924Board.org

[bureau13]

I recently added a Mittelmotor programmable distributor, timed the car, used a slight modification of the S2 base map they provided and set idle and idle mixture. The car was running the best it ever has since I bought it.

Today after fixing my AC (yay!) I took it for a test drive and I noticed at part throttle I was getting some bucking. My wideband was showing it was pretty lean...15-17 or so...when this was happening. Flooring it would settle it down, air/fuel ratio would be back to normal and it would pull nicely. Idle, the mixture seemed to have leaned out a little, so I adjusted for that, but the same thing happens. My idle mixture is now a bit rich, high 12s to low 13s, but I still lean out and buck at part throttle. It's not really driveable like this at all. Plus, I'm worried it's going to damage something.

Any ideas? I don't THINK it's a timing issue, although that is the most recent major change. But it was working fine, the map hasn't changed and the AFM reading lean makes me think fuel.
_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)

[morghen]

Vacuum leak?
_________________
https://www.the924.com

[bureau13]

It's possible. Wouldn't surprise me at all, really. I previously smoke-tested it and didn't find one, but I haven't done that since this problem started (which it just did). Would a vacuum leak tend to leave WOT conditions alone?

[Fasteddie313]

[bureau13]

Interesting. This sounds pretty likely then. When it first happened, I noticed I was showing lean at idle as well, so I adjusted up for that, but it's still happening under light throttle. It fits! Just gotta find it...I guess I know what I'm doing tomorrow

[bureau13]

I found one interesting thing. The idle mixture screw on my car has no cover. With it open to the air my AFR is around 13. With a cap I had laying around pressed onto it it went down to 12-12.5. That is a bigger difference than I expected, but unfortunately it didn't fix the part throttle issue, so I'm still on the hunt. Another area I've messed with recently has been boost control lines, as I've put a Tial wastegate and a boost controller inline with the top vent, but all of those connections appear tight, and boost behavior doesn't appear to have changed.
_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)

[Fasteddie313]

[bureau13]

OK, smoke tester for the win, I guess? First...quite a lot of smoke was coming out of the idle mixture adjuster, so I've put a cap on it. Hopefully it stays there.

With that closed off, I saw smoke coming from the vicinity of the turbo. Now, I was expecting to see a problem with the lower boot, but that actually looks fine. Instead, see the video here:

https://drive.google.com/file/d/1OF41_jiWyrjGoN0itdPFtCszGeCHbDFr/view?usp=sharing

That is from the bottom, slightly in front of the turbo, looking up. From the top, no video but it's not obvious from there exactly where it's coming from either, so maybe it's on the back side. But it pretty consistently looks like the vicinity of the turbo's internal bypass valve (?) if I understand this thing correctly. If someone could view the video and tell me if I'm wrong that would be helpful. And if I'm right, how depressed should I be? In a previous thread where I was getting no boost, someone had suggested that thing was stuck open, and I think it could be serviced in situ, but I'm not sure this would be the same type of operation. As far as I know, nothing should be venting to atmosphere.
_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)

[Fasteddie313]

Well it doesn't look like it's coming from the vacuum line elbow..

If it is coming from the pop off vlave gasket that would be intake side vacuum..

Can't really think of anywhere else there it could leak other than the big comp housing ring..

Video from the top maybe?
_________________
80 Turbo - Slightly Modified

[bureau13]

Intake side vacuum would still be a problem, wouldn't it? I thought anything after the AFM would be problematic.

I'll take a video from the top tomorrow. When I looked, I couldn't really see where it was coming from but another set of eyes can only help.

[Fasteddie313]

[bureau13]

A video from the top is here:

https://drive.google.com/file/d/1Ix1eFTnCTz-B4ZVnk0zB3wPyyU0xoOeO/view?usp=sharing

It's not too clear where it's coming from, but the clearest little puff you'll see is around the 13 second mark.
_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)

[bureau13]

...and here is another vid from the top, but from the other side of the charge tube:

https://drive.google.com/file/d/13-Og-pdnMICCOrNPa35krM1ZKOREnHej/view?usp=sharing

The more I look at these, the more I don't really think it's coming from that bypass valve. Looking at pictures of the turbo by itself, it looks like the back side of the compressor has a big plate that is bolted on, maybe that's what bolts the compressor housing to the rest of the turbo? Could it be leaking from there? (Maybe that's the compressor ring that was mentioned above?) Why would that suddenly start leaking? That seems odd. Is there nothing else in the vicinity that could be the culprit? I'm not at all looking forward to taking that turbo off...
_________________
-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)

[Paul]

I'd check the charge tube O ring.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

[Fasteddie313]