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When you measured the temperature across the cat, was the engine fully warmed up, and had the car been driven enough for the internals of the cat to reach ~600°F? Cat doesn't work until it gets that hot.
Previous owner could have damaged the cat if:
- he drove the car for a long period with misfires
- had a head gasket leak that caused silicates from the anti-freeze to deposit on the O2 sensors and then the cat (ding, ding)
- used leaded fuel at the race track
Yep, I actually measured right after the 20 min drive to Harbor Freight and back and at the car running while I measured.
Is it possible the lean condition could also cause damage to the cat?
So I'm not sure how bad the head gasket issue was with the previous owner, there was no mixing of coolant and oil when I checked. But definitely possible!
I'll check the resistance on the O2 sensors to see if there's anything weird going on with them. Particularly the driver's side.
Originally Posted by Injuneer
Not that simple. The lean condition can be any number of things that only affect one bank of the engine, and can be a “true” lean, or a “false” lean.
I don't think I have a vacuum leak I double checked the hoses when I had the issue where the car would die after a few seconds.
Injectors... it's possible. The fuel pressure gauge went from 43.5 psi to around 33 or so upon start. Something I can check, I'll have to pull the fuel rail and spray it on a piece of paper and look at the flow.
Leaky valve guides - not impossible but highly unlikely since it's a new engine but who knows
Faulty O2- I'm going to check the resistance on the sensors
Exhaust leak before the O2 sensors - I don't have a leak on the driver's side, I'm pretty sure. I had to undo the Y-pipe to exhaust manifold flange to remove the starter and had to replace a stud in the process. Tightened it down pretty good.
Misfire - no misfires that I can tell. The Scanner doesn't show any misfires or DTCs
The things I'll visit first are: O2 sensor testing and injectors on the Driver's side.
Cat will have to be replaced anyway and that should pass smog
I haven't read through your threads, but did you reuse your opti?
I bought my LT1 in CA, and the pre-purchase smog was fairly high on HC and barely passed on NO.
A few months later I figured out that the opti was bad.
The next time it was smogged HC, CO and NO were all much lower.
I did do the other basic tuneup stuff in there as well - plugs, wires, O2S.
You should be able to get it a lot cleaner. My numbers on the same test with 68k on the car were:
HC
9
6
CO
0.01
0.00
NO
429
404
Interesting.. I did re-use the sensor but I replaced the cap and rotor. Because the sensor is the original Mitsubishi and didn't have a problem.
But seeing as how my cat is totally busted, I'll replace that and also try to find out where my car is going lean and hopefully that'll fix it? I'll keep updating
If the A/F ratio closed loop control system is working correctly, and correcting for a true lean condition, it isn't running lean. Cats get damaged when fuel burns on them.... fuel + air, because of the catalyst, the fuel burns intensely, generating a lot of heat and damages the ceramic substrate that contains the catalyst. A lean system means more air in the exhaust than there is supposed to be, but there's no excess fuel to burn. Misfire is the worst case - lots of fuel and air being dumped into the exhaust.
It's the false leans that can cause damage as well, because the PCM is adding fuel that isn’t required. Fuel is going to burn and release heat with the trace oxygen that is released by the NOx reduction.
Explain how checking the resistance of the O2 sensors will tell you anything about them? They are not a resistance based device. The sensor works by exposing the outside of a zirconium thimble to the exhaust, and the inside if the thimble to ambient air. The difference in the partial pressure of oxygen in the exhaust vs. the oxygen in air generates a voltage that either adds to or subtracts from the 0.450 volt bias supplied by the PCM. Using a low impedance multi-meter can damage the sensor due to the current it passes through the sensor.
One thing I should mention. They accuracy of an O2 sensor can be destroyed by assembling them to the exhaust using non-conductive anti-seize. The sensors should come coated with conductive anti-seize. Accuracy can also be destroyed by foreign material like oil, WD40, etc. coating the outside of the sensor if the material blocks the ambient air flow into the thimble. Then there's the quality of the wiring, connectors, sensor heaters, etc.
The fuel pressure should drop 8 to 10 PSI when the engine starts, and the vacuum is applied to the fuel pressure regulator. Incorrect fuel pressure would affect both banks of the engine. Individual injector problems - dirty, sticking, weak magnetic coil, etc. If it comes down to injectors, swap them side to side, see if the problem follows the injectors. Same thing you can do with the O2 sensors (swap side to side).
Exhaust leak.... how about a faulty AIR check valve on the manifold? Warped flange not sealing tight to the head? Faulty gasket on the Y-pipe to manifold connection (can’t even remember if the stock exhaust even had gaskets there).
