TBI airfoil does not seat completely

[HallenTi]

In an effort to correct a minor hesitation, idle, and deceleration problem, I checked the forum and saw suggestions for cleaning the IAC and the throttle body. I removed the throttle body and the IAC and cleaned both. When removing the airfoil from the TBI unit, the screw broke off. I assumed that a previous owner had possibly "strong armed" is to a point of being ready to break off. I took the screw to a local bolt shop, and they gave me a 5mm x.8 x 50mm sockethead cap screw.

This new screw was exactly what a previous owner had used, but when securing the airfoil with the new screw, it stopped turning before the airfoil was seated correctly. I assume that it really isn't a 5mm x.8 x50 but I am stuck with it for now, and am going to order a GM screw and attachment piece (small threaded aluminum "cone" inside the TBI unit). My question is whether the airfoil being improperly seated now could cause my resulting high idle (2,000RPM cold or warm). From the look of it, since it is not a precision mating surface, and that it doesn't require as gasket, I feel that the minor seating problem wouldn't affect the idle in this way (I have even read in some posts that a few have run without the airfoil).

I assume that I either had an inoperative IAC unit and changed the fully-closed setting causing an air bleed, or more likely, I overzealously fooled with the IAC and caused it to fail. I plan to get a new IAC, but my main question is about the seating of the airfoil unit.

 

[HallenTi]

I have checked the flexible hose betweem the TBI and the airbox, and it seems fine. The airfoil appears to be the original unit that was shipped with the car (can't be absolutely sure, but is slightly different in shape from those "high flow" once pictured in the catalogs). Since you have suggested to just remove it, I assume that the "fit" of it is not critical, and I can leave it be for now since it is secure but not seated properly. I will check all the other hoses for cracks. I did check and the vacuum tube that connects under the TBI unit is secure and the rubber elbow isn't cracked.

 

[c4cruiser]

I wouldn't worry about the airfoil. It does not provide the HP claims the catalogs advertise. This is one of those things that would have been molded into the TBI unit if the design engineers thought it would have been beneficial to airflow.

 

[HallenTi]

I am not at all interested in a "high flow" unit, and never planned to change it (will keep the $40 in my pocket). I thought the same as you - they would have done it in the first place, if it were beneficial to the "average" owner, since they had to design it anyway, and it would not cost any more at the time. All I am wondering about is the current "gap" between it and the TBI unit, just for now, as like I said, I plan to get the correct screw and fastener as soon as I can. I just wondered if this gap changed anything as far as idle stability or speed. This was to see if it could have caused it, so I wouldn't needlessly replace the IAC if the gap caused the high idle. From what I have heard, it doesn't sound like this gap could cause any change, since people hav run without them entirely.

 

[Ken]

I don't think the manufacturer ever used an "airfoil" in the throttle bodies when they came from the factory; all of the airfoils you see are aftermarket items. If I am correct in that thinking, then it would just as well benefit you to remove the airfoil; no performance issues will arise if it is removed.

What you most likely incurred is the fact that the installer used a bit too much Loctite™ or the wrong type when they installed the airfoil. When you tried to remove it and it broke, you either left a piece of the screw in the hole or there is a glob of Loctite™ at the bottom of the hole preventing the item from fully seating against the throttle body face.

The idle should not be affected by the airfoil placement as long as the rest of the system is intact. Leaks in the intake stream will affect idle quality, as well as a few other problems.

And c4cruiser is right; the horsepower claims are, shall we say, a little on the high side. In fact, unless you are going 75-100 miles an hour it doesn't do you a bit of good.

Let us now how you fare with this problem.

_ken

 

[vettn94]

I'm kinda surprised no else has suggested this by now but have you tried to reset the IAC (min Air) and TPS?
My guess is that was your problem all along, especially after you took your TB apart.

 

[Ken]

:duh What is it they say when you assume something? You're right, we should have suggested that. :L

_ken

 

[newvette]

Just curious, when you say "reset" do you mean "adjust" or is there a procedure to just reset the TPS and IAC?

Thanks,
Mike

 

[Ken]

Adjust to the correct settings...

... AKA "reset".

