I am about to replace the distributor cap, rotor and plugs in my '93 944T with 97K miles. I have never done this before (on this or any other car) so I apologize if I sound like the newbie that I am. My car shudders during idle and fluctuates between 500-800 RPM. I have cleaned the throttle body, which did not seem to eliminate the problem. Someone else on this board suggested that I check the cap, rotor and plugs. Since I don't know when the PO did this work, I will probably replace them even if the old ones look OK. What I am confused about is whether I will need to check the timing when I am finished installing the new components. It appears from the FAQ that the later cars that have an RPM sensor do not require timing adjustments, using a timing light, etc. Is this true? The VolvoWorld site includes instructions for installing a new cap and rotor that includes using a light to adjust the timing by rotating the cap to the proper position and tightening the bolts, but I think this may be for older 740's. Will I need to do this, or can I forego buying a timing light? Is there anything else I should do in addition to plugs, caps and rotors (wires look fresh, no evidence of worn insulation, etc.). Anything else I should do to make my car stop shaking? I will have a Volvo mechanic do a new timing belt at 100K, but I want to do as many other things myself as I can. Thanks.
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Steve Van Steenhuyse ('95 944T, 95K)

Hey Hawk. I would also check the intake gasket for leaks by spraing a little carb cleaner around the gasket to see if the idle smoothes out a bit. try clamping the big vacuum line that goes to your brake booster from your intake at the intake side and check for idle quality, sometimes the vacuum canister or the little vacuum plug come off or develope leaks that can cause a bad idle.

There are so many performance symptoms in the FAQ it seems almost scary. Keep in mind all cars have issues at one time or another but the Brickboard also has most of the fixes.

You can check for voltage leaks in the wires. In total darkness, while the car is running look for blue flashes in vicinity of the wires. A fine mist water spray from a Windex bottle can expose the blue flashes a little better. This will not tell you if the wires are good, but it will tell you if they are bad.

A coupla things on the distributor cap and rotor: Label the wires before you remove them to maintain the correct firing order. There are three screws on the back of the distributor that holds it on to the distributor housing. Just loosen them with a wrench, 6 or 8? mm, and finish removing them with your fingers. You don’t remove the screws all the way. Look at the new cap and you can judge how far you have to remove the screws. The distributor cap locks into place on the distributor housing so don’t worry about it rotating. After removal check inside the cap for oil. There should not be any, a little could be ok, at least for a while, and a lot means the distributor housing needs a new seal. Of the four 940s, one cap had excessive oil and the other three were moderate. More on that depending on what you find. The ‘94-944 na, 140k miles, with excessive oil and crud in a totally worn out original cap did not exhibit any performance issues, even with a.080 gap on the probable original plugs. Check for cracks, though often not noticeable and excessive dirt/carbon which could cause electrical tracking. Note the angle of the rotor before removing it to make replacing it a little easier. The rotor is a tight fit and is a pain to remove. I have had to twist a flat blade screwdriver tip between the distributor housing and the rotor while pulling to remove it. Make sure the new one locks onto place. You can see the how the notch in the shaft should line up with the ridge inside the rotor mounting. When replacing the distributor cap, do not over tighten the screws. When replacing the spark plug wires, new or used, use a Q-tip to apply a light coating of silicone grease on the rubber ends to allow for easier installation and removal.

I cannot tell you what is causing your engine to shake. Take a look at the no history catch up maintenance and the FAQ's performance symptoms. You are starting a good place, though. Also at the top of the list are the vacuum hoses. I have slowly been replacing all the vac hoses with 3/16 fuel line hose, about a buck per foot. Its heavy duty, longer lasting, and less prone to cracking at the ends. I have been using small wire ties or spring clamps at the ends. Also check the short rubber elbows and straights that connect the plastic vac hoses.
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Tom F Four 940s, RIP '78 244

The FAQ is correct, you cannot adjust your timing. The spark timing signal is given from RPM sensor which takes a signal off of the flywheel.

If you haven't done so already, you should check your Idle Control Valve. It's the thing that is shaped like a cyclinder and is found under your intake manifold. Has a plug on the back side and two hoses going in to/out of it.

Pull this out and rinse it with carburetor cleaner. It may be gunked up with carbon or other mung. It shouldn't take long to do, you only need to remove the two hoses and unplug the electrical connector...and pull it out from the rubber holder.

Good luck,
Bean
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'80 242GT 91k, '94 945T 110k