2000 Chevy S10 P0446 and misfires

A check engine light recently came on for code P0446 on my 2000 S10 4.3L. I have one of the cheapo code readers that plug into the OBD port and that is how I read the code. The simple solution would be to throw a $30 evap vent solenoid at this but I am not sure that is the problem.

The truck recently went in to the shop for three things that might play into this, both at the same time. I had the fuel pump replaced. The fuel line coming out of the pump was leaking and spraying fuel all over the top of the tank. There was a noticeable odor of course and that is why the truck went into the shop. The dreaded intake manifold gasket had been leaking coolant for a while but was manageable. Then is seemed an injector was sticking open so I had both the intake gasket and fuel injection spider replaced. So it went in for fuel related problems at the tank and at the injectors. Obviously all the stuff on top of the intake had to come out.

The truck runs fine throughout 99% of driving conditions but has two obvious issues. There is a noticeable misfire at idle. You can hear that poofing sound out the exhaust while it is sitting and idling. It is fine under normal acceleration and driving. The other issue is at high throttle/high power conditions such as when accelerating quickly or passing. The engine will cut out and lose ALL power, creating a strong bucking feeling. It returns to normal when you let off the gas.

I have felt like the HVAC system switches modes such as from normal to defrost or the temp changes when climbing hills as well. I know these are vacuum controlled so am wondering where to start, especially considering all the variables that might be in play here.

I am not getting any misfire codes or pending codes for misfire. It has been driven several hundred miles since this started so shouldn’t be an issue with not enough start cycles or mileage.

I checked the vacuum lines and wiring connections . There weren’t any any obvious problems. I figure I will pickup a can of starter fluid to spray around the lines and see if I can notice a difference in how it runs to detect a vacuum leak. It seems to run fine when cold but starts misfiring and sputtering at idle once warmed up. It isn’t a high idle or anything with a lean misfire like you hear with vacuum leaks.

Is this possibly just the vapor canister purge valve? I figured this wouldn’t cause running issues and just create excess emissions. I have been told to just replace the gas cap because these are cheap and that might be the issue. The seal on mine looks fine but this might not be a bad idea. I don’t see this causing the misfiring I am experiencing though so want to narrow this down before throwing money at it.

The code indicates a problem with the vent system of the EVAP.

Not the purge system.


I concur w/post above, p0446 indicates a vent valve/circuit problem, not purge valve. Could be the prior work dislodged the vent valve’s electrical connector, or a vent related hose is being pinched. One reason that valve is used by the computer is to verify the truck’s evap system is not leaking fumes to atmosphere. I’m guessing a test vacuum is created in the evap system, then opening the vent valve to measure how quickly the vacuum subsides. If the vacuum (negative pressure) doesn’t subside towards zero quickly enough, the drivetrain computer thinks the valve’s functionality is suspect. Other possibilities of course, like the evap pressure sensor is inaccurate.

I don’t fully understand purpose of vent valve I guess. Can think of two purposes, but there may be others…

Purpose 1: During re-fueling fumes at top of gas tank are pushed into charcoal canister. The gasoline stays inside canister, and air cleaned of gasoline leaves canister (to atmosphere) via vent valve?

Purpose 2: When car is driving down the road gasoline inside canister fumes is pulled out (by engine vacuum) through purge valve into engine. In order for gasoline to exit canister and go into engine, air must come into canister to replace lost gasoline, and that air comes in through the vent valve? It seems like you’d want to have filtered air for that.

OK, I have a strange update to this situation…

I always clear the code at least once to see if it is what I call a “phantom code” or an actual problem. My girlfriend used to have a car like this. I would clear the code at least twice before taking it seriously. There were so many times the check engine light came on for random problems. You would clear the code and that code would never return but a completely different and unrelated one would show. That car was GM Daewoo garbage and is gone now. I figure in addition to all the other problems, something electrical must have been wrong.

Anyway, I cleared the code on this one. It showed right back up shortly after clearing so I figured I had a real problem and not just a “phantom code” as I call it. So, I went to the parts store and bought the vent valve/solenoid assembly. It looks like this is an easy swap but the weather has been crap or I have been busy so didn’t feel like messing with it. Anyway, the new part is sitting in the box and I can see that the code is no more and there is no more check engine light. It just went away on its own.