The TPS should be .54 (+/- .08v) volts at idle and around 5 volts with the throttle wide open.

The IAC resets itself if it was installed with the pintle at the correct depth (1 1/8 inches, or .28 mm) after the engine is started and then turned off.

The ECM uses the following information to control idle speed:

• battery voltage
• coolant temperature
• throttle position sensor
• engine speed
• A/C clutch signal
• P/N switch

_ken

 

[vettn94]

here is the procedure:

You will need the following tools and equipment:
1. set of Torx wrenches.
2. good digital voltmeter that will read voltages less than 1 volt.
3. paper clip.
4. small screwdriver.

There are two electrical components on the TB that you will be working with: The TPS and the Idle Air Control Valve (IAC). Make sure that the connectors for these two components are easily accessible and that you can easily disconnect the IAC.

You will also be working with the diagnostic connector under the dash. Remove the cover (if it’s still in place). Bend your paper clip into a “U” shape. You will be using the two top right hand terminals (“A” and “B”) in the connector.

- First step is to set the minimum idle speed. If nobody has messed with this on your car before, the set screw will be covered by a metal pressed-in plug. It’s located on the driver’s side of the TB. Remove this plug if it’s there.
You can do this using a punch or something with a sharp tip.
-With the IAC connected and the ignition OFF, stick the paper clip into the diagnostic connector from “A” to “B.” This grounds the diagnostic lead.
-Turn the ignition to the “ON” position without starting the engine. Wait 30 seconds.
-With the ignition still in the “ON” position, disconnect the IAC connector at the IAC.
-Remove the paper clip from the diagnostic connector.
-Start the engine and allow it to reach normal operating temperature. The idle speed will probably be really low, and you may have to coax the engine a bit with the gas pedal to keep it running for a while.
-If your car is an automatic, set the parking brake and put the transmission in DRIVE. If your car is a manual, leave it in neutral.
-Adjust the idle speed screw to obtain 400 rpm in drive or 450 in neutral.
-Shut off the engine and re-connect the IAC.

TPS:
There are 3 wires stacked vertically on the TPS. You will need to be able to measure the voltage between the two top wires. You can either buy a special harness connector that breaks these wires out (from Mid America or Ecklers), or gently pierce the insulation of the wires with the pointy prongs on your volt meter. You can also stick a paper clip into each of the two top locations of the connector and clamp onto the paper clips to measure the voltage. Whatever is easiest for you.

-Turn the ignition to the “ON” position without starting the engine.
-Loosen the TPS Torx adjustment screws.
-Set your volt meter to a low scale DC volt setting that will accurately read less than 1 volt.
-Measure the voltage between the two top TPS wires.
-Adjust the TPS by rotating its position until you get a reading of .54 volts.
-Tighten the Torx screws and recheck the voltage. Re-adjust if necessary to make sure voltage is right at .54.
-Turn the ignition “OFF.”

You are now in perfect adjustment on idle speed and TPS output. Start the engine.

 

[HallenTi]

It just hit me out of the blue last night what c4cruiser and Ken were saying...you were telling me that the factory didn't install *ANY* air foil.

I assumed (there's that word again! <g>) that there was a "stock" one, and the so-called "performance" one. I also think you are right that probably the wrong Locktite® was used, and thus caused the screw to break when removed. So, off it comes this morning! I was just afraid that it was a stock part, and since it wasn't installed correctly, might interfere with the idle and overall operation.

I did check the other CAC posts and got an overall feel for how to reset the IAC. I did follow the instructions but to no avail. I am afraid I may have ruined it when cleaning it, or it might have been inoperative before I took it off. Since I called around and found none available Sunday, I decided to try something myself, so removed the TBI and removed the bottom "air bleed" plate that houses the IAC, then manually adjusted the IAC and carefully screwed it in to check the seating of the valve (so as not to get it too far out and damage the seat). Once I was satisfied with the fit, I put it back on and the car idled much closer to "normal". I drove it briefly, but will check with the Chevrolet dealer this morning for a replacement IAC. But, at least I isolated what I believe the problem to be. Thanks all for the detailed information!

