Volvo 740 idle problem

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My 1987 Volvo 740 GLE, 150,000 miles, now have a problem. During Idle speed, or when I tap brake to reduce speed to idle, it frequently dies. And during idle speed, the engin's RPM behaves irregularly, suddently high and suddently low. All these problems didn't exist before I changeed timing belt.
Any help would be greatly appreciated!

[Kmaniac in California USA]

I do have a couple questions. Did you replace the timing belt yourself or did you have a mechanic do it for you? After the timing belt was replaced, was the ignition timing adjusted?

The poor idle performance after a timing belt change could indicate an undiagnosed failure of the crankshaft pulley/vibration dampener. Let me explain:

The crank pulley is also a vibration dampener. The center hub is connected to the outer pulleys (which contain the timing mark) by a molded rubber donut in between. When (not if) the rubber donut deteriorates, the pulleys (with the timing mark) will rotate on the donut and move the timing mark with respect to the center hub, most likely advancing the mark relative to its proper position. There is no way to test the condition of the vibration dampener or verify proper timing mark position without removing the crank pulley and comparing it to a new one.

I discovered this problem with my 740 when I replaced the timing belt recently. I mistakenly used only the timing mark on the crankshaft pulley to index the crank. When I put it back together, it would not start. I took it off and put it on several times with the same result before I considered that there might be a problem with my crank pulley. Fortunately, I had an independent, discount Volvo parts supplier in a nearby town, which had a new crank pulley/vibration dampener in stock. So I took my crank pulley to the shop, laid it on top of the new pulley, lined up the keyway slots in the hubs and found my timing mark was off from the new pulley by 48 degrees in the advance direction. Naturally, I bought the new pulley. NOTE: Some people might recommend buying a used pulley from a junkyard. I do not recommend this, because you might be buying another failed pulley and you have no way to check it.

To properly index the crank for a timing belt replacement, you first line up the timing mark to TDC before disassembly. Once you get the crank pulley off, you must verify that the keyway slot in the end of the crankshaft is aligned with a rib on the crank seal cover at approximately 11 o'clock. If the crank is not aligned with this rib, move the crank until they are in line. Only after the crank is indexed in this manner can you accurately index the cam and auxiliary sprockets and install the timing belt correctly. NOTE: This indexing procedure in not clearly explained in my Haynes manual. But using the pictures there and my explanation, you can figure it out.

If you find that the crank slot is not aligned with this rib, your timing mark on the pulley has shifted and the pulley must be replaced. You can verify this by comparing your pulley to a new one like I did.

I have a hunch that your crankshaft pulley/vibration dampener has failed in the same manner as mind did, though maybe not as far as mine did. If you changed the timing belt yourself and you used the timing mark on the pulley to index the crank without double checking the position of the crankshaft slot, your camshaft may be retarded one tooth or more. It will run with the cam retarded one tooth, but won't run with the cam retarded by three teeth (I know from experience). Also, if you found the timing mark advanced in the past and readjusted the timing by retarding it back, AND the timing mark has slipped on the pulley, the ignition timing will, in actuallity, be retarded from standard specification. Retarded camshaft timing in conjuction with retarded ignition timing could create the idling problems you describe.

The issue I encountered with the crank pulley/vibration dampener is probably more common than most people, even Volvo mechanics, might think. I am sure that many a car has been junked as a result of this “weak” design flaw that went undiagnosed.

Please let us know what you do and what you find. Let me know, too, if you have any questions.

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many thanks for sharing your experience. My timing belt was intalled twice by technicians. The first workshop had no experience with VOLVO, so I found it not properly installed. The second workshop who claimed expert in European car, replaced th timing belt, solved some problems left by the first shop, but this idle problems they can not figure out. Some times when I turn my car I can hear the screaming of the car, maybe the tension of the timing belt dut the crackshaft pulley, I will ask other techinican to check it.

Thank you again!