Intake Backfiring... Bad AFM?
 

Hello, on my stock 86 base, there has been a bit of backfiring through the intake lately... It usually only happens when turning the car on however... After turning the key, you will hear a pop and the car dies. After looking under the hood, you see that the AFM has literrally turned 90 degrees (like the backfire was strong enough to shoot it out of the intake piping, but the AFM was screwed in so it wouldnt go anywhere) and alot of the intake pipings are blown off and need to be reconnected (esp. the vaccuum line for the BAC solenoid.)

When I got the car, the little black plastic cover that guards the internals for the AFM was missing, so I figure that maybe the previous owner was screwing w/ the AFM and that caused some problems. So, I tried adjusting the AFM and that, although making the idle very high (1.5-3k), made the car driveable... The car has descent power.

Checked for vaccum leaks using carb cleaner but didnt find anything significant. Still, I removed the UIM and replaced the vac lines... Still the same issue...

I bought a used AFM off Fleabay, but the car wont run with it at all (just turns on and dies as if the AFM werent plugged in) so I figure that this new AFM is bad...

As for driveability issues, I noticed that after driving for 30-60min, the car will go into a "limp mode" where it wont seem to rev over 2200 rpm... Noticed that sometimes when that happens if I disconnect/reconnect the TPS the car will drive ok for a little bit... But will take a shit shortly there after, unless I allow the car to cool down again (which is weird b/c the car is only running at 1/4 temp since switching to e-fan... and before efan it ran alittle above 1/4...)

Also, when I took off the UIM, I saw that the coolant hose coming from the back of the throttle body was corroded causing so much blockage that the coolant wouldnt flow through it. So, I changed the hose and cleared the blockage (which had accrued both on the back of the TB and on the block.) Now, the coolant hose doesnt swell up and look like its going to explode anymore, but the BAC doesnt seem to work (I unplugged it while the car was running and it didnt make one difference in idle rpm.)

I bought the block off plates and they should be here soon, but besides this I am pretty stumped...

Sorry for the long text... Thanks for any help...

unlugging the bac and re-plugging it can short it out.... but that wouldn't cause backfiring in the manifold, i would also check the maf voltage just as prelim. but i think u have a leak between upper and lower manifolds, at the block, or at the tb... and i would also check them in that order

also check timing

Do you guys think my cats are clogged? Time to get exhaust...

Didnt know that could short it out... It says to do that in the FSM and watch the rpm, if the rpm doesnt change when removed then to rpl the BAC...

but the main problem is backfiring in your manifold... ur car won't blow up w/out a bac!!!!!!!

Check the compression. It is nearly impossible for a rotary engine to backfire with good seals.

Dont have a compression tester so I do a "poor man's" comp. test and it seems solid. Plus, the car drives good and turns right on... it just has a messed up idle... :wallbash:

Check timing, thats my first thought on the intake backfiring (its still weird, though.)
Also, does the s4 even have limp mode? S4's have a VERY basic ECU.

Problem solved... Turned out to be a really weird electrical issue...

Ok... So, once more I tried checking for vac leaks... None...

Adjusted tps and now it is smoother at a low rpm....

Then I adjusted the AFM and now it can stay at 800rpm w/o being choppy... But still wouldnt idle.

Then since, the FSM says that a poor idle could be caused by electrical issues, I changed the battery cables and terminals... Turns out that the cables were green on the insides... Now the car idles smoothly at 800rpm although I do need to change the alt/water pump belt b/c it looks worn also... (And it is wierd that electrical could have been a problem when it didnt look weird on the voltmeter... Maybe it isnt working properly...)

As for the backfiring (Dont know if its an actual backfire as it only seems to happen when I am in the car, but I assume it is b/c it is blowing off some of the vac lines...)
Since doing the earlier mentioned things, I have heard it backfire once, but it didnt
blow off any of the vac lines (is it getting better?)

Long story short: Its idling at 800rpm now... :)