Engine shakes at low RPM

[INF3RN0666]

Hi,

I've posted about this before but there's new updates to the situation. My dad has a 2000 impala 3.8L with 300000 KM on it. The engine causes the car to shake when it's idling in park or in gear at low RPM. The vibration (it's not a minor one) disappears as you add gas, but returns when you release the gas. The spark plugs and wires were replaced but that didn't fix anything. The EGR was replaced, but no luck. Mechanic thinks the engine is dying but it's been like this for a month now (7000 km have been added to it since that diagnosis). There's no loud sounds and the exhaust is fine. However, there is the smell of unburned fuel. Some people on the forum said it was a misfire. There's no misfire code when I scanned it last. Some people are telling me it's the coil pack. I have no idea what to do. Every mechanic keeps saying something different. Computer gives only 1 code --> Mass Air Flow sensor malfunction. I doubt that has anything to do with it. What do you think is causing this shaking?

Thanks for your time

[UncleBob]

never rely on a misfire code to be stored when there really is a misfire.

Some computers are very sensitive to misfires. Some computers are completely blind to them. This has to do with the parameters the factory programs into them. There are many OBD2 cars that you can literally disconnect 2 spark plugs and drive it and it will not set a misfire code.

You are discribing a misfire, regardless to what the computer says. The old-fashion way to narrow down the cylinder the problem is, is to use a pair of insolated pliers and pull wires one at a time while its running, and find the cylinder that doesn't change the idle quality when you pull it.

The much more accurate way is to find a shop that has an ignition scope. This will at the very least tell you which cylinders(s) are the problem. Depending on how educated the tech is, a lot of times the patern of the ignition will give you a strong hint to what the misfire is caused by also.

It very well could be a simple fix. It might be internal engine problems. I wouldn't recommend blindly replacing coils and other components, since as you have already discovered, that gets rather expensive. Spending $400 on parts to fix a problem that doesn't improve anything, makes the diagnostic fees from a good shop not look so bad

[UncleBob]

Originally Posted by INF3RN0666

Computer gives only 1 code --> Mass Air Flow sensor malfunction. I doubt that has anything to do with it. What do you think is causing this shaking?

I missed this when I originally scanned your message. MAF's can most definitely cause misfires at idle. If you could tell me what the code is exactly, I would be curious. Rarely does a bad MAF cause a MAF related code. Usually it throws a P0172/P0174 IE lean long term fuel trim codes

how many times has the computer been cleared of codes, and this code came back? More than once?

[obd2scannerman]

Originally Posted by INF3RN0666

Hi,

I've posted about this before but there's new updates to the situation. My dad has a 2000 impala 3.8L with 300000 KM on it. The engine causes the car to shake when it's idling in park or in gear at low RPM. The vibration (it's not a minor one) disappears as you add gas, but returns when you release the gas. The spark plugs and wires were replaced but that didn't fix anything. The EGR was replaced, but no luck. Mechanic thinks the engine is dying but it's been like this for a month now (7000 km have been added to it since that diagnosis). There's no loud sounds and the exhaust is fine. However, there is the smell of unburned fuel. Some people on the forum said it was a misfire. There's no misfire code when I scanned it last. Some people are telling me it's the coil pack. I have no idea what to do. Every mechanic keeps saying something different. Computer gives only 1 code --> Mass Air Flow sensor malfunction. I doubt that has anything to do with it. What do you think is causing this shaking?

Thanks for your time

Your MAF sensor measures the air flowing into your engine. Your pcm uses this info to determine how much fuel your engine needs. If this is wrong then your airfuel mix is wrong. Your Pcms Long Term Fuel Trim would tell you about the condition of your Maf Sensor.

Looking at your fuel trim could also tell you if you have an injector problem or not.

You could also as a quick test listen to your injectors at idle with mechanics stethescope and see if they click evenly. But a fuel trim report would tell the story on this car. Also the freeze frame stored with that code would possibly leave clues.

[UncleBob]

I agree that long term fuel trim can HELP diagnose a bad MAF, there are many things that can cause fuel trim to go out of whack

The uneven injector noise is a rather fascinating suggestion. How would a bad MAF sensor cause uneven bank fuel trims, assuming it has more than one?

[obd2scannerman]

I was going for multiple problems in the vehicle.

Misfire and Maf code. Possibly related, Possibly not.

[INF3RN0666]

Apparantly the code the computer returned told the mechanic that the MAF is faulty. He didn't tell me the code but I don't want to take any chances and replace the part randomly. Well thanks for your advice, i'm not sure what my next step will be but I'll keep you posted if you're interested.

[UncleBob]

a good next step would be call up the shop and find out what the code number was