I am not sure what happened or if I should replace the part or just leave it alone for now. I do have one idea… I was paying for gas at a busy station with cash so didn’t want to have to go back in for change. I figured out how much gas I thought I needed and gave them that much money so it would just shut off. Anyway, I come back out and start filling. I wasn’t paying a lot of attention but the pump didn’t turn off when it was full. Gas had just started to pour out the filler tube from being overfilled when I hit my money limit and the pump shut off. I know overfilling can damage the evap system and wonder if maybe gas that ended up where it shouldn’t have eventually vaporized and the problem resolved.

I am just wondering if I should return the part or not. I figure I will hang onto it for a few more weeks just in case but don’t know if I need to be worried about this or not.

Those vent solenoids can act up intermittently.

Hang on to the part for a week just in case the code comes back.

It use to be that way.

But with the new ORVR systems, there’s an overfill check valve that prevents that from occurring.



If it is working ok now, leave the old one alone. If it starts acting up again, you could remove old one and compare it to new one on work-bench. apply voltage to activate, you may learn the old one isn’t opening and closing consistently like the new one does.

The computer won’t allow a faulty vent valve w/o complaint b/c it needs it to work correctly to do the test that verifies the evap system is leak free. My OBD I Corolla has an evap system, but it has no way to test it is leak free on its own. That’s tested as part of the state’s emissions testing process. You are experiencing one the joys of OBD II … lol …

This is a year 2000. I consider anything OBD II to be pretty modern. Does the overfill valve protection apply to something 23 years old?

I will hang onto the part for a month or so. The truck doesn’t get driven everyday but will be used tomorrow so we will see.

ORVR has been mandated on all passenger cars (phasing in over the 1998-2000 model years



This is good to know. This problem is obviously unrelated to my overfill event. I am wondering about the fact I had the fuel pump replaced recently or maybe this is all a coincidence.

Anyway, I am thinking I might have an iffy crank position sensor. I talked to a few folks and they seem to think this might be the cause of my misfires at idle or under heavy acceleration.

I also notice it seems to take a little more cranking than any of my other cars to start. Again, this goes along with a crank sensor. I just assumed it was all the fuel system problems but the fuel pump, regulator, filter, and GM spider injector system were all replaced at the last service. It is maybe a little better as it doesn’t have to wait to build fuel pressure when it was leaking but still noticeable compared to anything else I own.

Again, I have NO CODES for the crank sensor or anything else now. I have a friend who wants to hookup his $1000 diagnostic computer for me so figure I will take him up on this offer. I don’t know what this will show if there are no codes/no pending codes for anything like this but it is worth a try.

I guess it could also be some strange electrical gremlin like the corroded solenoid wire on the 1997 F250 that drove me nuts with good voltage readings but that couldn’t handle the amps. Dealing with no so obvious problems like this is probably the most frustrating thing about having older vehicles around.

How do you know you are having misfires? By the way the engine runs?

Yes, you can hear it missing at idle. It seems to run fine under “normal” driving but if you really accelerate hard, it begins misfiring quite noticeably. I am wondering if that fact that is is likely not misfiring 99% of the time is why no codes are showing. Shouldn’t there be a pending code though?

Could be the engine is running overly lean. Symptom may be showing up more at idle and under acceleration. Suggest to obtain a fuel trim test. Not sure if a faulty vent valve could cause a lean mixture, but if you have a way to block it temporarily, might be worth a try. Note that blocking air from entering evap system could conceivably cause gas tank to collapse as fuel level dropped, so only a brief experiment. I’m guessing that’s not the problem; instead you have some sort of vacuum system leak, pcv system problem, or egr problem. Any weirdness in the brakes? If so, could indicate the power brake booster is leaking vacuum.

I would expect misfire codes, but the misfire-code algorithm may not set any codes until the problem has persisted for a certain number of miles, or it becomes more severe.

In the way of misery loves company, I noticed the other day my truck’s idle has become a little wobbly … lol …