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Symptoms point to vacuum leak

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QUESTION -- I own a 2000 Olds Alero with the 2.4-litre engine. I have an ongoing idle problem.

I got the computer and IAC valve replaced and the car still idles erratically. When in drive, it'll idle at about 1,000 r.p.m. and waver up and down when holding the brake. Then I'll put it into park and it will idle even higher and waver at higher r.p.m.! One morning I started the car and it went up to 2,250 r.p.m. before coming back down to 1,100 to idle at in park mode. The odd time it will idle normally. I checked for vacuum leaks at the shop twice and nothing was noted. The car idles normally still about three per cent of the time when stopped in drive. Could it be a sensor like MAP or throttle position sensor, or what else could it be?

ANSWER -- Idle speed on a fuel-injected engine is controlled by regulating the amount of air getting past the throttle plates and idle air-control valve. I know you have checked for vacuum leaks, but the symptoms you describe are typical of a vacuum leak rather than a fault with a MAP or throttle position sensor. Even if the idle air-control valve was sticking, the engine r.p.m. shouldn't go over 2,000 r.p.m. That means the air is getting in some other way.

The surge you describe is caused by the computer trying to compensate for the air leak. The computer closes the idle air-control valve to slow the engine but it overcompensates because the valve moves relatively slowly compared to computer commands. The r.p.m. then goes too low, so the computer then opens the idle air-control valve again. This happens repeatedly.

There are a few places you probably haven't checked for vacuum leaks. The crankcase ventilation system could cause the problem. There is an oil/vapour separator that is part of the ventilation system. A leak from the hose connections or a crack in the plastic separator could be one cause. The ventilation system depends on the rest of the engine being sealed properly. A leak at the oil filler gasket, on the dipstick seal, a valve cover gasket or even the crankshaft seals can allow too much air into the engine crankcase, which passes through the ventilation system causing the idle-speed problems.

It can be difficult to locate some vacuum leaks. One of the methods we use is to spray a little carb cleaner around the gaskets, hose fittings and connections. If there are any changes in engine speed at all, then that area is a suspected leak. Some technicians use propane from a propane torch to perform the same test. The engine speed will change and smooth out when a leak is detected. I prefer the carb cleaner or plain water to test the system.

QUESTION -- My 1999 Intrepid, which has 119,000 kilometres, was running fine until a couple of days ago. The only issue was that I got the transmission sensor replaced a couple of weeks ago after hearing strange noises from the engine and having the engine light come on. Last Tuesday, without warning, the car made a grinding noise upon start-up and to my dismay after getting towed to the Chrysler dealership, I was informed that the timing chain broke and the engine is now toast! It will cost me $5,000 to replace this engine with a used motor having 125,000 km on it, which I will not be doing. Upon research, it seems that this is a fairly common occurrence with the 2.7-litre engine. Can you shed some light on this matter? I will now be in the market for another vehicle.

ANSWER -- I have looked at a failed 2.7-litre engine when one has been apart and the oil passages that lubricate the cylinder head and camshafts are fairly small. If there are any restrictions in the oil passages, the camshafts start to bind in their bearing bores and can create enough stress on the timing chain to break it. Then the pistons come in contact with the valves and the engine needs major repair work. In my opinion, this is one engine that needs more frequent oil and filter changes to ensure the oil passages stay clean and unrestricted.

Jim Kerr is an experienced mechanic, instructor of automotive technology, freelance journalist and member of the Automobile Journalists' Association of Canada.

kerr.jim@sasktel.net

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