'96, 60,000 miles , fresh fuel, fresh mobil 1. Car passed idle test, but failed driving test with high NOx, everything else was good. I haven't driven the car since last June (New baby arrived and the vette took a back seat for awhile). Had a SES light when I stored it, however after a few miles of driving this year it cleared. I pulled the codes last night and got only one, a P0300, misfire detected. The light has not come back on. When the test was done the tech had the climate control shut off which allowed the car to get very warm as the fans do not come on until the car is almost overheating, could this cause the high NOx as the combustion temps would be very high? Before anyone says EGR don't bother as the LT-4 doesn't have it. Also spare me the "use search function" lecture I've duly searched and found nothing for LT-4 which is the only year with OBDII. I'm wondering if a seafoam treatment would help, or just a general tune up, plugs and wires. I'm not sure why I got the misfire code, as I've never felt the car run rough or miss. When the car was stored the fuel tank was on reserve, I put 3/4 tank in, so any moisture should be well diluted, however I may put a bottle of gas line anti-freeze in anyway.
My plan of attack:
1) Last night I cleaned my K&N and cleaned the throttle body
2) Tonight I'm thinking about the possibility of seafoam and then installing new plugs (Should I go with factory OEM heat range?). Do you have to change the oil after a seafoam treatment?
3) I'm going to put it up on the hoist and check the before and after temps of the converters.

Anyone else have some other suggestions or am I on the right track?

TIA

 

Gas stored over six months or so, goes bad. Especially if there is very little of it. Maybe fill it up and run some good fuel treatment through it. If you aren't tripping any codes and it is not passing emissions, that is odd to me. I can only think you had some gummy gas in the tank that still didnt run out yet. Just my 2 cents.

Good luck with it.

 

My car GrandSport LT4 experienced the misfire code also after winter storage (but I felt my car running rough). I put Seafoam in the gas and burnt off any old gas then refilled the car with fresh gas. The car ran fine I cleared the code and all has been well!
It is not time for my inspection but I'm sure it will pass.
In Canada do they run it on a dyno for inspection ? Here (Massachusetts) they just check OBD2 diagnostic port. And if the cars computer does not have any codes it passes emissions!

 

Yes, they put it on a dyno. I'm on the fence with the seafoam, don't know whether to go with in the tank or through the booster vac line. I know this subject has been beaten to death, but I'm still undecided. If you go the vac line route do you need to do an oil change after? If so that sucks because I just did the oil and filter 2 days ago.

 

you have to run these cars, can't let them sit.
take it out, and beat on it for a while.

 

Keep your failed inspection report in the car so that if you are pulled over. . . . you were on your way to the nearest inspection shop for a re-do

 

doesn't high NOx come from high engine temps?
do you have the tools to electronically adjust the fan on temp? I'd bring it down to 195 for the test or run the test with the ac on, and I'll bet the NOx reading comes down considerably

 

I think you're on the right track as far as the combustion chamber temps are concerned. Do you have an aftermarket tune in your car? This is a post I made last summer in the ZR1 section when a guy there was having problems with high NOx:
http://forums.corvetteforum.com/show...0&postcount=23
There are a few links in there too if you're interested in some good reading about emissions and the different chemical reactions occurring in the combustion chamber and catalytic converter. Good luck!

 

I suggested running the test with the A/C on but the tech wouldn't do it, he said it has to be done with no load. For the re-test I'm going to unplug the A/C compressor so the fans will run (I tried this last night and it didn't throw any codes). I have Lt1 edit for my other car but I don't have LT-4 edit yet to change the fan on temps. I am really thinking the whole reason it failed was because the engine was way too warm. The tech let it idle the whole time he was getting ready to do the test, so in total it probably sat there running for 1/2 hour and they wouldn't let me in the shop during the test (so I stood just outside the bay door). Needless to say I'm not going back to the same facility for the re-test, I'll pay the full amount and go somewhere else. The place I used to go to let me drive on and off the dyno and stand right there while the test was being performed. They have since changed ownership so I went elsewhere. I think I'll give them a shot to do the re-test, I called today and found out the same tech still works there.

 

60k miles on the original plugs and wires? You are sooooooooooooo in need of a tune-up. Bet you find that the majority of the plugs have lost their platinum pucks resulting in a huge gap and misfire issues.

