Looked around and didn't find a thread that addressed this precise problem, so here goes...

My '92 240 wagon is misfiring at idle. The miss varies pretty wildly in frequency and rhythm, but I can count on it to happen at every traffic light once the engine is warm (sometimes it happens cold too). It stops when I'm in neutral or park, or at least becomes unnoticeable.

fwiw it's done this *a little bit* as long as I've owned it, but it got really bad after I lost half my exhaust system on a cross country move and stalled out in Oklahoma. I convinced it to run again, finished the move, rebuilt the exhaust (not perfect, junk parts and still a bit leaky but better than it's ever been). Ever since, the miss has been about 10x worse than ever. Could easily be a red herring.

I am due for new spark plugs soon but I don't think it's that, I've inspected and tested them (by removing spark wires one by one) and they are contributing equally it seems.

Not sure where to start. Should I be thinking fuel system? filter? pump? Should I investigate the ignition further, new cap/rotor maybe? coils? sensors? Help me out folks, thanks

 

can you beg borrow or steal an old fashion clip-on inductive timing light? put it on each spark plug wire while its stumbling, and watch the strobe, see if the light stumbles when the engine does. if yes, then its an ignition problem, I'd be getting a new set of spark plug wires (bougicord are the best for 240's), bosch or volvo spark plugs, cap, rotor.

is your engine idle stable at 700rpm once warmed up? or does the idle go up and down if you go in and out of gear vs neutral/park ?

 

Thanks, I'll see if I can get my mitts on a timing light.

I have no tach but I can hear the rpm drop when I put it in park/neutral and go up when I put it in drive. Not by a lot but it is easily noticeable.

 

thats actually backwards, it should drop a bit when you put it in gear, but if the constant idle system is working, it should bounce right back to the same RPM with perhaps a bit more oomph.

if you have a digital multimeter that has a Hertz (frequency) scale, connect the meter between pins 15 and 1 on the coil, and read out in Hz... there's 2 sparks per revolution, so 700rpm idle would be 1400 sparks/minute, which would be 1400/60 = 23.3Hz. Or, putting it another way, Hz * 30 would be RPM.

 

In that case there's probably no real rpm change and it's the oomph that's deceiving me. But it definitely does miss much less, like almost never, when out of gear, so there is something going on there.

Sounds like I need to scrounge up some tools. I'll see what I can find out, thanks

 

I'd start with a "tune up". Mainly, do change those plugs--what you are testing is OK but the glitch you describe cannot be tested. Get some simple copper NGKs, gap them right, and I bet the problem goes away... Keep it simple!