Timing belt skips a tooth

hubrecht

Three months or so ago, the otherwise well-running engine of my 1998 Volvo S40 developed a rattling noise. Suddenly, without any apparent cause it began sounding like, what my mechanic called, “a typewriter”. I drove with the car a little more but decided to leave it along the road and call the mechanic on Monday. He said I should take the car to the garage for check-up. Concerned I asked him if I could still drive the car with that noise in front. He said yes.
After five weeks in the garage and numerous checks: fuel supply, electricity, hydraulic valves etc.they finally diagnosed the problem to be a defective timing belt that had skipped a few teeth and thus caused the engine to run wrongly timed. We purchased a new timing belt, adjusted all necessary settings and indeed the engine ran beautifully again!
For less than a week I drove the car for maybe 200 km when one day when I started the car I heard a “clunk” or somewhat heavy knock, after which the engine started running...... with the same “typewriter” sound again. I left the car at home and called the mechanic and explained. They came out and towed the car to the garage where they scratched their heads and tinkered with the timing, “discovering” eventually that the belt-tensioner did not work properly. After some deliberations we decided to replace it. After another week this was accomplished and the engine ran great again.
I used the car for just two days when the same thing happened: start, clunk and typewriter! The mechanics came out again, towed the car again and it is now at the garage waiting for any clever advice that may help solve this problem.
The mechanic was trained by Volvo and has good experience and Volvo tools, but has no clue what to do next.
Can you help? Please?
Engr. Huub J. Luyk.
Baguio City, Philippines

93tracker5spd

Hello! First, I am not a Volvo expert, but I do have many years experience dealing with automotive mysteries. Having said that, I would like to take a stab at this one as it intrigues me. My first thought because of something similar that happened with a Ford pickup, was the idler pulley checked as well as the tensioner? In the Ford I mentioned, I had a bad idler pulley that caused drag on the belt and made it heat up somewhat, and once good and warm allowed the belt to stretch just enough to keep jumping one tooth on the cam thereby throwing it out of time. Don't know if this will help, but it is something to look at. The only other thing I can think of would be worn cam bushings, but I'm sure your mechanic already thought of that.

thecardoc3

93tracker5spd said:

The only other thing I can think of would be worn cam bushings, but I'm sure your mechanic already thought of that.

You're probably leaning in the right direction. A loss of oil pressure, lubrication, could be at play here. Usually it just shreds the belt when that happens but there have been occasions where the net result is an occasional repeated skipped timing.