Question: catalic converter on a 1990 corvette

[Hib Halverson]

Going pupless will have no affect.

As to the main cat, I'm a little confused as to what your asking about. The oxygen sensor is ahead of the cat. The AIR goes into the middle of the cat. I mention that because, I'm sorry, but.I did not understand this...

I believe the computer compensates for AIR at the manifold anyways with regards to the O2 sensor.

 

[joshwilson3]

Going pupless will have no affect.

As to the main cat, I'm a little confused as to what your asking about. The oxygen sensor is ahead of the cat. The AIR goes into the middle of the cat. I mention that because, I'm sorry, but.I did not understand this...

Just wondering if there would be any problems removing the cats and leaving the rest of the AIR intact? I'm guessing not though.

With regards to AIR and the O2 sensor:

Yes, you have an AIR tube going to the cat. But you also have AIR going to both exhaust manifolds. I have read the computer makes adjustment to the O2 readings with regards to the extra air being injected into the exhaust manifolds from the AIR system. I just mentioned that as even though removing the cat would remove the AIR at the cat. It shouldn't affect the computer since the AIR would still be pumping in at the exhaust manifold.

 

[Hib Halverson]

I think the bigger question to answer first is does your State require periodic I/M tests?

If it does, you want to be careful about removing exhaust emissions equipment.

If it doesn't and you want to "cap-off" the AIR pipe going to the cat, you could do that without affecting the operation of the rest of the AIR system.

 

[boomdriver]

Is there any problems removing the cats, and leaving the AIR intact on the L98? I believe the computer compensates for AIR at the manifold anyways with regards to the O2 sensor.

confused:confused

L-98 ecm could care less what happens 3" past the o2 sensor and the A.I.R. system has nothing to do with intake air.

o2 sensor input to the ECM directs pump air to the cat when warmed up and time to "lite-off" or to the ex manifold, or wasted to the silencer.

I'd be more concerned about a state inspection than where the air goes when the cats gone...Seeing as how gutting the cat is'nt going to help anything, why breath extremely toxic dust just to preform an old school ritual?

replacing a clogged cat is $300...or less.
if there's nothing wrong with it, why bother? its not going to run better. More old school BS.

 

[joshwilson3]

confused:confused

L-98 ecm could care less what happens 3" past the o2 sensor and the A.I.R. system has nothing to do with intake air.

o2 sensor input to the ECM directs pump air to the cat when warmed up and time to "lite-off" or to the ex manifold, or wasted to the silencer.

I'd be more concerned about a state inspection than where the air goes when the cats gone...Seeing as how gutting the cat is'nt going to help anything, why breath extremely toxic dust just to preform an old school ritual?

replacing a clogged cat is $300...or less.
if there's nothing wrong with it, why bother? its not going to run better. More old school BS.

You must be forgetting about the AIR pumping air into the exhaust manifolds BEFORE the O2 sensor. There are 3 check valves for the AIR system. One for the cat, and one at each exhaust manifold. I've read the ECM makes changes with regards to the O2 sensor readings when it calls for air to get pumped into the exhaust manifold. As obviousy air getting pumped into the exhaust manifold would scew the O2 readings. It probably doesn't matter if you leave the AIR intact. I just pointed it out.

My interest in possibly removing the cat has nothing to do with performance.

 

[TedC]

Reducing exhaust back pressure (ex. removing/gutting the cats) can mess up the operation of the L98 EGR because it depends on a set level of exhaust back pressure to properly modulate the EGR valve. Possible net result, introduction of engine ping on light acceleration that doesn't go away with 93 octane. This is especially true of the late model MAF L98s that had aggressive (relative to early L98s) timing curves programmed in by GM.

 

[joshwilson3]

Reducing exhaust back pressure (ex. removing/gutting the cats) can mess up the operation of the L98 EGR because it depends on a set level of exhaust back pressure to properly modulate the EGR valve. Possible net result, introduction of engine ping on light acceleration that doesn't go away with 93 octane. This is especially true of the late model MAF L98s that had aggressive (relative to early L98s) timing curves programmed in by GM.

I get knocks on acceleration with my 89 even with a new negative EGR valve, running 93, and 6* advance timing. I don't know if 89 is early or late L98? The EGR valve is negative and relies on manifold vacuum to open.

