Back Fire at Start up

[arbita1]

I haven't driven my 911 in about 3 weeks. Yesterday when starting it up, for the first time ever there was a huge back fire (I think from the air box). When I opened up the hood to see if there was any damaged to the air box, the only thing I noticed was one of the air box straps was popped off. I have a pop-off valve installed in the air box.

After that it started right up and idled/drove fine. I did notice that the engine temp was higher than normal, but it was a hot day yesterday.

I searched, but haven't really found my specific scenario. I know that this is a problem that happens sometimes, hence the reason for the pop-off valve. What causes this? And is there a problem I should be looking for?

[KNS]

When your car is warmed up and at idle do the RPMs drop when you take the oil cap off?

[arbita1]

Originally Posted by KNS

When your car is warmed up and at idle do the RPMs drop when you take the oil cap off?

Yes. The last time I topped off the oil a few weeks ago it did.

[ossiblue]

Originally Posted by arbita1

I haven't driven my 911 in about 3 weeks. Yesterday when starting it up, for the first time ever there was a huge back fire (I think from the air box). When I opened up the hood to see if there was any damaged to the air box, the only thing I noticed was one of the air box straps was popped off. I have a pop-off valve installed in the air box.

After that it started right up and idled/drove fine. I did notice that the engine temp was higher than normal, but it was a hot day yesterday.

I searched, but haven't really found my specific scenario. I know that this is a problem that happens sometimes, hence the reason for the pop-off valve. What causes this? And is there a problem I should be looking for?

Generally, backfire through the intake is the result of an overly lean fuel mixture. In your case, having sat for three weeks may have allowed the fuel to completely seep from the injection system--possibly due to a minor leak in the check valve or fuel accumulator. Upon starting, the cold start valve injected fuel, the engine fired, but the pressure/volume of fuel had not built up enough to sustain a high cold start idle and the backfire occurred. Couple this with valves that may need adjusting or a small air leak in one of the many hoses/connections in your CIS system, and the likelihood of a backfire increases.

Naturally the above is just a guess, but it shows how these things can happen. The fact that your car idles correctly and runs correctly is a good sign that no damage was done by the backfire. It might be wise to inspect closely for air leaks and test your residual pressure.

[80spitfire]

I agree it sounds very much like it could be an intake leak. One of my 914s had a cracked carb boot and it would backfire almost every time you'd shut it off. I had to wait like a month for the boots to finally not be back ordered anymore and when I put it back together I found out that back fire ended up bending one of the push rods all out of whack. Definitely don't run it very much if the problem persists or you may get to put in a new push rod like me lol