Popping sound low revs

[180sx_vc]

hey guys, i have this popping sound when i accelerate and let go of the gas to make a gear change and also makes a popping sound when i decelerate. It only happens anywhere below 2800rpm. car is pretty much STOCK. Also here in vancouver, BC we have aircare and i failed just because of high NOx but this was when i had my timing advanced. Could both of this be related? lean mixture? vac leak?

MY CAR:
-clean/ tested injectors(370cc)
-new spark plugs
-new bosch o2 sensor
-coilpacks are good
-tps is good but has a slight stumble when u open the throttle too fast
-maf works
-timing is set to 10 degrees(stock)
-my secondary butterflies is T'd to the fpr
-high flow cat
-dual xspower exhaust
-stock sr20 smic and piping(ghetto rig)
The car runs awesome but since i drive alot in the city, im in the 2000-3200rpm range most of the time and the popping sound does worry me.

[boost_boy]

You either have some bad gas, water in your gas or the worst, leaking valves which means your valves are not seating correctly. Start with some fresh high octane fuel and maybe some sort of fuel system cleaner as it could be carbon on your valves that causing a few of them not to seal properly.

[TheMAN]

fuel treatments isn't going to do too much to clean out heavy carbon build up... the only good way is an induction service... GM top engine cleaner or seafoam is the way to go
major problem spot are the butterfly valves, because they are mostly shut during cruising

[180sx_vc]

Ive been running only 91 octane ever since i finished the swap, which was 1-2 months ago. Today i was driving and realized that the exhaust sound it makes isnt loud backfire or a pop .....its more like a gurgle sound(not loud). Also i took my car for aircare again with the timing set to 10 degrees. I still failed because of NOx, but the previous test my NOx reading was 1800 and max to pass is 1124. Now the NOx reading is 1418. Supposedly a lean mixture can cause high NOx, O2 sensor, a CAT, egr and carbon build up...i don't have a wideband so i can't really know if my mixture is wrong. I have a new O2 sensor, cat is good, no egr. For carbon deposit ill use some seafoam..

[180sx_vc]

fuel treatments isn't going to do too much to clean out heavy carbon build up... the only good way is an induction service... GM top engine cleaner or seafoam is the way to go
major problem spot are the butterfly valves, because they are mostly shut during cruising

Wouldn't the vacuum from the fpr close the butterfly valves?

[TheMAN]

exactly my point
when the butterfly valves are closed, fuel vapors pool behind the valve and eventually turns into carbon
unless you drove the piss out of the car all the time, there's little chance of the crap getting blown out from mundane driving

the only proof I needed to see was how much carbon there was in the butterflies when I took off the intake


anyway, what kind of high flow cat are you using? many high flow cats don't do much catalyzing at all and high NOx results, acting as if a cat is bad... worse yet, your typical cheap high flow cat don't last long at all

also, I hope you have the proper CA MAF in there or remapped your ECU to use another MAF (such as the KA) because if it's mismatched, it will give you high emissions

[180sx_vc]

so if i have the valves installed properly, this will still cause fuel vapors to pool..?? Time to use SEAFOAM YAY!!
i have a CA maf...
would a vacuum leak cause a lean mixture? im really leaning towards this idea because running lean does make the engine run hotter, so i would assume this would be or one of the reasons why my NOx is high. Also i don't know much about heat soak but would sucking hot air cause pre-ingniton and in return run hot? if it were a vacuum leak i would think its the intercooler piping, im not a 100% sure that my pipng is leak free, remember its a stock sr20 smic/piping which is ghetto rig

I also have a t25g, if this has anything to do with my situation....thanks for the replies...

[TheMAN]

a vacuum leak will definitely cause a lean mixture... exhaust leaks will cause richness
hot intake charge will definitely increase combustion chamber temps also... if you have no airflow over the SMIC, it's highly possible that part of your problem lies there

if you're going to seafoam the engine, make sure you disconnect the butterfly system... you want them to stay open that way thorough cleaning could be done at idle speeds!

[roast]

Hey, I see that you're battling aircare... I had a lot of fun making my Silvia pass emissions in Ontario... :P

I'd run 94 instead of 91, then do a proper vacuum leak check using carb/brake cleaner or propane, after that check the gap on your spark plugs, set it to the lower end of stock. It'll reduce NOX but increase CO, if CO is very safe you will be fine... if it's on the line, don't play with them...

I'd probably try sea foam next if you still fail... or do it (sea foam) anyways but make sure you're ready to burning through a few tanks of gas before going for your test.