I drive a 2005 Jeep Liberty with 140,000 km on it. I've taken over the car from my wife (who just used it to drive to and from work, about 20 minutes on the highway) and my job entails a fair number of start and stops throughout the day. A couple of months ago, after filling it up, the car failed to turn on. The battery had just been replaced and I'd had an oil change. The car would turn over, but the engine would not engage. Then, a few weeks ago, it started happening on a daily basis. I took the Jeep to the dealership and had them run their diagnostics and no code came up. They replaced the original spark plugs and that didn't fix the issue. A good friend said it seems like an electrical fault where a sensor is heating up (he said TPS - throttle positioning sensor) and not engaging to start the engine. I have no issues starting the car first thing in the morning when it's cold – it does seem that something is happening when the engine is warm and then, at the next turn of the key, something doesn't engage. Any ideas? – Trevor, Cambridge
There's a phrase nobody likes to hear from a doctor or a mechanic: "We'll have to run a few more tests."
But the mechanics I spoke to said they don't have enough details to figure out why your Liberty isn't starting once it's warm.
There are some likely possibilities – say, the fuel system isn't getting the right amount of gasoline into your engine – but then again, it might not just be a single problem.
"I would be suspicious of the fuel pump," says Stephen Leroux, an automotive professor at Centennial College in Toronto. "I would insist on running a fuel pressure test first."
Leroux also wonders whether the fuel filter – which keeps the dirt and rust in your gasoline from entering the fuel injectors – has ever been changed. A clogged filter could cause stalling and problems with starting.
Since it's not failing every time you start it, the key is to figure out the pattern, says Patrick Brown-Harrison, automotive instructor at SAIT Polytechnic in Calgary.
"We could roll the parts dice and eventually the culprit might be found," Brown-Harrison says. "The better approach is to document in detail what's happening and give that information to a diagnostic technician."
First, you need to record exactly what's going wrong – every time it happens.
"If you turn the key, keep track of how many seconds the engine cranked before it started," Brown-Harrison says. "Don't guess or estimate; time it."
So, write down for example: Rotated the key, the dash lights came on and the engine cranked for 15 seconds before it started.
Next, look at exactly where there's a problem – check to see whether anything else isn't working, even if it doesn't seem related to the starting problem.
Most importantly, keep track of when this is happening.
"What time did you first start the vehicle? How long did it run for? How far was it driven?" he says. "And then, how long did it sit before it was restarted? What was the temperature outside?"
"And again, all those need to be measured exactly. No guessing."
If it's something that happens regularly, see if you can figure out how to make it happen – say, you stop the car for a minute and then can't start it again – so you can show them exactly what's happening at the service centre.
You'll need tests, but at least they'll have a better idea of where to start.
If you have any driving queries for Jason, send him a message at email@example.com or contact him through Twitter: @JasonTchir