The old EGR valve was bad, but I never got a code. I'm thinking my EGR valve may not be working properly which I suspected could be due to too much backpressure. As the EGR valve opens and holds vacuum fine with the engine off, and when I then start the engine the vacuum pump goes to zero like it should. But with the car running, I can't pull a vacuum on the EGR valve. Which I should be able to pull a vacuum to open it and stall the engine. On my other car, I could pull a vacuum on the negative EGR valve with the car running or not running. My next step is to rig up a wire to the EGR solenoid, then ground out the solenoid with the car running to see if constant manifold vacuum can open the EGR valve with the car running.

 

[TedC]

Early C4 L98s 85-87 had a less agressive timing curves programmed into the EPROM. GM changed that in 1988 and 1989. I don't know about 1990, but in that year GM switched to a speed density system vs. MAF for tighter emission calibrations. SD didn't last long, but the L98 was replaced by the LT1 in 1991.

After testing my old (lazy) EGR against a new one, is when I finally understood how this EGR operates. The ECM manages the source signal of when and how much to open the EGR valve. You can verify this by installing a "T" in the EGR vacuum supply line with a long vacuum hose connected to a gauge and watch the needle pulsate during cruise conditions. Vacuum should drop, even under light engine demand. Part of EGR design is the use of exhaust back pressure that can override what the ECM is trying to do. Basically it creates a vacuum leak in a secondary valve which causes the EGR valve to close. Some test I ran... Manually open the EGR valve at idle. I used my finger to accomplish this. Not easy, but it opened just enough to confirm the ports were clear because idle turned rough. Next, check for EGR vacuum by running the gauge only to the EGR supply vacuum line. You should see the gauge pulsate rapidly at cruise when the engine is running closed loop. Next plug the EGR line and connect a vacuum pump only to the EGR. Run the engine in park and with a few quick pumps (one or two pumps may not be enough) the EGR should start to open and idle quality should drop. With the EGR forced open, when you rev the engine, back pressure causes the secondary valve in the EGR to open, which creates an internal EGR leak, and the EGR closes. My EGR was lazy. What I mean by that is that it required more vacuum to open when compared to a new one, especially for partial opening. I also experimented with the EGR by running a vacuum pump into the cabin so I could manage it on cruise conditions instead of the ECM. In closed loop it didn't take more than a few miles before the engine developed a ping with no vacuum provided. When I added vacuum on cruise the ping disappeared after a few miles. It convinced me how important the EGR system is to a propely running L98. I have found that L98s with the iron heads or those with aluminum heads with less aggressive programmed timing are not as sensitive to this. The net of all of this... Too much or too little exhaust back pressure can mess up the fine balance GM designed into this engine. On the topic of pinging, carbon build-up can cause this as well. My 1988 had leaking injectors and was all carboned up due to years of poor maintenance from PO. I installed new injectors, ran FI cleaner with Techron for a few months and sea foamed the intake system twice to eliminate that annoying pinging.

 

[boomdriver]

You must be forgetting about the AIR pumping air into the exhaust manifolds BEFORE the O2 sensor. There are 3 check valves for the AIR system. One for the cat, and one at each exhaust manifold. I've read the ECM makes changes with regards to the O2 sensor readings when it calls for air to get pumped into the exhaust manifold. As obviousy air getting pumped into the exhaust manifold would scew the O2 readings. It probably doesn't matter if you leave the AIR intact. I just pointed it out.

My interest in possibly removing the cat has nothing to do with performance.

Only does that in OPEN LOOP.
it makes no difference. Pump air to manifolds is just a place for it to go...open loop...meaningless to the o2 at that point in time.

Pump air switching to the Cat, is closed loop, the time when the 02 sensor matters.

You have an a.i.r. DIVERTER valve, as well as an a.i.r. SWITCHING valve. The o2 sensor does not react to the air in the manifold for tuning, its just used to help heat the sensor and cat; thats there to aid in maintaining the ex temp so the cat can light off, then air is SWITCHED to the cat (closed loop operation) and includes the o2 input at that point. Prior to closed loop, o2 input is none to zero (cold) as far as the ecm is concerned. It has to be hot enough and all other eng mngt has to be correct THEN the system closes those circuits and manages by sensor input. Until then, its an automated system designed to "stimulate" the cat and the o2 into getting hot enough to create the desired reaction. In other words, the o2 sensor has no say in the matter when the a.i.r. is being directed to the ex manifolds. As soon as it does, that air is then sent to the cat where it is then sent to manifolds or the cat as needed.

 

[joshwilson3]

Early C4 L98s 85-87 had a less agressive timing curves programmed into the EPROM. GM changed that in 1988 and 1989. I don't know about 1990, but in that year GM switched to a speed density system vs. MAF for tighter emission calibrations. SD didn't last long, but the L98 was replaced by the LT1 in 1991.

