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There is only vacuum (negative pressure) in the intake manifold when the throttle blades are not fully open . Actually, even when the blades are fully open (WOT), there is a small vacuum. The barometric pressure pushing air into the cylinders as the pistons drop is lost by pressure drop across the air filter, and frictional losses through the ducting, MAF, and throttle body. But at WOT the vacuum is very low.
There is enough data in the Scan9495 to allow you to calculate the vacuum. The “Barometer” column approximates the air pressure outside the engine compartment. The PCM reads the barometric pressure off the MAP (manifold absolute pressure) sensor when the key is turned to “start” and there is no air flow causing pressure loss between the air filter and the manifold.
At this point, you need to understand the difference between “gauge” pressure and “absolute” pressure. At sea level, the weight of the column of air (from the upper edge of the air layer above the earth to the ground) is nominally 14.7 PSI(A), or 29.92”Hg, or 101.4 kPa (kilo Pascals, a metric unit of pressure). The scanner normally reads the MAP sensor in kPa.
You can’t “feel” that 14.7 PSI(A), and a conventional pressure gauge can’t measure it. But it's there. Because the density of the air in the cylinders (and hence the mass) is proportional to absolute pressure, the PCM needs to know the MAP to calculate mass air flow (assuming the MAF sensor is not working and the fuel system has defaulted to speed-density). The pressure you read off a pressure gauge is only the pressure in excess of the 14.7 PSI(A) barometric pressure.
If you are looking at a scanner, or at a data log, subtract the barometric pressure from the MAP reading to calculate vacuum. At WOT, you might see MAP = 98 kPa, and BAR = 102 kPa.
98 - 102 = -4 kPa vacuum.
Now start closing the throttle blades, and at (almost) closed throttle idle the MAP will be maybe 32 kPa on a healthy engine. 32 - 102 = -70 kPa vacuum. The vacuum is there at idle because the piston is dropping, trying to fill with air, but the throttle blades are on obstruction to flow that is creating a 70 kPa loss of pressure.
EGR is not used on the LT1 at WOT..... there is too little vacuum to open the EGR valve.
I had a emmisions failure on NOx when I lived in CA. Maybe I can share some of the things I tried will help you. My '94 failed right after installing headers. This I realized lowered the exhaust back pressure enough to keep the EGR valve from operating as the '94 EGR valve requires a amount of back pressure to be to operate regardless of the solenoid command. I never found a fix for this but I see you are still using stock exhaust manifolds so shouldn't be a problem though could be your downstream piping has reduced restrictions - we don't know.
So one thing I tried was E85 and though it did lower the NOx it wasn't enough to pass yet. I think I should have ran a couple of tanks of E85 before retesting rather than the partial tank of E85 I used.
A good cat converter looks like your next step that's not to costly but I read the California laws have become extremely difficult to replace a cat in California now so you need to work with your muffler shop on that.
NOx is related to compression so if your new motor has high compression you need to find out with a compression test. If compression is high your options become limited to swapping heads with larger chambers or retarding the camshaft. But you need to do a compression test first.
Funny coincidence is now that I live in Nevada they don't test for NOx even though they use the sniffer test on OBD I cars. But I'm sure you don't want to move.
Good luck and I hope this helps.
Hey thanks for the tip. When you ran E85, how much of it did you run? Do you remember your before and after Nox levels after E85?
I just installed a new catalytic converter and it made no difference in Nox, actually my nox went up by a little bit but my HC and other levels went down. Smog tech is convinced its definitely something in the EGR system that's causing it because all other emissions is clean.
I don't have headers just a stock manifold and y-pipe, I think the cat-back might be custom piping + flowmaster but I'm not sure. The pipe that runs from the cat to the muffler is 3" diameter. The muffler however reduces down to the stock tip size and steps down in piping size.
Definitely I'm lacking exhaust backpressure but from what I'm not sure. If I had to guess, my money would be a leak somewhere pre-cat (maybe the pipe going back to the intake manifold) OR a blockage with carbon inside of the intake manifold. I'm hoping I find something.
My plan for this weekend is to start by checking for leaks. Im going to seal the end of 1 muffler tip off and the other I'm going to attach my shop vac and pull a vacuum on the exhaust system with the car off. Start listening and feeling for a hissing anywhere.
Also going to use my endoscope camera and snake it into the exhaust pipe going to the intake manifold and the intake manifold EGR passage as well. If I find blockage then the intake manifold is coming off most likely.
