erubin2000

A little background. I do all my own repairs and I'm stuck with smog on my 1990 LS400 here in CA.

I have low HC and CO. But failed NOx badly at both speeds. No check engine. So I'm looking at the EGR system.

I put vacuum on the EGR and engine stutters so I know the valve works mechanically and exhaust passage is open. I tee'd in a vacuum gage on the EGR vacuum line and see that I never get vacuum (in park, engine warm, at all engine speeds). So the EGR never opens. Problem, right?

The "Q" off the EGR-VM shows vacuum when I rev the engine so I know the VM is good. All vacuum hoses are good and tight.

Next I tested the EGR VSV. It closes with 12V and opens with 0V. This means it will operate the EGR when it's not powered (0V). The problem is that the VSV always has 12V to it during all my tests (engine hot, rev the engine). The VSV always getting 12V (closed) so it never opens to allow vacuum to the EGR.

Why do I always have 12V. Does the 12V always remain because I'm in Park or is there something faulty that can keep 12V on the VSV? Is that normal? The VSV only opens when 12V is removed. No matter how hot the engine or how I rev it I never get the 12V to release. Please help.

erubin2000

If this question is too technical for this forum can someone advise where I could send my question. Thank you.

RA40

iTrader: (6)

Welcome to Club Lexus. You've covered the necessary from your post and many instances this is a vacuum issue. Yamae may be more familiar with the electrical side of these systems to advise.

91LS400vgb

If the VSV is always getting 12 volts, then it may possibly point to an issue with the ECM.

Here is a guide for troubleshooting our EGR system, although it sounds like you've been doing everything correctly.
http://www.motor.com/magazinepdfs/092009_04.pdf

dicer

12 volts on the vsv will turn off the egr valve. It connects port E to the filter on the vsv . With zero volts it connects ports E to F and opens the egr valve.
If the ecu is not seeing the correct operating temperature the egr will not work. Verify the engine coolant temp sensor is doing the correct thing.
So if cold no egr, and really don't want egr too much at idle as that is a vacuum leak, the test rpms are 2500.

erubin2000

Thank you for the replies.

I'm going to put my odds with Dicer. Rather than blame the ECU it's probably an EGR related input sensor. The ECU is no smarter than the inputs feeding it. I've met too many mechanics that say "change the computer", That is rarely what is needed. Unlike a shop I dont have an incentive to replace a good part unnecessarily. Sometimes though, out of frustration or because the cost benefit vs the time needed for troubleshooting I will replace a part that may not be bad. I dont want to do that with the ECU.

The article 91LS400 gave me listed these as EGR-related ECM inputs:
throttle opening,
coolant temperature sensor
idle air inlet (IAC) activity

Does anyone have the ohm spec for the coolant sensor or other ways to test it. Also if that is not the problem should I test the other EGR related inputs to the ECU? Also location of the coolant sensor that informs the ECU to turn on the EGR (open the VSV) would be great to know. I dont want to test the wrong coolant sensor!.

91LS400vgb

I think you guys are on the right track. I was very busy when I wrote that post and should have been more clear, i.e., it points to an issue with the ECM not receiving the correct data input from the sensors or less likely the ECM itself. Anyway, my bet would be on the coolant temperature sensor. Here's the ohm spec for it



Good Luck

dicer

Also remove the egr modulator that has the 4 hoses going to it and clean the path for the large bottom hose.
I also have seen those modulators go bad. And they are not cheap new.

erubin2000

I go back to my original question:
"Why do I always have 12V. Does the 12V always remain because I'm in Park or is there something faulty that can keep 12V on the VSV? Is that normal? The VSV only opens when 12V is removed. No matter how hot the engine or how I rev it I never get the 12V to release."

I realized it was unlikely to be a faulty coolant sensor because I would have noticed some significant driveability problems. The ECU uses the coolant temperature for other things besides EGR activation.

So i hooked up my vacuum gauge between the VSV and the EGR. This time instead of just accelerating in Park I power braked in Drive, and guess what? The VSV opened and I finally saw 5" Hg vacuum to the EGR. So I've eliminated the EGR and the ECU control of the VSV as the problem. The car needs to be under load and not in Park for the ECU to activate the VSV. Under a "real" load the ECU activated the VSV! Well I guess we all learned something here.

Back to my problem, smogging the car. HC and CO are great, NOx is bad. Is 3-way CAT suspect? I don't want to throw an expensive part at this problem without clear cause. The car is not worth it.

Oil change is recent, cooling system fine, driveability and idle seem fine (no vacuum leak that I can notice) and no check engine light (this vehicle is pre OBDII). I'm leery of gas additives and other snake oil but maybe I should consider decarboning the engine or putting some race car high octane gas in the tank? I have some light pinging under heavy acceleration and load. I'm sounding desperate, I need to drop NOx by over 300ppm.

