After a Backfire, What Could be Wrong?
 

During an incorrect WUR testing procedure, I heard an extremely loud "SNAP" sound coming from the engine area. After that, the engine wouldn't start. Previously, the engine started, but ran w/ rich symptoms because the WUR wouldn't "warm up"/raise the CPs.

The likeliest scenario seems to be that I blew the airbox, as I had tried to start the engine w/ a CP that was now set too high for the temps . Even though I had a functioning popoff valve, the backfire apparently was too extreme.

Since I won't be able to do a partial engine drop to check the box until next week, I have several questions in the meantime:

1. What else can I check? The sensor plate and the rubber boot above seem fine, and I've felt aroud the box, components and hoses.

2. Could there be internal damage to the engine, say a broken valve spring or other component, that is causing the engine to fail to start?

3. Could the attempts @ restarting cause a hydro lock? Over time, will any excess fuel drain from the cylinders?

Sounds like what happens when you blow a CIS airbox

From my exoeriences I would double check around the pop off seal first. If its the toilet seat type especially. Mine had a hairline crack in the sealant holding it in (caused by a very loud backfire). It seemed ok and felt very secure but was sucking just enough air to screw things up.

Violent backfire
 

sorry for the repost but I couldn't find this earlier

http://forums.pelicanparts.com/porsche-911-technical-forum/434636-help-violent-backfire-now-engine-runs-rough.html?highlight=sailchef

99.9% - there's something that got blown off that you just can't see. When you find it you will say "oh, that's what it is". It's probably creating a huge vacuum leak in there. Keep looking around. With a popoff valve installed, it's unlikely that your box blew up, but you probably blew off a hose or something...

-Wayne

I just went out to check for hoses, etc. Still couldn't find anything, and all the airbox screws look tight. The popoff valve looks really secure too. I'm going in w/ a flashlight tomorrow and check further. Maybe there's a hairline crack somewhere I'm just not seeing yet. And I think I'll try sealing around the popoff valve just in case.

BTW what about my other questions:

Any chance there's an internal engine issue? A bent valve or broken spring?

What about hydraulic locking from excess fuel? Will the excess fuel eventually leak past the cylinders into the oil?

Thanks to fireant for posting the MM (I know) video in the "Air box help" thread.

Just a bump to see if there are any comments re internal engine damage and hydro locking.

recheck your popoff valve. Even if the bonder is still ok, make sure that the lead is closing all the way and that the o-ring seal under the lid is present and working.

regards,

al

Paul,

This may sound obvious, but don't forget to get a mirror and look in back (actually front) of the engine--between the firewall and the CIS. There are vacuum hoses there that could be cracked or blown off. Also, are you certain you are getting spark? I ask because even with a blown box, you should be getting some brief running from the cold start injector--unless that has been disabled also.

I had a metering plate get bent once even with the pop off valve in place. I've also seen them blow a hole on the bottom side of the air box. Good luck.

Thanks, guys. I noticed Davyboy removed/replaced his airbox w/ the engine in place. I'm going to try that first. If it gets too tricky, I'll do a partial engine drop.

You can remove one of the vacuum hoses and pull a vacuum on it (even by sucking). If there is a leak, it won't hold vacuum...

-Wayne

I'll try that. Thanks, Wayne.