jimq

my old 2.7 911S sounded like that when it had a bad cold start valve. I have no idea if a 964 has one tho.

Rocket Rob

I would double check your vacuum hoses. I had a VW bug with dual carbs and it would pop like that when it was cold (no chokes). The issue was the mixture was excessively lean which is slower burning. The flame within the cylinders is still burning when the intake valves open.

Another theory - Does it backfire when the engine is hot and cold or just cold? Could it be a bad cylinder head temp sensor? Do you have access to a scantool or hammer?

Indycam

Arcing in the cap sending spark to the wrong spark plug at the wrong time ?

altarchsa

Yea, what this guy said. (I think Indycam is one of the most knowledgable, logical and helpful contributors on here.)

Hope it's not a sticky or broken valve part. I'd do a compression test before driving it again, or it could look like mine in the pic below.

However,........ Other than the really bad backfire, it sounds exactly like mine did a couple of weeks ago after an engine rebuild and a cleaning of the intake system. But mine had only a mild and less frequent backfire. I cleaned the ISV thoroughly (for a third time) and removed / carefully reinstalled the intake. It ran beautifully on first startup. I think the problem was an intake leak at the breather hose. So, if above doesn't check out, don't forget the possibility of an intake leak. Those intake connectors and large hose connections are pretty easy to check.

3blackbirds

Thanks for all your help so far guys. I'm going to dive into it this afternoon. Your info will help me in what to look for. I hope I can get this worked out by our club's first autocross on the 18th in Hershey. If not, hopefully it will be ok for the swap meet. My wife and I will be at the entrance gate 9am-1pm on Saturday.

I'll let you know if I find anything...

Indycam

Broken distributor belt ?
Sending spark to one cylinder over and over and over ...

StanUK951

Hi, I had an intermittent misfire on my car for months which, after replacing coils, dist caps, rotor arms, etc. turned out to be either a dodgy o2 sensor or the cylinder head temp sensor. Not sure which as the garage replaced both at the same time but o2 sensor was most likely suspect. Just another couple of checks you can make... HTH.

3blackbirds

After checking all the things that could have been, those were fine. The intake manifold came loose only on the right bank. I put it back on and purrs again....

Thanks for all the help!

ian89C4

Glad it was an easy fix!!!! Congrats.

3blackbirds

Thanks. I ran through all the plugs and wires. Pulled the intake valve covers off to check the rockers and valves. I got it to hold a good idle but couldn't build the revs from the throttle body. Listened a little closer and the intake manifold wasn't seated right. I'm really not sure how that happened...but it's working fine now.

Indycam

A backfire will pop off the intake system .

3blackbirds

I understand that it would pop off the manifold from the pressure coming back up. I don't understand what caused it do that in the first place. Unless I missed something? Was it an engine hick-up? Once I fixed the manifold, it ran perfectly. Could something else cause it in the future?

Indycam

A tiny little bit that gets between the valve and seat and holds the valve off of the seat .
If it never happens again , alls good .