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1999 XLS with 134k miles. Starts with some hesitation and regularly 'lopes' when at idle. Have OBDII codes 0171 & 0174. I have replaced the MAF sensor, IAC valve, airfilter and plugs. I had it checked at Goodyear and they could not find a vaccum leak on the intake seals. They say it is probably the fuel pump. The pump has 65 psi when it is running, but when you turn it off it drops to 0 psi within 60 seconds. Does this diagnosis sound logical? Is there any other possibilities? I do not want to do the pump myself (I have done some and they are a pain), but I also do not want to drop $500 for them to do it. Any help would be appreciated.

 

I like to hook up a fuel gage and monitor the readings while i,m driving the vehicle. I tape the gage to the windsheild so that you can see the pressure while you are driving the vehicle and to see if there is a change in pressure when it is hesitating. also is it getting up to operating temp. Could have a bad thermostat. may even be a bad fuelpressure regulator.

 

I have a 96 Explorer with 120,000 miles and the 4.0. I was having similar problems with mine. The thermostat and Temp Sensor were bad. Both cheap parts, hope this helps.