81 Scirocco smog check fixes update

Throttle body off again. Throttle plates close but don't bind.

The idle air screw does not completely close the idle air bypass when screwed all the way in. I shine a penlight up the hole from the manifold side and see clearance around the beveled end of the screw in the venturi. There is no apparent damage to screw or throttle body, so this must be by design.

That leaves only the intake manifold as a source of 'false air', and the joy of a head removal just to find out for sure.

Can't figure why the manifold gasket would blow, and how it could be blown such that it doesn't affect one cylinder much more than others. The manifold is not loose and idle speed is low (~800 rpm) when starting cold.

nf

Are they adjusted like the article I cited?

You do not pull the head to remove the >Throttle body off again. Throttle plates close but don't bind.

Your cite: "Back out throttle plate adjustment screws so there is a gap between screw and stop lever." How much of a gap? What's the thickness and hardness of the paper stock? I simply gave it almost another 1/4 turn after I got it to the no-stick point. That's a bit less than 1/2 turn after barely touching the stop. In any case I'd already tried manually adjusting and couldn't slow down the idle.

I put the throttle body back in and tried setting duty cycle down to about

50-56 with dwell meter. With idle air screw all the way in warm idle is ~1300 rpm. (There is buzzing from the fuel distributor.)

I hooked up a hose and a piece of metal tubing and tried to get lots of propane close to the intake manifold gasket above the exhaust manifold. I didn't note any rpm increase. Maybe I still didn't get enough propane in there.

Are there different throttle bodies with different length idle air screws? If someone put in a shorter screw while testing, maybe because it had a better o-ring...

I don't see how excess air can be getting through the throttle body unless through the bypass. Maybe I should take a wad of chewing gum and stick it in the bypass. :-)

nf

I just took a picture of one of my TBs today similar to yours. I think some had different size venturis if a manual or an automatic. I did not see if that idle screw screwed all the way in, but I thought they did. I think that screw takes a seal and might need a tiny bit of lubrication. Yours takes a wrench, but others might need a screwdriver IIRC.

A backfire/pop might blow an intake manifold, but maybe it is that 'new seal' you put on that idle screw not allowing the screw to move enough. Did you lube that screw?

Spiffy clean with a light rub of Hylomar. The "new seal" is an o-ring, 5mm inner, 9mm outer diameter. The old o-ring had turned to hard plastic (like the o-rings on the injectors) and did not seal the hole. The o-ring is not stopping the movement, it's the edge of the o-ring seat hitting against the beginning of the threaded portion of the hole. I'll post a picture of the screw next to a ruler if you want to see what I mean.

It is almost impossible to see if the screw is sealing the hole in the venturi without shining a light up the air bypass.

nf

For a test unscrew the stop screw so it does not touch! What is the idle then? I will assume that the throttle plates are not bent!

Yes measure that idle screw and I can measure some of mine.

I just checked the throttle body that I have that is similar to yours. The idle screw fully seats!!! Probably your entire problem! ;-)

Thanks for looking!

Is there a plastic/rubber seat in your throttle body? (None in mine.)

I sure didn't replace that screw. I'm going to have a talk with my less-than-well-equipped-and-not-super-expert mechanic and see if he did. Here's the screw:

I tried unscrewing the throttle plate stop this morning. I left the mixture where I set it with dwell meter yesterday. The engine fired right up and idled a little below 1K without touching the gas pedal. After a few moments it slowed and pulsed a bit (I assume this is the cold start valve working). Then, as oil warmed to 80C, rpms rose to around 1200.

Your idle screw looks like mine (yours is cleaner ) but that seal is much fatter than mine. My seal is barely wider than the screw and it is flat but that could be from age. Is yours a new seal you bought from the dealer? Do they still carry them? Gee I should have taken a picture of the screw and measured it, but I was busy today servicing a boiler and a house radiator which occupied my mind along with my time! lol

There is no rubber seat that I can tell, it looks like all metal there. Take off the seal and see if it fully seats! Or did you try that already?

Hmmm...

maybe the OP should try removing the O-ring, seeing if the idle screw fully seats then... (you can't drive it like that obviously but it's a good test) or just buy a new, correct idle screw from the dealer (yeah, it'll be $20 for a darn screw, but still.)

nate

AFAIK, your seal used to be an o-ring. The dealer doesn't carry them, but the correct size is 5mm ID, 9mm OD. I couldn't find that size except in a

172 o-ring value pack. So now I have 23 of them. If you need a couple, let me know and I'll mail em to you.

Are you sure yours fully seats? I talked with my mechanic. Turns out he knows about the dwell meter trick and a number of other things I thought I might have outpaced his knowledge on (he does lack smog tools, obviously). He did not exchange screws in the throttle body. So, this is still the original. I've adjusted idle with that screw many times over the last few eons, and I'm certain that I never screwed it all the way in. It was never screwed so far into the hole as it is now (barely able to get a wrench on it). Much as I'd like the problem to be the screw, it isn't looking like a good candidate.

nf

It was probably pulsing as the oxygen sensor was starting to warm up and going into closed loop. The rpm change was probably the control pressure going up as the CPR warmed up from the electrical heater. It also gets warm with some oil heat but it takes about 15 minutes of driving for the oil to get very warm.

I see a kink in the vacuum advance plastic hose underneath the throttle body.

Good valve cover gasket?

Slack >

Yep............good test and very easy to do without having to remove the Throttle Body again. ;-) Hey you have 23 more O-rings if you need another! lol

Hmmm...

maybe the OP should try removing the O-ring, seeing if the idle screw fully seats then... (you can't drive it like that obviously but it's a good test) or just buy a new, correct idle screw from the dealer (yeah, it'll be $20 for a darn screw, but still.)

nate

I swear I replied to this message before, but it's never shown up... (I can only post to google groups because my laptop died a horrible death... I spilled crap in the keyboard *and* the backlight for the LCD died the same day...) anyway my thought is maybe the o-ring that the OP installed is keeping his idle screw from seating? maybe remove the o-ring see if you can kill the engine with the screw then, if so, then you know that the o-ring is slightly the wrong size and you need to find one that fits better

nate

hmmm and I thought I saw that message before nate!

I swear I replied to this message before, but it's never shown up... (I can only post to google groups because my laptop died a horrible death... I spilled crap in the keyboard *and* the backlight for the LCD died the same day...) anyway my thought is maybe the o-ring that the OP installed is keeping his idle screw from seating? maybe remove the o-ring see if you can kill the engine with the screw then, if so, then you know that the o-ring is slightly the wrong size and you need to find one that fits better

nate

Thanks, but that's not the case. there is a very solid 'stop' to the screw. I was able to torque it a tiny bit further, but that clearly burnished the edge of the brass 'lip' holding the o-ring and scraped away a tiny bit of aluminum where it touched.

nf

AH HA! So the screw is not bottoming out huh? I will try to measure mine so we can see if ours are different. ;-) BTW did you try to start it up and check the idle when you forced the screw in further?