94Z28 strong exhaust odor (not gas odor) - LS1LT1 Forum : LT1, LS1, Camaro, Firebird, Trans Am, Engine Tech Forums
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post #1 of 22 (permalink) Old 10-14-2018, 08:25 PM Thread Starter
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94Z28 strong exhaust odor (not gas odor)

I have a 94 Z28 with just under 68k miles and for the past few months have noticed a strong exhaust smell. Have ran the scan 94/95 program and have a .csv file created from driving the car a few miles. I do not know how to interpret the results however. No trouble codes or drivability issues. Have been unable to find a "normal" scan to compare with. Could anyone interpret the results of my scan? Have replaced opti with MSD unit as well as stock plugs wires and ignition complete. Also have added FIPK. This was done several years ago and has ran fine since. Only problem is strong exhaust smell.

Have never posted before and just introduced myself in the new user introductions forum. It looks like I can't add the .csv file for anyone to see.

Thanks much for any help!
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post #2 of 22 (permalink) Old 10-15-2018, 07:18 AM
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Is strong exhaust smell that of unburned gas, or is it a rotten egg smel?.
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post #3 of 22 (permalink) Old 10-15-2018, 11:27 AM Thread Starter
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Like an overly rich mixture, but not like raw gasoline. Have a scan94/95 report I made yesterday while driving the car for about 20 minutes but don't know how to interpret the results. Does anyone know what a "good" report should look like?
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post #4 of 22 (permalink) Old 10-15-2018, 04:35 PM
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I have a pdf file called "scanner readings and what the mean" I believe it will give you a handle on the info in your scans.

PM me with your email address if you want it.
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post #5 of 22 (permalink) Old 10-18-2018, 07:27 PM Thread Starter
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This PDF is very informative, however I am on a time constraint. I need to get my vehicle smogged and don't know if I will pass with the strong exhaust odor. I have looked at various results I got from about 30 minutes of running/driving time. I see variations between banks 1 and 2 O2 sensors, knock counts of 9081, timing which seems to vary drastically during my test between approx. 8-48 degrees. Despite this, the car is running great and NO DTC's reporting except CAGS which I eliminated shortly after taking delivery of vehicle. Disconnecting the plug on transmission was recommended by manager of St. Therese plant at the time. I could look in several directions by reading this report, but don't want to go the wrong way. Car has original exhaust system and O2 sensors with 68k original miles. The original Opti was replaced several years ago by an MSD unit and haven't had an issue since with smogging car. Only modifications done are to the ignition and air intake (coil, opti, plugs, MSD 6AL, MSD plug wires, and NGK plugs, K&N FIPK, Jet Performance Fan switch).

Bottom line is I could really use some help to decipher my results. Don't know what parameters to look at specifically, and what is considered "normal" or "abnormal". If I had any serious problems, I would think I would be seeing DTC'S.
Just concerned with strong exhaust and passing smog test. Thanks again for your help!
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post #6 of 22 (permalink) Old 10-19-2018, 04:59 PM
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you could be running rich, thus strong exhaust smell. Often this is due to a vacuum or exhaust leak pre 02 sensor. Since unmetered air is entering the motor the 02 sensor reads lean and PCM commands more fuel. This kind of condition long term will kill 02's

This usually is on one bank (bank 1 or 2) or the other (depending which side a exhaust or vacuum leak is) and high 02 readings on one bank would be a indicator of this kind of problem. readings that stay around 800+ all the time vs quickly fluctuating for 100-900


STFT (short term fuel trims) is another indicator of this as they would be 10-25 vs 0-5 max

running rich you will likely not pass tail pipe for emissions

cracked exhaust manifolds will cause this
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post #7 of 22 (permalink) Old 10-21-2018, 01:19 AM Thread Starter
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What I am seeing in Scan9495 is a variation between bank 1 and bank 2 02 sensors where it appears bank 1 after going into closed loop mode gets close to 0 on bottom of chart and the upper end of bars seems to average about 0.9 approx. On the other hand, bank 2 02 sensor doesn't seem to want to get close to 0, but seems to hover around .1 or higher on the bottom. Don't know if this is a problem, just seems like the 2 O2 sensors should be reading closer to the same. Could I have one or both O2 sensors bad? Have never tried to interpret this report before. Also, STFT for banks 1 and 2 look very similar within (for the most part) 120 to 140 (using the Scan9495 program). I have taken 2 pictures; one for the O2 sensors, and the second for the STFT charts for banks 1 and 2. Top chart in both cases is for bank 1. Exhaust system hasn't been touched, and this car was a factory order. Now has 68k original miles. I don't have a "smartphone" so don't get my messages or emails immediately. I do check messages daily from my laptop. Thanks for your help!