You don't have any codes for misfires because there is no misfire detection in the OBD-1 system. They had to add a crankshaft position sensor to the LT1 in 1996 to meet the OBD-2 requirement for misfire detection.
When you measured the temperature across the cat, was the engine fully warmed up, and had the car been driven enough for the internals of the cat to reach ~600°F? Cat doesn't work until it gets that hot.
Previous owner could have damaged the cat if:
- had a head gasket leak that caused silicates from the anti-freeze to deposit on the O2 sensors and then the cat (ding, ding)
^^^^, Op, 18k mi is not much mileage on a CAT...but if the above happened that will kill it and the 02 sensors.
Amazon typically has the better $ on AC Delco 02's. They come with a copper anti seize to apply to the threads
There may be other issues going on with your car (injectors) but consider the condition you got the car (blown head gasket)….the PO could have driven the car like this WAY longer than anyone should have causing secondary damage (02's, CATS, etc) let alone other issues on the car that needed attention (injector cleaning, vac leaks, etc)
As you resolve one issue the problems that were masked by a non running engine are becoming evident which you will have to diagnose and resolve.
That makes sense. The PO drove it for over a year in that state. Granted it was small distances to work and back but still.
Ill try switching out one side with the other for both the sensor and the fuel injectors. I'm going to get the cat replaced tomorrow at the shop and will check for exhaust leaks while I'm there as well.
Update: I installed a new catalytic converter this morning and re-tested emission. Same problem, NOx is still high.
The smog tech says I need to look into the EGR more carefully or possibly the air pump.
So here's where I'm at with testing the EGR system.
- The EGR valve holds vacuum, is new. No problem here
- EGR solenoid is good and creating vacuum that I can feel on my finger whenever I activate it using Scan9495's Actuator tool.
- Vacuum hose from intake manifold to EGR solenoid is good
However, when the EGR solenoid hose is connected to the EGR valve and I manually activate the solenoid using my scan tool, the engine kind of stutters but it's not that noticeable. It's slight.
I understand that when you put vacuum on the valve, it should cause the engine to stumble quite a bit because of the exhaust gases entering the combustion chamber at idle, possible even stall out the engine. Mine does not make that much of a difference, just a little bit. It also does not HOLD vacuum at the valve when the car is running and the valve is hooked up to my gauge.
So this tells me that potentially there is a leak or block in the exhaust recirculation pipe??
This one:
Based on the flow chart, if the EGR valve is able to hold vacuum by itself but unable to hold vacuum when the car is running, what does this imply? A potential leak in the exhaust recirculation pipe that's bringing in air?
The other option is the secondary air pump and I'm looking into that. I don't know much about that.
Lastly, I'm pretty sure I bought the correct EGR valve but shbox mentions there are positive backpressure ones and negative.
The one I bought is ACDelco 214-5083, which is supposed to be the special one for manual transmissions. Also the same one listed under rockauto.com
Can someone confirm if this is indeed the correct EGR valve? I'll start there.
If the tech mentioned the AIR (secondary air pump) he is not familiar with the LT1 system. Go back to post #8. It describes the difference between the 3rd Gen system, where the AIR pump injects air into the space between the first bed of catalyst in the cat, and the second catalyst bed. The 3rd Gen system uses a belt driven pump, and the pump runs and delivers air all the time. For the first few minutes it injects air into the exhaust manifolds, just like the 4th Gen system, to burn excess fuel from the very rich cold start A/F ratio (reduces HC and CO emissions) and generates heat that warms up the cat so it works.
The difference in the 3rd Gen systems happens when the cat is hot. The 3rd Gen system belt driven AIR pump continues to run, and a diverter valve stops the air flow to the exhaust manifolds, and sends it to the pipe on the cat. In the 4th Gens, the electric driven pump shuts down, and is not used again. The cat does not need injected air to function. There is no way the AIR pump can affect the exhaust emissions once the cat is hot, and the pump shuts off. And, the air that flows into the 3rd Gen 2-bed catalyst is used to oxidize HC and CO, not part of the reduction reaction that breaks down NOx to elemental N2 and O2.
The cat without the air injection pipe requires very precise control of the A/F ratio. This includes the PCM toggling the A/F ratio back and forth rapidly between slightly rich and slightly lean. The cat first reduces the NOx to N2 and O2. The cat “stores” the O2. Then the cat uses the residual O2 in the exhaust gas + the stored O2 to oxidize the HC and CO to water (H2O) and carbon dioxide (CO2). The entire process requires that the A/F ratio has to average 14.7:1. And the O2 sensors have to be responsive enough to switch rapidly back and forth between slightly rich and slightly lean. In OBD-2 GM added diagnostics for the O2 sensor switching (cross-counts), and O2 sensor response transition time. Unfortunately these do not exist in OBD-1.