Alright so update... spent the day troubleshooting. I'm kind of out of ideas and options here unless I missed something
I started by checking for exhausts leaks.. everywhere. Plugged both tail pipes and pulled a vacuum with a shop vac. Did not hear any hissing or leaks. I unbolted the corrugated exhaust pipe from the intake manifold to see if there's a blockage and nope. I was able to feel vacuum on my hands. Which means there's no blockage all the way from the tailpipe to the intake manifold. This pipe is clear.
But that wasn't enough so I now removed the vacuum and used my air compressor to push positive air into the exhaust system. Went around the entire exhaust particularly pre-cat area and used my hand to feel for air or listen for hissing. Everywhere I can, I sprayed soap water including both secondary air injection pipes, the top of the exhaust manifolds, the exhaust manifold-to-ypipe connections, and the corrugated exhaust pipe at the exhaust manifold connection. No leaks. I made sure to tighten down the y-pipe/exhaust manifold nuts anyway.
So currently at this point there are no leaks and there are no blockages up to the intake manifold. Which leaves the intake manifold passages only.
I used my endoscope camera.. here is the inside of the corrugated exhaust pipe going from the stock headers to intake manifold. No blockages, not really that dirty either:
I sprayed carb cleaner in here anyway as well as sprayed carb cleaner inside of the intake manifold where this pipe attaches to.
Next, I removed the EGR valve itself:
Here are the two holes for the EGR in the intake manifold.. the top one goes to the intake manifold from the EGR valve. Exhaust gases go into here. The bottom circular hole, is coming from the exhaust manifold pipe. When the EGR opens up, it transfers exhaust gases from the circular hole into that square hole. Well.... the square hole/intake manifold is clean as you can see in the video. The circular one was dirty but there were no major blockages. I have a hard time believing this dirtiness would result in nox levels of 1000+.
When I push air through the tailpipes, there is a strong rush of air that comes out of the circular hole so there are indeed no blockages ALL THE WAY up to the EGR valve itself from the tailpipe.
But I went and sprayed carb cleaner liberally down both holes and used shop towels to stuff it in as much as possible and pulled it out a few times.
Then here's where I noticed something interesting, I pulled my old EGR out of the trash. Unfortunately, I cannot tell if this EGR is even correct it's the one that came with the previous owner. Mine is a new ACDelco 214-5083, stamped 19209963. There is no N but according to rockauto and shoebox this should be the right one for my 1994.
New vs Old (right) New ACDelco has bleeder holes in the lower diaphragm Old EGR does not have bleeder holes New EGR's tube is longer. The metal casting is longer
The biggest difference is the bleed holes in the ACDelco and the old one does not have bleed ones. The 2nd biggest difference is the diaphragm travel size. The ACDelco's diaphragm has to travel 7mm in order to reach full vacuum. The one old only needs to move 3mm.
He had issues with backpressure and said he ended up plugging the bleeder holes.
I took 3 tiny pieces of Gorilla tape and blocked off all 3 bleeder holes in the EGR valve and installed it back no my car.
When I started the car up, I activated the EGR solenoid to 100% using Scan9495 and the car definitely ran like crap and stumbled. Idle didn't drop or die but it was a little more noticeable than before.
I don't know if it's the cleaning that I did or if it's the bleed hole plugging. Or if it's making any difference at all for real NOX numbers.
So that is where I'm at now...
There are no blockages or leaks. Pushing air through the tailpipes results in air coming out of the corrugated exhaust pipe AND the EGR port which indicates the intake manifold passage has no blockages. The EGR valve apparently is the correct one.
I have yet to find anything really dramatic that's noticeable such as a major leak or blockage, etc. etc. which makes me wonder if this car will pass on the 3rd re-try. Each time I test it it takes about 45 mins or so, it's really annoying so I want to make sure I have reasonable confidence what I did will have an effect on NOx levels.
I can try to test with the bleeder holes plugged or do it with it unplugged. I might try plugged.
I wish there was a way for me to test NOX so I don't have to go thru this smog ordeal only to find out if it passed or not.
I was going to go to Autozone to compare my EGR valve with another one but I think I have the correct one. So I guess the very last thing I am going to do is re-install the EGR valve with the bleeder holes blocked off, and observe the diaphragm visually when I use my scan tool to activate the EGR solenoid. If the diaphragm holds its vacuum VISUALLY, I will be convinced it's working. My MityVac might be toast or inaccurate.
Last concern, my datalog showed lean on bank 1 for long term fuel trim. Since I'm pretty sure it's not an exhaust leak, I'm thinking it's fuel injector(s). Before I pull the injectors I bought a bottle of Redline to run through the car to see if there are any changes to this. I know running lean can affect NOx levels but it's only running lean on 1 bank, could be a possibility there but I feel like at nox levels of 1000+, the EGR system is not working at all instead of just running lean.
