First, pardon me if this is a duplicate post. I thought I posted yesterday, but I can't find it on the list, so maybe I screwed up.
1996 Contour 2.0 liter. Car runs superbly at altitudes of 4000 ft and above (just drove cross country from S.L. to 9000 ft), but below 4ooo ft it has some hesitation off idle, and at light throttle settings surges somewhat (worst at S.L improving as altitude increases up to 4000ft when it's perfect).
At speeds above 50/60mpg there is no surging at any altitude. The idle is dead smooth at all altitudes (the engine is so smooth and quiet you almost cannot feel/hear it running).
The car has new plugs, and I ran Techron through fuel system to hopefully clean things up a bit, and it shows no engine related MIL (CEL) codes. Given the degraded drivability, I am surprised that a MIL light code of some sort is NOT indicated (the surging is bad enough at S.L. that it feels like a "soft" miss). I know the MIL and computer work because before I replaced the plugs, I did get a misfire code among others (since fixed). The car gets 33mpg at 70/75mph highway driving, so it can't be too far from right, though.
It is not a clogged fuel filter or low fuel delivery rate because the car will easily exceed 100mph on the level and pulls strongly and smoothly up steep Interstate hills at 70/75 mph.
I did not check the fuel pressure (I am now FAR from home and my serious tools). I have only a Haines manual with me, which does NOT show a barometric pressure sensor, but Auto Zone shows one on their parts list for the car. I also couldn't visually find one so am unsure if there even is one. Of course a bad altitude/barometric sensor could cause such a problem.
Any ideas? Fuel vapor canister purge valve? What have you?
Thanks, guys,
Bob
