924Board.org

[brealytrent]

Hey y'all,

After getting myself motivated, I've gotten quite a bit done on the 931 - The interior is all back together with everything in great working order including the power mirrors, wipers, windows and rear wiper (!). I also wired a speaker into the front factory location so at least there's some third dimensionality to the system besides the two behind the seats. I've also replaced the twenty year old rubber with some nice new cooper ones. They're a little on the wide side, but the deal was too good to pass up.

Anyways, to the issue I am here to ask you about with a long-winded explanation -

I'm having trouble setting the mixture on the 931 thanks to the lambda system. I'll start the car sometimes and get an extremely rough idle with a cloud of black smoke. Almost always when this is happening I'll find the frequency valve engaged. Disconnecting the lead to it will miraculously lead to a nice smooth idle. Plugging it back in while still running and it will not reengage at idle and still idle well.
Testing to see the effects of using no functioning lambda system, essentially running the car as a plain CIS system, showed how important the valve is: Without it engaged power is mostly linear but feels similar to an n/a car. The turbo spools but is not giving much additional power. My thoughts then are that when the turbo spools up the system becomes way too lean, necessitating the enriching of the air/fuel mixture with the help of the frequency valve. When plugged in, the car becomes a completely different animal. It still feels very much the same up to about 3000-3500, but that's when all hell breaks loose and the thing accelerates like a wild animal.

tl;dr What the title is addressing is how I am unable to get three things at once: 1)a nice consistent idle no matter when 2)lots of power 3)no hesitation when not on boost.

Since it is directly related to the frequency valve, I know it's either the computer, the oxygen sensor or the temp sensor. I'm looking for input from others to see if anyone else has had this issue?

Also some more recent pics of the car:







Ignore the dirty wheels/rims. Hadn't bothered to clean them from mounting yet!
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1977 Porsche 924 - sold
1980 Porsche 931 - sold
1984 Porsche 928 'S2' Euro - sold
2016 Ford Fiesta ST - sold
2009 Porsche Cayman

[Fasteddie313]

Sounds like best case scenario your mixture setting is too rich..

Find a dwell meter.
Find the diagnostic port, it's a round plug thing coming out of the firewall area under the hood at the back of the drivers fender where it meets the firewall right next to the drivers side.. You want to put your dwell meter on the black wire of the diagnostic port.

Run your car with the frequency valve operating until it's at operating temp..
Set your dwell meter to 4cyl and adjust your mixture screw to get the dwell at about 45% duty cycle on the diagnostic port.. This will also confirm your lambda system is functioning properly.

You want to check the dwell with the mixture screw hole covered, it is a vacuum leak, cover check, uncover adjust, cover check, repeat.. When your done the hole needs to stay covered.. I like to blip the throttle and let it come back to idle in between adjustments with the hole covered.

Check all over 10 times for vacuum/intake/boost leaks no matter the result you get from that..

If you do that and can't find any leaks and it's still running poorly your fuel pressures are probably out of spec or something else is wrong like bad injectors.. Time for a CIS test..
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80 Turbo - Slightly Modified

[Rasta Monsta]

Lambda computer unlikely to have the impact you are ascribing to it. The next two steps I would take:

1. Service the injectors (including cold start)
2. Smoke test the intake for vacuum leaks
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Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[brealytrent]

I tried the dwell meter as suggested, and the reading didn't adjust when mixture was changed. It stayed at 80 degrees the entire time....
_________________
1977 Porsche 924 - sold
1980 Porsche 931 - sold
1984 Porsche 928 'S2' Euro - sold
2016 Ford Fiesta ST - sold
2009 Porsche Cayman

[Fasteddie313]

sounds like WOT TPS switch stuck closed? Constant WOT enrichment via the frequency valve but not when you need it. Check your TPS microswitches for adjustment and function under your TB..

Your frequency valve should change its sound when you click the WOT switch if you're just jumping the pumps, if it doesn't change with clicking it try unplugging it and see if that makes it change sound/dwell/duty cycle..

Lambda fallback mode (no o2 sensor) is 45% IIRC, constant 80% sounds like WOT enrichment bit I've never measured WOT on a dwell, maybe I should..
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80 Turbo - Slightly Modified

[brealytrent]

Thanks FastEddie, you're suggestions set me in the right direction.

Using the dwell meter the number would sit at 43 degrees, then lean itself out to 60 degrees before jumping itself back down to 43 degrees and running horribly rich again. The WOT switch wasn't the culprit, but apparently a plugged oxygen sensor was.

Clearing the fins on the tip of the sensor and reinstalling helped end the computer system fidgeting so much with the mixture. now the dwell meter sits at 43 degrees with no issues. Driving the car now has shown a major improvement in linearity. Before there would be the sudden surge of power around 3500, and now the car is much more smooth when going on boost but pulls just as hard.

One issue just popped up now though: The odometer just decided to quit working I'm gonna try and find a shop that can fix it, as the last time I tried to open one of these I destroyed the bezel.

Again, thanks!
_________________
1977 Porsche 924 - sold
1980 Porsche 931 - sold
1984 Porsche 928 'S2' Euro - sold
2016 Ford Fiesta ST - sold
2009 Porsche Cayman

[Grenadiers]

You CAN fix this yourself~! Yes, removing the bezel is the hard part, but it's usually the little white gear that is non-operative. Check out this place:

http://www.odometergears.com/products/Porsche/924+76-88/76
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'83 944 Track car.
'88 924S Track car.
'89 944 Turbo
2004 Winnebago Vectra monster RV
2012 Jeep Wrangler
2014 Kia Soul
2001 Ford F350 powerstroke