Problem: Tank overfill, Intake, or Fuel Starvation?

[Joemm]

EDIT ** See my response below for the results & many thanks to those who offered suggestions. **



I'm having a problem and am wondering if it's because of gas tank overfill, my air intake system, or something else?

>>2001, MTX, 13,500 mi, stock except high flow (cone air filter) intake with cold air inlet tube to intake<<

The intake has been on the car about a month and cold air tube for about a week. The car ran better than ever after the install of both.

The night before a 300 mile trip, I broke the rules and really filled up the gas tank. No overflow just more gas than a normal fillup. The car ran fine for the 2-3 miles I drove it that evening.

The next morning, I immediately noticed the vehicle hesitating and surging at steady speed (20 to 85 mph). It starts and idles great. This surging lasted the whole first leg of the 300 mile trip. My gas mileage went up from it's normal 25 mpg to almost 30 mpg! This was not a trip computer error but showed up on the gas gauge also. No Malfunction Indication Light (MIL).

Thinking I might have bad "brand name" gas, I refilled with brand name premium. Before the return trip I unhooked the battery to reset the Powertrain Control Module (PCM). I disconnected the cold air tube from the air filter in case that was the culprit.

The exact problem resurfaced all the way home including greater than normal mpg. Again, no MIL.

The Mass Airflow (MAF) and Intake Air Temperature (IAT) senors and all hoses on the new intake are definitely hooked up correctly. At 13,000 miles could the cold air intake be screwing things up? Could a gas overfill cause problems for 600 miles? Could it just be a coincidence and something else went
wrong? Any suggestions before I have to put the stock intake back on and go to the dealership?

Thanks for any help.

[Blackcoog]

Fuel pump may be malfunctioning. It might also cause you to get better gas mileage if its not pumping what it should.

[madgeo]

did you have unleaded and put in a better kind of gas
it might be cuz you mixed the gas :goofy:
It might be your feul pump going out:disgust:

[eurocrisp]

Unlikely but could be an O2 sensor... If one's reading wrong and its leaning out your mixture a lot that retards the timing... Leaning out will increase gas mileage and improper timing would cause surging. I hear bad O2 sensors either: Throw a CEL immediately, die for about 2000 miles and THEN throw a cell, or never throw a cell at all while being completely dead.

[Joemm]

Here's what I did and the fix to the problem (see above).

To satisfy myself and avoid giving the dealership an excuse not to warranty the repair, I put the complete stock air intake back on. Sure enough, the problem remained exactly the same.

The hesitation/surging was caused by a bad Differential Pressure Feedback Exhaust Gas Recirculation (DPFE) sensor. The dealership stated this sensor is under recall (again 2001 Cougar). The mechanic found this during an Onboard Diagnostic (OBD-II) scan. The Malfunction Illumination Light (MIL or "check engine light") never came on!?

With no MIL and how poorly the car was running (really bad!), I would have bet the house it was the fuel pump as suggested above.

Anyway, thanks for the help and given what information I provided, you guys were right on the mark!

[Cougarsales]

yea, just had same problem 4 days ago. EGR and EGR sensor go bad on the 01's. bad surging at steady speed. easy part to buy and replace, for those of you who may have run out of warranty.

[ov3n]

DPFE could be going bad, it's hooked upto the egr system and will cause surging and hesitation if it fails.