Last edited by topfuelman; 09-16-2019 at 01:00 AM.
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post #8 of 22 (permalink) Old 10-21-2018, 08:20 AM
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O2 sensor readings aren't meant to read exactly the same between banks 1 & 2. The important thing is that readings of O2's are constantly changing. This indicates pcm is changing fuel input based on O2 readings.

On your 94, short term & long term fuel trims should be as close to 128 as possible. > than 128 means pcm is adding fuel because of lean O2 readings. < 128 means pcm is subtracting fuel because of rich O2 readings.

O2 < than 450 is lean exhaust O2 > than 450 means rich exhaust. When you look at a scan the pcm is adding and subtracting fuel many times a second. So the O2 readings are jumping above and below the 450mv middle point on a regular basis.

Again as BALLSS has said, look for vacuum & exhaust leaks. Your vacuum lines on your 94 are almost 25 years old. The intake elbow going into throttle body is a common leak Exhaust leaks are also common on older cars.

If you haven't done so, put a fuel pressure gauge on fuel rail & tape to windshield. Key on prime should be around 43 psi. Idle @ mid 30's. And other running conditions should give at least 40 psi. If fuel pressure is constantly low, pcm will provide lean mixtures. If pressure is constantly higher than 45 psi, pcm will give rich mixtures.

This is because pcm fueling strategy is based on a 40 to 45 psi fuel pressure. Lower or higher will give incorrect fueling as pcm is basing the amount of injector pulse time based on the 40-45 psi fuel pressure.
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post #9 of 22 (permalink) Old 10-21-2018, 01:33 PM
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OP

if bank 2 02 is reading high (always above 450) that side has either a vacuum or exhaust leak.

You could try swapping bank 1 & 2 02's to see if the issue transfers over to bank 1. That would indicate the 02 sensor itself is old and lazy. 02 sensors are a wear item. If they are the originals than put new AC Delco ones in.

Vacuum gauge is very handy detecting several engine issues if you know how to use one.

The "L" shaped vac hose that goes to the center bottom front of intake is worth checking along with all your other vac lines.

run your fingers along the back edge of intake manifold where it meets the block..any oil?

if so your intake manifold is the source of vac leak, fairly common
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post #10 of 22 (permalink) Old 09-14-2019, 08:25 PM Thread Starter
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Same issues and failed CA Smog test with NO high at 25 mph

Sorry it has taken so long to get back to my Z28 problems.l Thanks for everyone's help so far. Can't find any vacuum leaks. Have replaced both O2 sensors and EGR valve (it works when I test it in Scan 94/95 program making engine stumble). Have tested fuel pressure at idle is approx. 33lbs. and drops offf quickly to 28lbs when ignition is turned off. Can't drive car legally (no current registration sticker) so cannot road test now. Tried bringing engine up quickly to 2500 rpm several times and so no change in the fuel pressure from idle (33lbs). I have failed the California Smog Test twice now with high NO readings at 25mph (837ppm) and very low CO readings at same test speed (0.01%) Sounds like a lean condition and no matter how the engine is revved, fuel pressure stays at 33lbs.

Thanks for everyone's help!
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post #11 of 22 (permalink) Old 09-15-2019, 07:13 AM
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Fuel pressure at idle should be in mid 30's. But on all other operating conditions, fuel pressure should be at least 40 psi. GM uses a fueling strategy that relys on constant fuel pressure and changing injector pulse width to get proper air/fuel mixture.

