"Stock" 1991 K2500 5.7L V8 fails smog with unburned gas at exhaust.

[chevilex]

Just bought this truck from a curious, inventive fellow. He built his own carburetor out of mason jars for a home-brew small house generator.... On the truck, some things he's done well, and others have been questionable. So I say "stock" because I'm not entirely sure what he's done to it. For example, it's got catch cans and an oil cap breather, neither of which, from what I gather, is probably doing much. And it appears that he removed the fuel vapor canister.

I'm in California and he insisted on running the truck without a catalytic converter, skirting smog laws via registration loopholes. I'm trying to go legal. He handed me a brand new cat, but of course, it's not California legal so I have to get a new one anyway.

The truck runs fine. I can't tell if it accelerates well as I've never driven anything like it. But it starts relatively easily, idles just fine, doesn't hesitate, gets to 80 mph, etc. I've driven it some 250 miles. The check engine light comes on when I turn the key, so I know it works; it stays off while the truck is running. Just really smells like gas, all the time. Smog check passes everything except that it's got about 3X the legal limit of unburned gas in the exhaust.

The mechanic says he can do a "tune up" - he ran a compression test and it's all fine; he's suggesting new plugs, wires, and distributor rotor. Now that he's verified the cat is not California legal and discovered the vapor canister is not there, he's insisting that I get that handled before he does anything else.

Now that I've read a bit through the forum, I'm wondering if this thing even has an O2 sensor, and if not, could the previous owner have disabled something so the check engine light doesn't come on? Sure, I don't mind getting legal exhaust installed; just doesn't make sense to me to insist on doing that before doing a tune-up.

And what exactly is a "tune-up"? Let's assume all sensors are in order, and you put in new plugs, wires, cap, and rotor... does the computer have to "learn" the new behavior over some time, or will it instantly adjust the mixture and it'll be good to go? If I had the tools, I figure the last thing I'd do is go get a new cat put in - I'd do all the messing around with the one that I am being forced to dispose of anyway.

I'm wondering if this guy is actually diagnosing things or if he's just finding it difficult to tell me he doesn't actually want to explore this truck and its quirks any further.

Any opinions / advice / knowledge greatly appreciated.

 

[Schurkey]

it's got catch cans and an oil cap breather, neither of which, from what I gather, is probably doing much. And it appears that he removed the fuel vapor canister.

That can't be good.

He handed me a brand new cat, but of course, it's not California legal so I have to get a new one anyway.

"Aftermarket" cats were invented because the EPA (Federal regulations) wanted a less-expensive alternative to OEM catalysts. In other words, the EPA REQUESTED aftermarket companies make lower-cost catalysts. California--and those states using CA emissions laws--refuse to allow "aftermarket" lower-cost catalysts. You have to have an OEM-level catalyst which is hatefully expensive. Mind you, there are aftermarket companies that make OEM-level catalysts--they'll say CARB approved, and the price tag will make you stammer.

The truck runs fine... ...Just really smells like gas, all the time. Smog check passes everything except that it's got about 3X the legal limit of unburned gas in the exhaust.

NORMALLY, I'd say that excess hydrocarbon emissions is a result of misfire. But the total lack of a catalyst will have considerable effect on hydrocarbon emissions; as can an ancient O2 sensor.

The mechanic says he can do a "tune up" - he ran a compression test and it's all fine; he's suggesting new plugs, wires, and distributor rotor...
...I'm wondering if this thing even has an O2 sensor, and if not, could the previous owner have disabled something so the check engine light doesn't come on? Sure, I don't mind getting legal exhaust installed; just doesn't make sense to me to insist on doing that before doing a tune-up.

If this were me, I'd cram in the one O2 sensor it should have in the driver's side exhaust manifold FIRST. The only question is whether you install a one-wire (inexpensive) sensor; or go all-out for a newer, upgraded 3-wire and a wire harness kit that supplies power and ground for the heater element in addition to the wire that provides signal to the ECM. My '88 got the upgraded, heated sensor and harness. Most newer vehicles have them as original equipment.

He ran a compression test. That means he had the old plugs out, examined them, and decided you should have new ones. Why didn't he put new ones in since there'd be zero additional labor to do that? The only upcharge would be for the eight spark plugs.

Plug wires, and rotor? I'd tell him to put a new distributor cap on it, too--unless he's examined it already and found it to be OK.

Fuel filter, air filter, possibly a PCV valve, (I would remove the "catch can") and making sure the fresh-air inlet hose from the TBI spacer to the valve cover is in place. Then get rid of the vented oil fill cap in favor of a stock non-vented cap. The concern here is that the engine has excess blow-by, forcing fumes out the fresh-air inlet, and contaminating the air filter. Lastly, verify base ignition timing. Consider an oil change--maybe the oil has gasoline contamination in it.

And what exactly is a "tune-up"? Let's assume all sensors are in order, and you put in new plugs, wires, cap, and rotor... does the computer have to "learn" the new behavior over some time, or will it instantly adjust the mixture and it'll be good to go?

