Q: My 2004 Buick Park Avenue Ultra with 60,000 miles misses when restarted hot. The missing lasts for no more than 30 seconds. My mechanic is stumped. Any thoughts? Other than this, the car runs beautifully.

A: If the engine misfire occurs only during a hot restart, checking fuel pressure is the place to start. If the ready fuel supply in the fuel rail on top of the engine is overheated immediately after shutdown, it may percolate or boil the fuel. That would aerate the fuel with bubbles, causing a loss of fuel pressure at restart. As fresher, cooler fuel reaches the rail and injectors, fuel pressure comes back up and the engine smooths out.

Another possibility for a loss of fuel pressure on a hot restart is a leaky fuel injector or fuel pressure regulator diaphragm. This would also bleed down fuel pressure right after shutting off the engine with the resultant restart misfire. Even a sticky intake or exhaust valve or a heavy carbon buildup on a valve could cause this symptom. GM has recently recommended adding GM Fuel System Treatment PLUS to the fuel tank at every oil change to prevent this. SeaFoam is designed for the same purpose.

Because of the history of intake manifold gasket issues with the 3.8-liter V6 in your vehicle, I’d be a little concerned that cooling system pressure might be forcing coolant into a cylinder at shutdown, causing the initial hot restart misfire.

 

Q: My wife has a 2006 Chevy Tahoe with 109,000 miles on it. Recently after the car was driven on the freeway for an hour or so, followed by stop and go traffic, the oil pressure gauge dropped to zero and the low oil pressure warning alarm went off. She pulled the car over, shut it off and restarted — the issue seemed to go away. After the car was parked for an hour or so she went out to start it, the oil pressure gauge went to 40 psi for a few seconds, then down to zero. After an oil/filter change the oil pressure was a steady 40 psi at idle and 55-60 psi at highway speeds. Several days later after driving the car about an hour on the freeway, I put the car into park and the oil pressure dropped and the low pressure warning alarm went off again. Bumping the engine speed up to 1,500 rpm or so brought the oil pressure back up. Diagnosis from the Chevy dealership is that my oil pressure is erratic and that they think that dropping the oil pan and replacing an O-ring on the oil pump pickup is the issue, with a cost estimate north of $800. I suspect a bad oil pressure sender or the oil pressure sender is getting a bogus reading from sludge buildup on a screen between the oil system and the oil pressure sender. Your thoughts?

 

A: Temporarily installing a mechanical oil pressure gauge in parallel with the electrical oil pressure sender would help isolate whether the issue is with the sender/gauge or a mechanical issue with the engine. By symptom, there appears to be a bleed-off of oil pressure somewhere in the engine. At higher rpm the oil pump is supplying enough oil flow to keep the pressure up even with the bleed-off. But at idle, the bleed-off is enough to drop the oil pressure to dangerous levels. The somewhat intermittent nature of the symptoms may be due to variations in oil temperature and viscosity — the hotter the oil, the lower the pressure.

Where could this bleed-off be occurring? A defective or incorrect oil filter, which you’ve already addressed, the oil pump pickup O-ring or a missing or defective oil bypass valve in the oil filter mount — easy to check after removing the oil filter.

One final thought. Oil heavily contaminated with gasoline would be thinned significantly and generate lower oil pressure. Give the dipstick the sniff test.