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Colorado emission test woes for an '80 L20B


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Alright, so I've been driving my truck around for a while on temporary tags and finally brought it into emissions testing a few days ago and the results were unexpected since it ran so well. It failed and by much more than what I had hoped after hearing it had failed.


The official readings of the two part standing test:

                   Idle              2500rpm         Limits

HC PPM     528.9            206.5               400

CO%          2.74              2.65                 1.50

CO2%        12.11           12.84                  -

RPM           840               2588                1350



Since the test I have regapped the plugs, gone through the vacuum lines to make sure they were correct based on the diagrams I found and replaced those that were cracked, adjusted the timing back from since it was advanced all the way, adjusted the idle screw on the carb so that it wasn't nearly falling out like it was, discoveved my connector to my temperature sender had fallen off and checked my throttle cable positioning. Well, the issue now is that when it is warm outside, it feels like what little power available normally is halved, the idle is rough and inconsistent all the time, sometimes it will return to idle after coming out of a gear, other times it dies, I've had to advance the timing again to even get close to ten under the highway speed limit and there are random dead spots and surges all over the rev range. All of this wasn't there before with the incorrect vacuum line arrangement, leaking vacuum hoses and advanced timing, incorrect plug gap and an idle screw that was nearly falling out. The truck is original to Colorado and should have the correct jetting. Emissions hardware is all present I believe, and I'll take pictures a little later to confirm or deny that claim. Any thoughts/suggestions are welcome.

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The timing issues sound like this person's issue:





It's a more and more common issue that the advance plate on EI dists, particularly the matchbox ones, sieze up and no longer move.  Having the dist advanced was making it run at speed and under load, but now it's too far retarded because the dist isn't advancing automatically.


The high CO% is prossibly because the catalytic converter is plain worn out.  They only last about 15 years, so yours should be on the third one by now.  But you'll get all sorts of weird readings with a dist with broken advance mechanisms.

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Yeah, I looked at that thread when researching my issue. I did forget to check for free movement on the vacuum advance on the distributor when I was adjusting the timing and checking the cap. I assume by saying the catalytic converter only lasts 15 years, you mean it only lasts 150k miles, right? My truck is at 88k original miles since it was a farm truck owned by an older couple and with the way it ran, even though it didn't need emissions in their county, I thought it would pass the e-test easily. When I first retarded the timing slightly, the truck ran great but the next day, I was lucky to get to work on time since it was stalling every time I came to a stop, surging randomly and it was impossible to get above 60mph. I ran it home with same issues to see if it was consistent. The next morning, I capped off all my vacuum lines and drove into work with the only issue being a high/random idle speed but I'd think that was due to the loose idle screw. Today I was going to try to run it without the vacuum lines again but I seem to have gone from no oil spots under the truck to a spot about eight inches in diameter which is a bit more worrying.


Edit: Oil leak found, the oil pressure sender was loose. Also, a couple thoughts occured. The joint between the exhaust manifold and exhaust pipe is loose. Perhaps the catalytic converter isn't getting hot enough to function properly because of this? Then again, thinking about it, the truck may have low miles but that doesn't mean it sat idling quite a bit was used at low speed with a high rpm to compensate for being loaded down so the catalytic converter may very well be past its prime. That would not explain the erratic idle or why the truck does occasionally backfire at idle. Perhaps the distributor and anti-backfire valve are worn as well. With my luck everything is worn out and I've bought another money pit when I tried to buy a daily driver.

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While miles have a lot to do with it, catalytic converters break down over time too.  A farm truck's car will be toast in 20 years, irregardless of how few miles.  Especially if they were putting leaded fuel in it.


The oil leaks are from it sitting so much.  Now that you're driving it more than it used to be, the seals are all giving out.   You'll have to fix them as they crop up.

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datsunaholic, the Colorado E-test is pretty liberal, so don't you think that a well tuned, well running L20B will pass the E-test with a non-functioning catalytic converter? The fail numbers are honestly within earshot of where my '70 Chevy w/350 V8 was last time I had it tested. It of course has NO cats. I have often wondered if my truck has a functioning cat.. It's always run Unleaded gas, however.


As you linked, my dizzy WAS the problem. The difference between my first test and second test was dramatic. Give that thing a nice once over, because as I found online, mileage didn't matter with the plastic bearing cage in the dizzy. You can have a truck like mine with 183,000 or one like yours with 80,000. The cages simply get brittle and break..


When I test my truck, I drive it about 15 miles and then get there earlier in the morning. If you time it right, you can pull right in the bay pretty much. My strategy is what I used to use on the Chevrolet-

-Clean oil

-Clean Air Filter

-Plug Gaps checked

-Timing Checked (ALMOST all the time :o )- around 10 BTDC

-Idle at around 1000

-Premium gas in the tank

-15 mile drive


This has worked for me for a LONG time.. The only time I didn't check the timing? Yep, this last time. On one hand it is a drag to do this every year, BUT it probably saved my engine. I was getting detonating and chocking it up to using 87 octane. Well, I'm still using 87 now and getting NO pinging and close to or at 30 MPG. It was honestly a very cheap $15 diagnostic test.


