Intermittent Idle and boost problems
 

Recently my '94 850 Turbo has developed a rather strange problem. When I start the car, it almost always begins to idle rough after running for a few seconds. If I rev the engine, it will rev as if there is no problem. Occasionally it will be idling poor enough that if I cannot step on the gas when I put it in gear, it will kill.
Another problem that has developed is that occasionally when driving, the turbo will not boost. The gauge will move to straight vertical between vacuum pressure and boost pressure but will not go into boost.

From my own observations, these two problems seem to be related because if the car is idling rough, it will not go into boost. It is not always idling rough if it is not boosting, but if idling rough it does not usually go into boost. However, if during the course of driving, I can get the car to go into boost once, it will drive perfectly normal for the rest of that journey until I turn it off.

It may be worth noting that within the past 6 months I have replaced the fuel pump, rebuilt the turbo, and replaced the bcs with a tcv and replaced all of the hoses related to the tcv. The intermittent idle problem did happen to start sometime after the fuel pump was put in, but other than that these parts have seemed to have no effect on the symptoms.

Does anybody have any suggestions on things to troubleshoot?

Thanks!
-Greg

1.check for fuel pressure if you developed this problem after installing the pump.it should be about 40-45PSI on a warm engine.might have a defective pump
2. check your idle control valve if it is clean
3.check and make sure the throttle body is clean they can cause that too.
4. check and see if you dont have any additional bad hose that is causing a vacuum leak.which hose did you change on the car?

Thanks, I'll try 2 and 3 this weekend.

The hoses I changed were the three that attach to the BCS/TCV. I replaced them with silicone hoses from IPD. I found another hose, the same size as the others, that runs from the intake manifold to the turbo and replaced it today. I also cleaned the Idle control valve. These did not seem to change the idle and boost issues.