My 1991 240 (168k miles) has developed a TERRIBLE rough idle over the past year. For troubleshooting, I have checked/cleaned/adjusted replaced everything I could think of, once or twice: cleaned/adjusted throttle body, new TB gasket, checked/cleaned idle air control motor, new flame trap, new oil separator box, replaced several vacuum hoses, spark plugs/wires/distrib cap/rotor, replaced intake and exhaust manifold gaskets, used torque wrench to check manifold bolts, etc.. The rough idle remains, but if you increase idle, it smooths out.

A local Volvo specialist took a brief look. He used a squirt bottle to check for leaks around various gaskets and hoses, but couldn't find any. He "nose-checked" the fuel pressure regulator hose, and said it didn't SEEM to be leaking. When I reviewed my previous work, he gave me two options:
- 1) leave the car a few days; pay the shop to recheck my work; then replace items one-by-one (hoses, gaskets, fuel pressure reg, idle air motor) until the culprit is found; he also suggested swapping from NGK back to Bosch copper plugs!!!
- 2) since I have already gone this far, replace the last few parts myself!

[NOTE: Support your local independent Volvo specialists. They WANT your Volvo to run well, and will save time and money in the long run.]

I chose Option #2. So what are some other hidden sources of vacuum leaks? Should I just go ahead and replace the intake manifold (again), vacuum hoses and the fuel pressure regulator? What about the brake booster valve? Or is the idle air control motor the obvious culprit? Everything else seems to be working fine, so I cannot figure out the cause of this rough idle.

[[ southern fried volvo 1991 240 168k mi ]]