Problem with '01 Honda Civic on Cold Start

[roguerower]

I posted this a long time ago, but the problem went away, and now it is back. Car in question is a 2001 Honda Civic EX Coupe with 140k miles on it.

First problem I ran into with this car was when the bottom half of the catalytic converter heat shield rusted and fell off one day. No big deal but the top half would rattle ungodly loud at about 2000-2500 rpms. One day when I was getting my oil changed I asked if they could just take it off. They did, no big deal. Fast forward a couple of weeks and the CEL pops on. I take it back to the oil change station and ask them to code it. Turns out the O2 sensor had been unplugged. CEL goes out, keep driving the car.

After a while the car starts acting up. On cold start, the engine will start just fine. It will sit for about 30 seconds at idle (9k-1.2k rpms) and then the problem begins. The rpms will drop low, sometimes hitting maybe 100 or 50 and the car feels like it's shaking itself apart. As soon as I tap on the gas, the RPMs will sky rocket up to 2-3k rpm before settling back at idle (1k). Then it happens all over again. Drop down to low rpms, sit there for 5-10 seconds, pop up to 2k-3k rpms. If I hit the gas when the rpms drop, it'll pop out of its funk, idle, then repeate the same process.

It will repeat this cycle anywhere from 6-12 times before it runs perfectly fine. 10-12 times if it does it on its own, 5-7 times if I'm sitting on the gas revving it whenever the RPMs drop. After the engine warms up there isn't a problem and it doesn't do it if it's been turned on in the last 6 hours or so. The gas mileage has also been terrible, getting 28-31 gallons with very conservative use of the gas pedal. I should be getting 35 or so.

Any ideas? I have my bike which I use for my DD but I drive the civic every weekend for errands.

 

[LTC8K6]

Idle Control Valve?

Coolant Temp Sensor?

 

[BassBomb]

try re-learning idle

 

[Gibson486]

throttle body? Could be simple as needing to clean it, but as complex as having to replace it (well, it's not the complex because to clean it, you have to remove it anyways).

Since it happens on a consistent basis (after the car has started), it may even be a vacuum leak...

 

[roguerower]

*UPDATE*

Brought this back from the dead because of another issue that popped up.

Lately I've been getting bad mileage, around 25mpg for a car that should be getting at least 34 or so on a bad tank.

Talking with some coworkers who've worked on cars, they suggested that it might be a bad fuel pump/fuel filter.

Also, idle is sometimes different. At one time it could be 1k and another it'll sit around 600.

 

[exdeath]

Start it up cold and let idle and check if the EGR valve gets hot quickly. Does it act up like that more or try to stall following long periods of steady cruise?

Idle hunting/near stalling while everything else runs fine is usually a sign of open egr or vacuum leak. It would seem that fuel pressure and idle control are ok because it's able to maintain some control and keep it running.

 

[tbike06]

O2 sensor could be going bad, I had one of those go out and it killed my mileage. Also had intermittent CEL with it.