Loud "Bang" on Start

[DMECMAnderson]

I've got a new symptom... My car is a 81 with fuel injection.

The weather here has started to cool at night. After leaving the car outside all day while at work, a couple of times when it was around 40 deg, when I went to start the car at night to go home (after dark), the car would crank that I'd hear a loud "bang". Could be a back fire? Sounds like it's coming from under the hood rather than the tail pipe... After the "bang" the car starts and runs fine. Have I got a big problem? or is this just one of those things you live with.

[punes]

I use to have a 76 VW Scirocco that was fuel injected and did the same thing. It sounds like an explosion under the hood but the next attempt, it starts up fine with no issues...it did that for years and only in the colder months and I never found out the cause.

Frank]
79 Fiat Spider

[So Cal Mark]

I'd investigate soon! A backfire in the intake can bend the flap in the AFM, ruining it. Check the intake boot for looseness or splits

[Danno]

a backfire in the intake would mean timing, no?

[So Cal Mark]

more likely a lean condition

[dwcars0]

Or it could be the wires on the pickup in the distributor. They get old and fray. That is what caused my backfire and a bent AMF flapper. The car will work fine until the flapper sticks and then its just like you turned the car off. Not fun.

[bandit]

Add to Mark's check list any of the smaller hoses connected to the manifold
and the clamps use a screw driver on them and see that they are snug .


Check your spark plug wires as well when they start to leak on cold humid
nights may cause misfire .


Daniel

[bradartigue]

That sounds like my old 1981 before we rebuilt the head. Its a sticking valve.

When an intake valve sticks open the explosion blows back through the plenum and the big air hose. It also opens the AFM forcibly, which can blow the little safety door off the AFM (if it has one, early ones did not) and/or bend the door to the point that the AFM is junk.

It will typically only happen when cold and the oil is sludgy. You may want to try to use a thinner oil (such as 15W40 or 10W40), assuming you aren't already. You can also try and pour some CDC valve cleaner in there and it might help.

Important thing is, if it is indeed a sticking valve, to let the engine do the warming up. Start it without opening the throttle (which BTW is how fuel injected cars should be started) and let it warm up without applying a load or opening the throttle.

Plan for the head rebuild in the short term - you can revalve a good head cheaper than you can replace an AFM.

[DMECMAnderson]

I spent some time this weekend going through the hoses. Sure enough, I had one loose from a previous repair (fuel injectors) and I had knocked off the cap that was covering the place where the automatic transmission or a/c vacuum hose would go, since I have neither, this was capped off before. I must have knocked the cap off when also when I was working the fuel injector problem...

Still in a monitor mode, but car hasn't backfired since. Had a good weekend here for driving with the top down last Sunday.

I haven't visited the web site since the new software update. I like the new look

[So Cal Mark]

that's good news!

[laura79]

It's an old post but I have a similar problem on a '85 spider azzurra with fuel injection.Sometimes when I'm starting car it doesn't turn on and do a loud bang...then i turn on again and it start fine....what can I check?spark plug have 10000km,more or less, distributor cap seems fine...it's weird and if there is someone around they look scared at my car....