Mike Frye

As some of you may know, I've been wrestling with some vacuum problems with my car. I just did a top-end refresh and after it all went back together I had a serious vacuum leak.

I made up a tool to pressurize the system and I could hear air leaking from under the intake runners, but couldn't pinpoint it. It sounded like it was around the throttle body-air guide somewhere.

So I took it ALL back out and further rigged it so I could pressurize just the throttle body/air guide assembly including all of the hoses and ISV. Here's the jig (those are 3" plumbing plugs on the sides, about $3 each at the hardware store. They fit perfectly! ).



I've isolated the leak, but I don't know how to fix it. Air is leaking pretty fast from the side where the TPS belongs (I removed the switch for testing, but there's no bushing in there or gasket so I don't think the switch itself is supposed to seal) as well as the opposite side (although much less) by all the springs and where the throttle linkage goes.

My question is: Is this part serviceable? and if so, where can I get the bushings or whatever I need. The PET-6 doesn't show any parts in the TB, just the throttle body as a unit. Please don't tell me I need a new TB!

AO

They all tend to leak there. IIRC we experienced this when we pressurized Shanes car when installing the SCer in Green Bay. It shouldn't be enough to change the way it operates. Can you remind us what the operating issue was that percipitated this? Erratic idle? High idle? etc?

FUSE69

Presumably that would be leakage past the throttle plate pivot bearings.

Best I can suggest is to dismantle the throttle body, press out the bearings and head for a good bearing supplier... they will be able to sell you a replacement...

Unless of course this is just normal behavior....though from your description it sounds excessive...

Mike Frye

I had a hunting idle when I first started the car after the top end refresh. It was also running very lean when we tried to dyno it.

I did find a large vacuum leak at the base of the dipstick tube, so it's possible that was the real problem. In testing the system though, I kept hearing the air leaking out and from what Erik said, the system should be able to maintain pressure.

OK, so I'll probably have to disassemble it further. It seems excessive to me, it won't maintain even 5 psi, it just blows right out in less than 2 seconds.

Thanks guys. More work to do...

AO

Yeah that's maybe a bit excessive, but don't expect it to hold overnight. maybe 5 or 10 seconds at most. That throttle pivot bearing is usually pretty loose in my experience.

Mike Frye

Thanks Andrew.

When the whole thing was pressurized to 5 psi it was still all leaking out in less than 20 seconds, so I'm sure it's not right. For just this assembly I was expecting to pressurize it to 10 psi and have it hold for several minutes at least.

I'll have to get it apart and see what can be done in there.

AO

Ok... so 20 seconds for all (whole intake) that air to leak out... I think that sounds about right. I hadn't considered that the volume is significantly less with just the TB. Of course you can try replacing the bearing (assuming that's the leak), but don't surprised if the new one leaks the same. I think you might be chasing a ghost.

Mike Frye

That would suck.

So how come you were able to get your intake pressurized and Erik was able to crank his up to 40 psi and maintain it?

I can't imagine this much of a vacuum leak would be acceptable. Unless maybe it is acting like a valve and would hold vacuum fine but is leaking positive pressure. Maybe if I test it with vacuum and see if that makes any difference...


Did someone mention a bushing that's in there? Maybe instead of the bearings, it's just a bushing I can replace. Either way I don't think it will hurt to pull the assembly apart and see what I find in there.

AO

Depends on the word "maintain." In Erik's world, it means you put 40psi on the system and **** doesn't pop off! It does not mean that you can disconnect it and it holds pressure.

FUSE69

Thinking about it some more.... I'm not sure what type of bearings they are in the throttle body, if they are the simple 'needle roller' type, as in the throttle linkage (S4 & later) then replacing only those may not do so much.

Check the throttle plate shaft for wear... if it is scored or considerably worn, then replacing both may solve the problem.... and where do you get one of those?? I've got no idea!

As you mention, pressure can highlight a problem that doesn't exist.... as vacuum often has the tendency to help seal.

chrly924s

Just my $0.02, but having rebuilt several 944/951 TB's; I wonder if the rebuild kit would work here. I am getting ready to do the whole intake deal on my MY 83 and have been wondering about this. Those cars have needle bearings and several small orings. The orings turn to plastic and shrink over time. The kit is only $15-20. Just cleaning the needles and regreasing and replacing the orings would be about all we would need for this. That 944/951 TB kit had a bunch of model specific parts. You need to take the throttle plate out and slide the throttle shaft out to get to it.

Mike Frye

I didn't know there was a rebuild kit for this. That sounds like what I need. I'll check with the vendors and see if one exists.

Thanks for the tip!

doktor-t

Hunting idle you say. Did you check the idle air control valve (IAC)? It tends to build up oil residues (did I spell that right!?) in that valve and that makes the valve stick and not operate as it should. This is a common problem with many EFI cars and the 928 is no better. This valve suppose to do corrections many times each second to maintain a even idle. Carb cleaner or some brake cleaner would be the best to use. Maybe a small brush too. It should be able to move freely. I think it´s a solenoid (electric magnet).

If not filled with oil residues, perhaps you could try with another IAC just to be sure, it might be broken.

/Tobias

Mike Frye

Tobias,

Thanks, the ISV is brand new so hopefully that's not it. There's a procedure for applying voltage to test it's operation so maybe I'll do that before I put it back in. It wouldn't be the first time parts came in that were bad out of the box. Thanks for the suggestion. I have the old one which seemed to work fine so I can swap it out if that's the problem. My biggest problem with the whole thing is access. Once those intake runners are down and the injector rails are all in place it's damn near impossible to get in there to work on it, which is why I'm trying to test it all as a unit before putting it in there (this time!).

Mike Frye

SUCCESS!

I took out the throttle plate and shaft and looked in there and lo and behold! There's an O-ring in there on each end along with the bearing assembly. The bearings were fine, but the O-ring or bushing was split in several places on both sides and no longer sealing:




In an amazing stroke of luck, I happened to have some spare O-rings from the cam cover bolts and they fit in there perfectly. I lubed them up, pressed them in place with a couple of popsicle sticks, and then cleaned it all up and reassembled. Now it holds 5psi of pressure! (it might hold more, but the MAF kept popping out because it was only held in place with a cargo strap wrapped around it)


I recommend that if you're doing the top-end refresh to make sure all your plastic and rubber parts are new, these O-rings go on your list of things to replace.

Thanks Everyone!