IF the O2 sensor is lazy and you replace it with a new one, or the cylinders are misfiring and the 'tune up" fixes it, the computer may have to adjust the fuel trims and potentially the idle air control to get the idle speed right.

Not a big deal; the computer does this automatically and fairly quickly. It does this every time the battery is disconnected, for example.

 

[chevilex]

Thanks for the pointers, @Schurkey. I'm hoping I can get over to the shop tomorrow, talk to them about what they want to talk about, and move on to the next step. The truck has just been sitting now probably since Friday with no one working on it.

 

[chevilex]

I went to look at the truck at the shop. I think they're just trying to understand if I'm really going through with this or not. Good news all around; they are budget-conscious people. They did not put the old plugs back in (in fact, one of them broke when they pulled the wire off).

The O2 sensor is present, but probably ancient. The spark bugs were all black, so at least both injectors are currently dumping too much. While looking at the truck, it seems possible that the evap canister was removed to make room for a second battery. I was able to get the canister and all its associated parts from a nearby yard.

It's getting the exhaust done this week so we'll get to the tune-up part soon and find out how many bits and bobs need to get replaced.

 

[GoToGuy]

For older OBD 1 vehicles, when replacing the O2 sensor. There is a published service bulletin about new sensor replacement, older OBD 1. Aftermarket sensor technology has vastly improved since 80's 90's. And the older engine computers will not recognize how fast the new O2 sensors function and may throw a code. I use NGK O2 sensor's on my 91 C15, K1500, and K2500 as NGK builds these specific OBD 1.
I'm not waving the NGK flag, a quality product is just that. To me not having crawl back underneath or " WTH is that code still there!?". Is worth a few dollars. Good luck.

 

[Schurkey]

There is a published service bulletin about new sensor replacement, older OBD 1.

Got a link or bulletin number? I'd love to read that.

 

[Rocket Surgeon]

I too would like to see this bulletin.

I would also love to see good instructions on choosing and installing a heated type O2 (3 wire?) sensor where a 1 wire was originally. Note many sensor have 4 wires, two isolated grounds, a positive feed for the heater, and an output to the computer.

I know on some applications, California emissions vehicles received Heated oxygen sensors prior to the rest of the market, such as on 92 and 93 models.

Regarding sensors and branding, I have personally observed that earlier 90s original 1 wire ACDelco O2 sensors were made by Bosch, and the later heated ones were made by NGK. The later heated ones are superior, and this change over seems to be 93-94. Yes, many acdelco parts are rebranded. NGK is the same part, not to plug it, but it is real good.

 

[PlayingWithTBI]

I would also love to see good instructions on choosing and installing a heated type O2 (3 wire?) sensor where a 1 wire was originally.

In 2018 I did it with an ACDelco AFS74 heated sensor and this 3 wire adapter. The OE connector simply plugs into the adapter, I wired the power to the ignition switch and grounded it to the driver's side fender. It's still working fine.

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www.amazon.com/Michigan-Motorsports-Heated-Harness-Adapter/dp/B0141VHZ6U

Edit: Here's the sensor

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www.amazon.com/ACDelco-AFS74-Original-Equipment-Heated/dp/B000C9VTRC/ref=sr_1_1?crid=TR22NG3VZELT&keywords=ACDelco+AFS74&qid=1668007672&sprefix=acdelco+afs74%2Caps%2C1039&sr=8-1

 

[RichLo]

Edit nevermind, the title had the year and engine.

 

[chevilex]

Truck passed smog with:
- New California cat ($750 just for that). That was more visual than an actual chemical necessity; the old cat was new but just not CARB certified.
- New sparkplugs, wires, distributor rotor and cap, O2 sensor (they just put a one-wire unit in), and fuel filter.
- Evaporative canister from a junkyard.
- Stock oil cap to replace the vented one.

The catch cans are still there. But I'm starting to think that they're actually causing a restriction in the PCV path. Wouldn't surprise me since the filters look pretty awful.

The last owner had a cap on the dipstick tube because the O ring in the dipstick handle was gone. The mechanic thought that was unnecessary. Drove 15 minutes and there was oil dripped all over; it's coming out at the dipstick handle. The mechanic specifically took a whiff of the oil on the dipstick (with the engine cold) and said it was fine; after the 15 minute drive, the whole blade is splattered with oil and it smells like exhaust.

I ran the engine at idle with the oil cap off and don't feel any excessive gas coming out. The addition of a sealed oil cap, the catch can filters being dirty, and the leak at the dipstick handle is what makes me think the catch cans could be adding an unwanted restriction. Does this make sense to anyone else?

The other curious thing (which maybe is mormal) is that I went to the local Napa to get an air filter and they sold me something that's slightly too tall for the housing. There's about 3/16" gap between the lid and the wall of the housing. I figure it doesn't matter much as long as the filter seals against the housing and lid (a taller filter is better than one that's too short). I don't remember seeing any issue with that when I first put it in, but now, I see smoke coming out of that gap when I shut the truck off. Is this just normal exhaust from the recirculation system that you normally don't see when you have the right size air filter?

Here's a video immediately after stopping at a gas station.

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