Took our 1994 Toyota Land Cruiser for its annual emissions test. It passed the tailipipe emissions but failed on the gas cap pressure test. Drove 3 miles to a parts store, got a new cap and returned for a retest. Second test was on the same dynamometer with same technician. As you can see the results are very different. I'm pretty sure the gas cap plays no role in tailpipe emissions, just in gasoline vapor emissions. Anyone have any explanation? Emissions testing not reliable at all?

Limits:
HC--2.0
CO--20.0
NOX--4.0

Test one

HC--.7660
CO--16.5365
NOX--3.1477

Test two

HC--.4176
CO--7.6637
NOX--1.7369

Test one passes but pretty close on some numbers. Test two is clean as a whistle. Engine was at same temperature in both cases. No other differences besides a new gas cap and 6 additional miles.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Without doing it myself, and noting many other parameters, my gut would be that the cat was doing a better job on the second test. Possibly overall engine temp higher, and more consistent, ie; oil/coolant/cyl block/cyl head all equalized, and intake air also steadier. Those of course, all depend on how long the vehicle was driven before the first test, and how long after the first test the second was.

The gas cap, under most normal circumstances would not affect tailpipe emissions much, as you mentioned. Under some rather creative scenarios, the lack of a sealed tank could mean that evap purging was unrestricted, and hence able to flow more volume, thus affecting the fuel mixture. But that's pretty unlikely.

MV

Possibly but the engine was at normal operating temperature with the first test and only an additional six miles was driven. But cats doing a better job might account for it. The experience raises questions in my mind about the overall credibility of emissions tests. It would be interesting to do the same continuous tailpipe tests on vehicles like mine as were done o the VW diesels to see how indicative the dynamometer test actually is.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Well, the temperature thing is secondary to the cats, I agree that the cat is probably the main variable. The temp is actually best moderately by something akin to a moderate drive to the emissions shop, followed by the emissions test, then setting for a bit, then another drive... Pretty much just what you did. When you first start the vehicle, the cylinder head temp starts to climb first, (and fast), followed shortly by the coolant. Gradually the temps come up and bring the block along as well. The oil usually takes the longest to change temp. This would likely have all happened on your drive to the shop. The actual test (if it is like the dyno test formerly used here in CA), loads the engine and generates a fair amount of heat. After the test, the vehicle is usually parked for a bit while paperwork and so forth is done. During this time the hottest parts, usually the cylinder head and the coolant in it (which will actually heat up at first, since the water pump is no longer creating any circulation, instead it just soaks up the heat from the combustion chambers) will start to cool, by transferring their heat to the other areas. Another drive after this portion further circulates everything and equalizes the temps. This is good from the perspective of engine running and/or emissions, as everything now should be about where it is designed and engineered to be at its best.

The thing is, the driver usually doesn't know any of this, as modern instrument clusters buffer the data coming from the sensors, and display a "normal" reading over a wide variety of actual conditions. From a coolant temp of about 60* to 120* (Celsius, I've long since forgotten what temps things operate at in Fahrenheit...), the gauge will read a steady, middle of the range reading. So a driver might think the engine is warmed up, but it really takes a few cycles of the thermostat opening and closing before everything is really where it ought to be. And of course, depending on your ambient conditions, this might not happen at all. It isn't uncommon for me to see vehicles with no problems at all, not reach true operating temps without some sort of manipulation f the ambient temp is under about 40* or so (in Fahrenheit, oddly enough, I think of mechanical operating temps in C and weather temps in F...).

But this is all secondary, my money is on the cat(s) being hotter and/or "lit off" for the second test. To answer you main question though, I've found that the emissions tests do a fairly decent job of producing repeatable results. If things are changing drastically in the test results, things are usually changing on the vehicle.

MV

Does it have a VW ECU?

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2006 CL500
1969 280SE
2023 Ram 1500
2026 Genesis GV70
2007 Tiara 3200

Last month, when I took my 240D thru emissions, which measures only the exhaust particulate level here by means of passing it thru a device with two mirrors either side of the plume and I assume some kind of laser or other measuring contraption the first attempt was hopeless.

First he twit couldn't find and release the parking brake (kept messing with the hood release catch) so the engine kept lugging and stalling.

This apparently had some effect on the machine as they had to stop the test and re-boot the computer and open up the machine and re-calibrate it. Took about five minutes while all that was going on.

Then finally they managed to get it working and the car passed with 0.7 out of 20. But watching their performance left me feeling - not entirely confident - about the whole process.

As a matter of course I always take the car for a long and fast highway drive before going thru the emissions.

- Peter.

__________________
2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S

Of course this is relatively speaking.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

I took my daughter's 84SD for it's emissions test at the diesel testing station right after getting the Land Cruiser tested. It had been sitting for a year since my daughter is at college and I only drove it the 7 city miles to the testing station before putting it on the dynamometer. It was smoking like crazy but it passed. The tester told me to drive it hard before bringing it in next year.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Here's my hypothesis: The results of the first test are pretty close to the results of all the previous tests during my ownership. I haven't gone thru the old paperwork to confirm it, but my recollection is that the truck was never super clean and always relatively close to the limits. During the second test, the tech knew that the truck had just passed the test a half hour before (he had the paperwork in his hand) so rigged the second test so it passed easily. If that's true, then the system can be gamed pretty easily.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I was wrong. I looked at the old tests. The majority are closer to the second test results than the first.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

How long/far was it to the emissions center?

Here's some info on your numbers.

https://shadetreespeedshop.com/how-to-get-a-car-to-pass-emission-testing/

Hey. I reached almost 75mph at one point I'll have you know.

- peter.

__________________
2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S

Nice cross reference !!!

__________________
1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
********************
1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD