86 Suburban failed emissions with high NOx

[hotrodpc]

If its a 1500 its a half ton. Yes, yours will have Cats from the factory. 8600GVWR and above did not require Cats in 86.

[dcp]

Ok, I've been working on it this weekend and here's what I found and what I did. First, sorry Chengny that I didn't find the number yet to get a diagram but used your diagram for the AIR pump system and then found that GMC always puts a sticker in the engine compartment with the emissions related vacuum hoses, ERG, etc. Also, haven't done the seafoam cleaning yet, hotrodpc. Did find my truck has a cat as hotrodpc said so can't rule that out as a problem since the book says it is supposed to handle NOX as one of its functions. However, I did check out the AIR pump and make a lot of progress on the vacuum tubing as Chengny suggested.
AIR pump. Checked it out as suggested and it was pumping air just fine to both sides and the tubes past the check valves kept fairly cool. Seems like it works. However, I found four problems with vacuum. First, there was a severed vaccuum tubing that is supposed to go to a delay valve and then to the Trans. Conv. Vac SW. I'm not sure what that abbreviation means but I replaced a section of the tube to connect it. Second, There was a broken double T connector that used to be in the line from the PCV valve to the carb. Someone had simply connected the PCV valve to the carb with out the double T. When I say double T, it was supposed to have two branches off that line to two separate places. One was to the cannister (driver's side of the radiator) and the other was to the Decelerator valve, which was sucking air like crazy. I cut the vac line and T'd in both of these. Third, the EGR bleed solenoid vaccuum line that was supposed to be connnected to the back of the carb was hanging loose/open and the guy who put the rebuilt carb on had put a rubber cap on the port on the carb. I removed the cap and connected the EGR bleed solenoid vac line to the carb. Finally, the vaccuum line from the EGR TCC/purge TVS was broken and mostly open. I replaced that.
I would be interested in what you know about what all those vaccuum connections were supposed to be doing and what they would screw up. It may be my imagination but I thought I was getting some rumbling (like a bunch of little backfires/popping) when I rev'd the engine and let off the gas (in park) and it doesn't do that now. Given that two of vaccuum lines have to do with the EGR system I'm hopeful that the truck will do much better on NOX when I take it in this week. Thanks for all the suggestions. I'll let you know whether it passes.

[hotrodpc]

Fixing 4 leaks is going to make a significant differance somewhere, but for the vac alone. Then considering what items did you actually repair to operate properly now. Obviously you have the 700r4 trans for the Trasmission Converter Vacuum Switch. Also makes me wonder if your converter was even locking up. You may have just found enough to make it pass since you really did not fail by a huge margin.

[dcp]

Good news and bad news. Reconnecting the four hoses mentioned above helped me pass the NOX portion of the emissions test! Thanks for all the help on that! I also passed on the CO+CO2%, although the readings were a little higher. The readings for the 25/25 test were:

HC 149 (142 allowed)
CO 2.63 (1.74 alllowed)
NOx 758 (1881 allowed)
CO+CO2 13.9 (min 6)

The overall readings for the 50/15 test were:
HC 137 (145 allowed)
CO 2.04 (1.3 allowed)
NOx 1096 (2017 allowed)
CO+CO2 13.8 (min 6)

Now the bad news. Looking at the values above you can see that my HCs and CO% shot way up from my reconnecting these hoses (compare to the prior test values pasted below). My HC failed for the 25/25 test and just barely passed at 137 for the 50/15 test. Even worse was the CO%, which failed on both portions of the test even though they had been really low on the prior test (see below)!!! Help! What kinds of problems would do this? I know the carburetor could normally be an issue but why were the CO values so low before I reconnected these hoses? Any advice appreciated. For reference I’ve pasted the readings before I reconnected the different vacuum hoses:

My 86 C1500 Suburban 2wl drive 350cc failed emissions for the first time for high NOx. It failed on both the 25/25 and the 50/15 tests on NOx. The overall readings for the 25/25 test were:
HC 128 (142 allowed)
CO 0.11 (1.74 alllowed)
NOx 2396 (1881 allowed)
CO+CO2 11.1 (min 6)

The overall readings for the 50/15 test were:
HC 116 (145 allowed)
CO 0.11 (1.3 allowed)
NOx 2464 (2017 allowed)
CO+CO2 11.2 (min 6)

PS. Chengny, weight on my truck is 6800.
PS PS Hotrodpc. Book indicates it has computer but don’t know which one yet. Would this still be useful to know?
PS PS PS Anything else you guys requested and I missed?
Thanks!

[dcp]

I thought some of the high CO might be because the guy who put the rebuilt carburetor on adjusted the idle, mix, etc. with four vacuum hoses detached. When I looked at the carb I noticed that even when I warmed up the truck, the electric choke was still partially closed. I adjusted the electric choke so the Thermostatic Coil would have the choke totally open when the truck was warmed up. Could this be enough to have caused too rich a mixture or should I try to lean out the mixture as well?

[chengny]

I have lost track of this thread but one thing that I know about the electric choke is that there should be no gasket installed where it mounts to the carb housing. It is frequently found that the gasket is in place and that is wrong.

The choke heater is not provided with an external ground wire. The hot wire comes in and attaches to the tab. From there the only path available for current flow is through the coil, then the heater shell and then to the carburetor.

Using that gasket will act as a very effective insulator and the bi-metallic coil will not react as designed.

[dcp]

Hi Chengny. Thanks for input. I checked and there was not a gasket. I also checked the voltage crossing to it from the wire that plugs into it. It was ~14V so is getting a ground. Something is wrong though as it is not working. If I set the choke fully closed when the engine is cold, it never opens up completely. If I set it full open when the engine is hot, it never closes and when I try to start it cold the engine dies unless I kept the gas pedal depressed some. It does open some over time when the engine warms up. I guess I don't understand what the electric part does, that is, when it is supposed to kick in. The thermostatic coil seems to be doing the partial job. Is it possible that the electric choke is burned out? From my read of my manual it says the choke should open fully in 5min or either the electric choke is bad or there is no voltage to it. Looks like it must be bad. Thoughts? If not, I will replace it. Thanks again for all the previous help with the NOX. Finding the EGR solenoid bleed vacuum unplugged and plugging it back and the 3 other vacuum issues fixed the NOX great!
Best

[hotrodpc]

I think you are on the right track now dcp. High Hydrocarbons and Carbon Monoxide would seem to be a rich conidtion to me. Anyway, you can warm it up, take that adjust that choke very lean as in backing it off, get your emissions to pass then you can play with it and fix if you wish.

[leolkfrm]

when adjusting carbs for emissions, you adjust it for correct running and then lean it out! i think it was about 1 turn in

[dcp]

Thanks Hotrodpc, good advice. I did adjust choke to be wide open when warm. Will keep the gas going until it gets warm in the AM, then do emissions. Leolkfrm, wish I knew how to adjust the mix. From what I can tell, it is factory fixed and I have to take the carb off and turn upside down and hacksaw and break out a plug before I can adjust. Anyone familiar with that. Am I misunderstanding my manual? If I am, I need to find where to adjust. It's been a long time since I took a carb apart. Anyway, will try without the adjustment if it is that complicated first.
Thanks!