Keep us posted...You are so close, it will pass with a little work. One thing you should check since you obviously passed the VISUAL inspection. Check the flapper door on the air cleaner snout. It should be OPEN for this test. My little temp unit in the air cleaner is not working right, so I slice the hose in half and put a double ended metal rod in there to keep that door fully open. I guarantee that will reduce your numbers some. These little things add up, trust me.

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In 1980?  All my 1980 Datsuns were Federal, and all had cats.  '79, no so.  But then again I didn't have a 1980 truck.


Anyhow, for buzzbomb's question, yes, a well-tuned Datsun should pass without a cat, but a 34 year old 80,000 mile truck probably has the original carb.  That'd be a little iffy condition wise, time isn't good on the carb gaskets.  You'd be amazed how much difference a new cat makes in the numbers.  A new cat on my '81 went from 2.72% to 0.56 %.  No other changes at all.  Cat was collapsed internally, though, so it was causing excessive backpressure as well as being useless.  But it's not unheard of for new cats to reduce CO emissions by up to 80% vs not having one at all.  

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In 1980?  All my 1980 Datsuns were Federal, and all had cats.  '79, no so.  But then again I didn't have a 1980 truck.


Anyhow, for buzzbomb's question, yes, a well-tuned Datsun should pass without a cat, but a 34 year old 80,000 mile truck probably has the original carb.  That'd be a little iffy condition wise, time isn't good on the carb gaskets.  You'd be amazed how much difference a new cat makes in the numbers.  A new cat on my '81 went from 2.72% to 0.56 %.  No other changes at all.  Cat was collapsed internally, though, so it was causing excessive backpressure as well as being useless.  But it's not unheard of for new cats to reduce CO emissions by up to 80% vs not having one at all.  


My truck is also a Federal model WITH the original cat, also purchased new in CO. One thing I don't have is an airpump, and that too is original, and I've gone round and round with the folks at the smog stations about this. I almost failed visually until I whipped out the actual owners manual to prove it didn't come with an airpump.


I've wondered what signs of a failing cat are and now I know, so thanks datsunaholic. I guess higher CO is the tell tale sign with it. I'd be you're right about his carb too. It would have been better off running and not sitting. The gaskets are probably hardened up in his carb if it sat for any real length of time.


In Colorado, you need to spend $771 (some stupid number like that) on a pre-80? car to get a one year ONLY waiver. So even if he can just eek by, a few low cost mods to push it through would be worth  it. The HC's aren't off by a gross amount, and the CO could be bombed with a new cat- at least to get it passed so you can work on it and drive it, which is certainly possible. Honestly, you can't expect a vehicle that's been sitting to hit the streets after extended storage and become instantly reliable, as I'm sure you knew when you bought the truck. Things crop up as datsunaholic pointed out.


Keep us posted..

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Blocked off all the emissions vacuum lines and it runs perfectly. No stumble in the idle, no surging and no dead spots. Of course, I won't pass emissions this way since I'll fail visual inspection. The vacuum advance on the distributor does work smoothly and well in its operation. Did see something a little odd though, shouldn't my air injection system have an air check valve? My truck doesn't seem to have one. The reeds on the air cleaner still work though.

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No air pump.


The reed valve(s) on the air filter are a one way valve. They use the exhaust pulses to draw in small gulps of air into the exhaust manifold and this provides the oxygen needed to burn the emissions in the catalytic converter.

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So my reed valve, while functioning well enough to stand up to the air pressure of my lungs, might not be able to hold up to the exhaust pressure. So far the vacuum advance on the distributor works fine, the timing was set to Buzzbomb's the carburetor works just fine without the vacuum lines hooked up, the plugs are properly gapped. Elimating those, any other guesses as to why it's having such a hard time idling, backfiring and completely gutless when the emissions are hooked up? I've already decided to replace the catalytic converter on my next paycheck but there has to be something else wrong somewhere and I don't want to end up buying parts I don't need. The things that may not be working: PCV valve, thermo controlled vacuum valves (haven't had time to check them), EGR valve (moves by hand at least), BPT valve (I haven't researched this at all yet), the anti-backfire valve, the vacuum switch mounted next to the vapor canister and other vacuum thing next to the switch that I can't seem to remember what it does right now (vacuum booster?) Thank you all for bearing with me and my lack of knowledge with emissions hardware. I only moved to Colorado from Washington state nine months ago and have never had a vehicle that required emissions before.


