I recently moved to Australia for a one year stay and purchased a 1989 740 GL to use during my stay. The car has 86000 miles on it. So I find myself several thousand miles from home with an older Volvo and no manuals or tools. I've owned, worked on, and driven a 122 and an 850 turbo, but I don't really know what to expect in terms of performance from the 740.

The problem I have is with intermittent rough idle, and possible low power and poor fuel economy. I have read through the FAQ regarding engine tune, fuel injection, and rough idle and picked up a few tips there. However, I have some very specific and apparently somewhat unusual symptoms that I hope someone might recognize to help me pinpoint the problem.

When I test drove and bought the car it ran and idled beautifully. Within days, however, the idle became very rough and low, down around 300 RPM. However, after a long trip (>25 miles or so) at high speed, the idle would increase to about 600 or 700 RPM and was smooth. If I were to then turn the car off and restart it within a few minutes the idle would be slow and rough again. I don't think this is an engine temperature symptom, since driving around town for extended trips does not result in the same symptom and since the an immediate restart at the same engine temperature gives the poor idle. The problem got progressively worse until shifting from park into gear at idle would cause the car to stall.

I took the car to a mechanic for a service and explained the rough idle problem. He cleaned the TB, replaced the air filter, and adjusted the idle ($300). The result was that I had a higher but still rough idle (600 RPM) and a very high idle after driving for an extended hiway trip (1200 RPM). Cleaning the TB evidently did not help, and adjusting the idle higher only hid the problem.

I read through posts concerning rough idle section and became concerned about a manifold gasket or injector seal leak. I found some starting fluid and sprayed it about the manifold gasket and the injectors while the car was running to no effect. During the course of this procedure I discovered that removing a vacuum hose, pushing on any injector such that it would leak around the seal, or removing the AMM from the air filter housing would cause the idle speed to increase and become much smoother. It seems to me that the car is getting too much fuel or not enough air.

Has anyone experienced these symptoms or does anyone have an idea where to start to fix the problem?