Q: I've been having an ongoing problem with my 2001 Ford F-150. When the temperature's above freezing, the truck runs fine. Below 32 degrees, though, the truck runs poorly and misses.
I've taken it to a Ford dealership and three other shops, and none have been able to find the problem. I have changed all four oxygen sensors, the throttle indicator sensor, the fuel rail vacuum valve, and the cam shaft sensor. I've pulled the fuel rails off and blown them out clean. The injectors are fine, and I changed the fuel filter. I would really appreciate if you could point me in the right direction.
_Chuck Freeman
A: Yikes!
Upon reading Chuck's inquiry, I had far more questions than answers, so I gave him a call.
He lives near Edmonton, Alberta, in Canada and was enjoying minus 30-degree weather as we spoke.
Turns out, the truck runs fine above 1,200 rpm, so that ruled out quite a few of the ideas I had been chewing on. Chuck also told me he had indeed drained and refilled the fuel tank, which more or less ruled out the possibility that water had been freezing in a fuel line, filter or other component.
Chuck had also retrieved a pair of diagnostic trouble codes (P0171 and P0174) using a basic code reader. Those codes indicate a lean air-fuel condition affecting both banks of cylinders. There were no misfire codes pointing to specific cylinders. Another observation was the truck continues to run poorly regardless of engine temperature or run time and doesn't seem to idle too slowly; it's just rough.
With this being an idle-only type of problem and the trouble codes indicating either not enough fuel or too much air, I began thinking the issue could be a vacuum leak.
Chuck told me he had replaced all the vacuum hoses, and I explained how he might seek out a leaking component gasket by dousing the intersections with a squirt bottle of water and checking for a change in performance. The possibility of a sticking idle speed control valve was also discussed, but since the engine seems to run more rough than slow, I figured this would be a long-shot.
I also had Chuck check the hose leading from the air filter to the engine's throttle body for cracks. Air sneaking into the engine without passing through the mass airflow sensor (this part hides at the bottom of the huge air filter housing) could be an explanation for the lean condition and codes. A leaking hose or connection can also cause intermittent poor performance as the part twists and stretches due to engine movement.
After sleeping on it, it hit me: The exhaust gas recirculation, or EGR, valve on Chuck's truck might be becoming active when it shouldn't, which can really wreak havoc on idle quality.
This vacuum-operated device is controlled by a solenoid containing a filter that bleeds away unwanted vacuum. Should the filter become dirty _ or perhaps the dirt in its freezes _ the EGR valve would function almost continuously. The black plastic solenoid is mounted atop the engine, and the tiny foam filter resides beneath a squeeze-unsnap plastic cap.
I called Chuck back and explained this scenario.
Since four shops have worked on his truck without success and have drained Chuck's wallet extensively, he was receptive to my suggestion of purchasing an on-board diagnostic scan tool _ an Autel AL519 ($65 on Amazon), for example _ if the above checks don't pan out. Such a tool provides limited-but-useful engine performance data and is capable of detecting issues like a flaky mass airflow sensor or other odd system inputs.
My educated guess at what's causing Chuck's issues _ an overactive EGR _ would be unlikely to trigger the two trouble codes he found, so using the more-advanced scan tool to observe fuel trims and the exact time at which the codes actually set could be really helpful.
A scary thought is there could be two separate conditions occurring. A fuel pressure gauge such an Actron CP7838 (about $45 on Amazon) would point out any possible fuel delivery issues.
