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Hi car experts, I'm hoping you can help me fix my car. Below is a description of my problem, and things I have already tried--thanks, Beth.

My well maintained 1998 Buick Century, with 160k miles, is idling funny, but only when warmed up. If I start the car when it is cold, starts fine, runs great. If I start the car when it is already warmed up, like after filling up with gas, then it might not start on the first try. And once it does start easily, or may start and die, and once I get it started the idle goes way up and down. I've tried to fix it myself as I don't have a lot of money, and would like the car to keep running for a few more years. After reading, and trying lots, I came to the conclusion, since it runs so well at other times, that it might not be a serious problem, but a sensor that is giving the car bad info. I read to unplug sensors one at a time to figure out which one might be bad. When I unplug the MAF sensor, the problem goes away--I felt pretty proud that I had figured out what ails the car. I bought a new MAF, installed it, but the problem is still there. Aside from the MAF being bad out of the box, what might be my real issue? Could another sensor or problem be hidden when I unplug the MAF?

What I've tried:
Car has good compression. I tried what I saw on youtube, and sprayed carb cleaner on all the hoses while it was running, to check for vacuum leaks, this did not change the idle speed, and visually, despite their age, they all look good. I have put in new spark plugs and wires. The check engine light is on, but the folks at Autozone say it is only the evap system--I tried a new fuel cap, that didn't make it go away. Could the evap system cause idle problems such as this? And if so, why does unplugging the MAF make the problem go away?

The only problem that exists with the MAF unpluged, is when I first put it in gear to drive, the car loses power when I go to step on the gas, but only the first one or two times, after that car runs totally fine--is this what the car should act like with the MAF unplugged? But if I plug in the MAF, the car doesn't lose power when I first step on the gas, but once heated up and I restart it, it does the crazy idle.

Any help or thoughts are appreciated,
Beth


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contributions: 1961
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Location: Oregon
well, unplugging sensors to find one that causes a problem is not a very good way to figure out what is happening to your car. Evap codes could possibly cause a problem at idle. But there are many other things that could also cause a problem at idle.

Using a scanner, you could look at your fuel trims to see if the fuel mixture is running rich or lean. you could look at your coolant temperature sensor to see if it is reading too cold.

things like a leaking fuel pressure regulator can cause a problem like you are having. It can leak fuel through the vacuum line into the intake. When the engine is cold the engine needs a richer mixture so it would run fine. But when it warms up and the computer sees it being warm it cuts back fuel but it doesn't compensate for the fuel leak enough. or the coolant temp reads too cold and the engine runs rich after the engine is actually hot.

If the fuel trims show that it's running lean there could be a vacuum leak, weak injector (poor spray pattern) bad O2 sensor reading or weak fuel delivery.

a scanner will let you look at what the computer is seeing and reacting to.

The reason the condition goes away when you disconnect the MAF sensor is because you are turning off the computer's control over the system. Without the computer seeing how much airflow is entering the engine it no longer measures the fuel to make a balanced mixture.


   
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The reason the condition goes away when you disconnect the MAF sensor is because you are turning off the computer's control over the system. Without the computer seeing how much airflow is entering the engine it no longer measures the fuel to make a balanced mixture.


Ha. Darn it. At first I thought that the fact that the problem went away when unplugged, meant that it was the problem. Thanks for setting me straight. After I replaced it, I did wonder if unplugging it might have just masked something down the line--which as you state, was probably the case.

Quote:
But there are many other things that could also cause a problem at idle.


Thanks for pointing me in some other directions. I feel that this might be over my head. Can the scanner at the advance autoparts/autozone show fuel trims or is that something that is only going to be in a shop? How much does a good scanner cost? Would the other things you suggested be triggering a code? I went back and got all the codes:
po300 random multiple misfire

po102 maf (I've been unplugging it, so I am sure that is the issue)

PO341
Cam position sensor (I replaced this myself a year or so ago when autozone said that code came up, but didn't unplug the battery, so I don't know if this code is current either, do they go away on their own?)

PO442
Evap Emission leak

PO 440
Evap emissions system.

Also, is the coolant temp different than the temp gauge on my dashboard? Because that reads like it always has.

It is warm today in my state, so I'm going to try to replace the EVAP purge valve.

Thank you again,
Beth


   
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Additional info: I went to get the Evap Purge valve, but they didn't have it, which sucks, because tomorrow it will be cold again. Anyway, they ordered it. I checked the PCV valve, it was clogged with light brown sludge--I've read that this is bad, and most likely coolant, but could it be gas or something else? My coolant isn't low. I'm marking the overflow thingy, and checking the radiator wasn't low. Could it be gas causing this light brown stuff?

Thanks again,
Beth


   
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contributions: 1961
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Location: Oregon
yeah, it tough to check things without a god scanner on cars now days. you should have the codes cleared and see what codes come back.

Most evap codes won't affect driveability. But some can.

that's why looking at your fuel trims is so important. The fuel trims will tell you if the computer is adding fuel because it sees a lean condition or if is taking away fuel for a rich one.

when an engine has sit overnight a scanner should show all the temperatures to be about the same temperature. Coolant and the air temp should be within a couple degrees. That will tell you if the temp sensors are at least starting out the same. And they should match the ambient temp.

I use several different scanners. each scanner does something a little different. the prices run from several thousand dollars to over 10K. So having a shop scan your vehicle to see what is going on is usually not a bad price to pay, if you can get the information from them.

Have you checked your fuel pressure regulator for a fuel leak yet? Also, the engine in your vehicle has a problem with misfiring from dirty fuel injectors at times. Either one of those problems could cause a misfire code. A scanner will be able to see if your engine has a misfire counter and see if certain injectors are misfiring more than others.


   
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Thank you all for your help and suggestions. I gave up and bought a new (used) car. It is cold out, I don't have a garage, scanner, knowledge, money to try to fix my old car. I'm just going to try to sell it cheap, or if nobody wants it I'm told it has several hundred dollars in scrap value.

Thank you all again,
Beth


   
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contributions: 1961
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Location: Oregon
Sorry, I wish I could have been more help to you. Good luck with your new car


   
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