Engine problem

  • 4.0L
  • V6
  • 4WD
  • 200,000 MILES
Runs fine but sometimes after being in a store for fifteen minutes or so I start it and it barely runs, sometimes shuts off, but will even out if I gas it for a bit. The other day however, it would not start up again until after I had it towed home, and then it started. Also, when I am gassing it to even it out I have to press on the gas pedal twice the distance to get a normal rpm. I replaced spark plugs, and I am going to replace the fuel filter, but I have a feeling it is something else.
Do you
have the same problem?
Friday, June 23rd, 2017 AT 3:57 PM

1 Reply

You are right. Neither of those will cause the symptoms you described. The clue is the need to leave it sit for a few minutes with the engine hot. That is the classic symptom of a crankshaft position sensor or camshaft position sensor failing by becoming heat-sensitive. As long as you are driving, natural air flow keeps those sensors cool. When stopped for a short period of time the heat migrates up to those sensors and can cause one to fail. We call that "hot soak".

The place to start is by having the diagnostic fault codes read and recorded. The people at many auto parts stores will do that for you for free. Fault codes for these sensors do not always set in the short amount of time it takes for a stalling engine to coast to a stop, and they may no set just from cranking the engine. If that happens, a scanner is needed to see which signal is not showing up at the engine computer.

Where your description differs is you are able to get the engine started. That could mean one of those sensors is only developing a weak signal so far, or your computer uses a back-up strategy when one sensor fails. It could also mean there is some entirely different cause for this problem, but reading the fault codes is still the first place to start.

Ford also had a rash of coolant temperature sensor failures in the early 1990's. Its readings have a pretty big impact on fuel metering during start-up. The problem is there is a range of acceptable signal voltages, and while the voltage it sends could be wrong, as long as it stays within that acceptable range, no fault code will be set. For that you need a scanner to view live data. It will show the current signal voltage and the coolant temperature the computer has interpreted that as. You have to judge if that temperature looks right.
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Friday, June 23rd, 2017 AT 4:53 PM

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