After a Backfire, What Could be Wrong?

[Paulporsche]

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?

[Bill Verburg]

Sounds like what happens when you blow a CIS airbox

[sailchef]

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.

[sailchef]

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

Help-Violent backfire- now engine runs rough

[Wayne 962]

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

[Paulporsche]

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?

[Paulporsche]

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.

[al lkosmal]

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

[ossiblue]

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.

[Turboo934]

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.

[Paulporsche]

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.

[Wayne 962]

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

[Paulporsche]

I'll try that. Thanks, Wayne.