Poor idle, idles a lot worse when my foot is on the brake for a while, any ideas?

Maintenance/Repairs
1

This is in regard to a 2002 Daewoo Lanos with the 1.6L engine and automatic transmission. It currently has 91800 miles on it.

My car always has a slight miss at idle, and if I am stopped for a while with my foot on the brake, it gets a lot worse, and the engine shakes violently, but smooths out when I take my foot off the brake. Whether I leave the transmission in gear, or shift it into “P” or “N”, the same thing happens–after idling for about a minute with my foot on the brake, the engine runs really rough, and taking my foot off the brake lets it smooth out.

My engine does not smoke, or use any oil that I can see, however there is a TSB for the 2004-2005 Chevrolet Aveo, which uses the same engine regarding valve guide and valve stem seal wear. I had this vehicle looked at by a professional mechanic back in June who noted that the compression was about 130 PSI on two cylinders and about 150 PSI on two cylinders, and that the spark plug for cylinder #3 has oil on the threads and on the electrode. Other than the idle issue, this car seems to run fine, and has decent performance and fuel economy for a compact car.

Also, the check engine light is always on, unless I disconnect the battery and keep the fuel level topped off to prevent the evaporative emissions monitor from running. The codes are P0442 and P0456. I had a smoke test done, and the (alleged) leak was not found. There is no way to run bi-directional tests on this car without the factory scan tool, which no one has.

Any ideas for solving the problem would be appreciated. I am otherwise satisfied with this car, and don’t really want to deal with the hassle of trying to buy a different used car.

2

Have you checked for intake system or vacuum leaks?

3

My best guess is that the power brake booster’s vacuum supply from the engine is the source of the problem.

6 Likes
4

I agree with VDCdriver about looking at the power brake booster.

Try unplugging the vacuum hose from the power brake booster and plug the hose opening.
Then get back in your car, step on the brake again to see if the idle gets rough again.

5

Sadly, the brake booster is not the solution. I tried disconnecting the brake booster, which still had plenty of stored vacuum when I removed the hose, and plugged the hose by inserting the back of a Sharpie marker and closing the hose clamp over it.

With the brake booster disconnected, the brakes are of course much harder to apply, but the result is the same. Having my foot on the brake for a while makes the idle much worse, taking my foot off the brake lets it smooth out.

Some people on the Internet say that this type of problem could be a failing idle air control. A new one on Rock Auto costs about $60. Does this sound like a worthwhile investment, or is it likely something else?

6

Yes, it’s a worthwhile investment. You might check if your car has an EGR valve as well and give it a clean. Its a 2002, stuff is getting older. You have an EVAP system leak as well that you should have looked after.

7

This is puzzling.
How long does it take for the idle to get much worse, once you step on the brake?

And when you let go of the brake, how long does it take for the idle to get smooth?

And if you let it idle on its own, (without stepping on the brake), the idle remains smooth?

Thanks.

8

I decided to go to the local “you pull it” yard, and got a low-mileage used throttle body assembly, removed the TPS and IAC, cleaned it thoroughly with paper towels and carburetor cleaner, and reassembled it. I stopped at AutoZone and got a new throttle body gasket, although maybe it’s possible to reuse this type of gasket.

When I removed the throttle body assembly from the intake manifold, I noticed a huge amount of oil residue inside the intake manifold, much more than I would expect to see. I took some pictures, and then stuck a paper towel inside and cleaned up as much of the oil as possible.

Anyways, I installed this throttle body assembly with the new gasket and started the engine. It seemed to idle ok at first, but as it warmed up, I noticed a “stumble” every 30 seconds or so, and I could see the engine visibly shaking when this happens, and I could see by looking at the writing on the accessory belt that the RPMs drop real low and then recover.

I also noticed that if I take the oil fill cap off, lots of oil mist sprays out, more than I have seen on other engines, but the valvetrain looks clean (no sludge) and the oil level on the dipstick stays above the “full” mark. I also heard a hissing noise, which might be a vaccum leak somewhere, but unable to determine exactly where the noise is coming from.

Now, putting my foot on the brake or not, or having the transmission in gear or not does not seem to make a difference. Once fully warmed up, it idles better, but there is still a slight miss.

I was already advised by a professional mechanic who looked at this car that the poor idle quality may be due to the valve guide wear problem which these engines are noted for, although he could not prove it, and could not recommend such an expensive repair based solely on a guess. Also, he could not find any published specification for what the cylinder compression should be, only that the “service limit” is 100 PSI, and that the engine should be repaired or replaced if any cylinders fall below that value.

There is a device under the intake manifold which has a wiring harness. a fuel line going into it, and a nylon hose going from it into the throttle body. I removed the connection from the throttle body while the engine was running, and it revved up a lot, but once I plugged the opening on the throttle body, the engine went back to running how it was before. I believe this device is part of the evaporative emissions system, and perhaps there might be an internal leak which might also explain the evaporative emissions codes?

There is a procedure to reset the base idle speed whenever the IAC is replaced, so I will attempt that procedure and see what happens.

Any more ideas?