I have a problem with my 95' F150 5.0 5-speed. I don't have any check engine lights but the truck seems to loose a bunch of low-speed torque when it warms up. It feels like it's running too lean because I can feel it surging when I'm in a high gear at low rpm's. The problem might have started when I had to have the exhaust Y-pipe replaced after I drove over something on the road that crushed it. The replacement pipe had a single cat. converter instead of the 2 that the original had, but it fit perfectly. The only problem there was that the O2 sensor fitting was way down beyond the cat. converter and too far downstream for the O2 sensor to reach. I took it to an exhaust shop and they ended up cutting a hole in one of the down-pipes (as close as possible to the "Y") and welding in an O2 fitting. The truck actually ran a lot better at high RPM, probably because the replacement cat. converter flows better than the original, but I'm not sure. Anyhow, when I started loading dirt bikes and stuff in it, I really could notice the lack of torque after it got warm. I wasn't getting a check engine light, but I had it scanned anyhow and there were no stored problem codes. I was told by a mechanic that the computer runs in a closed-loop (or was it open-loop?) condition when the engine coolant is cold, where it ignores the various sensors and runs on a preset mixture and timing profile until the coolant temp. sensor tells it things are warm enough. At this point, the computer begins to monitor the o2 sensor, throttle position sensor, egr position sensor, knock sensor, etc. The truck has 117,000 miles now, so I replaced the plugs, cap, rotor and filters along with the the coolant sensor, but it made no difference. I replaced the O2 sensor, no change. As an experiment I disconnected the vaccuum line from the egr valve and it seems to run a little better but not much. This does cause a check engine light, but not for 5 or 10 minutes. I'm assumiung that's because the computer isn't monitoring the egr position sensor until that point because it's too cold. What I did notice was that the torque loss occurs at precisely the point where the check engine light comes on, so I think that as soon as the computer switches to open-loop, something tells it to go waaay lean on the mixture or maybe waaay retarded on the igition. I just can't figure it out. Could the fact that the O2 sensor is mounted in a different location than original be the problem? In the original Y-pipe, there was a small crossover tube between the headpipes where the O2 was mounted, so it "sniffed" all of the cylinders in an area where there probably wasn't much turbulance or whatever. I ran the idea past the mechanic and he said if anything, that would cause a rich condition. I'm stumped.