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

[2001ZR2]

If you are getting smoke out of the throttle body it's compression issues which your mechanic checked or the PCV system is messed up somehow.

The PCV system issue might be why the PO put on the vented cap and catch cans.

Wonder why the NAPA filter didn't fit.

 

[Donald Mitchell]

Bad valve stem seals. Look it up!

 

[Schurkey]

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.

There should be vacuum at the grommet for the PCV hose that leads to the air cleaner. This will be on the opposite valve cover from the one that holds the PCV valve. Or just pop the hose off the air cleaner nipple and check there. If you don't have (mild) vacuum there--if there's pressure at idle or fast-idle--you need to ditch the "catch cans" and/or fix the excess blow-by.

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.

Again--symptom of a defective PCV system, or excess blow-by.

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?

Yup. Ditch the catch-can(s)

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.

That air cleaner housing used to have a K&N filter...right? Is there still a rubber seal in the lid? Common for the rubber seal to rot away, and oiled-gauze filters seem to make that happen faster for some reason.

I see smoke coming out of that gap when I shut the truck off. Is this just normal exhaust from the recirculation system

There is NOTHING "normal" about that.

If that smoke is not coming backwards from the fresh-air inlet of the PCV system, I'd be looking for leaking injector(s) that still dribble after the engine has shut down.

 

[Busted Knuckle]

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.

It won't run right without an O2 sensor. CAT output is not monitored. I'd begin by changing the MAP sensor and all the hoses associated. Next, the TPS throttle position sensor. These are both critical. You must replace the cannister OR block the port where it connects to the TB. Make sure PCV valve is there and hose is good. If tuning up, check/set ignition timing per manual. (Must open bypass wire while adjusting).

 

[chevilex]

Bad valve stem seals. Look it up!

I let the truck sit for two days and started the engine. Zero smoke came out of the exhaust, which I read was a symptom of bad valve seals.

There should be vacuum at the grommet for the PCV hose that leads to the air cleaner. This will be on the opposite valve cover from the one that holds the PCV valve. Or just pop the hose off the air cleaner nipple and check there. If you don't have (mild) vacuum there--if there's pressure at idle or fast-idle--you need to ditch the "catch cans" and/or fix the excess blow-by.

On the driver's side rocker cover (which is the non-PCV side), there's a hose that goes to a catch can. From there, a hose goes back under the air cleaner somewhere (I didn't take the air cleaner housing off and couldn't see) - but it's supposed to go to the air cleaner extension. At idle, I got very little vacuum (if any) there, but not sure I'm supposed to given that it's above the throttle body and there's a gap between the cleaner housing and the lid.

I put a rubber glove on the outlet of that catch can and it does inflate - so gases are making it past the filter in that thing.

I haven't tried anything on the PCV side yet. I'm going to take my time and get the cleaner housing off and check the two gaskets on the extender while I'm at it - at some point the shop had the air cleaner off and had put all the parts in the bed of the truck, and I didn't know that when they asked me to raise the dump bed so they could look at the gas tank. So a few bits and bobs got lost in the parking lot (at least the wing nut did).

I understand that I should be able to shake the PCV valve and hear it moving around, and if not, I should get a new one.

To avoid splattering oil all over, I stacked a few fingers of a nitrile glove and zip tied them to all hell at the end of the dipstick tube - hoping they don't get launched next time I drive into town. At idle I can see a bit of pressure building there. My understanding is that this all points to blow-by, which wouldn't surprise me given that the bottom end has some 280k miles on it (the heads were redone a few years ago, and the transmission is also newer).

Can the severity of the blow-by show up in a compression test? All cylinders were up to spec.

That air cleaner housing used to have a K&N filter...right? Is there still a rubber seal in the lid? Common for the rubber seal to rot away, and oiled-gauze filters seem to make that happen faster for some reason.

Yes, perhaps it was a K&N filter - it looked filthy so I tossed it. I figured the K&N stuff on the housing were just fun stickers.

There is a seal on the lid, and it feels spongy, but maybe it's mostly gone - I'll see what they have at the parts store and see how thick it's supposed to be. I did find a photo from before I bought the truck and it looks like the gap was there.

There is NOTHING "normal" about that.

If that smoke is not coming backwards from the fresh-air inlet of the PCV system, I'd be looking for leaking injector(s) that still dribble after the engine has shut down.

Where's the inlet of the PCV system?

 

[Schurkey]

The fresh-air inlet for the PCV is on the air cleaner, or on the air cleaner riser. You said that hose goes to a catch-can, and from there to the valve cover opposite the PCV valve.

Scrap the catch-can on the fresh air side.

