[Alfa_Todd]

I’ve been trying to figure out what has changed to cause a popping / backfire to happen above 4k RPM when I take my foot off the gas. It doesn't happen below 4K. I looked on the board for some information and have come across some threads that say the popping sound is normal however this is an increase. I basically haven't had, or noticed, the popping...if it was there in the first place then it has increased dramatically. This is what I have done to try to resolve the issue however it's still making popping / backfire:

I noticed that one of the valve cover washers had deteriorated so I replaced all 4 of them as well as the valve cover gasket. I've noticed that I've got some oil leaking still however I don't believe that this is the cause of my issue. (Please correct me if I'm wrong!)

I then changed the plugs from Bosch +2 to Bosch +2. I didn't know how old they were so I decided to change them. Didn't fix the issue.

I then noticed that the spark plug wires were cracked and replaced them as well. The popping sound still prevails.


So I offer this question up to the board...where do I go from here? Timing off? Valve adjustment? maybe a valve seal is going? leak down test?

It's beginning to get out of my comfort zone, I'll tear almost anything apart and put it back together however messing with the engine is a bit different for me. I'd be able to do a leak down or a compression test however the others just aren't my cup of tea

 

[jclark]

Just what sort of Alfa do you have?

 

[Tifosi]

... and do you have any hariline leaks in the exhaust?

 

[Alfa_Todd]

I'm sorry, I've got an 88 spider.

Not sure about the exhaust I haven't checked it. I'm assuming that you mean just the manifold?

Thanks guys

 

[Koop]

I agree with the notion of an exhaust leak. When you close the throttle at that rpm, the mixture will go rich. There isn't enough oxygen for complete combustion so superheated hydrocarbon is introduced into the exhaust system. If it encounters free oxygen in the exhaust pipe it will combust immediately and make a "pop".

 

[Alfa_Todd]

ok, so off to look at the exhaust manifold. I'll make sure that the bolts are snug and then see if I can spot any cracks.

 

[Tifosi]

Manifold (*loose nuts, burnt gaskets or cracks, especially where the Y portion comes together into one pipe), any joints, (flanged gasket and/or lead dougnut), muffler holes, etc.

Right from one end to the other, though once you get past the center muffler, it's not nearly as likely to backplop as the fumes are pretty well dissapated by that stage.


*#4 at the head is kinda a common spot for the nuts to loosen a bit. Once they do (anywhere) the gasket starts to get blowby and eventually gets a hole cooked through it.

14-16ft/lbs on them. You're not likely to get a torque wrench in there, (no 'likely' about it, you won't get one in there, period), but the point is don't reef them too tight or you'll potentially rip the stud(s) out of the head, and then you will have problems.

 

[Elio Comello]

One other possibility is a belows leak in the Manifold Pressure Sensor.

It is located on the side wall behind the passenger seat (looks like a silver mushroom).

The S3 has no vacuum advance mechanism. The spark advance is set by the ECU from Manifold Vaccuum. It is common for the bellows to fatigue and leak, usually this is manifested by a "whistle" from back there that varies with manifold vacuum.

You can easily check if you have an MAP leak by isolating the small hose on the Intake plenum, that goes back to the MAP sensor. Remove it and give it a gentle suck. If you can draw air, this may be your problem. Toking your foot off the gas is causing a large change in Manifold Vacuum and hence an improper spark timing and or "popping".

Best regards, Elio

 

[Tifosi]

That hose would be the single one under the brake vac hose to the rear cylinder head side of the plenum (not one of the 3 up toward the front head side of the plenum) that heads toward the firewall then through it.

It may or may not transition to a semi-clear plastic line before passing through the firewall.

 

[Alfa_Todd]

Thanks guys!

I did notice that the two bolts on the #4 runner were a bit loose so I tightened them up a bit. I then inspected the manifold to cat and those bolts were so loose that I could move them with my fingers! I tightened them up a bit and took her out for a drive. The popping / crackeling /backfire noise is still there, just not as loud...which makes sence to me. I'm still going to assume that the issue lies in the exhaust manifold...if I get some time tomorrow I'll take the manifold off and inspect it a bit more closely....after I check out the manifold pressure sensor.

Thanks so much!

 

[101/105guy]

If the nuts were that loose, you may have fried an exhaust gasket. Tightening them now may not seal the gap.