View Full Version : Rough idleing GT

11-21-2006, 03:00 PM
Hey guys,

I know this question is for a GT, but I know there is alot of knowledge here. So I thought I'd see if you can help me out. My buddy has a '00 GT. He has a K&N FIPK on it and flowmaster mufflers. Other than that it's stock. The last couple of months his idleing has gone on the fritz. His idle will fluctuate between around 400 and 1800. But when he turns on his heater or a/c, his idle is still fluctuating but just doesn't go as high. I asked him if his MAF was in the same position that it was before he put the FIPK on, and he wasn't sure. So I told him to rotate 90* to see if it will smooth out. I also told him to disconnect the battery every time he does to see if it will reset. He doesn't have any leaks that I can see. I also told him to have his battery and alternator check for proper performance.

My questions to you guy's is, what else can he do to cure the problem? And do you have any ideas what the problem could be? Also how and with what would you clean the MAF with?

Any help would be appreciated.

11-21-2006, 08:50 PM
only problem is you can never get the MAF completely clean...I would just suggest replacing the whole unit...has he checked his plugs lately? might try swapping those out as well.

11-22-2006, 07:02 AM
Take a look at the Idle Air Control Valve, too. Are there any codes?

11-22-2006, 01:27 PM
I would agree with the others, IAC and plugs. The problem could be related to other things too.
When you start modifying parts on our cars things tend to go in the wrong direction and may just creep up on you. Sometimes having a tune done for the bolt on parts would prevent the ECU from attempting to correct itself back to its original operating parameters before the mods. From what I understand there is alot more to tuning than squeezing out more HP since it could also prevent bolt on parts to cause issues in the long run.

The cat-back more than likely is of no concern however would result in some changes in the A/F curve in the lower to mid RPM range. Changed done to the inlet air flow will cause some issues weather or not the FIPK kit is one that would effect anything or not unless the diameter of the pipe has changed if the wrong kit was installed. If all else fails to correct the issue, is it possible for your friend to go back to the factory intake set up to see if that corrects the issue?

If you notice that when the HVAC controls are activated and the RPM drops it may be related to the vacuum assist valves of the HVAC system besides the obvious like the AC compressor load or drop in voltage from the HVAC fan motor. Fluctuating voltage could cause issues and it may be related to the alternator or the diodes that are used to convert the AC to DC. That may be a link to what may be happening but hard to say if that would help to diagnose the issue. The fuel pressure is regulated by the vacuum created by the engine as well as the variable fuel pump. If for any reason there is a drop in vacuum there would be a corresponding drop in the fuel pressure. Fuel pressure is monitored and regulated based on the MAF measurements and speed of RPM as well as the duty cycle of the injector pulses. If the engine is running too lean or too rich at idle it will cause a stalling issue which could be due to a bad MAF sensor, poor air flow measurements, low fuel pressure, or unmetered air entering the intake. Poor response or sluggish O2 sensors could also cause similar issues at idle but will also be relavant at any engine speed under load or no load conditions.

If the MAF sensor incorrectly measures airflow that could lead to some issues (mild to severe). Sluggish response is typical of a dirty MAF sensor. Cleaning the MAF is easier when it is removed. Use only chemicals specifically created to clean them. Electrical parts cleaner should work and will not leave any oil or coatings on the sensor elements. Other things to consider is how clean is the back side of the throttle plate. If it is gummed up it may not be closing all the way. This would also indicate that the IAC may be in the same condition. Another check would be to remove the PCV valve while the engine is at idle and place your finger to cover the hole at the bottm. If the engine speed returns to normal after that it may be related to the PCV or grommet on the valve cover or one of the rubber vacuum lines. The position of the MAF sensor may be related only if there are sharp bends in the inlet pipe in front of the sensor or after the MAF sensor. Before going into parts and cleaning things..... consider the not so obvious.... Fuel filter, when was the last time it was changed? This is one part I always forget about.

If the IAC is operating within its limits there will be no codes generated. A simple test of the IAC would be to unplug the electrical connection to the IAC when at idle and the RPM should increase to around 2500 (unplugging the connector may generate a code so check the codes first before doing this for any pending codes). When plugged back in, it will return to typical speeds. Even if there are codes and if they indicate the IAC it may be related to air flow that is bypassing the IAC valve. There are several sources for air flow into the intake; vacuum lines, breather line to PCV line through the block, throttle body, and EGR system, or leaking gasket on the intake plenum. The low speed at 400rpm may indicate a leak since the IAC is attempting to choke off the airflow due to the 1800 surge in RPM. Weak fuel pressure or leaned out idle could result in surges in idle speed. Since at idle, the fuel pump is operating at a minimum as well as the fuel line at lower fuel pressure, a clogged fuel filter would reduce fuel flow even further.

The last item is the air temperature sensor. If it is connected in a bad spot on the intake pipe it may be missreading air temperature which could result in misguidded air/fuel regulation.

off the wall: Problem could be related to ECU and EEPROM memory. Since the ECU does parameter checks and builds up parameter tables for air/fuel ratio corrections it could have stumbled into safe mode if some of the data became corrupt or out of range. There is a fuse for the EEPROM memory I would check to see if that is intact. This is not a common issue but if the EEPROM does not operate properly or has become damaged, the ECU will operate engine management on default parameters. I am not exactly sure what would happen if the EEPROM went bad but I am sure it would not be good.

11-29-2006, 02:27 PM
Thanks everybody for your thoughts and suggestions. I was gone on vacation, so this is the first time I've seen any replies. I'm going to find out if he still has the old intake system and have him come over, and try swapping that out. And find out when the last time he did a tune up on his car. It could possibly be time for new fuel filter, plugs, wires, and etc. I know his car is not throwing any codes right now. We'll also check the PCV valve, and the IAC. The throttle plates aren't a bad idea either, I never even thought about those.

Thanks again, I'll get with him and see what we find out, and keep ya posted.