I don’t think your AIR pump has anything to do with the high NOx. Your cat replacement seems to rule out the cat as part of the NOx problem. I think there is something in the left bank A/F ratio control that may be contributing.
And the EGR flow needs to be considered. Simple to check the corrugated riser tube, and not likely it's plugged up, unless the previous owner was running the system extremely rich. But there is an internal passage in the intake manifold from the corrugated tube to the EGR valve, the pintle of the EGR valve and the hole in the manifold it moves in and out of, and the passages internal to the manifold that distribute the EGR to the cylinders. Any of those could be carboned up.
I'll have to check my old intake manifold, still sitting in my garage. I can’t remember the exact path of the EGR, and whether it combines with the idle air that passes directly through the hole in the throttle body, or whether it combines with the air that has passed through the IAC valve, and flows to the small hole in each intake runner.
Not sure I understand your comment about the EGR valve not holding vacuum when the engine is running and you somehow have a gauge hooked up to it. The negative backpressure valve is very complicated in my mind, and isn't clear to me how that reacts to constant vacuum, and whether the difference between pulsed vacuum and steady vacuum may explain the vacuum leakdown. I can’t really sort it out logically. Here's some info... maybe someone else can condense this into the Reader's Digest version so I can think my way through it.
If the tech mentioned the AIR (secondary air pump) he is not familiar with the LT1 system. Go back to post #8. It describes the difference between the 3rd Gen system, where the AIR pump injects air into the space between the first bed of catalyst in the cat, and the second catalyst bed. The 3rd Gen system uses a belt driven pump, and the pump runs and delivers air all the time. For the first few minutes it injects air into the exhaust manifolds, just like the 4th Gen system, to burn excess fuel from the very rich cold start A/F ratio (reduces HC and CO emissions) and generates heat that warms up the cat so it works.
The difference in the 3rd Gen systems happens when the cat is hot. The 3rd Gen system belt driven AIR pump continues to run, and a diverter valve stops the air flow to the exhaust manifolds, and sends it to the pipe on the cat. In the 3rd Gens, the electric driven pump shuts down, and is not used again. The cat does not need injected air to function. There is no way the AIR pump can affect the exhaust emissions once the cat is hot, and the pump shuts off. And, the air that flows into the 3rd Gen 2-bed catalyst is used to oxidize HC and CO, not part of the reduction reaction that breaks down NOx to elemental N2 and O2.
The cat without the air injection pipe requires very precise control of the A/F ratio. This includes the PCM toggling the A/F ratio back and forth rapidly between slightly rich and slightly lean. The cat first reduces the NOx to N2 and O2. The cat “stores” the O2. Then the cat uses the residual O2 in the exhaust gas + the stored O2 to oxidize the HC and CO to water (H2O) and carbon dioxide (CO2). The entire process requires that the A/F ratio has to average 14.7:1. And the O2 sensors have to be responsive enough to switch rapidly back and forth between slightly rich and slightly lean. In OBD-2 GM added diagnostics for the O2 sensor switching (cross-counts), and O2 sensor response transition time. Unfortunately these do not exist in OBD-1.
I don’t think your AIR pump has anything to do with the high NOx. Your cat replacement seems to rule out the cat as part of the NOx problem. I think there is something in the left bank A/F ratio control that may be contributing.
And the EGR flow needs to be considered. Simple to check the corrugated riser tube, and not likely it's plugged up, unless the previous owner was running the system extremely rich. But there is an internal passage in the intake manifold from the corrugated tube to the EGR valve, the pintle of the EGR valve and the hole in the manifold it moves in and out of, and the passages internal to the manifold that distribute the EGR to the cylinders. Any of those could be carboned up.
I'll have to check my old intake manifold, still sitting in my garage. I can’t remember the exact path of the EGR, and whether it combines with the idle air that passes directly through the hole in the throttle body, or whether it combines with the air that has passed through the IAC valve, and flows to the small hole in each intake runner.
Not sure I understand your comment about the EGR valve not holding vacuum when the engine is running and you somehow have a gauge hooked up to it. The negative backpressure valve is very complicated in my mind, and isn't clear to me how that reacts to constant vacuum, and whether the difference between pulsed vacuum and steady vacuum may explain the vacuum leakdown. I can’t really sort it out logically. Here's some info... maybe someone else can condense this into the Reader's Digest version so I can think my way through it.
Thank you for the explanation. I can rule out the secondary air system so that's good. I've been reading a while today about this. That PDF has been really helpful.
