Crankshaft Pulley – Timing Belt – Poor Idle and Drivability

[Kmaniac in California USA]

The recent struggles with my 740 revealed how the crankshaft pulley can degrade without obvious signs and how this degradation can affect drivability, curb idle, ignition timing and proper indexing of the crankshaft and camshaft sprockets during a timing belt replacement.

The outer edge of the crankshaft pulley has the ignition timing mark etched on the surface. This mark is used to set proper ignition timing and to initially index the position of the crankshaft during timing belt replacement. The crankshaft pulley is also a vibration dampener. The center hub is connected to the outer pulleys by a molded rubber donut in between.

This design creates a problem that will manifest as the crankshaft pulley ages. The accessories driven by the crankshaft pulley (alternator, water pump and air conditioning compressor) place a constant load on the rubber donut, trying to restrict the movement of the pulley. The center hub has a constant load from the crankshaft in the opposite direction. 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, advancing the timing mark relative to its proper position. There is no way to verify proper timing mark position without removing the crankshaft pulley from the engine and either comparing it to a new one or measuring the position of the timing respective to the keyway slot in the center hub.

Poor idle and poor drivability issues occur if the timing mark begins to advance from its proper position and the ignition timing is readjusted. If the timing mark has slipped from 1 to 18 degrees and the timing is readjusted in this condition, the ignition timing will actually be retarded and the engine will still run, but poorly.

Another problem occurs during timing belt replacement when the timing mark is advanced from its proper position. To properly index the crank, 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. Now, if you have the timing belt replaced by a mechanic who in not familiar with the proper crankshaft indexing method or is working for “flat rate” and skips the second indexing step, the crankshaft won’t be indexed properly. If the timing mark has advanced up to 18 degrees and the crankshaft is not indexed properly, it is possible that the new timing belt will be installed with camshaft one tooth retarded from its proper position, even though the camshaft timing mark is properly aligned with the mark on the timing cover. The engine will still run this way, but again, very poorly.

Verification that your timing mark is in its proper position will require removal of the crankshaft pulley from the engine and should be done during timing belt replacement. Place the crankshaft pulley with the front face down and the keyway slot on top. You can use either a protractor or a measuring tape. With a protractor, align 0 degrees with the center of the keyway slot. The timing mark should be at 160 degrees clockwise from the keyway slot. With a measuring tape, align the end of the tape with the centerline of the keyway slot. The timing mark should be 8-5/8 inches clockwise along the circumference from the centerline of the keyway slot. If either measurement exceeds these values, the timing mark has slipped and the crankshaft pulley should be replaced. Some people might recommend buying a used pulley from a junkyard. I do not recommend this, because you might be buying another failed pulley. I recommend replacement with a new pulley.

On my 1986 740 B230F, the first indication of a timing problem was noted during my biannual smog test. The technician measured the ignition timing at approximately 55 degrees BTDC. Further inspection of the engine revealed that the timing belt had not been replaced in quite a long time, indicated by the surface cracks on the outer rubber face of the belt. With the old belt still on the engine, I indexed the crankshaft by aligning the timing mark on my old pulley with TDC only and found the camshaft 2 teeth advanced from the mark on the timing case. Assuming there was nothing wrong with the position of the timing mark on the pulley, I indexed the crankshaft using the timing mark only and replaced the timing belt. The engine would not run. It was then that I considered there might be a problem with my crankshaft pulley. Fortunately, I had an independent, discount Volvo parts supplier in a nearby town, which had a new crankshaft pulley/vibration dampener in stock. So I took my crankshaft 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 43 degrees in the advance direction. Naturally, I bought the new pulley. With a new pulley and a new timing belt, installed correctly, my Volvo runs the best it has ever run in the last four years I have owned the car.

So, if you are experiencing poor idle and poor drivability, especially after a tune up or timing belt replacement or it has been this way since a recent purchase, your timing mark may have slipped and you just don’t know it yet. This issue with the crankshaft 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.

[billofdurham]

Chris,

A very good write-up, as usual. I have put it in the Repair Database.

Bill.

[Kmaniac in California USA]

This post has been brought forward for reference.

[Steven242]

I was just under the hood last night trying to figure out my poor idle and lack of power at the top end... I pulled the first spark plug and found my cam gear was retarded from the timing mark up top by about an eighth of a turn. it would run, but barely. So I pulled out my manual, spent an hour trying to understand what the meant by putting a pin in the timing belt tensioner and got to work. Repostioned the cam, and then it wouldn't fire. Spent another hour tearing it apart, found out the only reason the thing ran was cause the ignition was retarded to the same degree...

Anyways, got the thing running and started fiddling with it. I retarded the cam by only two or three degrees, and advanced the timing by a couple of degrees, and got a very smooth idle, and excellent bottom end power. Top end feels the same, but the bottom end of the revs feel like I gained twenty or so horses.

Anyways, I wish I would have seen this write up last night, would have saved me four hours.