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jazz guy · Joined: 26 Nov 2002 Posts: 434 Location: Colorado

I've been working on tracking down an intermittent misfire issue on my '82 931, and have more questions now then when I started. Here is the background:

The engine runs along fine, then all of a sudden it feels like a cylinder isn't firing. Sometimes it's just a liitle breakup, and other times it drops one cylinder all together. It happens more under a load; it seems to idle fine. I changed the plug wires, cap, and rotor, and no change. I thought maybe there could be some kind of fuel supply issue, bad injector, or possibly a head gasket, or...

To start narrowing things down, I hooked up my JC Whitney injection tester and got some ugly readings. The system pressure read 90psi and the control pressure read 90 psi, too. I blew out the return line, retested, and no change. This shows that the WUR is probably toast, but both numbers are high and I'm not sure what that means in my situation.

Any have ideas? Thanks for the help.

Cheers, Brian

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

Sure it's not the notorious loose DITC connection?
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Smoothie · Posted: Thu Sep 25, 2008 2:28 pm Post subject: Re: CIS Gurus?

924RACR · Posted: Thu Sep 25, 2008 10:14 pm Post subject:

With such a bad WUR (yes, it's junk, assuming as Smoothie mentioned that it is hooked up properly - but this is usually how I have seen them fail), it's quite likely that you're intermittently fouling a plug.

Wouldn't hurt to check compression and/or leakdown, just in case there's a leaking valve, but I doubt it...
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ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

I have extra WURs for sale...
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erstwhile owner of just about every 924 variant ever made

jazz guy · Joined: 26 Nov 2002 Posts: 434 Location: Colorado

Thanks for the quick replies. I did unplug, clean, and replug the ditc connections, and that seemed to make a noticeable difference. It still seems come off of full song at times, but is actually drivable now.

The idea of a plug fouling intermittently makes sense to me also. For the amount of miles on the engine, it is in amazingly good condition, but I know that one cylinder, while still just within compression specs, is on the lower end of the scale. That could explain why it would be susceptible to fouling.

As far as starting up cold, it definitely doesn't catch on the first turn of the key. It does start reliably though, usually on the 2nd try. Once it has been started in the day, it fires up quickly.

I'm pretty sure that I had the cis tester hooked up correctly, but I will double check my readings today. I know at one time, I located a helpful pic from our furry member's comprehensive pictorial library, that showed the tester connections. Thanks Smoothie!

I know where my initial confusion about the pressure readings came from; I was looking at the early 924 cis specs in the Haynes. I am still a bit confused though, because as I read the Haynes, I think it states that the high end of system pressure is 6.0 bar which is about 87psi. I'm probably okay at 90psi.

Moving forward, I'd like to understand the function of the WUR more fully. Has the engine been running in an overall lean condition because the control pressure has been high?

Cheers, Brian

ideola · Joined: 01 Oct 2004 Posts: 15550 Location: Spring Lake MI

If you have a later edition Haynes, there should be a supplement in the back that covers recent updates, and at least in mine, it has the correct specs for the 931 CIS pressures. Be sure you are checking against that and not the NA or Series I specs.
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erstwhile owner of just about every 924 variant ever made