Engine Pinging and Failed Smog test

[LSA]

So i failed the smog test out here in Ca, which i was a little surprised about because the car passed smog last november. It passed Hc and Co but failed Nox at both 15 and 25mph, at 15 it was about 40ppm over and 25 it was ~200ppm over.

The guy came back told me it failed and he heard the engine pinging, yeah pre-combustion of fuel all that good stuff. (i always put in 91) I've never heard my engine ping and I've listened for it how loud would it be? I read on that it sounds kind of like running a screwdriver over a chainlink fence, that tink tink tink tink sound. I've had this place do smog before on my other car so I think he's pretty honest.

So I grabbed a can of seafoam and am running it in the tank right now figuring maybe it's dirty injectors (and that would be the easiest fix) because the car sat for a while and well it's a 22 year old car. What's the reason behind why dirty injectors would cause engine pinging? Is it that it constricts the flow thus increasing pressure behind it and preheating the fuel or something else?

I really think the seafoam helped because it doesn't produce as much engine fumes anymore, before if I rolled down the windows while idling you could smell it strongly but now nothing.

[krystar]

if i remember correctly, a high NOx reading indicates too high combustion chamber temperature. you're probably running lean, which would increase temperatures and lead to the pinging.

pull yer plugs...if my guess is right, they'd be really white. i think you should be able to just use the FQS adjuster and dial in an extra couple points of fuel.

[Rasta Monsta]

Big vaccuum leak could cause lean running as well.

[LSA]

I'm hesitant to propose a vacuum leak because that was one of the last things worked on in the car, under the service records it lists all vacuum lines replaced withing 10 years of today i think it's closer to 5 (2004 or so) but I'd have to check the records and I'm at work, although they could have done a shoddy job, never know.

[krystar]

that doesn't mean that a line or fitting didn't break in the last 5 years....

check for vacuum leak first. but also see if your FQS is set for default factory setting.

[my1st911]

What were your co and hc readings. If they are "normal", it could just be carbon. If they are very low, its more likely lean, and your O2 sensor could contribute to your problem as well

[choinga]

Here's a question...if you have a wideband and at idle and at 15/25MPH you're snug in the ~14.7 range...is that a good enough indicator as to whether you'll pass?

[my1st911]

Originally Posted by choinga

Here's a question...if you have a wideband and at idle and at 15/25MPH you're snug in the ~14.7 range...is that a good enough indicator as to whether you'll pass?

Thats a good place to start. But you have to factor in whether you have carbon build up, and a functioning cat. Even with 14.7 afr, you are still relying on the cat to clean up all the exhaust gases, and if you have some carbon build up, your nox will rise to a level that the cat wont be able to keep within "clean" limits

[choinga]

Originally Posted by my1st911

Thats a good place to start. But you have to factor in whether you have carbon build up, and a functioning cat. Even with 14.7 afr, you are still relying on the cat to clean up all the exhaust gases, and if you have some carbon build up, your nox will rise to a level that the cat wont be able to keep within "clean" limits

Well, I don't have a cat - but got it to pass last year without one. My car was pretty stock then though - 3" SFR exhaust was about it. Since then I've installed a Vitesse MAF (w/ Piggyback so that should help me tune to get it right if I need to...hopefully), Tial WG and bigger injectors.

I plan on doing a clean oil change and my plugs only have about 1k miles on them so hopefully they should be fine.

My local shop can't do the inspection but they have one of the 'in the exhaust' probes that the DPS uses here to test - so I'll take it there to make sure I'm going to pass before even going but I didn't have the WB last year to see what was going on with AFR so was just curious...

[944Spec_bound]

Originally Posted by choinga

Well, I don't have a cat - but got it to pass last year without one.

If your in Calif too, instant visual failure and put on the evil polluter list for not having a cat

[my1st911]

Originally Posted by choinga

Well, I don't have a cat - .

Where are you from? Most states do the visual inspection of the cat. Is it gutted or gone. These cars do run prety clean on their own. I got my 85.5 944 after it sat in a friends garage for 7 years. I went right through inspection in NJ, which isnt as hard as CA I assume, but pretty tough anyway

[jlorda]

LOL massachusetts dosnt give a shiite , passed my 924s with black smoke and all