86 Suburban failed emissions with high NOx

[leolkfrm]

if it is still sealed then it should already set for,leaned out, since it was replaced i thought it had been adjusted.

[dcp]

You may be right. I don't know for sure that it is sealed. I can call the carburetor shop I bought it from last year. I bought it and had someone install it.

[chengny]

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.

IIRC, one of my first questions here at the GMCTF was concerning exactly this issue. I don't recall if we ever got it entirely figured out - but since then I have (through necessity) learned quite a bit about these trucks.

The thing that you are referring to is the electric assist choke (as opposed to a hot air choke). Since your truck has a very rudimentary engine control system there is no variable electric signal (except the dash temp indicator - which is not that type of circuit) that can be tapped to modulate the choke. Instead, it is actuated by oil pressure.

Working on the assumption that there is a direct relationship between oil viscosity and the need for an enriched fuel/air ratio at cold start, and yet also a need to lean it out as soon as the engine is warmed up, the design engineers specified the installation of a second oil pressure sender. This one is entirely independent of the one that drives your dash mounted oil pressure gauge).

If you look behind your distributor you will see two oil pressure senders. The one with two electrical connections is the one that controls the choke heater. Without getting too technical - when the engine is started oil pressure develops and passes through an orifice to press on the diaphram, which in turn closes the switch. Thus upon the establishment of oil pressure, straight battery voltage is applied to the choke heater.

With the application of this unrestricted battery voltage the ceramic disc begins to heat up and act upon the bi-metallic spring. This spring is what then assists the choke in opening up as the engine warms.

If you have a warning light on your dash board that says CHOKE, it is wired in series with the switch and heater and is actually the same as a low oil pressure warning light. So if you get a CHOKE indicator light while driving - shut the engine off until you determine that there is really oil pressure available to the engine!

Remember, the choke heater is there to help open the choke after the engine is started. The choke will close by itself whether or not it has power. I usually check my choke by putting the entire carb in the refrigerator (normally 40 F) for an hour or so. When you remove it and pull on the throttle linkage it simulates the act of pushing your foot on the accelerator and the choke should snap shut.

The electrical system is described in the attachements.

Wow - sorry for the seminar.

[hotrodpc]

Oh crud, you still have those on there? Be careful about removing those. Some states will fail a vehicle if those are removed and the carb was altered. I have a feeling you'd be OK in your state. CA would fail you at one time if they were removed. You manual is correct. You pretty much have to break or cut them out. If that vehicle has gone this long with them in there, and always up until now, geez, I call that impressive. Very Impressive. They been in there for 23 years and never needed a carb adjustment? WOW !!!

[crabtruck]

dcp I am completely impressed by your tenacity.
Staying on top of this while all the rest of life's chaos takes place.

Probably you did this, but I did not see above, so I'll ask:
PCV ? air filter ? plugs,wires, cap checked and clean?

[dcp]

Chengny, Hotrodpc, and crabtruck: replies for each of you below:
Chengny, really appreciate the choke seminar. ; ) I'm VERY detail oriented so it didn't seem long to me. (as you can tell by some of my posts above) Thanks also for the attachments, which I read and were helpful. I saw in my manual what you said about the electric signal coming from the oil sensor, but I didn't realize it was separate from the one that goes to my gauge on my dash. The guage on my dash is not reading correctly (it doesn't move!), I think because the oil sensor has gone out for it. It went out about 5 years ago and had to be replaced but I didn't do it myself. I haven't gotten around to replacing it again. So the fact that I have 14 volts going to the electric 'assist' choke says the other sensor is working. Will look for it behind the distributor (can follow the wire also, I assume, from the choke assist) to educate myself. As for putting the choke in the refrig, it is in the 40's when I wake up here so have tested. Before I adjusted the electric assist choke position, when I manually opened the choke (since assist was not working) with truck off and then turn throttle at carb, choke sprung closed. Now that I have set it to be full open when warm, it doesn't 'spring' very far-only half closed. Will replace choke assist. With your input I NOW understand whether the heat is coming from to affect the thermostatic coil!!! I was wondering how the heat would transfer very fast from the engine to that choke hanging off the carburetor!!

