2003 Ford Mustang V6 3.8L gets around 12mpg and seems to shake a bit when started cold and has a bad rough idle when warm

03mustangv6wh

Member
Apr 5, 2019
8
1
13
19
Virginia Beach
Hey everyone I've tried so many things to get this fixed. When I start it up in the mornings the car will shake a little bit, but when I drive around for a bit let it get up to temp and run it for a bit. Then I'd stop at a store or something for around 15 or so minutes and I'll come back to start my car but the rpms will drop down to about 500-700 and want to die out. I recently cleaned the maf sensor, put a new cai, new radiator, new water pump, oil change recently, new spark plugs, new spark wires, ignition coil, (my alternator was tested and it failed) new alternator, ran seafoam through gas tank and through the throttle body (not at the same time and not within the same gas tank). I'm out of ideas of what to do next. Any help would be great, I've touched just about every bolt in my car it feels like. Also only has 75k miles
 
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wmburns

SN Certified Technician
Aug 14, 2009
5,390
423
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Houston Texas
Do you have an ODB2 scanner? If you did it might make short work of your problems. At the very least it will give access to vital trouble shooting information.

Take a look at the ODB2 mode 6 misfire data to find out WHICH cylinders are misfiring.

ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/

Are you POSITIVE that the complete charging system is in tip top shape? Not just a new alternator?

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-forums/resources/howto-perform-charging-system-voltage-drop-test.56/

Troubleshoot IAC idle problems 1996-2004
https://www.stangnet.com/mustang-forums/resources/troubleshoot-iac-idle-problems-1996-2004.13/
 

Bsharp21

Member
Jun 12, 2019
8
1
13
26
michigan
need to check the MAF, MAP and IAT sensors. the pcm utilizes all of these to know how much fuel is needed in the cylinders, if any of these are faulty or dirty you will get an improper reading which will lead to idle and driveability issues
 

Onesick99GT

Active Member
Nov 20, 2018
287
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Dayton, Ohio
Go to your nearest auto parts store and have them run the codes. Write the codes down and post here. It's completely free to have them do this. Then we can provide more help. We have no idea what's going on and making guesses without more information is pointless. If I had to assume you probabaly need to replace your intake gasket. Air entering past the throttle body will cause your pcm to read lean and add more fuel causing horrible gas mileage and low rpms at start. Could even make it stall. Couple that with a vacuum leak and you will easily have a rough running car. The intake gaskets should be replaced every 50 k miles in my opinion. Most don't go bad before 100 k miles but preventative maintenance to keep the car running great reduces issues for other problems to arise down the road. In your case fouled spark plugs from too much fuel or bad oxygen sensors repeat combustion in the exhaust. Just to name a few examples.