Hello,

I just bought a 2002 Cavalier Z24 with the 2.4L engine.
It is idling at 1500 and seems to be burning a lot more fuel than I would expect.

I got about 16mpg without getting on the gas very aggressively.
I am idling about 1500RPM.
The long term fuel trim is -18. I read that it should be between -10 or +10 else there is a problem. The number gets even smaller (< -18) when i raise the RPMs.

Here's what I have done:

-Sprayed carb cleaner around intake manifold, sensors, vaccum hoses to check for vaccum leak
-Disconnected IAT, IAC while running - no change in rpm.
-Disconnected MAP sensor while running - engine quits (is this normal?)
-Used code scanner at auto parts store - no codes listed.
-Replaced TPS sensor - no change.

Questions:

-What kind of mileage do you guys get driving around town?
-Does the LT Fuel Trim indicate it is trying hard to reduce the amount of fuel the vehicle is getting (i.e. running rich)?
-What should I try next?

OBD 2 Scanner Data attached.

Attachments

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20141219_180217.jpg (61k)

High idle is normally a vacuum, leak.

I'm leaning towards a bad IAC. That is what controls the idle anyways.

idle fueling is map based, so disconnected the map sensor will likely kill the engine.

I usually grab spare sensor's like the IAC from the junkyard to have for these kind of times.



FU Tuning

Ok, i changed the IAC valve with a new one, but no change in behavior.

I have an o2 sensor i could put in next.
I should probably crawl under and see if the exhaust is stock or not?

Since idle is MAP based, should i think about changing the map sensor? At 1500rpm (idle) my manifold pressure is 7.7"hg. Is this normal or does that indicate part of the problem?

Thanks

Update:

Problem: Idles at 1500 rpm. Burns 22mpg on the highway and 16mpg around town.

What i have done:

-New Intake Gasket
-New IAC Valve
-New MAP Sensor
-New TPS Sensor
-Cleaned Throttle Body
-Cleaned PCV port
-Replaced various vacuum lines

I notice there is a bit of air blowing out of the oil fill hole when i remove the cap. It also has had a fair amount of oil in the intake. I noticed the oil was slightly high on the dipstick. Does this mean i have a lot of blow-by or maybe someone overfilled it? Maybe I just have a tired engine with low compression? It has 190K miles. Would low compression cause high idle? I will try to find a compression tester next.

Forgot to mention i also changed the O2 sensor.

Does it always idle at 1500?

I have seen the throttle cable get stuck on the butterfly not allowing the throttle plate to shut all the way.

Idle fueling is map based, but does not control idle.



FU Tuning

It consistently idles at 1500rpm. However, it isn't a stuck cable because i disconnected the TPS sensor while it was running and it went to 900 rpm until i restarted the engine. This was probably due to it running in open loop after i introduced a fault?

I'm having this vary same problem, but it started after I put a header on my 02 ld9. When I start my car up in the morning or after a full cool down it idles at 2800 rpms. I'm also throwing 2 cel something about miss firing and something about the high idle.

Interesting. I know that when you change exhaust back pressure, you typically need to change fuel delivery. I don't know if that means you need a chip to richen your mixture or what?

Well I got 31.5MPG HWY in my z24 and 23.5mpg in town.

Maybe you have a stuck injector? Is it blowing black smoke? Is it manual or auto? If it is auto does the RPM stay the same if it is in drive?

Also have you checked your Air filter?

I have the same issue. Replaced the same parts except for the intake manifold gasket. Spraying carb cleaner around the top part of the intake manifold flange chokes the engine idle. So I'm not sure if its the intake manifold leaking at the block or possibly the cam tower covers.