1990 LT5 emissions fail

[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.

[Racinfan83]

Not trying to be a smartarse - but are you sure you even have to pass emissions with a 1990 model car?? We have emissions in the Metro area here - but in Mo the car has to be a 95 and up. The test here is also an OBD test that plugs into the ECM to see if there are any codes, they don't do the "run test" anymore. You might want to check and see if you are exempt???
And I have also seen cars with ZERO emissions equipment on them that are well tuned pass a run test with lower emissions then a car with a ECM and full emissions. I proved it to one of my professors in College. SO - I guess what I'm saying is that it may not be running "great"....

[Franke]

Thanks Racinfan. In Colorado's front range counties the requirement is that cars newer than 1982 have to pass the IM240 enhanced emission test. Perhaps i need to run it out for awhile and then use new fuel to see if there is a dif. I can't believe the cats are bad at 55,00.

[mike100]

NOx is very sensitive to ign timing, combustion temps (EGR help this), coolant temp, and fuel mixture. The air/fuel ratio at which NOx is minimized is a very narrow range and the oem calibration strives to keep it there during light loads and cruise conditions.

On mine, I found that changing out both of my 22 yr old O2 sensors did the trick and lowered the NOx significantly- like nearly half as much output.

[Franke]

Thanks Mike, I thought about the O2 sensors as well. I might try that before going through the beating i will take on the cat converters.

[SoCalZR1]

Are you sure it is 3.5 ppm. I just ran my 90 through California smog, no small accomplishment, and it passed NO. We are on a "dyno" and it was 388 PPM at 15 MPH and 306 at 25 MPH. Max allowed is 541 and 498 respectively.

I did swap O2 sensors last time through. Also make sure they have a fan running through the radiator if it is a hot day or your NO will shoot up. And the usual clean tank of gas, of course. I squeaked by on HC right at max, by the way.

Mike

[Franke]

Hi SoCalZR1,
The standards out of the book from Colorado is 2.000 HC gpm, 20.000 CO gpm, and 3.5 NOX gpm. I will run out this tank of gas, replace the O2 sensors and refill before I retest again. I will also run this car for 10 to 15 miles before taking it in. O2 sensors are alot cheaper than cats. AutoZone sells them for $44.95 and they are mfg by Bosch.

[SoCalZR1]

I see, gpm vs. ppm is all. Just different units of measure.

Sounds like a plan. One sensor is easy and the other, I forget which, is a PITA. But like you said, cheap swap of old parts. Good luck!

Mike

[Schrade]

Welcome onto the boards there...

Hows the burn temperature? Hotter burn means higher NO's, so you want it to burn a little cooler. This will also boost HC's however; what was the HC count? WAY under limit???

You're only 10% over limit on NO's; if you can bring down the burn temp a little, you should be ok on it...

[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.

[Schrade]

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]

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]

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]

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!

:cheers:
Tom

[Franke]

Thanks TOMTOM. I agree and will replace the O2's at the same time. I really don't like the way chevy did the pcv setup but it is what it is. I may redesign it with solid metal pipe back behind the motor and shorter sections of rubber hose. When I pull the plenum, I will replace every vaccumn hose under there. Also good idea about the zip tie too.

[tpr60]

I did the metal pipe assy. to replace the pcv hose, worked great. There is a thread on how to do it in the net registry files.

[lakemeadzmonster]

A change to 10-30 Amsoil or any synthetic and to be sure run a bottle of Amsoil P.I. thru your system will lower it that much no worries

[Franke]

All, just an update. Replaced all injectors, O2 sensors and all vaccumn hoses under manifold and some externals as well. The ones under the plenum i replaced with 3/16 inch brake lines and new short rubbe pieces to connect them. Brake line bends well and i had no problems. The Lterm trims are now sitting at 128. (histogram shows good fuel mix across the rpm range from idle to 5000 rpm vs. map output). Before repair the left side was 115 and the right was 130. Thanks to all for your great advice.

[Hog]

Sounds like you got it all under control now. Does it run any better at light throttle?

peace
Hog

[batchman]

dealer ... believe the cats are failing and $4000.00 dollars could fix it...L98 engine exhaust

Am amazed, and then again not, that a dealer would prefer to shotgun $4k of the wrong parts rather than diagnose your actual problem.

Cats only catalyze HCs. While failing cats can lead to mixture problems that can show up at NOXs that's really only on an OBDII car where the after-cat sensor is getting out of range. I've not looked specifically at this recently enough to recall but I'm pretty sure these cars only have an O2 sensor before the cat so a failing cat that is not blocked won't have an effect on mixture, or anything other than HCs for that matter.

Glad you've got it running better and hope that clears your issue!
- Jeff

[Franke]

Actually Hog it is smoother idle and quicker response. Light throttle is very smooth and it feels good to be able to drive it like a regular car and then when i get on it it doesn't hesitate. One other note is that i discovered my map sensor to have a low output voltage after i replaced the injectors. I got a code 34 on a test run and the MAP was about 1 volt low at idle according to the FSM. Replaced the Map and all is well. Going in for emission next week - should pass ok.

Jeff, you are correct there is only 1 O2 ahead of the cats. I'm thinking of going back to the Chevy dealer to explain what they need to do to diagonse emissions issues. They probably ran the DTC codes only and that is not enough. My car didn't show any DTC's as it was barely out of compliance on NOX.

[mike100]

.... My car didn't show any DTC's as it was barely out of compliance on NOX.

That's because the original O2 sensors were not completely accurate anymore. As was mine. I had a 50-60% reduction in NOx after O2 sensor replacement (no other changes). NOx production is very fuel mixture sensitive. Also 3-way catalysts do indeed handle oxides of nitrogen.

[Z51JEFF]

I see, gpm vs. ppm is all. Just different units of measure.

Sounds like a plan. One sensor is easy and the other, I forget which, is a PITA. But like you said, cheap swap of old parts. Good luck!

Mike

I believe its the Pass side that a pain to replace.Its not so much the sensor but getting the plug to connect takes some work.

[SoCalZR1]

I believe its the Pass side that a pain to replace.Its not so much the sensor but getting the plug to connect takes some work.

I think you are right. It was that connector that was the battle!

[scottfab]

Am amazed, and then again not, that a dealer would prefer to shotgun $4k of the wrong parts rather than diagnose your actual problem.

Easy to see why. They probably have a senior tech that is 20 something right out of WyoTek and is only given 30min to troubleshoot.

Cats only catalyze HCs..

Cats do indeed convert NOX also (to nitrogen and oxygen)
However, you're correct that with an upstream O2 sensor the cat is not likely the problem.

While failing cats can lead to mixture problems that can show up at NOXs that's really only on an OBDII car where the after-cat sensor is getting out of range. I've not looked specifically at this recently enough to recall but I'm pretty sure these cars only have an O2 sensor before the cat so a failing cat that is not blocked won't have an effect on mixture, or anything other than HCs for that matter.

Glad you've got it running better and hope that clears your issue!
- Jeff