 

[HallenTi]

PS: I meant that it "hit me" BEFORE I signed on and read their replies...don't want you to think I read them and still didn't "get it!" :eyerole

 

[Ken]

Originally an e-mail from HallenTi
You mentioned 1 1/8", and the book says "no longer than 1 1/8" as if to say that anything up to 1 1/8" is fine.

Would you suggest I set it to EXACTLY 1 1/8" before installing it then, rather than just a range "up to" 1 1/8".

I hope you don't mind that I posted the response here rather than in private HallenTi, but it will benefit more of the GP if we discuss it here.

According to the GM Service Manual for my '87:

If pintle will turn, continue turning until 28 mm (1 1/8") is reached. Return spring to original position, with straight part of spring end lined up with flat surface under pintle head.

IMPORTANT!
No physical adjustment is made to the IAC assembly after installation. IAC valve resetting occurs after reinstallation on the vehicle, and is reset after the engine is started and then the ignition turned "OFF".

My interpretation of that means to set the pintle at exactly 28 mm (or 1 1/8 inches). I can't tell you the part number because I don't have a reference book for those, but the dealer parts counter should know which pintle you require, and have exactly what you need. Good luck.

_ken

 

[HallenTi]

Thanks to all for the good and very useful information. When I ordered the IAC from the Chevrolet dealer, I mentioned that the book I have (Haynes) says there are three types (single-taper, dual-taper and blunt) but he said that his parts information only showed one type, so I went ahead and had him order it. When I picked it up, it was exactly the type I needed (dual-taper) and the only difference was that the body of it was made out of plastic rather than pot-metal as the original was. The pintle moved in and out much more easily that the original, so I adjusted it to exactly 1 1/8". It went on just great (I have had the TBI off and on so many times, I am getting good at it!). The car now idles and runs MUCH better. I also removed the air foil and might see if I can sell it on eBay for a bargain price, since I won't be using it.

 

[Ken]

_ken

 

[maxm]

Did that cleaning today. It was messy. My IAC was 25 mm. I did not adjust it to 28 mm. Does it mather ? The engine ideld god before. I left the TPS in it´s place. Was not able to start it upp, no battery. How fragile is the TPS and IAC ??
Max
and a happy new year to you all...

 

[HallenTi]

Max,

I was very careful with my IAC when I took it out (and I too didn't fool with the TPS). I tried moving it very gently in and out, and was able to do that, but it was rough and "jerky" (best way to describe it) so I assume that it was in need of replacement. The brand new Chevrolet one I got was quite smooth and moved in and out more easily. I think it is does idle fine once you get the battery connected, that you're home free. I knew right away that mine was not adjusting itself (after cleaning and reinstalling the original) as the idle speed was about 2,200 to 2,500. The new valve was just the ticket for me and corrected it very well. I hope yours just needed a cleaning and a replacement isn't needed due to the cost. Tim

 

[maxm]

Hallen Ti
How much for the IAC in USA ?
I have my CD player in the USA for repare so the dach is a mess. It will be back enyday. I will conect the battery as soon as i get the CD back.
While i had the TB off i did the TB coolant bypass. I djust plugged the hoses. I have also clened the radiator, it was very messy. The thing is when you reed on this forum abaut things to do the list gets longer and longer. So even if there is no problems you want to fix it enyway. Hopfully the vette is even better wen the spring comes. We have a very bad winter here in sweden this year so it is dificult to start the vette in the garage.
Max

 

[HallenTi]

My IAC valve, through the local Chevrolet dealer was $98 plus tax. I have seen then in several mail-order catalogs for less, and if I wasn't in such a hurry could have ordered one. All the local parts stores could have ordered me one for less, but at the time I was under the impression that there were three pintle types, and since none of the parts stores ask me which type, I was afraid that they would order me the wrong part and I would have to hassle with that, then finally get the right one from the dealer.

 

[maxm]

Parts are a litle more expensive in sweden, the IAC costs abaut 130 USd.
vettn94 instruktions were easy to follow. It feels safe to have this forum wen there is something wrong with the vette.
Al in al i find the vette reliabel, last summer we made a tripp araund europ. It was 5000 km thru germany, austria, italy, france and back home.
Max