924Board.org

jpab924 · Posted: Wed Jul 14, 2004 5:34 am Post subject: oh oh, wheres my boost?

Installed a vdo vac/boost guage I pilfered from my bae car. Brought the plumbing and guage home and installed it into the brake booster line. The check valve for the booster is in the booster and not in the line, so the guage plumbing is before the check valve.

Its reading vac. at a steady 20-22" at idle and no boost whatsoever under load...0...nada. Vacuum does drop down under acceleration. Where should I start to look? It should be hooked up correctly if its reading vac.

I mentioned in a prev. post that the car has a slight miss/bog/hesitation under sudden accel. and clears up at 3-3500 rpm, which I thought at the time was the turbo kicking in....maybe not. Fuel mix is cw rich and ccw lean correct? I did turn it in cw and it ran a little smoother at sudden acceleration, but this shouldn`t have a bearing on boost pressure...Timing was set at factory specs (21 Deg) at 2k rpm. But was done before I turned in the fuel mix.

Any thoughts? Thanks....JP

emoore924 · Joined: 13 Apr 2004 Posts: 2822

Assuming you want to see what the turbo is doing, wouldn't you want to hook the sensor up to the manifold somewhere. There is no boost in the brake system for your gauge to measure (you'd be pressurizing your brake boost system if you did, think about it...)

I would hook into the manifold just before the tb. I can send you a pic of mine if you want. Just PM me.

Smoothie · Posted: Wed Jul 14, 2004 7:12 am Post subject:

Yes, cw to richen, ccw to lean.

Do you hear the turbo spinning? - kind of like a little jet engine sound..

Apparently you do have the gauge hooked up right - if on the wrong side of a check valve at the brake booster, you wouldn't (at least shouldn't) see vacuum drop much on accelleration. Take a look around for any other check valves that might have been added between the manifold and your gauge connection point.
Here's where mine's hooked up (the thin plastic line into the metal "T") -
This location shows both vacuum and boost.
<click>

Either the Pop-Off valve (aka recirc valve) on the turbo or the wastegate may be stuck open... Hearing a click when you blip the throttle? (a click would indicate that the pop-off valve is not stuck).
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

jpab924 · Posted: Wed Jul 14, 2004 8:46 am Post subject:

Emoore, I followed smoothies advice per a previous post and tied it into the brake booster line. It was hooked up like this on my bae parts car. I even took the vacuum splitter (common part on 78 924s) with the 2 take-offs that splices between the booster and manifold. And hooked it up exactly like that system was.

Smoothie, in your pic, the plastic vacuum take-off (where you have the vac. line going to the tee) was not present in my car. That is where the one off of the 78 with the 2 vac take-offs came in. It goes directly from the vac. tee to the guage and the other one is capped off. You have it split 2 ways. Apparently you had to get a tee to supply vacuum to another source that was hooked up at that point before right?

Ok, anyways I have had this rattle under the car since I got it. I figured it was a heatshield or something in the exhaust. I couldn`t get a jack under the front end for a while since the trans was out for over 3 weeks and the ass end of the car was way up in the air. Well I just took a looky underneath and the wastegate is the cause of the rattle. (rather loud listening to it with the stethiscope)

So apparently it seems I may have an issue with the waste gate. So if the gate is stuck open, There will be no boost....hmmm

Smoothie, I did recheck for any added check valves...none present. My brake booster check valve is like yours in the picture. On my 78s` it is located in the line. Also pressurized the line to the guage and it does read boost. I also don`t hear the turbo spinning like you mention smoothie....no jet engine sound.

Is the wastegate something I can re-build? What parts are necessary, and what should I expect to encounter?

Smoothie · Posted: Wed Jul 14, 2004 10:18 am Post subject:

Yes, I do have a second T there - a plastic one with cloth braided lines just to the right of where the turbo gauge is T'd in. That second one goes to a pressure switch for my homemade water injection - at something like 5 psi this pressure switch closes and water gets sprayed into the intake. That's a story for another time though.

As far as rebuilding the wastegate... Mine has failed twice over the years in the opposite way yours has (if it has)(and there's a good chance it has)(You did verify the click sound to be sure the recirc. valve isn't stuck, right?) Anyway, the wastegate in my case developed tears in its' diaphragm and that resulted in the wastegate staying closed all the time which resulted in boost continuing to build until the overboost cutoff switch was activated and shut down the fuel pump. In your case, with no boost, the wastegate would be stuck open instead. That could be caused either by the valve (which looks very similar to a regular engine intake or exhaust valve) being frozen stuck in the opened position or the big spring in there (whos' job it is to close the valve) is broken. I don't know how common it is for that spring to break, but if broken it would explain both the rattling sound and the lack of boost pressure. In my case, I just had to replace the diaphragm both times. To determine if your diaphragm is torn, it makes sense that you could pressurize the control line to the wastegate and listen carefully - if air can be heard leaking at the wastegate you need a new diaphragm. (Or simply look it over if/when the wastegate's disassembled.) Obviously need a new spring if that's broken.. As far as the valve - like I said it's similar to a regular intake/exhaust valve with a valve guide and seating surfaces. I'd imagine any place that rebuilds cylinder heads could rebuild the valve part of the wastegate, but wait around for more opinions on that. Someone else has probably had it done.

One other possible scenario that might cause a rattle and leave the valve opened if the spring isn't broken - the end of the valve stem has a threaded hole for a screw that holds the valve to 2 large metal disks that reside on top and bottom of the center-end of the diaphram (in other words, a screw and 2 disks hold the diaphragm to the end of the valve stem). The spring pushes on the inner disk to shut the valve. If that screw's backed itself out and fallen off, the valve won't get pulled shut by the spring and there'll be a few parts loose to rattle around in there (namely the screw, disks, and the valve as well).

