Yes Its Another High Idle Problem


New Member
Jan 5, 2018
Hey guys, Ive been lurking on this forum for awhile trying to figure out this issue but Im at the point where I have no idea what else to do.
So Ive got an 01 Cobra that has the 1500+ idle surge problem. Im pretty sure Ive done all the fixes that I found here. I replaced the TPS and IAC. I changed out every vacuum line I could find and I did a vacuum test on the manifold with no leaks found.
Where Im at now is I bought an IAC Restrictor and installed it which resulted in the car idling at 1600 nonstop. So I widened the holes to 7/16 and now the car struggles to stay running when its cold. Once it warms up it wont try to die but I still have the idle surge when driving and now Im throwing P0171 and P0174 codes.

I am at a loss with this. I really wanna push this car off a cliff at this point so any advice would be helpful. The only mods on the car are a CAI, Accufab TB, Catless X-pipe with flowmasters.
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Stangnet Facilities Maint Tech... Er... Janitor
Admin Dude
Jan 4, 1985
Box behind Walmart
I would begin by pulling off the IAC restrictor. It didn't come on the car so it's not necessary to solve the problem.

You have both banks showing lean. Have you replaced O2 sensors yet? What is your fuel pressure at idle?

It sounds to me like you have a failing fuel pump, regulator, or clogged fuel filter.

More troubleshooting will keep you from throwing parts at it.

CAI, Accufab TB, Catless X-pipe with flowmasters

All of this is suspect.... All of it.

What method was used to tune out the pre and post cat O2 sensors?

What CAI are you using and where is the tube bend in relation to the MAF?

Is the TB correctly sealed and vacuum routing been confirmed?

What's the condition of your PCV valve?


New Member
Jan 5, 2018
I took the restrictor back out which made the codes go away. I dont know why restricting the IAC would cause those codes though
Its a JLT intake with the maf moved into the fender well. Im not sure about my PCV valve. I'll swap it out with a new one and spray out my maf. The car doesnt have a tune on it as far as I know. As far as the TB goes there arent any leaks from the gasket and the vacuum lines on my car are routed the same as my co workers cobra.


SN Certified Technician
Aug 14, 2009
Houston Texas
Once it warms up it wont try to die but I still have the idle surge when driving and now Im throwing P0171 and P0174 codes
It seems to me that the source of the idle problem is likely the SAME as the cause of the outstanding DTC codes.

It also seems to me that IF taking the IAC restrictor out makes the DTC codes go away THEN there almost has to be unmetered air entering from somewhere associated with the IAC. Or something has changed from then to now (during the work done).

Do you have an ODB2 scanner that is capable of reading operational PID's? If so this could make short work of your problem. It would be handy to know such things as:
  • RPM's
  • MAF flow
  • Fuel rail pressure sensor
  • IAC duty percent
  • TP mode (closed, part, WOT).
Here's some information on an affordable Windows based unit.

ForScan ODB2 scanner w ELM327 USB

Troubleshoot IAC idle problems 1996-2004
P0171 - System to Lean (Bank 1)
P0174 - System to Lean (Bank 2)

Air Measurement System
  • MAF sensor (contaminated, damaged, malfunctioning, etc.)
Fuel System
  • Fuel pressure regulator (leaking, malfunctioning, etc.).
  • Fuel filter plugged, dirty.
  • Fuel pump (weak, check valve leaking, etc.).
  • Leaking/contaminated fuel injectors.
  • Low fuel pressure or running out of fuel.
  • EVAP canister purge valve leaking (when canister is "clean").
  • Fuel supply line restricted.
  • Fuel rail pressure sensor (incorrect reading).
Air Induction System
  • Air leaks after the MAF.
  • Vacuum Leaks.
  • PCV system (leak, valve stuck open, etc.)
  • Improperly seated engine oil dipstick.
Exhaust System
  • Exhaust leaks before or near the HO2Ss (exhaust manifold gasket, mating gaskets, etc.).
EGR System
  • EGR valve tube/gasket leak
  • EVR solenoid vacuum leak
Secondary Air Injection
  • Damaged/malfunctioning secondary air injection system (mechanically stuck valve).
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New Member
Jan 5, 2018
Well I got it fixed. I ended up just going with the nuclear option and replacing everything. I swapped out all the intake gaskets, hose clamped every vacuum line put a new pcv in and installed a different maf. I am still not sure what caused the issue but the car runs normally now so Ill leave it at that.