After testing my old (lazy) EGR against a new one, is when I finally understood how this EGR operates. The ECM manages the source signal of when and how much to open the EGR valve. You can verify this by installing a "T" in the EGR vacuum supply line with a long vacuum hose connected to a gauge and watch the needle pulsate during cruise conditions. Vacuum should drop, even under light engine demand. Part of EGR design is the use of exhaust back pressure that can override what the ECM is trying to do. Basically it creates a vacuum leak in a secondary valve which causes the EGR valve to close. Some test I ran... Manually open the EGR valve at idle. I used my finger to accomplish this. Not easy, but it opened just enough to confirm the ports were clear because idle turned rough. Next, check for EGR vacuum by running the gauge only to the EGR supply vacuum line. You should see the gauge pulsate rapidly at cruise when the engine is running closed loop. Next plug the EGR line and connect a vacuum pump only to the EGR. Run the engine in park and with a few quick pumps (one or two pumps may not be enough) the EGR should start to open and idle quality should drop. With the EGR forced open, when you rev the engine, back pressure causes the secondary valve in the EGR to open, which creates an internal EGR leak, and the EGR closes. My EGR was lazy. What I mean by that is that it required more vacuum to open when compared to a new one, especially for partial opening. I also experimented with the EGR by running a vacuum pump into the cabin so I could manage it on cruise conditions instead of the ECM. In closed loop it didn't take more than a few miles before the engine developed a ping with no vacuum provided. When I added vacuum on cruise the ping disappeared after a few miles. It convinced me how important the EGR system is to a propely running L98. I have found that L98s with the iron heads or those with aluminum heads with less aggressive programmed timing are not as sensitive to this. The net of all of this... Too much or too little exhaust back pressure can mess up the fine balance GM designed into this engine. On the topic of pinging, carbon build-up can cause this as well. My 1988 had leaking injectors and was all carboned up due to years of poor maintenance from PO. I installed new injectors, ran FI cleaner with Techron for a few months and sea foamed the intake system twice to eliminate that annoying pinging.

I did T in a vacuum pump to the EGR solenoid and valve to verify EGR operation while driving. Which that checked out good. Though, I can't pull a vacuum on the EGR valve with the car idling. Which means something is going on. My guess is too much backpressure as it did run rich for a while. Which could have clogged up the main cat. There are no pre-cats.

I did run several bottles of Techron cleaner, and did a couple of upper intake cleanings with sea foam which didn't change the knock on acceleration.

I'm thinking about dropping the timing to 4* to see what that does. I was thinking of removing the cat and getting a straight pipe welded in it's place. I figure that would be cheap to do just to see what it does. I don't have emissions and had removed a cat from another car many many years ago with no problems.

 

[TedC]

To stop the pinging, you would need to drop timing more than a few degrees. I wouldn't go down that path. Too much back pressure could cause this, but I would think your car wouldn't run properly at higher RPMs. First confirm that the idle runs poorly when you force open the EGR using your finger at idle in park. If not, you have a plug in the EGR port(s). Next, run engine until hot (closed loop). Attach vacuum pump direcly to EGR (no "T") with engine off and pump it up so that the EGR is opened fully. Start the engine. What happened to the vacuum reading? Did it hold or drop to zero? Didn't you install new injectors last year and were showing a lean condition with a scanner?

 

[joshwilson3]

The vacuum drops to zero when I start the car and have vacuum on the EGR. I fixed the lean condition a while back, but I had previous knock counts when it was running rich. With your car running at idle, are you able to open the EGR valve directly with a vacuum pump? I believe you are supposed to be able to, which is how they tell you to check to see if the EGR ports are plugged.

 

[TedC]

When you say open the EGR valve at idle with the car running, I assume you mean that the EGR supply hose is not connected to the EGR. If that is the case, then yes, I can vacuum pump the EGR and the valve will open. Idle quality drops. At idle, the exhaust back pressure is not sufficient to open the secondary EGR valve. If I rev the engine I can increase exhaust back pressure to the point that the EGR secondary valve opens and pressure drops quickly to zero. I've found that this style EGR takes a few more quick pumps to start the valve opening process. Also, verify that the EGR holds vacuum, and that the secondary valve is not leaking. This is why I prefer to force open the EGR first using my finger to first verify the ports are open.

An idea... I don't think an exhaust restriction that causes higher than normal exhaust pressure is the only condition that could mess up the EGR operation. If there is a part or full restriction on the EGR port to the heads, couldn't this also create a similar condition? If it is a partial restriction, then the idle quality would drop, and mislead you into thinking the ports are fully opened.

