off idle stumble/stall/misfire?
 

After a 10 year absence I am finally back in a camaro.

I just purchased a 98 convertable with 98,000 miles. When I purchased the car the fuel level was just above 3/4 full and the check engine light was on.

When I test drove the car the engine was "cold". After buying it I noticed right away that once the engine warmed up to 185 or so after a few miles of driving it started "miss firing or stalling" just off idle.

The engine kind of stumbles. If you hit the gas it takes right off and clears up. I checked the codes and they were a P0440 and P0442. I cleared the codes and the check engine light has yet to re appear. But I noticed the the more I drove the car the stalling was less and less. In fact at about a half a tank of gas it all but stopped, just doing it when I would leave a stop light every now and then, and when it would do it again it was only after driving for a while when the engine was warmed up. So I filled it up and again and the missfire and stumbling started again at almost every stop light. I ran that tank of fuel out with the stalling at almost every redlight getting less and less as the fuel level went down. Not going away completely, but getting better as the level gets to about below 3/4 tank or so. Today I fill her again and its the same thing.

stalls, stumbles miss fires just off idle
when you floor it the engine clears up
stalling happens less below3/4 tank but does not completely disappear
I do not over fill the tank, when it clicks I stop
The car idles fine and pulls hard for a 6banger
Never runs over 180-185 degrees, 50-60 psi oil pressure, gets 17.2 mpg city on 87 octaine
check engine light has not come back on after 400 plus miles of driving.

I have basic hand tools and a code scanner. is there someone else who had this problem before?

Thanks in advance

scann for stored trouble codes. check fuel pressure, could be high. check fuel pressure regulator. pull vacumm hose off and see/smell for fuel. how long from last change of plugs? how old are and how do the plug wires look. hows the air filter look? probly be a good idea to have the injectors cleaned and flow tested.

You have a leak somewhere in your evap system. For starters, as a cheap possible fix to try, see if you have access to a known good gas cap. Swap yours out, and see if that changes anything for the better. Read all about it....
P0440 Evap Emission Control System Error: OBD-II Trouble Code: P0440 Evaporative Emission Control System Malfunction
P0442 Evap Emission Control System Leak Detected (small leak): OBD-II Trouble Code: P0442 Evaporative Emission Control System Leak Detected (small leak)

Did you do any kinda tune up? Change fuel filter? Plugs wires... The basics? Go to a shell gas station and fill with hightest octane, and see if it acts the same below the 3/4 mark.

^ At 98k miles, changing plugs, wires, and filter probably wouldn't be a bad idea if it's never been done. But that's not going to fix his evap system leak.

Thanks guys. I have not started changing anything yet. I changed the oil and filter the day after I bought it but other than a visual on the plug wires I have not done anything. I looked at how hard it was going to be to change the plugs last night and I have to say I am not looking forward to it. Its tighter than my c4 vette. I sealed the gas cap yesterday so I know that is not the problem. I am starting to wonder if the fueling up was just a fluke and it was going to start acting up anyway if I would have killed the engine then re started 10mins later.

As I said I filled it yesterday and it is still on full. It ran fine yesterday evening and was running fine this morning unit I stopped in the store to get a pop and talk with the chick behind the counter. I had let the car running for about 5mins or so then when I went to leave it started to sputter but it was not bad. This is an intermitten thing which leads me to believe that it is something electrical. The temp down here in south louisiana has been 95-99 degrees with a heat index of 115 at 100% humidity. At 6am this moring it was 81. What I was trying to describe by saying that it seemed to do it more when the engine was "warm" was that if I drive it on a hot day, full sun shine for 20mins or longer thats when it seems to act up. The engine temp is fine. It never gets over 185 or so. Think it could be heat soak for the coils? Also keep in mind that the codes that were stored in the car when I bought it could have been caused by a loose gas cap. Once I cleard the codes it has not come back on with over 400 miles of driving.

it sounds like its doing it when it goes to closed loop, most times that would mean a sensor or sensors are not working correctly. if its not a sensor and its to do with heat i would look to the coils or the fuel pump. if its not a sensor and has nothing to do with heat i would suspect a lose or bad connection or wire to a sensor, coils or plugs.

I drove the car around today and I took some time to try and nail down what was going on. I have determined that it has the "miss" all the time. When at a stop light if you quickly snap the throttle to 1/3-1/4 thats when it stumples cold or hot. if you take off easy it does not show or if you floor it. It is a very minor miss if it is cold, but after driving for a while it gets worse. I did a visual and I noticed that on the driverside of the engine there are 3 hoses wrapped in foil, one of them if you pull on the connection will slide back and fourth and the one right next to it will not. There is no evidence it is spilling any fluids. This may be unrelated but it is something I noted. Everything else looks ok. I do plan on changing the plugs and wires but I plan on bringing it in to a shop. As tight as it looks I just dont feel like fooling with it.

Riddle me this, if I were to remove the gas cap would it just set a code or run like crap? My scanner took a dump yesterday andI had to beat on it to power up, after taking it apart and not finding anthing I tossed it. I dont want to set a code by trying this myself if someone else has done it already. also is there a way I can bypass the evap system to see if the miss clears, i kno wit will set a code but at least it will tell me if it is the MAF or ignition.

leaving the gas cap off should not make a difference but will most likely throw a code.