Side note: While I still remember, does anyone have two of the things that the tailgate latches onto for sale or picture  of them so I can make my own? I'm tired of using two bungee cords as functional parts of the tailgate.


Edit: Thought it worth mentioning that it has a brand new air filter, I've cleaned the carb by spraying carb cleaner through it. Also, the pre-heat hose is missing and I've been lazy to replace it considering it's summer right now.

Edited by Viror
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Are you using a vacuum gauge? Not sure how much wrenching you've done, but on an old car, a vacuum gauge is a super good tool to have. Did you check the distributor vacuum CAN to make sure it's holding pressure? I would think that lack of advance from this would make it be a dog on the highway.. A worn out PCV valve can cause a low idle speed and erratic idle. Cheap to try, and it probably needs it.


Are you within the 10 days for a free retest? Why not just jerry rig the vacuum hoses and make it look they are connected (got bb's or a little bit bigger ball bearings?), set the carb to as lean as idle as you can get with the vacuum gauge, retard the timing a little more, jack up the idle to 1000, put in some super, change the oil, and see what it does? Worse thing you can do is fail again and have to spend another 15 after 10 days is up anyway?. If you do this, at least you know where you're at with the carb mixture? I think an L engine will pass this test in Colorado with NO smog stuff as long as it's tuned right, personally. A well running (and stock) V8 with NO cat should be able to pass the test at those numbers. If you're going to replace the cat anyway, why not give it a shot? At least the free retest will tell you where you're at. It might just eek out a pass at "lean best idle" as thought of in the traditional sense.


Here is a resource that you can use to help you.. It may not have ALL the stuff an '80 has, but it has most...




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The advance canister does hold vacuum quite well and the idle isn't erratic when all of the emissions hardware with the exception of the PCV system. I would expect my truck to be slow on the highway if it was all new. 92hp, a four speed and the aerodynamics of a brick don't really lend themselves to high speed on level ground, let alone hills. The oil was changed about 500 miles ago when I had to remove the engine to replace the clutch. I also replaced the rear main, oil pan gasket, coolant and gear oil in the transmission, transfer case and differentials while I was there. I'm going to run it through the test again after work with the emissions capped in a stealthy way. Worst case I don't pass and order in a few things. I still have time on my temporary tag.

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I am removing my entire emissions system from my 1980 L20b 720 2wd. I will probably list it in it's entirety for $75+ shipping. It's a California truck, so it is more than sufficient for Colorado's smog laws. I live in Redding, so Im just going to register my truck up in Ashland so I can avoid smog and build my motor up to 125hp or so.



I also didn't read the whole thread, so maybe you have already fixed the issue and I am typing nonsense lol :-)

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Naw. I can drive a vehicle registered in Oregon and still retain my California residency and license. I need a PO Box in Ashland $39 for 6 months, a vehicle inspection $7, plate fees $24 (plus $30 for the Oregon Wine Country plates), and $77 for registration. I also have a business license in Multonomah county to work and pay taxes for said work in Portland which is $50 per year plus a few minor fees (and business licenses in CA and WA too).


If you don't plan, and don't know the law, a local California cop will potentially give you a ticket. Heck he can write one anyway. You just prove you conduct business and/or live in both places.

Kinda like direct tv registering trucks in NV and working in Cali. Uhaul does it too. Most west coast rigs are registered in AZ even if they are only rented locally in Redding, CA.

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K_trip - the project just kinda fell into my lap. Its actually a simple build.


As far as smog goes, last November Cali almost passed a new exemption law that would include everything up to around 1982 or 83. It lost 4 to 5. Last time it lost 8 to 1, so hopefully it will be passed next election. Arnold doing away with the 30 year rolling exemption really sucked.

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Yeah I like clean air too. I plan on building a motor only with the pcv valve. I have to machine an adapter for the intake manifold (unless there is an aftermarket adapter out there).


Some call registering a commercial vehicle out of state cheating. I call it basic profit/loss accounting. I also have a 1 Ton E-350 registered out of Port Townsend, WA. I have used it to work in 5 states and it lives in Redding. When you pay commercial taxes local cops dont really ask about the smog components under your hood. They see you are working, and they go mess with idiot tweekers instead.


The van is registered out of state because the 1 ton fees are crazy here in Cali along with the lack of smog ;-)

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Sorry I didn't post this last night but I sort of just collapsed when I got home. Okay so here are the results with everything blocked but the PCV valve:



                          2500                   Idle                Limits

HC PPM             289.3                  866.6             400

CO%                  2.83                    6.02               1.50

CO2%                13.35                  11.01              -

RPM                   2541                   748               1000


So another another fail. At least I have a comparable baseline though. Here's some pictures of the truck:








Proof of Colorado origination:



Another potential problem is the previous owners "fix" to the exhaust pip from the stock muffler back. It's tiny!


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