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Boost leak from idle adjustment screw.

maxitoman007

Member
Joined
Feb 15, 2017
Location
Calgary, Alberta, Canada
After performing a boost leak test on my 1987 Volvo 740 turbo I found that air is leaking from the thumb screw used to adjust idle speed on the throttle body. Does anybody know how to seal this? Are the threads supposed to be air tight (Teflon tape?) or is there some kind of internal seal/o ring/gasket that I can replace in order to fix this issue.

I was thinking of somehow adding a nut to the thumb screw that could be used to clamp an o ring against the threads to try and seal it up. Anyone have any other suggestions?
 
I believe there should be a lock nut on this already, but I'm not a 7 series fellow, so I don't know.
 
If your talking about the black plastic knob, there is an o ring. It's the same one as the dipstick o ring.

Yes that?s the knob I?m talking about, does the o ring go on the end somewhere? I can?t see it fitting on the screw itself because it?s so large. And the dipstick seal is so small. Or am I just imagining that the dipstick seal area is smaller than the screw.
 
Removed the thumb screw and there is indeed an o ring albeit mine looks like it is in quite good condition. Might have to look for a generic one that is just a bit thicker.

OE 947114 0-ring size:
11 mm (7/16"?) OD
6.7 mm (1/4"?) ID
2.4 mm (3/32"?) thickness

Same as dip stick o-ring.
 
If I remember, it takes a little convincing/force to pull out and push in the thumb screw with the o ring on there. Did yours pull out easily after the threads? Maybe the o ring dried out and shrunk a bit?
 
OE 947114 0-ring size:
11 mm (7/16"?) OD
6.7 mm (1/4"?) ID
2.4 mm (3/32"?) thickness

Same as dip stick o-ring.

Measured mine up using calipers and got those exact measurements so why is air leaking past?

Edit: found a place in my home city that sells o rings in various sizes. I’ve measured the groove on the screw and determined that it is 3.2mm wide with the segment having a diameter of 7mm. The store I’ve found sells o rings that are 3mm thick with a 6.5 mm inner diameter so I’m thinking one of those might seal it up right.

If I remember, it takes a little convincing/force to pull out and push in the thumb screw with the o ring on there. Did yours pull out easily after the threads? Maybe the o ring dried out and shrunk a bit?

Came out quite easily once unthreaded to spite the o ring looking to be in great condition, hence why I’m looking for a bigger o ring.
 
Last edited:
Update: picked up one slightly bigger o ring today and another, the one closest to the original. Guy who ran the seal shop told me that plastic parts like to expand and contract due to heat and that overtime this can change the size of the part just enough for it not to seal. He also explained, after inspecting the original o ring, that it was “squished” and was smaller one way than the other making a not so optimal seal. The slightly larger o ring ended up being too large and I ended up using the new, close to original sized o ring. Took some wiggling and lubrication but eventually I got it threaded in. Performed a boost leak test again and this time much less bubbles there but still some small ones. Removed the screw and wrapped the threads with Teflon tape. This combined with the new o ring managed to seal it up to the point that by the time I reach around 5psi using my bike pump, something else is leaking that’s not that. Hopefully this week or this weekend I’ll be able to shoot down to the gas station, spend a couple bucks and utilize their compressor to finish off my boost leak testing.

On a side note, I discovered a puddle under my car today, of brake fluid. Clutch slave seal is leaking again. Thinking I might go to my new found seal supplier and see if he has something that’ll seal her up good too!
 
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