I have a 1988 Sentra with the 1.6L fuel-injected engine, manual transmission. Here's what's happening (it's kinda long-winded so please bear with me): the car starts and runs fine up to about 2000 or 2500 RPM (guessing based on engine noise). At that point, it starts oscillating between that RPM and what sounds like idle about twice a second: "vroom...vroom...vroom...vroom", like the throttle is being pumped, even though the accelerator pedal is held at a steady pressure. If I press the gas pedal more, it just keeps doing it (the revving/idle oscillation), not sounding any different, until I let it go back to lower RPMs, where it runs fine again.
Another strange thing is that it seems to be RPM-related, since if I'm in, say, 4th gear, I can floor the pedal and it accelerates fine up to about 40 or 45 MPH, which is the speed it goes when the engine is at about 2000-2500 RPM, at which point the oscillation starts again (makes for a mighty jerky ride, too). It doesn't happen all the time though; sometimes the car runs just fine and I can rev the engine all the way up to around its redline (no tach so can't say for sure) without the oscillation starting again. I've not been able to pin down the specific conditions that cause it to happen. Sometimes it works fine, sometimes it has this problem. I thought it might be temperature related, but it's happened on cool and hot days, with the engine both cool and hot as well.
I've replaced the air and fuel filters and changed the oil in the last couple of days trying to pin this problem down, but it hasn't helped. I'm assuming it's not a timing issue (slipped or loose belt or chain) since it doesn't happen constantly.
In an attempt to track down the source of the problem, I've taken off the cover for the air filter and looked down into the intake as I pull on the accelerator cable manually. When I run the engine up, I can see the butterfly valve move with the throttle, and that valve does not oscillate when the problem starts happening. Instead, I can see what must be the fuel/air mixture coming from the injector housing (I think that's what it is) in the middle of the intake throat, and it looks almost like that is what is fluctuating. I.e. the throttle valve stays constant but the injector seems to be oscillating between putting enough fuel/air in and not enough; I can see the gas running across the surface of the butterfly valve, and the amount seems to oscillate.
If anyone has some ideas on what to go after next, I'd sure appreciate it. I'm hoping to avoid having to take it into a mechanic. Might it be a problem with the injector (dirty injector maybe)? Clogged fuel lines? Some kind of air intake problem? I have a replacement PCV valve, but haven't installed it yet (can't seem to find where it goes on the engine... :-/). Anyway, any advice would be most welcome.
Thanks, ~Sam