Bosch IIIs... You should not need to increase fuel pressure over stock to address a lean condition with these injectors. Are you sure the EPROM was not reprogrammed for larger injectors? My BLMs are excellent using the same injectors with stock fuel pressures.

 

[Hib Halverson]

"TecC"....interesting discussion about the L98 EGR system.

 

[joshwilson3]

When you say open the EGR valve at idle with the car running, I assume you mean that the EGR supply hose is not connected to the EGR. If that is the case, then yes, I can vacuum pump the EGR and the valve will open. Idle quality drops. At idle, the exhaust back pressure is not sufficient to open the secondary EGR valve. If I rev the engine I can increase exhaust back pressure to the point that the EGR secondary valve opens and pressure drops quickly to zero. I've found that this style EGR takes a few more quick pumps to start the valve opening process. Also, verify that the EGR holds vacuum, and that the secondary valve is not leaking. This is why I prefer to force open the EGR first using my finger to first verify the ports are open.

An idea... I don't think an exhaust restriction that causes higher than normal exhaust pressure is the only condition that could mess up the EGR operation. If there is a part or full restriction on the EGR port to the heads, couldn't this also create a similar condition? If it is a partial restriction, then the idle quality would drop, and mislead you into thinking the ports are fully opened.

Bosch IIIs... You should not need to increase fuel pressure over stock to address a lean condition with these injectors. Are you sure the EPROM was not reprogrammed for larger injectors? My BLMs are excellent using the same injectors with stock fuel pressures.

I've got some more tests do to on the EGR valve since I can't open it at idle with my vacuum pump. But I thought this was a good read on the EGR:

http://www.tomco-inc.com/Tech_Tips/ttt27.pdf

I don't think the EPROM was changed. I have a ton of receipts which includes the injector swap, etc. And nothing was mentioned about the EPROM. Plus the car ran rich and was fouling spark plugs. I think the PO just bought the wrong injectors as they were listed as injectors for 85-93 Corvettes.

Bosch III 22#, I had to raise the FP to 55 psi, and it still was running slightly lean. I put in Bosch III 22.9#, and I had to raise the FP to 48 psi, and the BLM's are staying mostly in the safe range in the 130's. I believe I could put in Bosch III 24# and run it close to the stock FP at 41 psi.

There were a few others who had problems with running lean with the Bosch III 22#. Though, most people don't run scans when they swap injectors. If you swap injectors, I recommend running a scan to check your BLM's.

 

[TedC]

Yes, this is an excellent article that helped me understand how a negative pressure EGR works. I ran into a situation where the EGR was lazy. Lazy enough that it wasn't flowing sufficient EGR into the heads when requested, but it was opening and responding (had me fooled for weeks). The FSM and this article were just a guide. It took hours of my own testing to isolate the cause, which was much easier once I had a new EGR in my hands to benchmark. I installed Bosch IIIs 22 lbs from JIC based on Jon's suggestion. BLMs are excellent at stock pressures.

Additional infomation: My 1989 with stock EPROM programming has Fuel Injector size set at 22.36 lbs/hr at 40 PSI. If the Bosch IIIs pump out 22.0 lbs/hr at 40 PSI, then the delta is less than 2%. The stock Multecs ran a little rich at WOT, which according to Gordon Killibrew was part of the design to help cool the cats under extensive power enrich driving.

 

[boomdriver]

There are many things that can cause a "lean" condition besides a new set of Bosch-III 22lb/hr fuel injectors...
MAF calibration...air leaks, valve seals/guides, o2 sensor TPS...just about anything. You don;t get more fuel by turning the pressure UP. You can get a little less by dialing the pressure DOWN, but it don;t work the other way...

 

[joshwilson3]

There are many things that can cause a "lean" condition besides a new set of Bosch-III 22lb/hr fuel injectors...
MAF calibration...air leaks, valve seals/guides, o2 sensor TPS...just about anything. You don;t get more fuel by turning the pressure UP. You can get a little less by dialing the pressure DOWN, but it don;t work the other way...

That is true. There are other things that can cause a "lean" condition. I know some people have had a problem with aftermarket MAF's and running lean from it.

Though, there isn't much you can do when you rule out as many things as possible.

When you turn the fuel pressure up, what causes the BLM's to go down? I had always read more fuel pressure increases the amount of fuel that gets sprayed. I do know when I was running lean with the Bosch III 22#. The ground strap on my spark plugs and my O2 sensor were white. I raised the pressure to 55 psi to get my BLM's as far down into the safe range as possible. Replaced spark plugs. And the ground strap on the spark plugs haven't turned white. I've since put in the 22.9" injectors and was able to lower my fuel pressure some.

