Failed emissions test

[delco1946]

My mechanics sniffer is broken so he can’t help diagnose this. All I know is that my truck failed emissions with 12% co (limit is 3). Even my mechanic said this is sky high. My truck has been sitting in a shop for ten months but before that had just had the carb rebuilt, new cat, new vacuum lines, new egr, runs reasonably well etc.

High co seems to mean it’s running rich.

I don’t see any mention in the manual of sensors - does an 81 have an oxygen sensor? Another common failure from generic lists also seems to be an engine temp sensor but I don’t know if my truck is that “smart”. Also sounds like I should check my thermactor system, which I thought worked but I can’t recall if I ever really tested it.

Where to start?

[Rusty_S85]

High Co = running rich. On a carb setup could be high float level, incorrect jet size, stuck choke, high fuel pressure.

High Hc = unburned fuel in exhaust

High NOx = running lean or faulty egr system

Question is when it failed on the Co was it the idle portion of the sniffer test or the low speed/high speed test?

If it failed the idle portion but passed the low speed/high speed test then you need to look at your idle circuit on the carb. If it passed the idle portion but failed either the low speed or high speed test then you need to look at the primary fuel circuit. If it failed on both the idle and the low speed/high speed test then you need to check the choke, float level, fuel pressure.

an O2 sensor would be on electronic systems with computers. California did have them earlier than the federal mandate for the other states I cant remember the exact year but I know federally you wouldnt see feed back carbs/electronic sensors and EFI till around 1984/85. Based off your tag with you being in Oregon your truck unless it was built for california sale should be a federal emission truck and for a 1981 it wouldnt have a computer it wouldnt have a O2 sensor and it would just be a basic traditional carbed truck with DSII ignition system.

[Gary Lewis]

High Co = running rich. On a carb setup could be high float level, incorrect jet size, stuck choke, high fuel pressure.

High Hc = unburned fuel in exhaust

High NOx = running lean or faulty egr system

Question is when it failed on the Co was it the idle portion of the sniffer test or the low speed/high speed test?

If it failed the idle portion but passed the low speed/high speed test then you need to look at your idle circuit on the carb. If it passed the idle portion but failed either the low speed or high speed test then you need to look at the primary fuel circuit. If it failed on both the idle and the low speed/high speed test then you need to check the choke, float level, fuel pressure.

an O2 sensor would be on electronic systems with computers. California did have them earlier than the federal mandate for the other states I cant remember the exact year but I know federally you wouldnt see feed back carbs/electronic sensors and EFI till around 1984/85. Based off your tag with you being in Oregon your truck unless it was built for california sale should be a federal emission truck and for a 1981 it wouldnt have a computer it wouldnt have a O2 sensor and it would just be a basic traditional carbed truck with DSII ignition system.

I agree with Rusty. But will add that a clogged air filter will also raise the CO levels, but it would have to be really clogged to be up that much.

So I'm going to guess it is the choke as it might have stuck shut or not have been properly adjusted after the rebuild. (I don't change the choke's settings in a rebuild if it was working correctly.)

As for a computer, it should have one if it has the California emissions system. The pages below are from the 1981 EVTM (Documentation/Electrical/EVTM/1981 EVTM) and show that the CA-spec 302 and 351, which was an M in '81, had a computer. I don't know where it was on an '81, but in '82 they were under the driver's seat. And there is an O2 sensor, engine temp sensor, MAP sensor, etc. Any of those can cause problems.

[85lebaront2]

I agree with Rusty. But will add that a clogged air filter will also raise the CO levels, but it would have to be really clogged to be up that much.

So I'm going to guess it is the choke as it might have stuck shut or not have been properly adjusted after the rebuild. (I don't change the choke's settings in a rebuild if it was working correctly.)

As for a computer, it should have one if it has the California emissions system. The pages below are from the 1981 EVTM (Documentation/Electrical/EVTM/1981 EVTM) and show that the CA-spec 302 and 351, which was an M in '81, had a computer. I don't know where it was on an '81, but in '82 they were under the driver's seat. And there is an O2 sensor, engine temp sensor, MAP sensor, etc. Any of those can cause problems.

Gary, I agree that 1981 CA spec 351M would have a feedback carb, now the question comes which one? A relatively standard 2150 or a 7200 VV? 2150 could have a bad power valve, 7200, good luck, those need a bunch of specialized tools to set up properly.

Check that the EVAP system is working correctly and the canister isn't saturated, PCV in good order. Sometimes doing an oil and filter change can bring it back into spec. All this makes me glad I don't have to deal with this.

[Gary Lewis]

Gary, I agree that 1981 CA spec 351M would have a feedback carb, now the question comes which one? A relatively standard 2150 or a 7200 VV? 2150 could have a bad power valve, 7200, good luck, those need a bunch of specialized tools to set up properly.

Check that the EVAP system is working correctly and the canister isn't saturated, PCV in good order. Sometimes doing an oil and filter change can bring it back into spec. All this makes me glad I don't have to deal with this.

Hadn't thought about what carb it might have. Yep, if it has a VV then there could be a problem. And you are right, a bad power valve would certainly cause the CO to go through the roof.

