Fuel mileage and rough idle [Archive] - GrandAmGT.com Forum


View Full Version : Fuel mileage and rough idle

12-02-2010, 07:42 AM
Where to start........ I'm not a mechanic but can do simple repairs. I have access to a lift in a garage sometimes. Bought the car two years ago with 188,000km. I think the rear struts and LIM gasket were replaced. Since then, I've changed the ignition switch, both front calipers, idle air controller, air filter, blower motor resistor, thermostat, and not sure how many front rotors. Also had to replace every bracket for the rear bumper after a snow plow tore it off. I still have to change both front struts (driver side strut stuck) and possibly the rear (one squeaks over bumps), both headlights (cloudy), plugs and wires, fuel filter, all brake rotors and pads, rims (got two bent alloys, getting steel for winter, lots of roadwork and potholes here), wheel hub, something else.........

When I first got her, she was quieter than a kitten but had lots of power. You couldn't tell she was idling if you were standing beside her. You couldn't feel she was running if you put your hand on the hood. Now I notice she's gotten louder. The pulleys seem like they're squeakier, like dry on grease or something. You can hear the sound of a chain moving like the chain on an old bicycle. There's a very faint ticking noise even after she's warmed up. You can only hear it with the hood open. I've also noticed that the car seems to be slowly losing power.

When I'm parked on a small slope facing up the hill and the car is being held in park, you can really feel the rough idle, like she's slightly missing every second or less. You can feel it through the whole car. When parked on level ground, it's not as noticeable. The RPM's are constant though and don't waver two much. Maybe ~30 RPMs or so if you really look closely at the needle. You can also hear a faint whistle when accelerating and it's in sync with the RPMs. The higher the RPMs, the louder and higher pitched the whistle. It's hard to tell but I don't think the whistle is coming from the engine bay, maybe behind the dash? I've heard of vacuum leaks but would like to know more about them.

My front rotors are warping every two months. I'm going to have to invest in a set of expensive high quality ones. I'm getting tired of the vibration on the brake pedal. Also having issues with the wheel speed sensors. I changed the driver side hub and the lights went out. A month ago, the lights started coming on whenever the front right tire hit a pothole or manhole. Now the lights are on constantly. I'm thinking maybe a dirty plug or hopefully not, a bad sensor.

Anyway, the problem I've been having lately is horrible fuel mileage. It's all city driving which is bad enough, but lately the car is just guzzling gas. No difference in driving, maybe a slight loss of power. The SES light came on right around the same time. I got someone else to check and instead of telling me the code, they said it was an Evap Emissions code or something and said it was my gas cap before he deleted the code. Research has shown there to be a lot of different Evap codes that mean different things. I'm getting it re-scanned today by someone else in a few hours. I will post back with the code.

So what I'm wondering:

1. There are two kinds of rotors here. One is $35 and the other is $99. I was told the more expensive rotor won't warp as quickly. Is that true?
2. Why is the motor getting noisier? Besides normal wear and tear, is there any way to quiet it? Any recommended maintenance work for this issue?
3. The *slightly* rough idle problem? I'm guessing plugs but any ideas?
4. Fuel mileage issue? I suppose you need the code to diagnose it but any ideas on possible problems maybe related the Evap Emissions?
5. Possible causes of slow power and fuel mileage loss over the last couple of years? Front O2 sensor?

12-02-2010, 11:01 AM
im willing to bet the poor gas milage and lack of power and rough idle all could be a combination of a bad O2 and or cat convertor.

12-02-2010, 11:02 AM
I have a universal fit cat convertor for sale that i purchased for my grand am before i ended up writing it off if u need one.

12-02-2010, 04:30 PM
Got buddy to put it on a scanner and it had two codes. He couldn't see the second one but he doesn't really know how to use the scanner. I assume it was the same code pending. It was a 0442 Evap emissions. I read up on it and the easiest fix is to replace my gas cap, which I'll do soon. If that's not it, then I would need a smoke test done somewhere, which costs more money. It also said something about a small coolant leak. I don't know if that was the second code or part of the 0442 code. I don't see anything leaking.

I really don't see how a small leak in the Evap system could cause such a big drop in fuel mileage. Maybe if it was combined with a faulty O2 sensor.

A bad catalytic converter can cause rough idle? I have noticed the exhaust is louder then when I bought the car. Seems to be louder right behind the engine, maybe in the engine bay, and almost at the back of the car. Not like your muffler rusted out loud, but loud enough to tell which end of the car it's coming from.

I drove for maybe 30 km today and burned almost $10 worth of gas. Mostly highway driving. Doesn't seem right. At that rate, city and highway mix, a full tank would only net me about 150 km maybe.