If your fuel pressure is low, the pcm does not know this. It will give injector on time based on 40 psi. Because the pressure is about 18% lower than it should be, the mixture that pcm commands is not being met. O2 sensors are showing a lean mixture. So pcm commands more fuel. O2's still tell pcm that mixture is too lean. So pcm commands even more fuel.

You need to find out if fuel pump or regulator is defective. Or maybe fuel injectors are leaking. You said in earlier post that pressure fell off quickly when key turned off. This can be bad check ball in fuel pump, bad regulator, injector leaks, leaks in fuel lines, or in tank fuel line leak.

Once you get a handle on problem, when you go for inspection, change the oil & filter right before going. This helps in testing. High NOx means combustion process is too hot. The lean mixture, because of low fuel pressure, contains too much O2. This produces a hotter fuel burn & increased NOx.

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post #12 of 22 (permalink) Old 09-15-2019, 03:14 PM
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Can you post a copy of your .csv file. I'll take a look . High Nox burns your eyes, you should see your EGR Duty Cycle (on your .csv file) going from 0 to 100% when RPM is between 1000 and 2000 under power load about MAP of 50.. EG: 25 MPH w/throttle on the test dyno. The EGR should cool the cylinders and slow NOx generation. Lean mix as previously said could cause cyl. heat problems.

+1 on fix the fuel pressure.S/B 41-47 lbs with vacuum hose removed from regulator, simulates WOT low/no vac . Should drop 3-10 lbs with vac line hooked back up at idle high vac. If you running at 30 then your injectors are not adding enough fuel (lean) and the PCM is bumping up the LT trims (BLM's) to add more fuel.. Also check valve in the pump and the regulator valve should hold pressure for 5-15 mins easily when car is shut down (mine holds overnight). Here is some info on how to to check pressure conditions. ref shbox.com 4th Gen LT1 F-body Tech Articles

I'm Bob 1994 Z28, A4, cam, 1.6 RR's CAI, LT's, stall, cat back. Custom tuned. 2007 GMC Envoy, tuned. 2015 Kia Sorento, bone stock.

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post #13 of 22 (permalink) Old 09-16-2019, 01:08 AM Thread Starter
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Can't send a .csv file due to forum restrictions

Would love to post the file, but even when zipped it is over the 97.7k limit (at 146kb) for a zipped file. Tried to send as a .csv file, but was not allowed due to file type!!! And yes, the exhaust is strong and burns our eyes! My wife even complains as the exhaust permeates the ceiling in our garage and enters the house. Is there another way to post the file?

Today, I pulled the vacuum line off the fpr and got approx. 43psi at idle at the gauge. When I turn key to on (run) gauge reads 38psi for about 3 seconds then I hear something like a solenoid click and the pressure drops to around 33psi almost instantly (with line attached to fpr). The "clicking" sound accompanies the fuel pump cutting off.

Car runs great and we drove it about 160 miles and changed the oil before trying to smog it early this year. The three main complaints are 1. Strong exhaust odor 2. High NO readings at 25mph test (causing a failure to pass) and 3. Low CO% at both 15 and 25 mph tests (o.02 and 0.01 % respectively).

Is there an LT1 expert in the San Francisco Bay Area you know of? This project is really dragging on. I could get a temporary permit from DMV to bring to a shop.

Love the car and it only has 68k miles on it. Am sending a picture of car; a factory ordered 6spd. Medium Patriot Red Z28 Convertible.

Thanks much for your help and interest; car has been out of commission for almost a year now.
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post #14 of 22 (permalink) Old 09-16-2019, 06:54 AM
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If file is too big for our site, join a file sharing site like "mediafire" or any of the other file sharing sites. You can then upload file to the site. Then link it back to our site. I'm about out of ideas on your other problems.
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post #15 of 22 (permalink) Old 09-16-2019, 12:46 PM Thread Starter
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Mediafire Link

Mediafire link: http://www.mediafire.com/folder/u2aoobegmh97q/Documents

These files go back to 10/14/18 before I changed both O2 sensors and egr valve.

Looking back at my old smog test reports, it seems that the NO numbers have slowly been creeping up as the years have gone by.

Thanks again for your help!
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