[delco1946]

Hadn't thought about what carb it might have. Yep, if it has a VV then there could be a problem. And you are right, a bad power valve would certainly cause the CO to go through the roof.

Thanks all! Rusty - we never even went past the idle test . This was at idle, and it was early in the morning and my truck sat in line for a bit so there’s always a chance it wasn’t super warmed up, but that seems like a stretch .

When you say a problem with the idle circuit, are you just saying the idle screws? Sorry it’s been so long - there’s a screw that controls air/ fuel mixture too right? It’s idling at good rpms so I don’t think I’d want to change that Willy nilly….

This is indeed Oregon and my truck was built in Ca but was sold from Seattle so it was never a CA truck. I’ve never once seen anything computerized on this truck so that would makes sense that I don’t have O2.

I should add it’s a motorcraft 2150. The oil, filter, and PCV shouldn’t be old/ bad but I should change all that I guess since it was sitting for so long. It was certainly dusty/ filthy after that time.

[delco1946]

Thanks all! Rusty - we never even went past the idle test . This was at idle, and it was early in the morning and my truck sat in line for a bit so there’s always a chance it wasn’t super warmed up, but that seems like a stretch .

When you say a problem with the idle circuit, are you just saying the idle screws? Sorry it’s been so long - there’s a screw that controls air/ fuel mixture too right? It’s idling at good rpms so I don’t think I’d want to change that Willy nilly….

This is indeed Oregon and my truck was built in Ca but was sold from Seattle so it was never a CA truck. I’ve never once seen anything computerized on this truck so that would makes sense that I don’t have O2.

I should add it’s a motorcraft 2150. The oil, filter, and PCV shouldn’t be old/ bad but I should change all that I guess since it was sitting for so long. It was certainly dusty/ filthy after that time.

Also, if memory serves I paid a shop to rebuild the carb - they did a bad job ( would barely run after) so I rebuilt it myself. I replaced the accelerator pump but I don’t think I ever changed the power valve which looks to be under the carb sorta. Unless they replaced it, it’s likely old.

[Gary Lewis]

Also, if memory serves I paid a shop to rebuild the carb - they did a bad job ( would barely run after) so I rebuilt it myself. I replaced the accelerator pump but I don’t think I ever changed the power valve which looks to be under the carb sorta. Unless they replaced it, it’s likely old.

Yes, the power valve is probably at the front/bottom of the carb and the cover is held on with 4 screws.

On some of the carbs the cover has a fitting that is for vacuum and that goes to manifold vacuum. Not all 2150's have that, but one of mine did. And that vacuum hose is important as w/o it the PV won't work.

[Rusty_S85]

Thanks all! Rusty - we never even went past the idle test . This was at idle, and it was early in the morning and my truck sat in line for a bit so there’s always a chance it wasn’t super warmed up, but that seems like a stretch .

When you say a problem with the idle circuit, are you just saying the idle screws? Sorry it’s been so long - there’s a screw that controls air/ fuel mixture too right? It’s idling at good rpms so I don’t think I’d want to change that Willy nilly….

This is indeed Oregon and my truck was built in Ca but was sold from Seattle so it was never a CA truck. I’ve never once seen anything computerized on this truck so that would makes sense that I don’t have O2.

I should add it’s a motorcraft 2150. The oil, filter, and PCV shouldn’t be old/ bad but I should change all that I guess since it was sitting for so long. It was certainly dusty/ filthy after that time.

In that case then it will be something that effects both circuits. Could be clogged airfilter which doesnt seem likely as to be that clogged it would cause a issue with driveability you would notice. Could be a stuck choke, could be a improperly set float where it is just flooding the carb out with fuel, could be excessive fuel pressure over powering the needle and seat flooding the carb out. Could even be trash in the fuel bowl that is sticking the float even.

[delco1946]

Thanks all! Rusty - we never even went past the idle test . This was at idle, and it was early in the morning and my truck sat in line for a bit so there’s always a chance it wasn’t super warmed up, but that seems like a stretch .

When you say a problem with the idle circuit, are you just saying the idle screws? Sorry it’s been so long - there’s a screw that controls air/ fuel mixture too right? It’s idling at good rpms so I don’t think I’d want to change that Willy nilly….

This is indeed Oregon and my truck was built in Ca but was sold from Seattle so it was never a CA truck. I’ve never once seen anything computerized on this truck so that would makes sense that I don’t have O2.

I should add it’s a motorcraft 2150. The oil, filter, and PCV shouldn’t be old/ bad but I should change all that I guess since it was sitting for so long. It was certainly dusty/ filthy after that time.

In that case then it will be something that effects both circuits. Could be clogged airfilter which doesnt seem likely as to be that clogged it would cause a issue with driveability you would notice. Could be a stuck choke, could be a improperly set float where it is just flooding the carb out with fuel, could be excessive fuel pressure over powering the needle and seat flooding the carb out. Could even be trash in the fuel bowl that is sticking the float even.

I am trying to test the thermactor system and after reading lots and lots of material I still can’t make heads or tails of when air is supposed to be injected into my exhaust manifold. At idle, is air supposed to be going into the EM? Or not?

It appears that I have an early CT style system with no vacuum at idle going to the “do hicky” on top of the belt driven pump.