Dear Tom and Ray:

I'm the original owner of a '94 Nissan Sentra with 125,000 miles on it. It came equipped with the 1.6-liter engine that has thus far been outstanding, except for its lack of power. I am a private investigator and spend approximately 12-15 hours a day in my vehicle, much of the time with the engine idling and the air conditioner on. But in the past 25,000 miles, I have begun to experience valve clatter when the engine has been running for a while. It only happens when I've been sitting for extended periods of time with the air conditioner running, or when I'm engaged in extended stop-and-go city driving with the AC on. It never happens under normal driving conditions, even on hills or with a load in the car. I do all my own repairs and maintenance, and I have always changed the oil every 3,000 miles. Do you know what's making the noise? -- Frank

TOM: I think I do, Frank. But first, I need to correct something. In your letter, you said it doesn't happen when you have "a load in the car." The correct expression is to "impose a load on the engine." Having "a load in the car" implies that you're transporting my brother.

RAY: In any case, Frank, I think you have SLOP (Severely Low Oil Pressure).

TOM: Here's what's happening. When you sit for an extended period of time -- staking out a perp, eating pound after pound of pistachio nuts -- your engine gets hotter and hotter. Running the air conditioning only adds to the heat in the engine.

RAY: And when the engine operating temperature gets really high, the oil gets thinner and the oil pressure drops. Yours is dropping low enough that you get engine noise. Why is your pressure dropping so low? It could be that you have a weak oil pump or that your engine is wearing out and the spaces between parts are getting bigger (this would not be unheard of after 100,000 miles).

TOM: So you have a few choices, depending upon how committed you are to this car and how much money you want to spend. The cheapest solution is to try a thicker oil. A 20-W50 weight oil may solve the problem, at least for a while.

RAY: But if you're looking to get more than a few months out of this car, you can spend a few hundred bucks and install a new oil pump. If the problem is being caused by a weak pump, you'd notice the exact symptoms you describe.

TOM: And if a new oil pump doesn't solve the problem, then you need an engine rebuild or a new/used engine. But before you go to that extreme, make sure the rest of car really deserves a new engine. Have it thoroughly checked out by a mechanic you trust before dropping that kind of money on it. After all, think of all the pistachio nuts you can buy for the price of a rebuilt engine.