If the bleeder holes are blocked, it effectively bypasses any need for exhaust backpressure. This MAY present an issue for drive-ability such as a stutter or hesitation due to the EGR not closing fast enough but would not have a problem with the EGR opening. It's actually better from an emissions standpoint. The exhaust backpressure control built into the valves are there so that the EGR does not come on at weird times because in order for the EGR to be efficient and not affect driving, you wouldn't want it to come on when you're at idle or not moving, etc. But it's not entirely necessary
This should fix the NOx levels and get the EGR working. Will go for a 3rd test after the weekend
Remember.... the elevated LTFT's are the PCM's response to lean O2 readings. If the system is working correctly - no false leans - the elevated LTFT's have corrected for the lean condition. If the O2 lean readings are “false”, the PCM would be adding extra fuel, creating a rich condition. With a rich condition, HC and CO go up, NOx goes down. Your HC and CO results are not elevated.
I downloaded the 15-minute driving log when you posted it. But other things got in the way. I'll try and review that log ASAP.
Hey Cardo0, actually you don't have to disassemble the EGR to do that. There are holes in the pintle inside of the EGR where the shaft is but you can block the holes where the lower diaphragm is.
So, I'm feeling good about it. I think I solved it but I won't know until I test again.
Cleaned the intake manifold passages one more time and did a test.
Scenario #1) EGR installed as-is with bleeder holes open to atmosphere - Activated EGR solenoid. What I saw visually helped explain what was going on. The diaphragm was moving in and out, unable to hold full vacuum. The way negative pressure EGR works is the intake manifold negative pressure will pull the shaft down and close the EGR without enough exhaust backpressure. So the EGR solenoid is pulling vacuum but the intake manifold is continually trying to close it, essentially not letting the EGR valve open all the way or hold vacuum.
This is why I noticed "some" stumbling at idle but not a whole lot. And this is also why I was able to get some vacuum on my gauge but unable to hold it at idle.
Scenario #2) EGR installed with all bleeder holes plugged - The theory behind this is essentially bypassing the EGR exhaust backpressure control unit, eliminating exhaust backpressure as a prerequisite for the EGR valve to open up. Only the EGR solenoid would be responsible for EGR valve operation now.
With this setup, when I activate the EGR solenoid to 100%, the diaphragm will suck up ALL the way and hold vacuum. That is the difference.
Datalog differences between scenario #1 and scenario #2 showing Engine RPM and EGR Duty Cycle %:
EGR bleed holes open
With EGR Duty Cycle at 100%, there is very little RPM change at idle. You can hear it stumble a LITTLE bit (kind of pulse) but because the EGR valve is unable to remain open to let exhaust gases back in, this is not effective at re-routing exhaust gases back into the combustion chamber and thereby reducing Nox levels. EGR Duty Cycle 50% or 100% really makes no difference here. RPM never really made it below 700-750 I think
EGR bleed holes closed
You can see that now with the EGR valve PROPERLY engaging and opening, the engine RPM is responding to the influx of exhaust gases that are making its way into the intake manifold. Even at EGR Duty Cycle 50%, there is a lot more RPM variability, causing a rougher idle. RPMs fluctuated from 800 down to about 450.
Based on these tests, I can hopefully hypothesize that the EGR will work as it should and reduce NOX levels at my next smog.
Now, I thought about the reason why a negative pressure EGR valve was used in the first place. Given that the EGR only comes on while the car is moving, not at idle, not at WOT, it seems like detecting for exhaust backpressure is an unnecessary mechanism particularly when you can visually observe that the EGR valve has no problem closing, or lagging to close. Therefore the EGR solenoid run by the computer should be sufficient enough to make the EGR system work properly without the engine stumbling or stalling. But then again, I haven't driven it long distances or did any major tests in this state but I don't think there will be any issues.
Also, fixed the glove latch mechanism today
If you have a glove latch that doesn't work i.e. doesn't engage the mechanism, check to see if the spring shown below has slipped off the plastic notch and behind/top where the red arrow is. My spring slipped and was there and it took me going to the junker to take a look at a good one to see what was going on.
