Expert: Roland Finston - 7/26/2011

Question
QUESTION: runs fine cold, when hot, can't accelerate, it bucks/hesitates & only does about 20mph. I felt the problem happen gradually. Idols fine, when hot turn off, it starts right back up.
What is your opinion/ experience?
Thank you

ANSWER: Hi Bob,
I am not familiar with the truck line, but try to use the ignition key to get any fault codes: "on-off-on-off-on and leave on" doing that in 5 seconds or less elapsed time. Then watch the check engine light, which remains on, to see it begin to flash, pause, flash, etc. Count the number of flashes before each pause. Then repeat to assure an accurate flash count. Then tell me the counts in order of appearance and we'll go from there. Tell me which 2.5L engine this is: 4 cyl or 6 cyl v-6?
Roland

---------- FOLLOW-UP ----------

QUESTION: Roland,
I have followed the directions, and the check engine light stayed on for a couple of seconds then just turned off. Nothing else happened.
2.5L 4 Cyl

Answer
Hi Bob,
I believe that you might take a look at the engine coolant temp sensor which is located on the driver's side of the thermostat housing. The plug has tan and black/light blue wires. The resistance across the sensor with the plug removed should start out cold at about 4,000 ohms and then as the engine warms up it should drop gradually to 700-1,000 ohms at operting temperature. You could also check the sensor wires which may go to pin 2 and pin 4, respectively, at the engine control 60-pin plug.
It appears that the self-test procedure is either not provided or not functioning. But if the sensor is not decreasing its resistance as the engine warms then the mixture would not be adjusted toward the leaner position is needs to function with a warm engine.
Roland