Cranky engine may be a result of vapor lock

Q I own a 2005 Pontiac Vibe 1.8 liter 4-cylinder with about 70,000 miles on it. It has an intermittent problem after the car has been run until it is fully warmed up and then sits for anywhere from 10 to 75 minutes. The car has an extended crank time and takes some cranking to get it started. When it finally starts, it kind of sputters, and sometimes I even have to give it a little gas to get it going. The problem all but disappears in the winter, but it's slowly showing up again. The problem was much more pronounced last summer. I had a similar starting problem in 2006, around 32,000 miles, and had the fuel pump replaced by the Pontiac dealer. The dealership said it needs to see the problem to diagnose it. I don't have the time to try to get the car to act up on command at the dealer. What is causing this and what can I do about it?

A From your description, it appears that the engine is somewhat flooded when you try to restart it warm. Next time this occurs, try holding the throttle wide open as you crank the engine -- be prepared to quickly release the throttle if the engine starts. Wide-open throttle (WOT) while the engine is spinning at 200 to 300 rpm cranking speed commands a "clear flood" mode from the engine management system. The fuel injector pulse widths are cut in half and the fully open throttle maximizes the amount of air entering the engine to help clear the excess fuel. If WOT while cranking helps the engine to a quicker start, you've confirmed a flooded condition.

There are three classic causes for a partially flooded engine on a hot restart: vapor lock, an internal fuel leak or a faulty sensor input to the computer. Because your vehicle suffers this symptom more in hot weather, vapor lock tops the list.

Vapor lock, or fuel percolation, occurs when temperatures near the fuel climb high enough to boil it. When this happens, the ready fuel supply becomes aerated with vaporized fuel, causing a loss of fuel pressure at the injectors.

When you try to start the engine, the injectors don't deliver a solid, well-atomized spray of fuel into the intake port. Instead, fuel spits and sputters from the injector, failing to properly vaporize, and it floods the combustion chambers. Remember, only fuel vapors burn, not liquid gasoline.

Try letting the engine idle for 60 seconds before shutting it down, then pop the hood latch to let the hood open to the safety catch position to release some of the hot air trapped under the hood.

Also, try different brands of fuel to see if any specific brand might be less prone to percolation. And finally, make sure that the air conditioning condenser and radiator are clear of dead bugs and debris, the engine is clean and the cooling system has been properly maintained. Those steps help keep under-hood temperatures down.

A fuel pressure leak-down test would identify an internal fuel leak from the fuel pressure regulator or an injector, and a scan tool would identify a faulty signal from the coolant sensor or mass airflow sensor.

Q I have a 2003 Hyundai Santa Fe four-wheel drive with 95,000 miles. I am getting some rear-end noise but don't know if it is tires, bearings or transfer case. The noise varies a lot depending on road surface, but when I am on smooth asphalt at 30 to 40 miles per hour, I get a "wa-wa-wa-wa" noise. The rear tires are BF Goodrich Long Trail T/As -- the original tires with a significant amount of tread remaining. I replaced the front tires last year. Do you have an opinion on this and a recommendation to a good mechanic that can diagnose this? Also, when do you recommend changing shocks? And how do you know when new shocks are needed if no discernible leaks are occurring?

A This is a trick question, right? That "wa-wa-wa" sound is very likely from the rear tires, reminding you that you never rotated them. I suspect their tread surfaces have developed a somewhat irregular wear pattern.

Swap the rear tires to the front for a test drive. I'll bet the noise follows the rear tires to the front of the vehicle. The only solution is to replace these tires with a set perfectly matched to the front tires.