I've got a 2002 2.5RS, 5MT, 97k miles, head gaskets, timing belt, plugs, wires, and clutch replaced around 78k. Only mods are a cold air intake and an aftermarket muffler welded onto the stock tailpipe.
Last fall around 80-85k miles it had a couple separate stumbling/misfire issues and I got those resolved by replacing the knock sensor and upstream O2 sensors at different times. Knock sensor is Autozone, which research showed me is the same as OE, and O2 sensor is Denso, same as OE. I put in a new WIX fuel filter in that timeframe as well. Those fixed all issues up till now.
A couple weeks ago I was getting on the highway and at 3-4k RPM in 4th gear and it sort of hiccuped, felt almost like the clutch slipped instantaneously, then went back to normal. It's done it intermittently since then and definitely isn't the clutch, but a very brief power cut or misfire, then normal operation again. Yesterday I noticed when I was sitting there idling it'd occasionally misfire or something at idle. Maybe once or twice a minute, just a single mild pop or thud that can be heard and felt over the normal idle. Not severe, but noticeable. Then getting on the highway it started bucking, shuttering, popping, and cutting out as I tried to accelerate away from the on ramp. I've since discovered that if I coast in any given gear and let the engine wind down and compression brake, then get back into the throttle it does the same thing, and starts bucking and shuttering. I assume this is a misfire. If I downshift and raise the revs it stops. Under normal acceleration from a stop it doesn't seem to do it, just if I've decelerated or coasting without hitting the clutch or shifting. Maybe it'll get worse and do it all the time, but it hasn't yet.
No codes are being thrown, and nothing shows up on my cheapo scanner. I tried unplugging the front O2 sensor to see if the misfire/bucking went away(it worked last year when the sensor was bad) and besides causing the check engine light to turn on, nothing changed. I checked the plug wire connections at both ends and all looked good and clean.
Any ideas what's likely to be causing this? Hopefully my specific issue has been seen and fixed by someone else on here. I assume some sensor is going bad, but without a CEL being tripped I don't know where to start.
As always, thanks in advance for any help.
Last fall around 80-85k miles it had a couple separate stumbling/misfire issues and I got those resolved by replacing the knock sensor and upstream O2 sensors at different times. Knock sensor is Autozone, which research showed me is the same as OE, and O2 sensor is Denso, same as OE. I put in a new WIX fuel filter in that timeframe as well. Those fixed all issues up till now.
A couple weeks ago I was getting on the highway and at 3-4k RPM in 4th gear and it sort of hiccuped, felt almost like the clutch slipped instantaneously, then went back to normal. It's done it intermittently since then and definitely isn't the clutch, but a very brief power cut or misfire, then normal operation again. Yesterday I noticed when I was sitting there idling it'd occasionally misfire or something at idle. Maybe once or twice a minute, just a single mild pop or thud that can be heard and felt over the normal idle. Not severe, but noticeable. Then getting on the highway it started bucking, shuttering, popping, and cutting out as I tried to accelerate away from the on ramp. I've since discovered that if I coast in any given gear and let the engine wind down and compression brake, then get back into the throttle it does the same thing, and starts bucking and shuttering. I assume this is a misfire. If I downshift and raise the revs it stops. Under normal acceleration from a stop it doesn't seem to do it, just if I've decelerated or coasting without hitting the clutch or shifting. Maybe it'll get worse and do it all the time, but it hasn't yet.
No codes are being thrown, and nothing shows up on my cheapo scanner. I tried unplugging the front O2 sensor to see if the misfire/bucking went away(it worked last year when the sensor was bad) and besides causing the check engine light to turn on, nothing changed. I checked the plug wire connections at both ends and all looked good and clean.
Any ideas what's likely to be causing this? Hopefully my specific issue has been seen and fixed by someone else on here. I assume some sensor is going bad, but without a CEL being tripped I don't know where to start.
As always, thanks in advance for any help.