There should be mild vacuum at idle or fast-idle on the HOSE that connects to the air cleaner riser. The vacuum is regulated by the PCV valve in the other valve cover; drawing through the entire crankcase until the vacuum reaches the hose that connects to the air cleaner riser. That draws fresh, filtered air into the crankcase to remove fumes and boiled-off water that's condensed in the oil. The fumes and water vapor are what collect in the "catch cans". There's no reason to prevent the fumes and water vapor from entering the intake manifold unless there's an unreasonably-high amount of them. If there's an unreasonably high level of fumes and/or water vapor, SOMETHING IS WRONG inside the engine that should be fixed, not ignored.

If I understand this right, you've also got a catch-can on the hose between the PCV valve and the manifold vacuum nipple on the TBI unit. Scrap that one, too.

THEN test for vacuum at the hose connected to the air cleaner riser. You should have mild vacuum there (plug the hose with your thumb at fast idle, when you release it five seconds later, you should feel a whoosh of air go into the hose.) If not, you have excess blow-by which may or may not show up as low compression on a cranking-compression test.

Wild Guess: The previous owner put two catch-cans on the PCV system because there was excess blow-by, and he wasn't interested in fixing it properly. MAYBE you can solve the problem by soaking the rings with GM "Top Engine Cleaner". Probably not, but it might be worth trying.

At any rate, REMOVE THE AIR CLEANER LID and have someone else shut off the engine. Verify whether the smoke is coming from the nipple on the air cleaner riser (blowby) or from throttle plates/IAC passage (leaking injector or coolant entry into the manifold.)

 

[chevilex]

Alright - the smoke when I stop the engine has ceased. It coincided with me capping off the dipstick tube, so maybe oil was splattering all over the top of the engine. Not sure if it was that, or something the shop spilled something which has now fully evaporated.

Here's my understanding of the crankcase ventilation system in its stock configuration:

You must be registered for see images attach

Air comes in through the passenger side. The intake is in the air cleaner extension, i.e. just below the air filter. It circulates through the crankcase, through the PCV valve, and into the TBI. The driving force is suction at the TBI, i.e. the pressure below the throttle plates is lower than in the filter housing.

At idle, the vacuum is high, which is supposed to pull the check valve in the PCV valve closed (not sure it's supposed to close fully, or just restrict flow).

Below is the configuration on my truck right now:

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As per @Schurkey's suggestion, I started checking flow at the hoses. I disconnected the hose between catch can "A" and the air cleaner extension. At idle, air is flowing *toward* the air cleaner - this is the wrong direction.

I have a new PCV valve in hand and will get deeper into it this weekend. But so far, after some 40 miles of driving, I can see that Catch can "A" is filling faster than "B". To me, this implies the air flow is in the wrong direction even under load.

I will say - I like the catch cans from the point of view of helping diagnose things like this. But I still worry about the additional restriction they're introducing, unless the filter elements are religiously being cleaned.

 

[Schurkey]

Yes, you have correctly determined the airflow through the PCV system, other than the operation of the PCV valve itself--which is only somewhat important for our purposes. The PCV valve regulates the airflow based on pressure at the PCV valve outlet. Low pressure (high vacuum) causes the valve to restrict flow to a reasonable level that won't upset the fuel/air mixture delivered to the cylinders, and won't create an unacceptably-high idle condition. Moderate pressure (lower vacuum) allows the PCV valve to open; the lower pressure still draws a reasonable amount of air through the PCV valve. High pressure at the PCV valve outlet closes the valve to prevent a backfire from traveling into the valve cover/crankcase; potentially causing a fume explosion.




As said before: There's really just two possibilities. (You could have BOTH, of course.)

A. The engine has so much blow-by at idle that even a fully-functional PCV system can't handle it all; the excess goes "backwards" into the air cleaner extension/riser. This is ecologically-sound in that all the blow-by and crankcase fumes are still run through the combustion process rather than vented directly to atmosphere. However, it still indicates an internal engine problem--the combustion gasses are not appropriately sealed and directed to where they should be going.

B. The PCV system has failed, or partially-failed. It's so restricted that proper evacuation of the crankcase is impossible without changes/repairs.

GET RID OF THOSE DAMNED CATCH-CANS, at least temporarily. See how the system performs as engineered by GM. If you don't have excess blow-by--so that there's mild vacuum at the fresh-air inlet hose at idle and fast-idle--you've determined that the engine likely does not have excess blow-by; and you won't need the catch-cans. You can take them back to the previous owner, maybe he can make a carburetor, or a flux capacitor, or a space-age vacuum cleaner out of them.

If removing the catch-cans and verifying the PCV valve is correct and functional, does not produce enough flow through the PCV system to promote vacuum at the fresh-air inlet...you've got engine work ahead of you.

Note that under heavier load, there may be more blow-by in the crankcase than the PCV valve can evacuate; that's why the other end of the system is directed to the inlet of the throttle body via the air cleaner riser/extension. The excess still gets burned in the combustion chambers rather than vented to atmosphere. In a properly-sealed engine--one with good rings/cylinder walls, and decent valves/valve guides, you'd only have back-flow under heavy throttle conditions. Back-flow under ordinary, day-to-day driving indicates excess blow-by.