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AUTO Q AND A: Troubleshooting a mysterious stall

Q: I have a 1990 Volvo 740, automatic. It starts just fine, but I will be driving down the street and it will just stall. I will pull over, turn the key off for approximately three to four minutes, and then it will start again. It may run fine fo...

Q: I have a 1990 Volvo 740, automatic. It starts just fine, but I will be driving down the street and it will just stall. I will pull over, turn the key off for approximately three to four minutes, and then it will start again. It may run fine for a few days and then it will happen again. Do you have any knowledge of this or what might be causing the problem? The car has approximately 118,000 miles on it and I'm the third owner.

A: Ugh! Your situation is a tough one, as are most intermittent problems. In a follow-up Q&A discussion you indicated the problem seems to happen at almost any time, regardless of vehicle and/or outside temperature, the stalling typically occurs while the vehicle is moving -- rather than stopped, and the engine cranks fine but fails to start -- until it is good and ready.

It's going to take a sharp and patient technician to get to the bottom of this, but you may be able to help narrow the search with some exploration. In a nutshell, the fault may be limiting or terminating fuel delivery (fuel pressure/volume or fuel injector operation), ignition spark (weak spark, no spark, or radically mistimed spark), or a handful of bizarre but unlikely other causes. Since the engine runs well under all conditions some of the time, it's reasonable to rule out most mechanical causes, along with a restricted fuel filter.

A good place to start is the check engine light. Does it come on prior to the engine actually quitting? If so, a hugely informative diagnostic trouble code may be waiting to be accessed. Pre-1996 vehicles have limited but useful on-board diagnostics, and a check for DTCs sure wouldn't hurt. Checking fuel pressure/volume and fuel pump consistency requires somewhat expensive tools and skillful hookups, so we'll steer around this and look for a couple of easier things to check. Fuel injector pulses are fairly simple to verify, either by touching the tip of a long screwdriver to an injector and your ear to the handle, or by temporarily installing a noid light -- an inexpensive test light designed to clip on to the end of an unplugged injector wiring harness connector. A clicking sound or flashing light, as a helper cranks the engine, verifies the presence or absence of injector pulses -- a useful piece of information.

Ignition spark can be verified similarly, by installing a simple spark tester to the end of one of the spark plug wires and clipping the tool to engine metal. This inexpensive tool simulates a spark plug under extremely demanding circumstances. If spark snaps across the tool as the engine is cranked, you have enough spark for two cars! If not, available secondary ignition voltage is deficient or absent.

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These test results will help your tech zoom in on the problem area. Let's assume there is neither spark or fuel injector pulses during a no start episode. I'd look towards the crankshaft or distributor sensor, depending on the vehicle. If there was no spark but the injectors were working, a close look at the ignition system control module and connections would be in order. Be really careful to keep hands and clothing clear of rotating or hot parts, and don't attempt these tests unless you feel confident you understand them. With luck you may be able to cut your technician's diagnostic time -- and headaches -- by a third.

(Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose. E-mail under-the-hood@earthlink.net . He cannot make personal replies.)

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