924Board.org

[Ants]

OK so we have decided (from previous posts) that my wastegate could be replaced, (one is on order)
Would a problem here also explain why just recently my tick-over has become lumpy, and i seem to be getting more of a back fire/rumble when changing down approaching a junction for example.

Ants

_________________
1980 931 Euro spec, "you know the one with the power, without repeaters or those daft bumper extensions."

[ This Message was edited by: Ants on 2002-06-21 06:44 ]

[Smoothie]

I don't get backfires or rumbling while downshifting. I wonder if your bypass valve (aka - "pop off valve" or "anti-backfire valve") is stuck. Have you checked that? It should make a loud click (or clack) when you open and close the throttle quickly.

[Rick MacLaren]

Aha!

That's not a wastegate symptom. That's vaccuum. Ricks Maxim: Hesitation is either air, electrical or gas. (But then again, what else could it be?).

Check the rubber booty that is beneath your fuel distribution unit, above the turbo. The sucker cracks and erodes. It's rubber, and hot as hell. You'll have to remove the fuel distribution unit thus:

1. Unscrew three bolts around the FDU that hold it in place. (10 mm)
2. Take off your air box. Filter, etc. Place to one side. Don't take off the hoses, they're flexible. (all 10 mm)
3. Remove and plug the coolant line. (clip)
4. Unscrew your gas cap. Now, remove the 2 gas lines at the FDU. (17 mm I think, if not, 15 mm). Also, reach beneath the FDU and remove the clip with a screwdriver or (I think) 6 or 7 mm socket.
5. Swing the fuel distribution unit to one side. Remove hoses with wrench (15 mm? if not, 13 mm). Same with the hoses. Be careful, they're plastic and brittle. I'm replacing mine with stainless tomorrow.
6. Now, reach down way beneath where the fuel distribution unit used to attach, with a screwdriver. Take off the second clip at the turbo housing.
7. Remove yourself and have a smoke. If you and your garage haven't yet exploded, return to garage.
8. Reach down to remove the rubber booty. Have a look. Wriggle it around and look at it. Anythin' odd?

Takes 2 hours.

Hoo ha! I love archery! I'm betting this'll hit the target. Lemme know if this reveals a worn booty. Also check the top booty. The symptoms of a screwed booty will mimic a stuck wastegate, without the 'lunging', but also will add backfiring on downshift, rumbling and hesitation at load.

Cheers.

BTW, you know, vaccuum problems like this can totally destroy your engine if you put the nuts to it.


[ This Message was edited by: Rick MacLaren on 2002-06-20 04:35 ]

[Ants]

Cheers Rick i will have a look this weekend, and see if that could be it, i'll let you know.

[Rick MacLaren]

An alternate test is this, and I haven't tried it but it should work in theory:

Doncha love theory?

Ok,

1. Fill a spray bottle with gas.
2. Start your car. Ensure the rest of the place is quiet.
3. Spray the gas around your intake, just once. Feel the RPM rise?
4. Now mimic that action around any intake hoses that you think are suspect. BTW, this is nearly impossible for this little booty I've outlined above. It is too far down beneath the FDU to be safely accessible with a spray bottle.
5. If car RPM rises, and you don't explode, you've got a vaccuum leak. Alternatively, if you explode, you probably sprayed gas on your exhaust manifold, in which case you deserve what you get.





[ This Message was edited by: Rick MacLaren on 2002-06-20 04:39 ]

[Smoothie]

Good catch Rick. Glad you finally found your gremlin. I haven't seen that one on my car yet - guess I should expect it within about the next 2 years.

Ants - There's got to be a reason why the bypass valve is sometimes called the anti-backfire valve. It's real easy, so definitely check that before you go disassembling things...

[Rick MacLaren]

Yeah that valve can also cause problems. How can you assess that Smoothie?

[Smoothie]

It can be done right from the drivers seat, but to hear it best open the hood ("bonnet" for those with carriageways) and rev the engine right at the throttle body and release it. You'll hear the click/clack with each sharp turn and release of the throttle and it will come from the area of the turbocharger - obviously...

[Rick MacLaren]

By the way, that rubber booty should have been replaced at engine rebuild time. Same with ALL the hoses, not just the ones that looked worn out.

I know that click clack, as I've heard it on a pal's 944 turbo. Haven't heard it on mine yet.

Notably, with the booty problem, one symptom that you can use your ears to detect is that the turbo spool up is a tad louder than usual. Makes sense too, as the sound can carry out through the booty easier with a hole in it.

[Ants]

Thanks again

Thanks all the same but i don't fancy spraying fuel anywhere near my engine, i just don't trust myself

BTW is it easy to remove the front valance, i'm gonna get some ramps (last time i tried i scratched the valance on the ramp gradient) it would be a good time to clean my oil cooler too

Cheers

Ants

P.S. my wastegate just arrived

[Ants]

Ok new wastegate looks pretty good.
Now forgive the idiot talk but i don't know the technical terms for these things (i'm a scientist not a engineer dammit )

Should i be able the push the plunger thing manually ? the plunger chamber (with the air cooling fins) looks a little rusty but not bad (i'll clean that up). Inside the chamber its black with deposits inside but again not too bad.
How can i clean this out??
Attached to the plunger chamber is a larger silver thing about the size of a large fist with Porsche written on it and a kind of nut with a hat ontop of it Whats this??

On the whole it looks pretty good

[Smoothie]

Do you have it apart so you can see the spring and diaphram? It would be good to have a look at these and make sure the diaphram is in good shape - there'll be a fold around the circumference of the diaphram. They wear and eventually tear at that fold, so check that area.

[Ants]

So do i take those 2 sections apart from each pther, the silver cylinder and the air finned plunger chamber??

[Smoothie]

Someone else should jump in here - it's been a long time since I had mine opened up and don't recall every detail about it... From memory, there were about 6 or 8 bolts around the circumference that needed to be removed and the spring is in there, so you have to hold it together as you take out the last bolts or it will jump apart on you. Be careful with it.

[Smoothie]

The more I think of it, the more unsure I am of the internal construction of the wastegate. I'd like to do a drawing of it - or if you can post pictures, could you do one? If not, and you wouldn't mind, describe the internals, then I'll do a drawing and post it, then you could check it for correctness, then I'll redraw, you look, etc. etc., until it's correct.

The piece on the top of 924T wastegates is a vent. On some other cars this can be used to attach a second control line that counteracts the first control line to increase boost. That was discussed in an earlier post last year.