cause of afterfire....

[89IROC&RS]

hey all, still trying to get the damn IROC to pass emissions. I posted before so ill give the quick overview thus far.

1989 IROC-Z MAF TPI 5.7L with a 700R4 tranny. Engine has about 160k miles, tranny has less than 2k miles.

Mods - Edelbrock TES headers, Flowmaster American thunder 3in catback exhaust, high flow cat. 26lbs injectors, and a stage two B&M kit with a 2600rpm stall converter, also replaced the timing chain due to wear and have yet to put 1000 miles on it.

I took her in to run the emissions, and the HC's and NOx's were high, but CO was about nothing. Im blowing lots of condensation out the tail pipe, so i know im getting a good burn, so im guessing im running rich... and hot. and on top of that, ive got after fire (backfire in the exhaust).

What ive checked - TPS, golden, .4v at idle to 4.8v at WOT, with a smooth progression. checked my IAC, it moves back and forth smoothly (but is dirty as hell). ive put on knew cap, rotor, wires, spark plugs, and even swapped the distributor for another one. with no effect on the problem. ive opened up the exhaust ahead of the cat, which has less than 2k miles on it, in the off chance that it was plugged, still afterfired. ived attempted changing timing, ive swapped IAC's with no effect, ive cleaned the entire intake, checked the EGR for operation, and replaced the EGR activation switch, Ive set the base timing to 0* with the timing connector disconnected, and set the idle by running out the IAC, unplugging it, and adjusting the idle speed screw to about 500rpm. With everything hooked back up, it idles around 750rpm.

The car is running GREAT, throttle response, speed, idle, are all fantastic, it just has that annoying afterfire, and the weird emissions report that makes it look like its running lean AND rich.

For the afterfire, im guessing ive got a header leak, that last bolt near the number 8 cylinder was hard to get to, and i dont think i ever torqued it down.

Now, what im wondering is, would the header leak throw off my emissions? or is it possible that the afterfire (which is pretty severe) is triggering the knock sensor, and screwing with my timing, throwing off my emissions?

any thoughts?

[89IROC&RS]

got it.... ive got a header leak somewhere.

air getting in to the manifold causes the afterfire by mixing with the fuel. also fools the o2 sensor into thinking that the engine is running lean, and richens up the injectors, and the coup de gras, the cold air handicaps the cat's ability to process NOx by overloading it with oxygen and cooling it below its optimal operation temperature.

and HAZA, the high HC from a fooled O2 sensor, High NOx from an air conditioned Cat, and afterfire from the cold air mixing with the rich exhaust.

its funny how it makes sense once you think of it. ive been banging my head against this for MONTHS.

and now that i think about it, i never did tighten down that damn bolt back by the number 8 cylinder.

anyway, hope yall are doin well, i dont have enough time to breath these days, otherwise id putz around and shoot the shit with you guys.

-erik

[bag91]

Originally Posted by 89IROC&RS

got it.... ive got a header leak somewhere.

air getting in to the manifold causes the afterfire by mixing with the fuel. also fools the o2 sensor into thinking that the engine is running lean, and richens up the injectors, and the coup de gras, the cold air handicaps the cat's ability to process NOx by overloading it with oxygen and cooling it below its optimal operation temperature.

and HAZA, the high HC from a fooled O2 sensor, High NOx from an air conditioned Cat, and afterfire from the cold air mixing with the rich exhaust.

its funny how it makes sense once you think of it. ive been banging my head against this for MONTHS.

and now that i think about it, i never did tighten down that damn bolt back by the number 8 cylinder.

anyway, hope yall are doin well, i dont have enough time to breath these days, otherwise id putz around and shoot the shit with you guys.

-erik

WOW lol Im glad you got all that advice lol. Anyways hope you pass/ passed with flyin colors

[wrightz28]

Hey Erik!

Your TPS setting should be higher, .52-.54 closed is the premo spot. You're dirty IAC is cause and effect fron the higher NOx as I'm sure you know.

I have yet to get headers for the GTA but it's running the same exhaust and it does the same thing as well just so you know.

[Genopsyde]

If your theory ends up not being correct, I would suggest looking at the o2 sensor itself as the culprit. good luck, and stop back soon.

[Genopsyde]

and just to add...My RS passed echeck with very good numbers (way below the limits) with egr deleted and A.I.R. deleted, a mail order tune from pcmforless and a universal hi-flow cat. Not that any of this applies to you, but I figured I'd throw it out there.

[FormulaLT1]

Not to get off topic but how was your PCMforless tune Geno?. I have heard hit or miss stuff with Alvin who does the pre 94 chips but Bryan who does most of my work is usually dead on.

[Genopsyde]

it ran a bit rich, but the bosch o2 sensors are known to cause that as well. I also kept getting a map sensor code. otherwise it was great.

[mikeemon]

or... move to MA. We now have no emissions test on 1995 and earlier. That is about the only reason to move TO here. Plenty of reasons to move FROM here.

[89IROC&RS]

lol, well i am about to register the damn thing in richmond, no emissions down there. I wanna keep the car, but it seems everything i fix leads to another thing down the line. I managed to track down a header leak where i thought it was on the back of the passenger side manifold. Only way i can figure to fix it is to pull the engine put the headers on, and put the engine back in.

i also found that despite having a functional EGR valve, my EGR solenoid seems to be faulty, because im not getting any vacuum to the valve. so i hard wired manifold vacuum to the egr to hold it open, and took it to work to have the emissions run, and see if the egr being held open made me pass.

heres the fun part. as i passed the chevy dealership (purely by chance) i lost power. the engine started to stutter and stall, and i managed to get off the road and into a parking lot before it got really bad. seemed to be running very very very lean, because every time i pressed the throttle it wanted to die. id let it idle and it would run but wasnt happy about it. then it strait up died. i let it sit and cool down (it wasnt overheating by any means) and it started up. i just drove it to the dealership and admitted defeat. Im having them throw the tech 1 on it and scan the sensor data in real time and see if there is something obviously wrong, cuz im at my wits end.

no error codes, and everything that i know how to check with a multimeter has come back fine. set the idle correctly, set the timing correctly, so on and so forth. So for now, just waiting to hear back from the dealership to see what the problem is....... hopefully. on the plus side, she was running AMAZING up until she decided she didnt want to run anymore lol.

[Morley]

You need to get a laptop and cable/software so you can look for yourself.
I've made several cables for myself and others and Craig Moates' software (ECM 851) is free and does a great job. Note, you'll need a laptop with a serial port.

[89IROC&RS]

i forgot about good ol moats engineering.... *smacks forhead* i hand planned on learning to burn my own PROMS anyway, might as well look into being able to do my own diagnosis. thanks for the reminder morley

[Genopsyde]

let me know when you get all that set up, I'll need you to burn me a free prom for a twin turbo 406 that i'm dreaming up.