Does the CAT perform much better with NOx when it goes from operating temp to super hot after a hard drive? I'm looking for over 300ppm improvement! I must confess that although I kept the car idling the 15 minutes while waiting my turn at Smog I had not driven that hard or far before hand. Nonetheless CO and HC were real low and I only failed NOx so could I expect a big drop in NOx after a hard drive and the unfortunate need to wait in line idling and waiting my turn which will allow the CAT to cool? I need to drop NOx about 300ppm. BTW I have an IR temp gun. Should I measure temp before and after the CAT? But my HC and CO are great. Any troubleshooting tips from a tech?

Thank you.

cobalt91

You mentioned light pinging under heavy load. What is the condition of your fuel system? What is the maintenance history for your vehicle? What do the spark plugs look like? The coolant temperature sensor that talks to the ecu maybe causing a rich or lean condition. Check this before thinking about blaming the catalyst.

fixmiester

You seem to have a pretty good understanding of the mechanical operation, but I'm not sure about your knowledge regarding the emissions control process: HC and CO are products of incomplete combustion, and are handled by the cats--when they reach proper operating temperature. NOx is a byproduct of high temperature combustion (which may lead to pinging), and in your car, it is reduced by introducing exhaust gases back into the combustion chamber, which lowers the combustion temperature--reducing the amount of NOx produced. The cats have little to do with NOx reduction here.

So, if you have a leak in the EGR tube near the firewall, you will be recirculating fresh (oxygen-rich) air into the cylinders, instead of oxygen-depleted exhaust. Not good; NOx cannot be reduced enough to pass emissions tests.Since this hasn't been mentioned yet, I suggest you look there.

Steve

timmy0tool

iTrader: (7)

i too was going to suggest checking the EGR pipe to make sure it is not leaking as it is a very common occurrence. also, why not just replace the ECT sensor, as it's cheap and easy to do, just to eliminate that factor?

i recently went through this exercise of trying to lower NOx, albeit on a friend's 92 honda accord with 260k, and what i found was that cleaning the EGR passages helped. i also changed the plugs and retarded the timing a bit, and made sure the coolant is up the right temperature before performing the smog check (fans turned on at idle). i understand, possible apples to oranges, this may or may not apply to the LS400.

erubin2000

Turns out the car is not pinging. Sorry about that misinformation. It's my wife's car and she mentioned pinging under load but I believe she was actually hearing suspension clunk when driving uphill in our potholed LA alley. Best to always do the road test onself.

I sprayed around the engine with carb cleaner to look/listen for a vacuum leak. I found the PCV grommet leaking because of a crack in the plastic grommet. Could that cause my high NOx? It's sucking in more outside air than it should. In any case I'll replace it (BTW the PCV valve is clean and good). But I wonder if I should go back to smog with only a replaced grommet for the repair? I'm failing only NOx. It's about 300ppm over. But O2 is 0.1% or lower on the smog test. Does O2 level tell me anything here?

The EGR opens under load like it should and the engine almost stalls when I vacuum activate it at idle. The EGR holds vacuum so diaphram is good. Nonetheless could the exhaust passage be somewhat restricted with carbon and should be cleaned? I doubt it, as I said the EGR seems to function fine and the engine almost stalls at idle when I open the EGR. I need to move on from the EGR and it's control, they test out fine.

I want to check out the EGR pipe for leak but cant see it from topside I dont think. Engine is right up against firewall and cant see/reach down too far. Get under the car to see it? But my O2 at smog check was 0.1% and 0%. Can I rule out cracked EGR pipe or does O2 measurment mean nothing here? Is it useful or not for troubleshooting? Would I expect to see higher O2 levels (along with a high NOx) if I had an air leak into the EGR pipe?

But my first question is: can a leaking PCV grommet cause high NOx levels (over the max allowed by 300ppm)?

python

iTrader: (4)

Better off changing the pcv and the hose while ur changing the grommet

paulo57509

I'm not an emissions expert. But I struggled with trying to pass CA emissions testing for years on three of my vehicles.

I don't think the leaking grommet is causing your high NOx condition, assuming it's being caused by a lean condition. However, I would discount a lean condition because of the O2 reading. If the mixture were lean, you would see excess O2 (much higher than 0.1%).

I would think that this would eliminate the possibility of the exhaust tube sucking in fresh air (I think the exhaust would leak out instead of air leaking in anyway).

It wouldn't hurt to decarbon - is this a high mileage car? But I don't think it would provide a 300ppm drop in NOx. The only time I've seen this happen is when I installed a new converter.

This is an apples to oranges comparison, but just to illustrate how dramatic a new converter can affect the numbers. This is my 440k mile '87 Legend before:


The Legend after I installed a new converter:


I've also seen this kind of drastic change in test results in my '92 Chevy Lumina as well; I killed the old converter because I lost a coil pack. It doesn't take much.

Edit: I just looked back at the post where you attached the image of your test numbers. Your HC and CO numbers are damn good. I'd take a closer look at your EGR passages and look again for vacuum leaks. Does the car have a lot of miles or driven on a lot of short trips (excess carbon build up in the combustion chambers)?