Off-idle stumble

[kenwa]

A few weeks ago, my Corvette started stumbling/surging at light throttle application. It only does it when the engine is thoroughly warmed up - running for 20 minutes or more with oil temp over 200. When cold/cool the engine runs perfectly. It is most noticable when taking off smoothly from a stop, like in city traffic. In first gear as I take off, the engine pulls fine at first, but when rpms hit about 1000-1200, then engine suddenly exhibits a loss of power. Also, when following traffic at about 40 mph with rpm about 1200, it "surges", kinda like a light bucking you would get pulling a trailer. The car runs fine at highway speeds. All symptoms go away above 50 mph. Also, it runs fine with the engine in park, with me holding the rpms in that 1000-1200 range, no missing. Here's what I've investigated:

1. It's not the MAF. I've got two, one stock with screen, one without screen. No difference between the two.
2. Last fall I needed a water pump, slight seepage. At that time the shop opened up the opti to see if it was damaged. They said it looked like new and put it back on.
3. I pulled the #4 plug to check the general condition. Looked like new. The wire had 1000 ohms resistance.
4. I looked inside the throttle and saw some oily residue, I guess from the PCV system. I opened the throttle all the way and wiped out as much stuff as I could with a clean cloth, off the throttle plates and bores. It doesn't seem to have improved the situation.
5. No change whether the A/C is on or off.

Any ideas?

Thanks,
Ken

 

[SSTibet]

Same here....

I have a very similar problem here but mine does not happen consistently. It happens only some of the times I drive my Vette. It is always when engine is warm, never ever when it is cool / cold... I am also smelling fuel odor when that happens. I am suspecting contamination / water in the fuel.

I had the car checked with the diagnostic computer and everything is running fine except for right hand O2 sensor; my SES light comes on all the time after driving some miles and it is the O2 sensor doing it. I don`t think that is relevant though for the rough idling problem.

Any comments???

 

[hAZcAT]

O2 sensor will cause bad idle, at least in fords, in the air densty (spelling is not my strong suit), tomorrow I am going to change the spark plugs O2 sensors and go over the vacuum system really carefully, as well as change the air filter, the fan belt, and check the EGR, pcv, change the air in the tires:L, throw some injector cleaner in the gas tank, if I can get a fuel filter I will replace it too. and of course report back.

 

[kenwa]

I took it to the shop and they found a bad EGR valve. It was causing a vacuum leak when it was active. When the valve was closed (idle, cold engine, etc) everything was fine.

It's fixed now. Could have been worse.

Ken

 

[SSTibet]

I had my O2 sensors replaced last weekend . there are 3 sensors on the car; one on the driver side (before the cat) and 2 on the passanger side (one before and one after the cat). I had read somewhere in CAC that the O2 sensor on the passenger side, which is after the cat, does not emit any signals to the PCM, I replaced it anyways (I would replace this one with an O2 simulator if I could find one here - there was not any in the spare part shops)...

This new mechanic has the diagnostic computer and he told me that all 3 new sensors are now functioning - he had also checked them before. I also fueled up the car from a good gas station and put some injector cleaning compound in the gas. So far so good, no CES fault light or engine stumbling... Hope it goes on like this.