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Discussion Starter · #1 ·
Hello All,

I have a stock 1994 Trans Am LT1 Automatic. I just replaced the wires and plugs. I replaced the ignition coil a few years back. Its an Accell. Has stock opti. Mechanic suggested it may be opti and/or 02 sensors. Readings are the following:

HC 15 MAX 58 Tested 128 Fail
HC 25 MAX 34 Tested 105 Fail
CO 15 MAX .34 Tested 0.27 Pass
CO 25 MAX .32 Tested 0.17 Pass
NO 15 MAX 483 Tested 193 Pass
NO 25 MAX 494 Tested 128 Pass

So hydrocarbons are through the roof. Would a bad CAT be the underlying cause? Or is he right and might be the opti or 02 sensors? Any insight would be appreciated
 

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Hydrocarbons are produced by over rich fuel mixtures. Try doing a running sensor scan with a obd 1 scanner. See what your fuel trims & o2 sensors are reading. As well as other sensors.

A trick many people do to reduce hc is to change the oil right before testing. But this only helps if hc failure is minor. You need to find out why fueling is rich.
 

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Hydrocarbons are unburned fuel. CO is partially burned fuel and from air/fuel ratio, dirty oil, etc... High HC‘s can be caused from lean misfires, misfires, engine wear, ignition problems, etc...

Your max allowed values seem quite low to me. What state are you in? Also, what are the two tests being performed? Is the “15“ test done at 1500rpm and the “25” test done at 2500?

You should pass without a cat for HC, that’s not root cause. The vehicle must be at full operating temperature- take a 15 minute drive and make sure your at operating temps- hot oil and hot exhaust.

Has your cap and rotor been changed?
 

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Discussion Starter · #4 ·
I am in CA so super strict. rpms were 1321 and 1296. I had changed to a chinese knock off (I was broke at the time) and when I was having issues with spark I replaced the original. I couldnt pass smog before and I decided the original might perform better than a new chinese knock off. So, I switched to old original stock opti. Also ran sea foam and always pass in the gas tank. Burned an entire fuel tank and put in fresh 91 octane right before testing. Drove on freeway for 30 minutes and left it running while I waited to get smoged.
 

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Sounds like you were doing all the right things. You may need a good tech to help diagnose how everything is working (compression test, leak down, power balance test, check temps pre/post cat, monitor O2 cross counts if lazy and staying a bit lean, etc..). There could be some underlying issues but sure you just want to get through the test. Air injection system helps light the cats and there are some things that can be done to help reduce emissions. Higher idle speeds (I’m still a bit confused on the difference between the two test speed ranges you failed ) and retarded ignition timing. This was easy to do on older carb’d stuff.
If your cat is shot, the HC’s will eventually kill a replacement unless cause is addressed.

We use to do idle and dyno testing here but light load steady state. Certain vehicles did idle (1100 rpm max) and 2500 rpm no load sampling.

HTH
 

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Discussion Starter · #6 ·
If this may help here is the scan log from 94/95 program. Mostly idle with periodic revving. Let me know if this may help at all. I see the egr doesn't seem to do anything as far as % in the grid. Is that normal? I waited until the car was warm before I started the program
 

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So I cannot pass smog at all I got a 1995 Impala ss I just bought really nice car never gave me problems at all only had it like 4 weeks now took it to emissions passed everything but HC readings was 1.4701 the limit was 1.2000 so went and changed oil changed Sparkplugs had a coolant leak sitting in line waiting at emissions 2nd time had to leave and went and replaced water pump thermostat and radiator came back to emmisons and passed HC but failed CO reading was 15.7165 and limit was 15.000 so went home made sure K&N air filter was blown dry with air compressor it was kinda dirty put it back on and returned to emissions and what do you know I failed everything this time no lights on the dash come or have came on there is only 150k on her all the hoses are new under hood I bleed the coolant after replacing the water pump and thermostat too the car was backfiring for the kid who sold it to me but the day he sold it to me I filled her up full tank of gas and was pressing the pedal to floor enjoying her n she quit backfiring hasn't backfired on me once and now when I start it it kinda hesitates I put my hand at the end of the right Exaust while car is ideling and good flow coming out but the left one not as good air flow coming out like it's trying to breath idk if that matters but figured I'd ad that in here I checked all the fuses and relays there all good no Exaust leak had shop check that they said it was good so idk what to do I'm omw to go buy me a OBD1 scanner now maybe it'll bring up what's wrong with it and one more thing it your abs speed sensors snapped on the front left and front right wouldn't it come on the dash lights or something ??? Mines are snapped and no dash lights have came on im very confused any info or help would be appreciated
32339
32340
32341
 

