worn bady you can have a machine shop bore out the throttle shaft bores and install oilite bushings in the housing to fix the worn bores.

leesvet

Because you have PROVEN that it is NOT a cable problem, then it has to be binding in the shaft bushing OR the blade contacting the bore.

There is not enough vacuum in this universe to pull those blades open. Vacuum only reaches 29hg.....that's a finite amount of force. Its physically impossible to pull any harder than that. The spring is many many times able to overcome that amount of force. Its a TINY surface for vac to pull against.

That's TWO things that can be eliminated: vac and cables.

There are only 3 possible explanations.

Ever heard of occums razor?

http://pespmc1.vub.ac.be/OCCAMRAZ.html


Curviet:

interesting possibility ... I wonder if it is possible to 'tweak' the TB enough to cause binding without cracking the aluminum body...?

Daveinrenton

I've shelved the "vacuum pulling open" idea and am convinced it is vaccum pulling against the entire assembly enough for the shaft to "catch" a burr in the axel bore. This theory also accounts for the "sticky" action when I handle the throttle manually when the engine is running and the other thing that drives me nuts, a non-smooth throttle. This speaks volumes on the build quality of this product. If the minor vacuum is enough to pull the axel to the bore walls hard enough to expose a bore so poorly finished as to cause sticking, it makes me very desirous of finding other choices in manufacturer for this item. There just don't seem to be many choices available. Has anyone bought an aftermarket TB that they can swear by? I've decided to pull out my old TB to work on the shaft/bore issue and see if I can resurrect it. If so, I'll swap it with the BBK and then do the same to it. I'll report results.