My recently purchased '93 944T with 96K idles very rough, especially when cold, and every half-minute or so, it seems to briefly "shudder" and then even out again. This is not noticeable while the car is in gear. There is a significant amount of vibration during idle (this can really be felt in the driver's side door); it is less noticeable while in gear, but seems to still be present. The on-board tachometer shows idling at about 600K-800K rpm. I'm looking for a cause and a solution:

1) The Volvo mechanic that performed the pre-purchase inspection noted that the center muffler is weak (you can really tell this at highway speeds). Could the poor muffler performance affect this? Will replacing the exhaust (with the Volvo kit, I know) solve the idle problem?
2) Do I have a fuel system problem (dirty throttle body, injectors, etc.)?
3) It appears that there is about a 3/4 inch clearance between the oil pan and the front crossmember. Could the vibration be caused by poor engine mounts? Will fixing the idle problem solve the vibration?
4) The other problems identified by the mechanic were the breather box (I have tried to find it, I know where it is supposed to be but I just cannot see it behind the exhaust manifold, so I can't tell if there is an EGR in front of it) and the turbo oil return tube. I don't have any more detail on these problems. Could bad O-rings on these cause a vacuum problem leading to the idle shudder?

If it matters (this shows what a newbie I am), I have replaced the ATF and changed the oil since I purchased it 4 weeks ago. I have also replaced the oil cooling hoses that were identified as weak.

Sorry for the barrage of questions. I live in an area remote from a dealership and I am still deciding which local "Euro imports" mechanic I will be able to trust, so if there is anything I can do myself to solve the problem, I will give it a try. Thanks.
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Steve Van Steenhuyse ('95 944T, 95K)

Some very good info from the previous post for you to consider.

You should take a close look at all your vacuum lines. I had some rough idle and hesitation with my "new" 940 purchased a few months ago. I replaced ALL vacuum lines, the spark plug wires, and high tension wire. There was a substantial improvement in smoothness of idle after replacing the vacuum lines (many looked ok, but were loose when examined closely). The new wires increased power.

One last thing. Check the distributor cap for cracks, cleanness and burns.

1) The Volvo mechanic that performed the pre-purchase inspection noted that the center muffler is weak (you can really tell this at highway speeds). Could the poor muffler performance affect this? Will replacing the exhaust (with the Volvo kit, I know) solve the idle problem?

Not likely, unless there is significant blockage/backpressure, what does he mean by "weak"? Rusting out? Loud exhaust?

2) Do I have a fuel system problem (dirty throttle body, injectors, etc.)?

Good possibility the TB is dirty, go after it first, the gunk can hold the throttle valve open, thereby exhibiting the problems you refer to

3) It appears that there is about a 3/4 inch clearance between the oil pan and the front crossmember. Could the vibration be caused by poor engine mounts? Will fixing the idle problem solve the vibration?

3/4" sounds about right, bad mounts will have the oil pan almost if not touching the crossmember, I'd go after the idle first and see if the bad vibes disappear

4) The other problems identified by the mechanic were the breather box (I have tried to find it, I know where it is supposed to be but I just cannot see it behind the exhaust manifold,

It is below and to the rear of the INTAKE manifold, driver's side, about the size of your fist.

so I can't tell if there is an EGR in front of it) and the turbo oil return tube. I don't have any more detail on these problems. Could bad O-rings on these cause a vacuum problem leading to the idle shudder?

When you clean the TB and breather box, you may need to ream out the small hose nipples for the vacuum lines (maybe even replace the lines) with a small drill bit or stiff wire. They tend to clog up along with the TB getting dirty, be very careful with the plastic Y-adapter between the breather box and the manifold as it needs to be cleaned but can be brittle (a yellowish color is a brittle danger sign).

If it matters (this shows what a newbie I am), I have replaced the ATF and changed the oil since I purchased it 4 weeks ago. I have also replaced the oil cooling hoses that were identified as weak.

Sorry for the barrage of questions. I live in an area remote from a dealership and I am still deciding which local "Euro imports" mechanic I will be able to trust, so if there is anything I can do myself to solve the problem, I will give it a try. Thanks.
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Bob Kraushaar '94 945T, '91 940T(back from retirement), '88 240, '84 242Ti, '94 F-150, '88 300TE, '89 560 SL, '68 Shelby GT-500 KR