1990 LT5 emissions fail

[Schrade]

Originally Posted by Franke

Thnx you for the welcome. The HC is 1.2913 and the limit is 2.000. Thats approx 1/2 the limit. CO was 15.1407 and the limit is 20.0000. So i'm good on that too. Some history on this car is that i've owned it for 5 years and the nox has been slowly creeping up. In 2007 with 53389 miles on it the nox was 1.282 and the state standard was 6.0000. In 2009 with 55294 miles on it the nox was 2.6949 and the standard was 3.5000. CO State changed the standard sometime between those two years. In May of 2011 with 55933 mi on it nox was 3.2886 and the standard was 3.5000. today it failed with 3.9183 nox.

Yup - as I guessed - WAY under limit on HC's - proportionally speaking.

You're burning just a little to hot. And it probably will NOT show in the engine coolant temp either.

Did they happen to give you the scan data, with ALL engine parameters? With THAT, some of the guys on the boards here could pinpoint the fix in short order.

If not, call the inspector, and see if they can get it for you. If not, call a speed shop. Then post it here.

Short of that, you have to reduce the burn temperature slightly somehow...

[scottfab]

Originally Posted by Franke

Can anyone tell me why a 1990 LT5 would fail emissions with 3.9ppm Nox? The car has 55,000 miles on it and runs great. The max Nox limit in this state is 3.5 ppm. The HC and CO are well within limits. Some emissions people tell me to check the EGR but I don't think the LT5 has a EGR. The local Chevy dealer thinks the cats are failing but can't go any farther. Any help would be appreciated.

You could be running lean. Did you dump out any codes that maybe set? Like maybe 44 and/or 64 ?
See the bottom of page 6E3-B-6 in your FSM.
Not likely at all it's the Cat.
O2 sensor(s) very possibly.
In any event you're so close to passing that another try with the car when it's not running as hot could pass. Let it cool before getting in line.

[Franke]

Thanks Schrade and Scottfab. Coolant temp is about 200 degrees. There are no codes set as per the chevy dealer and they didn't say much about anything except they believe the cats are failing and $4000.00 dollars could fix it. When I pressed them further on the parts availability they said they could get 1993 parts to fit. Then they gave me a parts diagram they used to get the pricing and I realized they were using the L98 engine exhaust specs. I explained the difference to the service guy. So much for the experts. Anyway I'll try the O2s first as I am close to passing. Maybe the O2's are a bit dirty or something. I know that these can be tested out of the car as well as in it. They produce about 1 volt DC when properly heated.

[mike100]

Originally Posted by Franke

Thanks Schrade and Scottfab. Coolant temp is about 200 degrees. There are no codes set as per the chevy dealer and they didn't say much about anything except they believe the cats are failing and $4000.00 dollars could fix it. When I pressed them further on the parts availability they said they could get 1993 parts to fit. Then they gave me a parts diagram they used to get the pricing and I realized they were using the L98 engine exhaust specs. I explained the difference to the service guy. So much for the experts. Anyway I'll try the O2s first as I am close to passing. Maybe the O2's are a bit dirty or something. I know that these can be tested out of the car as well as in it. They produce about 1 volt DC when properly heated.

Just buy new oxygen sensors- the bosch branded ones at Autozone worked great and had the correct lead length.

My orig made-in-germany units passed HC's and CO's pretty well, but a small error in accurate o2 content still throws off the NOx a lot. Even good cats can't scrub too much extra NOx. Try the sensors before you try to buy some used catted manifolds.

[Hog]

Rock Auto has 25176708 in stock for $47 a piece for a Delco part.

I would go with Delco over Bosch narrowband sensors. And certainly just replace the sensors no testing needed, esp. if they are 24 years old now.

peace
Hog

[Franke]

Thanks to all who posted here. I appreciate it and will check for vaccumn leaks, coolant temp issues, and O2 issues. (lean conditions) If i find the fix i will post it at a later date to let all know. There is always good info from the ZR-1 folks here.

Thanks again.

[Franke]

An update to this problem. After some testing of the PCV system, vaccumn, Fuel pressure and fuel injectors i found the following: The bottom PCV hose had 5 splits in it. The vaccumn reads good everywhere i could test and is 14.5 at idle steady. The multec fuel injectors have some problems. Of the 8 primaries cylinder #5 reads 10 ohms cold and 6.5 ohms hot. All the others are approx about 13 ohms hot and cold. On the secondary side i read cylinder # 3 at 10.6 ohms cold and 7.6 ohms hot. Cylinder # 4 is way off at 1.5k ohms cold and 1.7k ohms hot.
Looks like its time to replace the injectors. This could be why i am running a bit lean i think on the primaries.

[mike100]

^^^ Vac leaks will do it too. Easy to work on the PCV tubing after you pop the plenum off. I would still recommend new o2 sensors. All repairs will contribute to a smoother and cleaner running engine.

[Coupe89]

Got to ask. What chip are you running?

I did not have a stock chip when I failed emission testing.
Put a stock chip in and passed

[tomtom72]

Originally Posted by Franke

An update to this problem. After some testing of the PCV system, vaccumn, Fuel pressure and fuel injectors i found the following: The bottom PCV hose had 5 splits in it. The vaccumn reads good everywhere i could test and is 14.5 at idle steady. The multec fuel injectors have some problems. Of the 8 primaries cylinder #5 reads 10 ohms cold and 6.5 ohms hot. All the others are approx about 13 ohms hot and cold. On the secondary side i read cylinder # 3 at 10.6 ohms cold and 7.6 ohms hot. Cylinder # 4 is way off at 1.5k ohms cold and 1.7k ohms hot.
Looks like its time to replace the injectors. This could be why i am running a bit lean i think on the primaries.

Hi Frank & welcome to the asylum! Yea, those numbers on the injectors are not good. It's time. Also, the vac leaks like Mike said are very important as on a 90 they are a bunch of small ones that add up to cause an even leaner condition. I have to agree on the O2 sensors. I did mine at 17 yrs and got an immediate improvement in NOx, but I had already done the vac leaks & injectors and I was still very close on the NOx till I did the O2's.

You want maybe to look at Jerry's or Limey's PCV replacement parts for the hoses, and soak the valves in carb cleaner to make sure they are free. Check the double connector just under the TPS as that one is most likely very loose, I put a zip tie around mine & a dab of silicone to help seal the old tired rubber.

I'm so glad NY state did away with the roller test in my area! That was always a PIA for the car. Oh, not to be a wise guy, but turn the key OFF. I used to forget to do that!


Tom