Hey everyone-
I own a 1990 Volvo 240 with a manual transmission and have been trying to fix the problems that 30+ year old cars are bound to have. Recently, I noticed that the car will jerk rapidly many times while accelerating or even just driving at a constant velocity while on the freeway. I thought about dirty injectors or worn spark plugs, but what interested me was that this issue consistently only happens after I've been driving for about 30 minutes; before that it feels normal. While cleaning/replacing the injectors and replacing the plugs should probably be done regardless, I was curious if anyone had an idea as to why this issue only occurs after a good bit of driving. I'll also note that I listened to the injectors through a screwdriver and they all sound normal at idle- perhaps they would anyway even if they're dirty though, I'm not sure.
Thanks for reading

 

how old are your spark plug wires? How do the plugs look? Sounds like its heat related - but that not really a help when diagnosing the root cause. A good starting point is a "stage 0" tune up - new rotor/cap/wires/plugs and consider a new coil. Make sure your plugs are correct for your car (good ol copper cores work best). Next is to spray around for intake air leaks/check vacuum lines etc - including around the injectors (I assume you have 30 year old o-rings?). It could be fuel related, ie fuel pump or relay start to overheat due to the steady demand or a spark issue (if you've ever had a coil pack fail, you'd notice it at highway speeds or accellerating)

 

I would clean the throttle body too, cant hurt.