FWIW I have the same EGR (M6 EGR w/holes in it) 383, cam, headers, larger (2.5") CAT back exhaust and my NOX, and Co% is very low. Always has been. The HC are "close" to MAX allowed but still under so I pass
A mechanically sound motor and good tune...so if either is not solid you will have emission issues
1. Lean Fuel Mixture. Either not enough fuel or to much air is being added. This causes high combustion chamber temps. Can be several causes for either
2. Bad EGR System
3. Bad CAT but less likely on EGR equipped cars
4. High engine mileage where carbon build up has happened
5. Engine running at high temps (bad cooling system)
Ok so here's an update. Blocking the bleed holes lowered Nox by like 100 points, not much. It's still around 9xx something. So I'm going to revert the valve back to stock. I found a post-cat exhaust leak right where the rear axle is. It must be recent and the clamp must have worked its way loose so I'm going to get that sealed tight.
I don't know if this leak is enough to reduce exhaust backpressure by that much, I don't think so. It's very small only see it with soap water.
The EGR still will not hold vacuum and engine will not stumble with bleed holes unplugged. There is either a major leak somewhere I missed or something else is going on.
I do believe it's possible to have a reason related to combustion (i.e lean condition) that creates high NOx so I'm not ruling that out, but currently my EGR system is not working as its should so I need to address that first. It will not hold vacuum even manually on a vacuum gauge with the car running.
There are only 2 possibilities for this... too much negative pressure in the intake manifold or too little exhaust backpressure. Those are the only two factors that contribute to whether or not the EGR valve can remain open according to the design of the negative pressure EGR valve.
Other possibilities would be exhaust leak before cat (I couldn't find any), unmetered air somewhere that making its way into the intake manifold causing it to run lean. But if it's pre-o2 sensors, the o2 sensors should pick that up and the PCM should compensate for the lean condition. My datalog shows long term fuel trim to be about +7% which isn't all that high/lean.
Every time it's smogged, the AFR was 14.7. It's just the Nox /Egr system.
I'm going to take it to a muffler shop and have them professional check for leaks from front to back but a lot of these shops around me don't know much about the lt1s.
It's puzzling that even with the EGR bleed holes blocked and the EGR system theoretically putting exhaust gases back into the CC, that Nox was still high. I don't understand that. And if my engine was running that lean, I feel like datalog would show in the LT or ST fuel trims. The engine has no driveability issues either
EDIT: I just called the performance shop that rebuilt the engine and yeah everything is bone stock. I have the piston part # also, it's just a regular flat top piston that's forged. I feel like it would be hard to accidentally build a higher compression engine, you'd have to do something with the pistons or the heads, etc.
The exhaust for the LT1 isn't anything special and a good shop should be able to locate a leak if there is one.
I would also recommend getting the car tuned. Even though everything went back to stock, you can still likely get some extra performance and possibly find your NOX issues with data logging and a knowledgeable tuner.
Took the car to a muffler shop. A different one from the one that did my catalytic converter just so I can get a second opinion. They used a stethoscope to check for leaks from front to back including the stock headers.
No leaks before the cat.
This doesn't make any sense to me. At this point if there's a shop that knows LT1s I'd hand it over but I don't know of any.
I am trying to use deductive reasoning to see what kind of parts overlap there was with me and the previous owner since he basically had the exact same Nox results on a different engine.
I'm left with:
- O2 sensors
- Check valves in the secondary air system
Smog tech said first thing he'd do is just replace the O2 sensors. If they are not getting bad but not at total failure, they will be slow to read.
He also said other possibility is that the secondary air system is injecting air in the system when it shouldn't, and that can cause false readings. But I imagine if the secondary air injects air when it shouldn't, the fuel trim would reflect this.
Injuneer, would you consider these O2 sensor reading discrepancies abnormal on either side? It appears Bank2 reads low/lean on start up and Bank1 reads high around 800mv+ until about 2 minutes into the start up, they both converge. There are moments where B1 and B2 O2 sensors are off by a bit, more than your guide suggests of 20-30mv. Sometimes 1 is at 800mv and the other is at like 200mv. But only few moments in between.
But regardless, almost all my datalogs show Bank2 (purple line) is almost always on the low side on initial start up compared to Bank 1.
I'm still working on your data log. You have elevated left Bank LTFT's in all the cells the engine operated in, in the data log. As engine load goes up, the LTFT's go up, and in the max cells the right bank LTFT's start to elevate too. I am not located where I can look at them right now, but as I recall some of them go into the 140’s. That's 10% extra fuel. Anything outside +/-5% suggests a problem.
Are those plots from the same 15-minute driving data log I downloaded above?
Another comment on the 15 minute driving log - you are only scanning at 2 frames/second or less. You need to get above 5 frames per second. Slow scan masks the data. The best logs run close to 10 frames per second. Particularly when looking at how fast the O2 sensors are switching.
A lot on my plate right now, but still looking at that log.
11-08-2018, 01:54 PM