I hope the turbo is good (spinning - not frozen). My guess is that it is good and the poor running you're experiencing up to 3500 rpm is a result of the intake airflow being quite restricted, then the turbo starts spinning, gradually driven by the intake airflow (if the wastegate were closed as it should be, of course the exhaust would be driving the turbo). Once the turbo gets spinning sufficiently fast, the intake restriction is lessened and you're running better... How's that for a scenario?

If it were frozen, you'd have real pathetic performance all the time.
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

jpab924 · Posted: Wed Jul 14, 2004 1:25 pm Post subject:

Ok, so basically I have the guage hooked up correctly. (I`ve stared at that pic you submitted a couple of times on other threads, and wondered where the hell that other vacuum port went) Water injection....yeah I remember reading a thread a year back or so when you guys were talking about it.

I`m convinced by the rattle that there is something wrong inside of the wastegate. Now as far as pulling it apart, can it be done on the car. It doesn`t look like it will be much fun to pull out and repair. Can I access the internals without pulling the whole unit out? Just for a quick peek to see what is broken?.....JP

jpab924 · Posted: Wed Jul 14, 2004 1:36 pm Post subject:

Smoothie · Posted: Wed Jul 14, 2004 2:21 pm Post subject:

Sorry, but the wastegate has to be removed to get a look inside. There are 6 or 8 bolts around the perimeter that have to be removed and when the last bolt comes off, the thing goes "BOING!"

and the top housing flies up about as high as a two story house propelled by the giant spring inside.
Just kidding. That only happens if you do it wrong. The right way is to have it on the ground, kneel in front of it and hold the top down with one hand while removing the bolts with the other. Seriously, keep it under control as you remove those last bolts.

The recirculation valve works independently of the wastegate. It's controlled by a large vacuum line from the bottom of intake manifold runner #1. When the throttle is closed and vacuum is high, the recirc valve gets opened. When the throttle's opened vacuum drops and the spring inside the recirc valve closes it. Either the opening, the closing or both make a loud click sound that I can usually hear from inside the car. Pop the hood and operate the throttle from there to hear it better. It's an integral part of and cast into the compressor housing of the turbo - easily visible on the front of the turbo.
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

Smoothie · Posted: Wed Jul 14, 2004 2:34 pm Post subject:

Make sure the recirculation valve is operating before you go and take the wastegate apart because if the recirc valve is stuck opened, that will leave you with zero boost too. Only thing I'm not sure of is if you should be hearing the little jet engine sound from the turbo if the recirc valve is open. In any case you should be hearing the recirc valve clicking on sharp throttle openings/closings.
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

Lizard · Posted: Wed Jul 14, 2004 3:28 pm Post subject:

what I recommend that you di is to drop the exhaust off the turbo (you will need to do this ANYWAY to get the wastegate out) once you have this out the try to spin the turbo by hand if it spins freely can you wiggle the shaft left right up down at all if yes then you need to replace the turbo or have it professionally rebuilt, if no but it spins smoothly then your wastegate could be to blaim but I dont see exactly how it could be causing no boost at all, I would recommend testing a boost only guage between the throttlebody and turbo intake compressor and see what you get.
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3 928s,

emoore924 · Joined: 13 Apr 2004 Posts: 2822

10-4 on the vac line tap clarification. Never thought of it. Thanks.

-nick · Joined: 16 Nov 2002 Posts: 2699 Location: Cambridge, MA

The exhaust manifold-to-wastegate J-pipe has problems breaking around the top joint. If it's bad enough you won't get much/any boost (it would be just like having the wastegate stuck open). I would check that out first. I suppose the question is- are you hearing a lot of exhaust noise?

-nick

jpab924 · Posted: Thu Jul 15, 2004 3:21 am Post subject:

No nick, no exhaust leaks.

Ok, I poked around looking for the recirc. valve and think I know where it is. There is a hard line from the #1 intake runner (underneath) that goes under the alt. under the t-belt to a hose that connects to the air intake side of the turbo unit and looks like it has 4 bolts on each side holding down two covers. Is this it? The only other lines I see are on the midsection of the turbo which are oil feed and return lines.

Front end is up in the air, and I don`t hear any clicky when quickly accelerating. Just alot of rattling from the wastegate pipe/wastegate.

I need to drop the front suspension/ crossmember to do some repairs anyways. It should make it easier to reach some of those hard to reach nuts and bolts.

Is the wastegate unit difficult to remove after dropping the exhaust?
Are gaskets and parts hard to find from on-line vendors ? I can only imagine.....JP

Smoothie · Posted: Thu Jul 15, 2004 3:45 am Post subject:

Yes, you found the recirc/pop-off valve. It's possible to check it out and fix without removing the turbo. Find "Pop-off Valve, Removal and Installation" under http://www.924.org/techsection/technical.htm for some detailed instructions by Zuffen.

It's been a while since I last removed the wastegate and don't remember details of it, but from what I recall it was just tight quarters. Needed a socket wrench with multiple wobble extensions and a u-joint for some nuts/bolts, a combo wrench for others. I've a vague recollection of disconnecting the wastegate control line at the wastegate - that was a bit tedious.
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

-nick · Joined: 16 Nov 2002 Posts: 2699 Location: Cambridge, MA

The rattling around the wastegate has me curious. Has anyone ever seen a broken wastegate spring?? That's the only way I can imagine the wastegate popping open easily enough to keep the turbo from boosting. If this is the case, then I'm not sure the pop-off valve would clack or not when you blip the throttle.

-nick