'78 911SC: Low Idle "Bounce" & Backfiring has "magically" cleared (?)
 

A month ago, I fired up the lump (cold) and it went "POW!" similar to that of an Anchor Brand 1 1/2" firecracker, (you remember those: "Do not hold in hand -- Lay on ground, Light fuse, Get away quickly).

For the last four weeks, after starting, the idle would fall from its cold 1800rpm level to its "normal" 900rpm, but then would strangle down to 400rpm, only to bounce back up to 900rpm repeated. Tapping the throttle during this bit would emit backfires, (not as loud as that first one, but I could literally "feel" them through my shoe on the pedal).

Now I've just come in today from a 15-minute drive, and the condition has cleared, no idle bounce, (steady again at 900rpm), nor backfiring whatsoever.

Of course, I'm expectant that whatever was wrong may recur: Any ideas of what's going on would be appreciated :) Something electrical perhaps, (plug cable?), or a temporary "clog" in the fuel injectors?

Best to the "group".

Classic symptom of your oxygen sensor relay going bad. Remove your oxygen sensor relay below your passenger seat. Try to start your car, if the problem returns replace your oxygen sensor relay (<$20.00). Good luck...

Thank you Everforward. Will investigate. Cheers :)

you dont have an O2

do you have a pop off valve in the air box?

usually the surging up and down means its too rich. but with the loud backfire IF you have a pop off valve and since it seems to have cured itself, the pop off valve may have not seated properly.

no pop off valve, you may have cracked the air box. but for wom reason it has stopeed leaking and is running fine now. if so, dont worry, it will run bad again.

the thermo time valve (TTV). if that stopped working it would go very rich and surge like it has been doing. loss of power to the TTV and warm up regulator(WUR) will make it surge.

tapping the throttle and backfiring is lean, which goes back to an air leak.

one more option.
sitting may have allowed the plunger in the fuel dist to get stuck. this would make it run lean and pop, then it could free up and run fine.

Also... temporarily clogged return line ?

You blew an organic carbon life(less) form out the tailpipe thanks to hard driving.

Thank you T77911S, prebordao and Jdub.

Correct, there is no pop-off value, however this is all revealing info from you expects towards resolving the issue. (And Jdub likely nailed it the best). Cheers Mates!

you need to put in a pop off valve

You need a pop off valve, and maybe a new airbox (check it for cracks, your backfire probably cracked it)

my 80 did almost the same thing .. wound up being the connector to the O2 sensor ... found out after i failed smog and after i paid 75 bucks for a new O2 sensor .. fixed with a 10 cent spade connector

up until 80, they did NOT have an O2. so he does not have one. this is why i think the 78-79CIS was the best one ever made.

i fugure he has blown his air box too, just waiting to see if he comes back with its not running right

~~UPDATE~~ Pricey but a great ending, (estimate $1,700, final $2,016 including oil service and a few other things). As was pointed out, the airbox indeed did crack and was thus replaced, with the addition of a pop-off valve. But there was a fabulous cherry on top as a by-product to having this done: the oil-return line that is integrated in there above the engine was cracked and split, which explains why I've been suffering with what we thought were multiple oil leaks, (and thus the addition of a fire extinguisher between the front seats -- just in case). Now, since completing the R/R for the airbox, new oil-return line, new oil sensor switch and a few other benies from my benevolent mechanic, I've put about 60 miles on the car with zero sign of smoke nor stench of burning oil. Sometimes in life, ya get lucky :)

Thanks to all of you who took interest in this issue (!)

expensive! but glad to hear you have a final fix. that always feels good to have that load off.