Little Trucks, Big World

Hi guys,

Well, it's again time to get the shaft in Cali, especially Los Angeles.

My truck runs very well, but it IS borderline on the HC and NOX. Smog from 2 years ago passed by a hair, but it was so close I'm not holding my breath on it passing this time.

So I have done the following already...

Rebuild the carb, new O2 sensor and the computer IS cycling properly in closed loop.
New thermostat, new vacuum hoses, distributer is rebuilt and all new ignition components for a proper tune up. EGR and PVC valves are ok and working properly.

What I really need is someone with a gas analyzer to properly tune the truck. The star stations around here will only test, and of course no one want's to help you actually tune up your vehicle.
They'd rather have it fail so you can get into the Cali run around.

Does anyone know anyone that can look at the truck with an analyzer to actually tune it properly without running a full smog first? I may need a new cat as well, but can't know that without the proper tests. My goal has always been to keep the truck running stock and correctly, but it seems that no matter how carefully I keep it maintained, it is getting slowly worse.

Any help would be great, as no one really knows how to work on the z24 without making things WORSE.


Views: 207

Reply to This

Replies to This Discussion

Air your tires up to high hell, skinny street tires that are the proper size helps too. Beat the shit out of it before smogging, a hard run right before testing helps. When you go to the smog station have your buddy sit in  the truck and keep the rpms above idle until the tech comes out to get your truck. Those are a few things I know other than the obvious stuff

 That sucks that your HC and NOX are high... when you lower one the other generally goes up, you are running out of wiggle room.

 This Star station shit is really pissing me off...

I believe there are some shops that have a smog exemption plan that is good for 2 years. CASH only I think...  :)

My truck get's run freeway most of the time, and I run it pretty hard. Last time I took the cat off to look at it, it was clean as a whistle, so no carbon deposits. I never drive slow enough... LOL.

As to the cash excemption, no one is willing to risk their licence and the fees any more. I don't blame them at all, and I'd rather have the truck run clean to begin with. If I recall, a member here got busted for that. While his help was greatly valued, the price he paid wasn't worth it.

I know a lot of people hate the hitachi carb, but I know it well and have not had any issues with it in 29 years other than rebuilding it twice JUST to keep it clean and running right. My truck actually goes like hell, for a stock Z24....

But I can only go so far without actually having a gas analyzer to tune it. And that's the problem.... I have no feedback of what I'm doing. Pity a gas analyzer is so expensive.... You damn near HAVE to own one in this state.
Did a compression test this morning. 190, 190, 185, 185. Plugs look great.
This engine has 170k on it, btw.

Timing is dead on. Egr valve is working, but I think I'll remove it and have a look anyway.

Ran some water through the carb, but as I thought.... No carbon at all came out the other end.
I drive it too hard to get any deposits.

Still, it idles a LITTLE rough, and that's likely where the elevated numbers come from.

I'll keep looking. Smog is due in a month, so I have a little time.

Marc: I believe you are entitled by law to have a smog run done without a failure counting against your vehicle.  It's called a pre-test.  See here:


Note a place doing a pre-test may charge you as much as an actual test.  That said, I asked a smog place about a pre-test when I had just got my 720 (PO didn't smog it; I was aware when I bought it), and they hooked me up with a partial for free.  Maybe just feel around, or if not, pre-test costs less than failing and ending up in the gross polluter bucket.

Also, this looks like a really good reference for Cali smog:

That said, I know an _awesome_ shop that specialized in Datsuns, but they're up here in NorCal.

Let me know if you have other questions.  Cali folks have to stick together.

When I took some auto classes at Mira costa community college down in Oceanside years ago we had all the smog equipment at our disposal. My car at the time was smog exempt, but it woulda been cool to have if I needed it. I think Steve's onto something with the right turn, just make sure you do it with you left hand. Good luck brother

if the vehicle is "star test only" even the pre-test is recorded.... so I have been told

Steve: I've heard the same thing.  What the smog guy told me is that they transmit it either way, but if you fail during a pre-test, it doesn't go on the vehicle's record.  Of course, given that they have it, they could turn around N years down the road and say "hey, actually, we're going to pay attention to these now."

That said, for the immediate term, if Marc doesn't have any other options, the pre-test is still a good option.  Especially if the tech cuts it off before the test actually completes (but if doing so, I believe the machine doesn't give a printout)

This is Los Angeles. ALL smog is Star only here.

And yet, the 2 million 2 stroke leaf blowers and weed whackers for some reason don't pollute.
In fact, they freshen the air with their oil stink.

That's California for ya.

So I decided to replace the EGR valve anyway. I think after 29 years, even though WORKING, it may as well. I'm suspecting that my CAT is tired and will get a new one as well. One thing the spark plugs told me is that the engine may be running a bit too lean, so I'll look into that.

This truck runs like a clock, it has not even ONCE stopped on me in 29 years. Stone reliable, or it would seem I got lucky with this one. I'll be dammed if Cali squeezes me out of this one.
I can always tune it to burn pure alcohol for a test.

I actually entertained having it converted to propane just to give Cali the finger.

So I'm ordering the EGR valve and see if the idle smoothes out any.

Stay tuned.


            CO2    O2      HC                          CO                             NO

 15mph 14.7    0.4     max 95 meas 24      max 0.92 meas 0.06    max 1101 meas 379  PASS

 25mph 14.8    0.3     max 75 meas 22      max 0.82 meas 0.05    max 1151 meas 259  PASS

Still on the original carb and catalytic converter.  BLOW ME CALIFORNIA!

It turns out the truck was actually running too lean 2 years ago.  I riched up the idle mix screw a bit and crossed my fingers.  Changed the EGR valve, the truck runs like a bat out of hell, but still shakes a bit at idle.  I'll let it shake.

Woah, those are some serious numbers.  Good work.

this brings a tear to my eye... so beautiful

Goes to show you that the Z24 is perfectly capable in it's bone stock setup.

It's always been tuned to run leaner and leaner in Cali, but that was actually wrong. I had to fatten it up to balance the numbers and give the cat something to burn. And man, does it go on the freeway...

In open loop the numbers are scary high, but as soon as you close the diagnostic connector and the feedback takes over, down they drop to what you see.

I am needless to say, THRILLED.

I was about to replace the cat... Dodged a bullet on that one.

Keep in mind, that's on that POS 29 year old Hitachi carb everyone hates so much.
I rebuilt it twice in it's life.
I did buy a fully rebuilt from a reputable shop 2 years ago, figuring it was time enough. It failed smog so badly, I could have just poured raw fuel into the open intake. I put my original back on and passed, but high numbers. It was too lean, it turns out. Didn't realize that until I really studied up on the inter relationship of the various gases and the color of the plugs.

That was an AHA moment. Opened the idle mix about 2 turns and bingo.
The original mistake was tuning the mix open loop. That didn't leave enough room once it went closed loop, as it went even leaner.

The clue was the 4.5% oxygen left over. Now it is 0.3 to 0.4 which it should be.

So there you have it. Learned something new, and the truck really hauls ass now.
I just cannot believe how sensitive this engine is to a little tune.


© 2019   Created by TigerRacing.   Powered by

Badges  |  Report an Issue  |  Privacy Policy  |  Terms of Service