Hotrodpc, I just called the carb place where I bought the rebuilt carb at last year. He says the rebuilt carb should not have the plug, whereas the original did. I never adjusted the old carb but it is possible that my brother did. He bought the truck new in 86 and I got it from him at ~120,000 miles in 95. It has ~240,000 miles now. So I'll have to figure out where the adjustment screw is if it doesn't pass emissions today. If I had time I could lean it out but I've gotta run to get to work soon and I'm curious if the choke assist might be the issue by itself.

Crabtruck, thanks. Weekends are the only way I make progress! Good advice. Had checked. PCV is clear, air filter is almost new, and wires were new last year and look in good shape. A couple other things that may give some higher hydrocarbons are 1) the heated air pulled off the manifold will bring some oil fumes into the carb. I have oil leaking from the seals and when the engine is hot some of the oil fumes suck into the air intake. Question, would it hurt my emissions to block that intake (only the part that brings in heated air)? Second, truck burns some oil but I warm it up before inspection to let the rings seal and there is no grey exhaust at that point.

You guys are incredibly helpful. Thanks for sticking with me on this. Hope to have some good news!

[dcp]

PS When I say oil leaking from the seals, I mean from the valve cover gasket. Sorry.

[dcp]

IT PASSED!!!!!! Thanks very much to ALL who helped, but especially to Hodrodpc and Chengny who gave lots of good advice, some really useful diagrams and info, and a few seminars to educate me!!!! ; ) I've learned a lot. Short summary of emissions results: CO way down, passing with flying colors. HC just barely passed on 25/25 but much better on 50/15 test (both were down from before). NOX went up, which makes sense, but still well within limits. For those interested in the details, below are the emissions results from this morning. Also, below that is a brief (for me, at least) summary of what I did to get NOX down and then get CO+HC down with your help.
The readings for the 25/25 test were:

HC 142 (142 allowed) (got 149 last time before adjusting the choke)
CO 0.19 (1.74 alllowed) (2.63 last time before adjusting the choke)
NOx 1281 (1881 allowed) (got 758 last time before adjusting the choke)
CO+CO2 11.9 (min 6) (got 13.9 last time before adjusting the choke)

The overall readings for the 50/15 test were:
HC 120 (145 allowed) (got 137 last time before adjusting the choke)
CO 0.10 (1.3 allowed) (got 2.04 last time before adjusting the choke)
NOx 1446 (2017 allowed) (got 1096 last time before adjusting the choke)
CO+CO2 11.5 (min 6) (got 13.8 last time before adjusting the choke)

Brief summary of changes made and results:
At start of post, everything was ok except NOX was much too high. Checked vacuum lines as suggested and found five breaks/lines missing. Fixed these, including EGR solenoid bleed valve, cannister to PCV-carb line, Decel valve to PCV-carb line, line going to a delay valve and then to the Trans. Conv. Vac SW, and vaccuum line from the EGR TCC/purge TVS. Result: NOX was great, but CO went through the roof and HCs were too high at 25/25 emissions test (25mph). Also, idle seemed higher. Found electric assist choke was not opening choke all the way so adjusted so was wide open when engine was warm, but only half closed when engine was cold. Only took ~2 min running after starting before didn't die (since choke wasn't fully closed had to keep gas pedal depressed some for 2min). Went to get free emissions recheck at same place and PASSED!!! Plan to replace electric choke assist (since had proper voltage coming from oil pressure sensor) in future.
You guys rock!!! Thanks so much!

[hotrodpc]

Congrats !!! Awesome. I figured, backing that choke off would sure make enough differance to get it done since you were not that far off from passing. You did a fine job !!! And much appreciated for the updates. Thanks for taking the time. This thread will valuable to many, and you even titled it right. Some mark a thread, Help, or what do I do? With no description of the problem. A thread like that is not likely to be found. This one will be.

[crabtruck]

Originally Posted by dcp

would it hurt my emissions to block that intake (only the part that brings in heated air)?

If you are talking about the stove pipe exhaust to air cleaner, I would keep it.

Very nice job describing the problem and providing good details on progress.
That is a huge help for all future readers.

Kudos to chengny and hotrod for comments throughout.