AggieVette'03

In the last couple weeks my engine surges back and forth from 350rpms to 800rpms, with it usually in the 450-700 range. If I keep the pedal in just a hair to get in to 800rpms it is fine and holds steady (assuming I hold the brake in to keep from hitting the person in front of me at the light). I have also noticed that my city mileage has gone from about 18 to 16, and hwy from 24 to 20. It doesn't seem to be running rich, and I have been told it is a vacuum leak, but when running with the hood open in the driveway, there is no hissing and I can't find any cracked or broken vacuum hoses. Any help is appreciated.

Corvette0096

Sounds like you have a dirty or bad IAC,Or T/B is dirty or Vacuum leak.

scorp508

How is it for the first 3 minutes when cold? Fine? Sounds like it could be when the ECM goes closed loop and the O2 is messing up.

AggieVette'03

For the first three minutes or so it is fine (getting out of the neighborhood at stopsigns, etc.). I did have the front Y-pipe off just before this started happening and wondered if I damaged the O2 sensor in the pipe, but when I asked about it, I was told by three people to check my vacuum lines. How do you check your O2 sensor. The check engine light only comes on when it is unplugged, no warning lights have been on in the last two weeks. What is the AIC?

scorp508

If it is fine and then freaks out I'm going to suggest replacing your O2 sensor. When your car starts it automatically goes into "OPEN Loop" mode. The ECM ignores the O2 sensor because it is not up to temperature yet and cannot be used for readings.

These are the time required for Closed loop to occur on my own car, yours should be similar:

Coolant is under 100*: 300 seconds
Coolant is 100-124*: 206 seconds
Coolant is over 124*: 66 seconds

Once this time is acheived the car goes into "CLOSED Loop" mode and starts using the O2 sensor readings to adjust the fuel.

The easiest way to test for a failing O2 sensor is to go full throttle. At full throttle the O2 sensor is also ignored. If your car is acting all really funky at partial throttle, but pulls clean and hard under full throttle then the O2 sensor is junk.

At idle you can also attach a scanner unit and monitor your O2 readings. At idle in closed loop the readings should cycle from 0.999mv to 0.000mv over and over and over if the O2 sensor is good.

IAC stands for "Idle Air Control". It is a little pintel on a motor that moves back and forth allowing a certain amount of air to bypass the throttlebody blades so that when they are closed the car still gets enough air to idle correctly.

dwestsr

:iagree: Right on target, Scorp.

AggieVette'03

Well..I replaced the oxygen sensor and disconnected the battery for 20 mins to reset the computer, and nothing has changed, so I guess I will buy 50 feet of vacuum tubing and replace all of it. If that is the problem, and one line is leaking somewhere I would imagine the rest of the 19 year old tubing is ready to go as well. If that doesn't help then who knows, the computer gives me no codes.

scorp508

It doesn't make any sense to me for the car to run fine at first. If there was a vaccuum line causing issues it should be there 100% of the time. Its also possible that the O2 sensor lead itself is fried.

Anybody else have ideas?

legs

Experiencing a similar problem, although waiting at a light or in traffic it will sometimes die. It will always start right back up but embarrasing to say the least. I've replaced the plug wires, cap, rotor, and coil but it still happens, although it runs much better when accelerating. 02 sensor ?
good luck

BILLYD

2 thoughts, well 1 thought & 1 question
Thought = plugs ok ? I had 85 that had fouled plug on drivers side & that kept o2 sensor flooded with fuel & terrible lean condition
Question = If computer runs car in open loop for 60 seconds after warm start, I'll get full advance regardless of ping and full rich regardless of o2 sensor for 60 seconds??


thanks
Bill
:confused:

korvette4u

i would also check egr control valve is working.. it between the fuel rails