Hey everyone, thanks for the help.

1986 Volvo 245, A/T, 125,000 miles

Noticing the last few weeks that when I abruptly take my foot off the gas from acceleration (say, going uphill to a stopsign), the engine almost stalls, then catches and continues to idle. Most noticeable in reverse (parallel parking, etc), but that could just be my imagination. Also noticed a pretty significant drop in mpg. Engine doesn't seem to be idling as smoothly as normal, but it isn't completely missing or even idling really poorly... just different. Engine has a bit of shake.

What's been done recently: Had a mechanical check done by german auto on vehicle 250 miles ago, they replaced a leaking vacuum hose. Replaced spark plugs, air box tstat, air filter, PCV valve, air intake hose, ATF flush 2500 miles ago. Valves adjusted, fuel filter replaced, cam seal replaced 30,000 miles ago. Timing belt replaced 40,000 miles ago.

Just noticed that distributor cap, rotors, and plug wires haven't been changed in the last 60,000 miles... perhaps this is the issue? Any other ideas?

Much thanks.

 

You are on the right track: do the plugs, cap, etc, see what happens. It does sound like you may have a missfire... 60,000k, they are overdue...

 

Make sure you have nice blue spark on all four plugs and after that see if you are getting good preassure from your gas line by checking your scraeder valve and if yours does not have a valve take the line off to make sure your getting strong and consistent gas from your line. You'll put a small jug at the end of ur line and view that. If it seems like your losing power and not idling strongly and without tapping the pedal to stay idling start looking at your ignition control module. The module controls spark and spark timing and could be cutting out at times or failing when warmed.

 

Also what did you mean by "continues to idle"? Like it gets shaky then goes to an idle or after it smooths out your idle past the smooth out point is a regular non jerky idle? A bad if module can also lead to pistons being flooded out partially or fully. If your driving hard enough after it floods out you'll burn that excess fuel off preventing you from seeing g flood symptoms. The flooding hurts MPG too.

 

Thanks for the replies. By "continues to idle" I mean that after it smooths out the idle past the smooth out point is a regular non jerky idle.

The thing that's stumping me is why the car almost stalls when I abruptly take the foot off the pedal... seems to drive fine with foot on pedal, and once it recovers from almost stalling it maintains idle (albeit a bit shaky and strange sounding). A failing ignition control module could cause this?

Thanks again

 

a sticky idle air control (IAC) valve could do this, so could a sticky throttle and/or badly adjusted throttle position idle switch..

When you take your foot off the throttle, idle position switch closes, then the ECU monitors the RPM and pulses the IAC to let in more air to keep the speed up.. it should idle smoothly at about 700 RPM. with the IAC disconnected, the idle of a warmed up engine in neutral/park, with all accessories off should be about 400-500 rpm, thats the base line.

btw, Volvos have no PCV valve, they have a 'flame trap', instead.

 

update: In the last week I've replaced distributor cap, rotor, plug wires, and engine mounts. Still no improvement. I did find a big bolt with accompanying nut in my splash pan when I pulled it off to do the mounts. No idea how a bolt AND nut could possibly have come off together unless it held some plastic piece somewhere that broke, or the shop that did the mechanical check dropped a random bolt into the engine.

Engine still shakes pretty significantly at idle, and during cold idle has a mechanical knocking sound. Also noticed a faint metallic ringing at higher rpms. Gas mileage still crap.

The timing belt hasn't been replaced in around 45K... that's my last "regular maintenance" task to check off the list, I have a feeling it won't make a difference though. I'll look into the IAC... is that something I can clean, or should I just replace? I'll clean the throttle body. Thanks pierce for the help and correction on that PCV/flame trap