It's explains what I was trying to understand: There's some mechanism that is closing off my EGR valve prematurely, resulting in too little EGR flow. Might explain why my engine runs slightly on the hot side which I combated through the use of cooling fans. (Also makes sense that the previous owner had the same issue on a different engine)
Now I know it is the negative pressure inside of the intake manifold at the bottom of the EGR valve is overcoming the vacuum from the hose at the top of the EGR valve. This is why even when I hand pump vacuum from my Mityvac when the car is running, it is unable to hold vacuum and unable to hold the EGR open. It will build but drop quickly like something is fighting me.
And when the car is off, it will hold vacuum no problem because there is no negative pressure coming from the intake manifold to the bottom of the EGR valve.
The part that the PDF didn't explain is, what causes vacuum/negative pressure in the intake manifold? I don't understand airflow and vacuum all that well.
Is it the exhaust gases? Would carbon build up or blockages in the intake passages increase vacuum? I assume so since I imagine it'd be like sticking something inside of a straw, the vacuum pressure would go up.
I can't find a picture or diagram of the intake passages but if there was one, I'd have an idea of how to clean it. I'm going to have to pull the EGR valve off and spray some heavy doses of carb cleaner down the hole of the intake manifold as well as disconnect the corrugated pipe and spray down there.
I wish I had known this before because I would've cleaned it much better when the engine was out of the car. Live and learn.
but the good news is, I love driving this car. Took it for a spin today for about 20 minutes after I got the catalytic converter installed. Gotta love V8s..
Ok I'm looking at shoebox's flow diagram and trying to think about this logically and I think I figured it out. Well I have a hypothesis, only one way to see if I'm right.
That PDF helped clarify a few points.
Basically the intake manifold is always in a state of negative pressure. It's sucking in air from the throttle body opening and down the runners into the engine. Which is why it also has vacuum applied at the little hose going to the solenoid.
There has to be a break in this negative pressure enough to where the vacuum on the EGR valve hose can lift the diaphragm open against the spring. Otherwise it will suck back down. And it does this through the help of exhaust gases.
I was trying to build vacuum using my Mityvac and wasn't able to hold it for long because the negative pressure inside the intake manifold is too great, it was basically sucking the EGR valve closed through the mechanism described in the PDF (using the bleed holes, pintle, etc.)
In order to keep the EGR open, the intake manifold negative pressure has to be relieved enough to where the vacuum hose from the solenoid can maintain it open. This is where exhaust gases come in. Exhaust gases from the corrugated pipe is positive pressure coming into the intake manifold. Positive pressure meets a negative pressure state in the intake manifold, and now it starts to level out, allowing the vacuum hose from the solenoid to be strong enough to open the EGR valve.
Once there is not enough exhaust gases (i.e. low RPM) the positive pressure (AKA exhaust backpressure) into the intake manifold is gone, allowing the intake manifold to return to a negative pressure state. Which closes the EGR valve.
So my theory now, is that my car has a lack of backpressure from the exhaust, i.e. lack of exhaust gases being routed back to the intake manifold.
Which can either be a leak OR a blockage in the corrugated pipe or gasket area. If there's a leak, exhaust gases will escape into the environment before making its way into the intake manifold. If there's a blockage, exhaust gases are unable to get to the intake manifold either. Or there's a blockage in the intake manifold where the exhaust gases come in.
I think the plan of action is to spray some soap water on the gasket area of the corrugated pipe to see if there's a leak. If there are no leaks, then I'll spray some air down the pipe to see if there's a blockage. I'll also remove the EGR valve itself and spray carb cleaner down the ports of the intake manifold particularly the exhaust side.
Remember that any leaks in the exhaust pre-cat could also be the issue. Have you checked all of the exhaust for leaks? Stock manifolds have been known to have micro-cracks that can cause this type of issue.
Remember that any leaks in the exhaust pre-cat could also be the issue. Have you checked all of the exhaust for leaks? Stock manifolds have been known to have micro-cracks that can cause this type of issue.
I just re-did the exhaust flange that goes from the exhaust manifold to the Y-pipe so I know those are done right. I haven't checked the exhaust manifold to head connection or the secondary air pipes yet.
What's the best way to check? The space is so tight do you just put your hand down near the exhaust manifold when the engine is cold and feel around?
I'm looking at the EGR valve and the corrugated exhaust gas return pipe and the space is so tight, this job is going to be a total bitch. I don't know how I'm going to clean back where without pulling the intake manifold off. I might have to get a really small kid's toothbrush and a long straw
Space in these cars is by far the biggest challenge. most work on the back of the engine is done blind.
My exhaust is so loud and shaky I can't hear anything lol but I'll give that a try.
yeah tell me about it, I had to re-do the banjo bolt gasket on the back of the head.. trying to not lose the $10 washer while threading it into the hole blind was a challenge.
11-05-2018, 09:47 PM