 

They are not originals. I changed them when I got the car. I've put approx. 15k miles on them in 5 years. I was asking if I should be running a different heat range.

 

Do both 1/2 in the tank 1/2 through a vacuum line. Check and clean your MAF I have seen that code caused by a dirty MAF. Just hope you are not starting to get a flakey opti another source of misfire as well as the ICM.

Looky here what I found with the search function

http://forums.corvetteforum.com/show...ghlight=p+0300

http://forums.corvetteforum.com/show...ghlight=p+0300

http://forums.corvetteforum.com/show...ghlight=p+0300

And there are plenty more if you search P 0300 in the archives

 

Well it failed again, even worse this time. Failed everything in the driving test by a mile. Here is what I've done so far in this order:

1) Cleaned K&N and TB
2) Seafoam in the intake and gas tank and drove for 1 hour at highway speeds. Then added 3/4 tank of fresh fuel.
3) Changed oil and plugs to NGK TR55 platinum's
4) I had a random misfire code P0300 before I did all this but have not had it since.

The car actually seems to be running worse since I did all of this, it smells rich. The old plugs didn't look too bad, nice color, all pucks still there, gap was good. I noticed the biggest difference after I changed them (rich smell), but I am getting no new codes. I'm getting it scanned at a buddy's place next Wednesday, any suggestions before then? The car has decent power in first gear but seems to bog a bit going into second if I get on it. I'm going to do a thorough check for VAC leaks tonight, although I don't think there are any. Any constructive comments are welcome. Thanks

 

Try posting the numbers from your emission test...kind of hard to give constructive comments without any facts. Post the limits as well as actual for HC, CO, and NOx. If they give CO2 and O2, post that also. If your buddy has a 5 gas exhaust analyzer, post the results from that too. The combustion process is a chemical reaction and the chemical composition of the exhaust gas reveals what's going on in the combustion chamber. If you can get a sample before the catalytic converter analyzed too, that's even better.

 

HI

May be worth checking (or replacing the ) temp sensor, when mine went it sent the emissions all over the place, but did not set any codes

Barry

 

Some great troubleshooting here. (i'm being sarcastic).

Why don't we skip the seafoam, snakeoil, or whatever else and figure out why it's running lean.

What was the code when the check engine came on last year?

If you can't tackle it, have a mechanic fix it. Making fans run constant and all that is nonsense. Fix the problem. (probably egr).

-- Joe

 

Just a couple thoughts:
Run it in a dark area and look for any sparks from shorting plug wires.
The gas smell could be from a bad injector--any gray smoke out the exhaust?
The LT1/4s were designed to run hot in order to reduce emissions, so higher engine temps, as long as not over 240*F is actually a good thing.
A bad ECT sender on the water pump would keep it from going into closed loop, and it will fail in open loop. Is your digital temp gauge working correctly?

Like I said, just a couple things to check.

 

If it is running worse after plug change, did you check to make sure all plug wires are seated properly?
Check the EGR system and your o2's.

 

NOX sets in with high combustion temps - not high coolant temps. It's controlled by EGR and scrubbed up in the CAT. Absent a specific code for this circuit, it's probably not EGR.

A/c operation is exempt from Federal and State Emissions - most don't/can't pass with it on due to the fact that it requires lower coolant temps and more fuel (to manage the load). Without either, you're not going to have cold air out of the vents at idle (if it'll idle at all).

General misfire codes usually set with dirty or plugged up injectors. In CA, some Years are covered under a special 200,000 mile warranty which was traced to MTBE in the fuels. GM issued the extended warranty to avoid a recall. If you're out here, check Bulletins to make sure it's not covered (or if it is, it's on the Dealer if the injectors are full of crud).

I'd guess you've got a lean misfire which would occur if there was water in the fuel and if it was stored without a full tank, that's a good possibility. If you don't want to guess as to why it failed, scan it and post the data. Otherwise, siphon off some fuel from one of the lines and let it sit - any water in it will settle out and you'll know what to do from there.

 

The LT4 does not have an EGR, but injector problems could cause a lean condition and high combustion temps. It would be nice to see the test readout to see the other values. Any chance the cats are gutted?