940 Non Turbo

The latest information about causes to a fluctuating idle is the brake booster and air cond vacuum line. (I've tried many other stuffs but to little avail).

This was related to me by the parts dealer who seemed to be quite conversant with Volvo's cars. I've checked with the FAQ and the archive about brake booster but I couldn't determine if my is shot.

Basically, I tried pumping the brakes 10-15 times as suggested before starting the engine. When the engine got started, yes the pedal depressed further. I held the pedal for another 15 second or so to see if it goes further in but held still. However when I released the pedal a couple of inches higher and then pump again, I do notice that there is some resistance and I had to exert a little force to depress the pedal. I tried a similar exercise on my 244 (1981) and notice that after the engine has started and when I released the pedal a little bit, I could move the pedal down and up without much resistance. Now, which is the problem booster.

One other observation, whenever I pump the brake, I could hear hissing sound at around the pedal area. Initially I thought I had a booster leak but after trying it on other cars, I notice that all the booster gives hissing sound on depressing the pedal although the hiss may not be as loud.

Also, when I repeatedly pumped the brakes with the engine on, rpm decreased from 900 to 500 on both cars. I tried it on two other cars of different make, toyota and mitsubishi, and noticed that on repeated pumping of the brakes, rpm shot beyond 1000. Why the different observation. Could it mean that both my volvos have problem at the booster?

Another cause brought up by the parts dealer that could cause poor idling is the vacuum line linking to the air cond compressor. Can anyone tell me which line is this so that I can check for leak and where is the compressor to begin with?

Thanks all.

Laozong

The small vacuum check valve which prevents vacuum loss in the booster when the brakes are applied could be shot. The real test here is to disconnect the booster line, plug it, and test for idle abnormalities. If the idle smoothes out, then you've found the problem. Same with the a/c line.

Thanks steve for the advice. I've got a problem though. Where exactly is the air cond vacuum line?

Thanks

As I recall (and it is ten degrees outside, so I have not checked this) it tees off the brake booster vacuum line.

I've checked but there is no branching out from the brake booster vac line. The vac line from the brake booster is a 0.7 inch hose so I could not have missed any T-joining it. It is a direct line from the booster to the intake.
Thanks for your help and if ever you have the opportunity to check this out, I'll appreciate it.

Thanks