I've been chasing an erratic idle problem for the last 75K...now have 225K on the car. The problem is erratic. Sometime when I stop at a light the idle is as smooth as a kitten. Other times it's noticably rough. What gives? I've done normal tune-up stuff, changed out both fuel pumps (needed it anyway), hunted vacuum leaks, verified the mix, changed the wires, and fixed the airbox thermostat.

Any ideas? The problem seems to get worse in hot and humid weather.

-Ted

I had a similar problem on my 1978 260. I had tune ups and the fuel pressure regulator changed over the years. It was more of a problem during the summer months. I looked at the spark plugs and they were the cleanest spark plugs I had ever seen. The problem was eventually solved when Volvo changed the fuel pump relay.

/// Did you repace the injector 0-rings (non turbo). Check the battery cables. The battery acts as a large filter for the alternator output. If the battery is switched in and out of the circuit by either the negative battery cable or the positive system wire failing you can have this hard to find trouble. The test is cheap, almost free. Bypass the system wire with a jumper from the bolt on the battery clamp to the distribution rail. Bypass the negative battery cable with both sides of your jumper cables. Drive and test those cables.

Ted -- in tracing vacuum leaks, have you isolated the vacuum braking system as well? I'm told that's a sneaky source of vacuum leaks.

My '81 does much of what you describe. Have you ever replaced your control pressure regulator (also called warmup regulator)? I was advised to replace it to clear up lumpy idle problems, and I installed a "known good" used unit (new ones are filthy expensive). It significantly improved matters, but the lumpiness, which comes and goes, isn't entirely gone. I may end up getting the original CPR rebuilt. Maybe next spring...

Rob

Rob,

Thanks for the info. I'm not sure if the brake booster has a vacuum leak...any idea how to check? I tried spraying carb cleaner around it.

On the control pressure reg, any idea how to test this other than parts swapping?

Thanks,

Ted

Ted

Have you changed the injector seals. My 82 was doing the same thing until I changed them, big improvement. Just spray some carb cleaner on them when the car is at idle and see if it the idle is disturbed. That would be a clear sign for replacement. Good luck.

Mario

It did have some minor vacuum leaks in the injector seals which I fixed...but it didn't completely clean up the idle...Thanks