2000 Mercury Mountaineer Hesitation Running Lean

Tiny
MTEBALDI
  • MEMBER
  • 2000 MERCURY MOUNTAINEER
  • 5.0L
  • V8
  • AWD
  • AUTOMATIC
  • 194,000 MILES
2000 Mercury Mountaineer 5.4L throws code P0171 P0174 hesitation, lack of power, lurching. If I stop shut off the car and let it sit for 20-30 minutes it starts back up and runs better. But this only takes place when car is under load going up hill when the outside temperature is ~80deg F. This has happen several times in the last couple years. The hesitation is like the EGR valve is open but I if that were true I would throw a code for excessive EGR flow P0402. When outside temperature is <80 there are no issues driving under a load.
1. Disconnected the vacuum line to EGR and drove vehicle 9 miles received a code P0401 insufficient flow which I expected because the EGR vacuum line was disconnected. So this told me the that the PDFE module was working along with the Vacuum Solenoid that opens to operate the EGR valve is working also.
2. Replaced fuel filter, vacuum checked and cleaned EGR valve all good. The EGR was very clean.
3. Checked fuel pump pressure with gauge connected key on engine off 5psi was reading. Turn key off then wait 10sec then turn key on again pressure increases by 10psi each time 10psi gains to 60psi.
4. Bled down pressure in fuel rail then started engine and pressure gradually increased to 60psi. Left engine running and disconnected the line from the fuel dampener rear of engine. 5. Shut down engine then repeated fuel pump pressure test. Key on engine off, pressure jumped to 60psi immediately. Bled down pressure in fuel rail then started engine and fuel pressure jumped to 65psi. So I am not sure if I might have a possible bad fuel pump or dampener.
Wednesday, March 18th, 2015 AT 11:48 AM

3 Replies

Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Fuel pressure should be 55-65 psi key on engine off. It may be a partially blocked egr system or the valve isn't opening enough. It cold be bad O2 sensor/s a vacuum leak, fuel injector leaks or problem, pcm, or electrical circuits going to O2 grounding out or open.
Was this
answer
helpful?
Yes
No
Wednesday, March 18th, 2015 AT 12:20 PM
Tiny
MTEBALDI
  • MEMBER
  • 3 POSTS
1. Fuel pressure is 60psi
2. If EGR valve wasn't open enough wouldn't there be a CEL code P0401?
3. Bad O2 sensors would produce a CEL code?
4. Checked Vacuum lines and intake for leaks all good.
Grounding issue whether O2, electrical circuits or a PCM issue would be all the time not just under certain environmental conditions?
Was this
answer
helpful?
Yes
No
Wednesday, March 18th, 2015 AT 5:20 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
The 171 and 174 can be set due to partially clogged egr passages or valve and won't set a code. Other than what you have. The O2 sensor is not eh ground to pcm it means the O2 sensor is grounding out someplace either a wire burnt through or the O2 sensor bad itself. It can also be an air leak like a grommet on engine as well as duct work leading to engine between maf and throttle.
Was this
answer
helpful?
Yes
No
Thursday, March 19th, 2015 AT 5:46 AM

Please login or register to post a reply.

Sponsored links