 

[TedC]

Increasing fuel pressure can increase your fuel ratios in open loop, limp mode and enrich (power) mode because the ECM uses preset parameters. In normal closed loop operation, the ECM is going to reduce the FI pulse rates to maintain proper fuel mixtures using sensor data. Bosch III injectors have excellent atomization, even better than the stock Multecs that were some of the best injectors of their time. I know you spent endless hours trying to isolate the real cause of this lean condition. I would like to suggest starting a new thread, outlining the situation and the numerous steps you have taken to try and resolve it. Even if they were dead ends, it can help in isolating the real issue. There is way too much expertise on this forum to let everyone sit on the bench when your vette is not running right.

 

[boomdriver]

That is true. There are other things that can cause a "lean" condition. I know some people have had a problem with aftermarket MAF's and running lean from it.

Though, there isn't much you can do when you rule out as many things as possible.

When you turn the fuel pressure up, what causes the BLM's to go down? I had always read more fuel pressure increases the amount of fuel that gets sprayed. I do know when I was running lean with the Bosch III 22#. The ground strap on my spark plugs and my O2 sensor were white. I raised the pressure to 55 psi to get my BLM's as far down into the safe range as possible. Replaced spark plugs. And the ground strap on the spark plugs haven't turned white. I've since put in the 22.9" injectors and was able to lower my fuel pressure some.

True...up to the rated output of the injector. Not beyond.

regarding the plugs...observe the insulator, not the electrode. The insulator is where carbon deposits will remain somewhat after they burned. The electrode tips are subject to what the engine was doing at the instant that the ignition was cut.
Back in the cave man days of "jetting" a carb, if you wanted accurate plug readings at the speed you would be racing, you had to kill the ign at that rpm and load, then coast the rest of the way clutch-IN so the engine stopped at that instant and preserved what was happening in combustion when it was killed. That gave you a snap-shot of what was happening at whatever rpm you shut down.
Electrode tips are more likely to collect residues from additives or even be varnished because they are metal and not porcelain like the insulator.

Another possible "fix" or cause, is the spark plug heat range. Nobody understands that anymore. Everyone just buys whatever plug is listed as appropiate for the yr, make and model they have. A 'true' tune will have a plug thats appropiate for the driving conditions and that motor. If city driving is most of what the car does, it may need a plug that has a deeper insulator that allows it to stay hot and burn cleaner,. although that may be too hot for sustained high speed. Carefully consider the facts before running hotter plugs. If you see your plugs too white now, you might be able to run a step colder and get the results that you want. There is lots of good info on this at the various plug mfg websites. Google to the rescue..

So much of this has so many variables, that without knowing most of them its almost impossible to say if a white insulator is good or bad in your engine...but most likely its normal. These engines run hot, the ign is very hot, and the emissions and stock tune is lean. Thats exactly how most people think they got some more HP with a chip or some other add-on, is just a bump in fuel. Thats all well and good but it ruins the emissions and also shortens the life expectancy of the engine. If temps are stable, and performance is normal, there is nothing wrong with slightly lean on the street.

"Normal" plug color ranges from clean white ( hi-speed runs, hi rpm cruise, high ambient temps) to lite brown in slow speed traffic, low rpm, cold ambient temps, short trips. Ideally, if your driving conditions are somewhat mixed, some hiway and city driving, on a nice day @80 degrees F, then I'd expect to see a "bone" white, or "off-white" as near perfect. The slower or shorter the drive the darker the color, but we're still talking shades of white...no where near brown.
NGK used to have a great color chart for carbed engines that could be applied to V8 engines with EFI and HEI. The only copy that I found in a web search was a scanned copy of one that somebody had on their shop wall for 30 yrs....a little faded but you could still see the difference in the 40 or 50 plugs, side by side. They still have a chart but its only a doz or so examples. Not as detailed as it used to be.

In this case with a pretty much stock motor, you're possibly looking for too much air or the false signal indicating too much air. My motor is bored slightly (357) has intake work and exhaust, head work and other misc enhancements and I'm running 26lb/hr @ 48-50psi and running a hair rich when babying the car around in city traffic.. (truth is gas prices), so 22's should be fine with a stock motor.

Did'nt intend to write a book, but wanted to throw some ideas out there so you had some more info to work with.

Update Fri:
found this link to more good reading on plugs, colors and the meaning of life.

http://www.summitracing.com/experta...lugs-101-Understanding-Spark-Plug-Heat-Range/