It's always the GD gas cap!
Oh joy, P0401 is back... (and how 80's fail emissions)
- Thread starter nakman
- Start date
"Test 1: Passed all emissions checks. Failed gas cap. 
- especially since I had a gas cap in the back of the truck. But they wouldn't let me just throw that on and recheck the cap; I had to rerun the whole test."
It's clearly not about emissions. It's about money and compliance.
- especially since I had a gas cap in the back of the truck. But they wouldn't let me just throw that on and recheck the cap; I had to rerun the whole test."It's clearly not about emissions. It's about money and compliance.
rover67
Rising Sun Member
Assholes. Also, Strawberry Chapstick?
If you apply sufficient strawberry chapstick to the fuel cap gasket you'll pass. It just feels weird.
rover67
Rising Sun Member
At first, but I promise you'll get used to it.
Inukshuk
Rising Sun Member
Passed! Did the usual new gas cap and 2x red Heet.
The lack of line was epic!
The lack of line was epic!
60wag
Rising Sun Member
- Joined
- Aug 23, 2005
- Messages
- 2,629
I had my '96 80 tested a few months ago. And for the first time in the 16 years I've owned it, I got a fast-pass. It was on the rollers for less than a minute. When they cut the test short and pulled it off the rollers, I thought I had a gross failure but then the tech said "you passed!" I haven't done anything to the truck that would have affected the emissions. I'm guessing it was due to the variability of the testing hardware. And no, the exhaust sucker didn't fall of the tail pipe - I still had high CO2 measurement. Many years ago, the sucker fell off the exhaust on my 60, they passed the truck but all of the numbers were suspiciously low.
So with my recent emissions failure I checked some things out today. The EGR modulator is not behaving per the FSM. But if it has failed wouldn’t that throw a P0401?
Failed step 6 here.
Also failed step 2 here
1. Replaced spark plugs, cap and rotor.
2. Adjusted timing. It was slightly retarded, maybe 2.5 deg instead of 3. I almost left it but wanted that notch dead on the 3.
3. Checked the EGR modulator per above. Still head scratching that one. Might buy a new one just to try and have on hand.
4. Washed the air filter. I think I’m done with the washable air filter. I think it’s cool and all but I think I will just buy new regular ones when needed.
5. Checked the EGR valve. Works like a champ.
6. Checked the temperatures of the cats. Outlet is about 150 degrees higher than the inlet, so that seems good. EDIT 3/28/25: On second check, inlet is about 600F, outlet of cat 1 is about the same, maybe even cooler, and outlet of cat 2 is 700F. So It seems cat 1 is dead.
7. Put some Lucas fuel injector cleaner in and ran up 285 to Bailey and back.
8. Checked spark plugs wire resistance. Well within spec.
I dunno
Failed step 6 here.
Also failed step 2 here
1. Replaced spark plugs, cap and rotor.
2. Adjusted timing. It was slightly retarded, maybe 2.5 deg instead of 3. I almost left it but wanted that notch dead on the 3.
3. Checked the EGR modulator per above. Still head scratching that one. Might buy a new one just to try and have on hand.
4. Washed the air filter. I think I’m done with the washable air filter. I think it’s cool and all but I think I will just buy new regular ones when needed.
5. Checked the EGR valve. Works like a champ.
6. Checked the temperatures of the cats. Outlet is about 150 degrees higher than the inlet, so that seems good. EDIT 3/28/25: On second check, inlet is about 600F, outlet of cat 1 is about the same, maybe even cooler, and outlet of cat 2 is 700F. So It seems cat 1 is dead.
7. Put some Lucas fuel injector cleaner in and ran up 285 to Bailey and back.
8. Checked spark plugs wire resistance. Well within spec.
I dunno
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I have an extra modulator if you want it? See if anything changes.
After all that I added some HEET to the tank, drove C-470 for a bit, and tested again after having to wait in 45 minute line. Results worse than before. CO was almost high enough to fail this time. Going to have to dig deeper.
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I think I might set up an appointment with these guys to see if they could help me figure out my issue:
I wish I had the answers. My recent experience was/is the opposite of yours - high CO, but low/normal HC and NOx. New O2 sensors seemed to resolve the high CO and I managed to keep the P0401 suppressed for those two tests. But with my recent round of additional PM (cap, rotor, plugs, wires, PCV, cleaned TB, bunch of new hoses, new vapor canister, EGR temp sensor clean), the frequency of my P0401 codes has gotten worse. Solenoid and EGR tests are next on my list. Haven't tested the EGR VSV yet but I did replace it 10-12 years ago.
It does seem that your numbers would point to an EGR issue so it is odd that you've not seen any codes. What about carbon build up in the EGR port on your intake? Or did you take it off and clean it when you replaced the VSV?
It does seem that your numbers would point to an EGR issue so it is odd that you've not seen any codes. What about carbon build up in the EGR port on your intake? Or did you take it off and clean it when you replaced the VSV?
Have you tried hooking up an OBDII diagnostic tool to it to see if it will pull any codes? Also, going back through this thread and re-reading everything, other folks were having the same issue as you and they ended up replacing both O2 sensors to get their vehicles to pass. Not sure if you said when replaced yours last?
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Thanks Matt. I would be interested in knowing if you need to replace the EGR VSV again. I think the original lasted about 10 or so years on the green 80.
On this white 80, my EGR valve worked fine when I was testing yesterday. And I have not tested or removed the EGR VSV since I am not throwing any codes. When you say EGR port on the intake, do you mean the small pipe that passes through the intake? I checked that with a small wire and it was clear.
I "replaced" it with the one from the green 80, but they both behaved the same during testing so not sure if they're both bad, or both good? Again not throwing any codes, and if the modulator was bad I would think it would be throwing a code. I have a new one on the way (thanks @RayRay27 but I just decided to order a new one) so I will do the same FSM tests on that when it comes and see if there is a difference. I wasn't really expecting the NOx to get any better with what I did yesterday, but I was hoping that the HC would come down so I would only have one problem to deal with instead of two. Was also wanting to see if what I did made any incremental difference, so that when I do figure this out I can have a better idea of what the smoking gun was.
One thing I was thinking about today was that my boost gauge shows vacuum but no pressure. It's been like this for a while. I figured the gauge was bad but now I am wondering if somehow there is a leak in the charge piping or intercooler that is preventing pressure from building even though the turbo is spooling up. I would think the performance loss would be noticeable though, and would also manifest as unmetered air when not under boost.
I did hook up the OBD2 and there are no stored codes. I am going to check the O2 sensors and make sure they are within spec. I am not sure if/when these O2 sensors have ever been replaced.
DaveInDenver
Rising Sun Ham Guru
Would unmetered air cause mixture (I'd think a lean burn) issues or a detonation, which those should throw codes?