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O yea my temp gauge is still going up above the half way mark but it'll stop when almost to hot and go back down or when I start driving it goes back down under half way mark to cold then bounces back up to half and both fans are working also
 

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Doing a running sensor scan with an obd 1 scanner could help you spot what's wrong. Make sure your O2 sensors are changing values rapidly. Fuel trims on or close to 128 are important.

Make sure temp sent to pcm by coolant temp sensor is close to gauge temp. But map, maf, intake air temp, and other sensors are important also.

Make sure you have no vacuum leaks or exhaust leaks before the cat. Unmetered air will cause pcm to add more fuel.

Make sure steam pipes attached to rear of cylinder heads are actually there, and they are not clogged with gunk. The steam pipes are very important to the reverse cooled lt1. Also make sure they're hooked up to the right fittings at front of engine.

Is your ignition system up to par. New plugs, wires, cap & rotor can work wonders. Also, if your going to live with an optispark equipped car, you need a PC based oscilloscope to make sure low & high resolution opti pulses are the correct 5 volt square wave.

An oil change right before testing can help. As with a new fuel & air filter.

Also make sure egr system is working correctly. Egr valve can get carboned up and not work correctly. Don't forget to check electrical part of egr system.

A factory service manual for your car would help you a lot. We have f-body manuals at the following link, but your impalia ss has some changes from f-body. So you might want to order one from Helms.
www.mediafire.com/?40mfgeoe4ctti
 

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Yea see when I step on gas I'm not only hearing Exaust I'm hearing a ting type of noise coming from left front side and that Exaust for some reason is sitting lower then the right one I'm thinking it's eather the O2 sensors or the intake temp sensor I changed the cooling sensor to a brand new one but had to switch it back to old one because my transmission light came on for some reason on dash and it kept dieing Everytime i tried to back outta garage soon as I put old one back in it went back to normal Opti in excellent condition shop said so idk it is original flowmasters Exaust that's on there too same Exaust for 20, years lol but shop said it's good too nothing wrong with Exaust so again idk only thing I can think of is the sensors
 

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Lt1 engine temps should be above 200 degrees. The engine was designed to run hot in order to reduce emissions.

Make sure you have a thermostat of the correct lt1 type installed.

When diagnosing problems with sensors, don't forget that wiring, connectors, splices, power wires, & ground wires can be the problem.

That's why I recommended you to buy the factory repair manual set for your car. They have full wiring diagrams, a driveability & emissions section, and other helpful info.

For a scanner, I recommend the GM tech 2 china clone available from amazon. It's about $300, but out performs all the other scan tools on the market.

95 f-bodies have the 16 pin obd 2 type aldl in them. Does your 95 impala have the 12 pin or 16 pin aldl?
 

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32347
32348
this stuff is the truth if you can't pass smog use this I just put a full tank and poured half the bottle in and went straight to emissions to test it and passed this the real deal
 

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Discussion Starter · #14 ·
Ok, so finished doing compression test and all cylinders hit around 145 dry. So, I started thinking what else could cause this issue. So, I turned my attention to my fuel injectors. Now here are the hard questions. Its hitting 38-39 PSI primed with engine off. Now, sometimes it stays at 35-39 indefinitely and other times it will leak over time. 1st time I tested it dropped to 23 PSI after 15 minutes, 21 after 30 minutes, and 18 after 45 minutes. Is that normal? I've heard some people say it should stay around 35-39 and others say it will leak over time. If it is supposed to leak over time at what rate should it leak? Any help would be appreciated.
 

