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bpu699 bpu699 is offline
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Join Date: Apr 2007
Location: wisconsin
Posts: 2,539
Some more followup...

Posted this by acident in the 911 forum, rather than here. Oops.

1988 930. Some running issues as posted in another thread. Car initially ran great, but occasionally went extremely rich, and died. Randomly. Then, ran great again. Changed the ignition to MSD, seems to have helped. Car doesn't cut out and die any more.

Then, started to get really rich at idle all of a sudden. AFR slowly went to <10, and it stumbled. With the new MSD, it doesn't die anymore. Above 3000 rpm AFR's perfect, ran great. Disconnecting the 02 sensor "fixes" this, with afr's of 12.5 at idle, 14 at cruise, and drops down to 11 or so at full boost. Prior owner must have had the same issues, as the 02 was disconnected prior.

Also, fuel pumps seem louder than I remember.

I thought this was an 02 lambda issue, but folks recommended I check my fuel pressures. So, I did.

Not sure what to make of it:

FUEL PUMP DELIVERY (Reference is 1500cc/30 seconds)…… 1600cc/30 seconds if measured after 1st fuel pump. 2100cc/30 seconds at line from fuel filter to distributor (after 2 fuel pumps)


CONTROL PRESSURE FLOW (Reference 160-240cc/Minute)… 200 cc as measured at the line from the fuel distributor to the WUR.

SYSTEM PRESSURE (reference 6-6.7 BAR, or 88-99PSI): I got 103PSI, but that might be within the error limit of the gauge.

CONTROL PRESSURE COLD (reference 23 psi at 50 degrees, up to 45 PSI t 90 degrees)….I did it when it was 50 degrees outside. got 40 psi for about 3 seconds (then WUR makes a squealing sound), then it slowly climbed to 55 PSI within 10 seconds. This is with the WUR power supply disconnected, and the car ice cold)

CONTROL PRESSURE WARM (reference 55PSI )…. I got 75 PSI after about 2 minutes, and then it plateaued…

I repeated it again, the next day...

40 psi cold, 55-60 psi warm...

Ok, so what does that mean and does it explain any of my issues? Clearly the warm and cold pressures seem off? But I thought a high control pressure makes a car run lean???

Is there an issue here, or not??? Remember, with the 02 sensor disconnected the car runs PERFECTLY. Its not lean, or rich, doesn't backfire, etc. It does stumble when first turned on for about 5 seconds until the idle evens out...

Is this a fuel issue, that the 02 lamba system can't overcome? Or is it just an 02 lamda issue, and the fuel pressures are just unrelated???

I calibrated the HF gauge against 2 other gauges, and it was spot on...

As always, any help appreciated. And hopefully this helps someone else too!

Plan to check the WUR filter next, and back flush the fuel lines... Odd. Car runs GREAT with 02 sensor diconnected...

JFAIRMAN...can a malfunctioning frequency valve cause any of this? The valve is LOUD. I thought I was hearing my fule pumps or CDI buzzing in the cabin... Now I suspect its the FV... Its muh louder than the fuel pump...


Last edited by bpu699; 11-11-2014 at 12:36 PM..
Old 11-11-2014, 12:33 PM
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