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Hello All,

I have a stock 1994 Trans Am LT1 Automatic. I just replaced the wires and plugs. I replaced the ignition coil a few years back. Its an Accell. Has stock opti. Mechanic suggested it may be opti and/or 02 sensors. Readings are the following:

HC 15 MAX 58 Tested 128 Fail
HC 25 MAX 34 Tested 105 Fail
CO 15 MAX .34 Tested 0.27 Pass
CO 25 MAX .32 Tested 0.17 Pass
NO 15 MAX 483 Tested 193 Pass
NO 25 MAX 494 Tested 128 Pass

So hydrocarbons are through the roof. Would a bad CAT be the underlying cause? Or is he right and might be the opti or 02 sensors? Any insight would be appreciated
. So if you can get this at AutoZone or advance autos this shit works my LT1 wouldn't pass emissions same readings as yours basically i filled my tank a little more then half then threw whole bottle in and went straight to emissions and passed but if you don't wanna use this I would check your filter replace it and get a oil change and go straight to emmisons also could be your maf sensor there like 40$ on carparts.com n they ship quick within 2 dats....the shop diagnosed mine and said it was my 02, sensors or my both my catz too but it wasn't try that stuff first before you go buy parts if it don't work it says you get your money back double so yea
32364
 

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Fuel pressure does leak down over time. There's no normal rate but if you still have 23 psi after 15 minutes, your fine.

Abnormal leakage woulb bleed down to 0 after a minute or two. But, if it makes you feel more secure, pull your fuel rail out and watch injectors to see if they are leaking.

And while fuel rail is out, check spray patterns of injectors while helper cranks engine. Be advised that gas is dangerous. So have catch cans under injectors and a fire extinguisher at hand.

Injectors should have a nice full well atomized spray pattern.
 

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Doing a running sensor scan with an obd 1 scanner could help you spot what's wrong. Make sure your O2 sensors are changing values rapidly. Fuel trims on or close to 128 are important.

Make sure temp sent to pcm by coolant temp sensor is close to gauge temp. But map, maf, intake air temp, and other sensors are important also.

Make sure you have no vacuum leaks or exhaust leaks before the cat. Unmetered air will cause pcm to add more fuel.

Make sure steam pipes attached to rear of cylinder heads are actually there, and they are not clogged with gunk. The steam pipes are very important to the reverse cooled lt1. Also make sure they're hooked up to the right fittings at front of engine.

Is your ignition system up to par. New plugs, wires, cap & rotor can work wonders. Also, if your going to live with an optispark equipped car, you need a PC based oscilloscope to make sure low & high resolution opti pulses are the correct 5 volt square wave.

An oil change right before testing can help. As with a new fuel & air filter.

Also make sure egr system is working correctly. Egr valve can get carboned up and not work correctly. Don't forget to check electrical part of egr system.

A factory service manual for your car would help you a lot. We have f-body manuals at the following link, but your impalia ss has some changes from f-body. So you might want to order one from Helms.
www.mediafire.com/?40mfgeoe4ctti
Changed new radiator new sparkplugs new water pump new thermostat got air bubble out of coolant had shop check egr vavle said it was good all my sensors are working good they said it's idling good new filter now it's starting to backfire though I'll be on highway 5mins later when I go to pass it studders losses power and backfires oil light flashes then goes away transmission light comes on and stays on I pull over restart car and the tranny light goes away if I step on gas it backfires 3 times n light comes on again if I drive it with tranny light on it don't backfire at all but don't wanna ruin the car so I'll shut it off wait 5mins turn it back on let it run for ,couple mins and she's back up to running perfect no problems
 

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To be honest everyone says the catz yes it's same exhaust that's been on car for 20, years but I think it's the fuel pump honestly because I was low on gas and it started backfiring so I went and put full tank hasn't back fired yet on me and the other day after I put gas it started backfiring and the car started rumbling like it was Exaust so idk I know u can feel the heat from under car just by standing next to it that shit been getting too hot
 

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And shop checked Exaust no leaks or nothing and just replaced to new MAF sensor just cause so idk tf but I'm bout to replace everything on car honestly lol that's how tired I am of dealing with it haha
 

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If I turn air conditioning on my car temp gauge goes down but if I turn the heater on it goes up more too still going over half way mark unless air conditioning on
 
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