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Engine stumbles likely a spark-plug problem

Questions & Answers

Question: I have a 2003 Hyundai Sonata with the 2.7-litre V-6 engine and 170,000 kilometres on it. When idling in drive, it feels like the engine is randomly stumbling, one sharp stumble at a time. It does not do it all the time, and it seldom does it with the air conditioning off. When it does this, the car feels as if it wants to lurch forward slightly.

Now it has started to stumble when accelerating from a dead stop, but not all the time, and only for about four to six stumbles. The transmission fluid was changed at 120,326 kilometres and the spark plugs at 90,124 kilometres. The check-engine light has not turned on and the auto parts store checked and found no codes. I think it could be a bad spark plug or bad plug wires, but wouldn't the check engine light be on? I do most of my own car repairs myself except for the big stuff, but I do not even know where to start with this.

P.C.

Answer: You may be correct in suspecting the spark plugs and wires. Try misting them with a light spray of water to see if it causes a momentary idle roughness. A P0300-series misfire fault code will result if the crankshaft position sensor sees more than a two per cent variation in r.p.m. between any two consecutive cylinders in the firing order. In other words, if a cylinder does not fire, the crankshaft will slow down momentarily until the next cylinder fires. That sets the code.

Because there is no checkengine light and no fault codes, the stumble may be a slightly more progressive change in r.p.m. because of a low idle, carbon buildup, slightly low fuel pressure, too much exhaust gas recirculation flow, vacuum leak or a slightly weak cylinder. Make sure engaging the air conditioning raises the idle speed roughly 50 r.p.m., as it should. A slightly higher idle speed may well eliminate the stumble. The specs call for 700 r.p.m., plus or minus 100 r.p.m., at idle in gear. In park or neutral with the A/C on, idle speed should be 870 r.p.m., plus or minus 100 r.p.m.

Hyundai service bulletin 0436-015 calls for reprogramming the engine control module to eliminate intermittent hesitations while driving.

Q: I am in the air force and stationed overseas. My friend has a 2006 Suzuki Forenza with air-conditioning problems. The local shop on base in Turkey recharged the system, with no improvement. They told her that the compressor was bad. The compressor clutch engages, and the compressor makes no unusual noises. Both the highand low-pressure lines heat up when the compressor is engaged. Unfortunately, I don't have a set of gauges here. Is it possible to have overpressured the system when they recharged it, and would that cause the A/C to not blow cold?

P.A.

A: Yes, but not immediately. Most refrigerants used to recharge an automotive A/C system contain a dye that will leave visible stains if it is leaking from the system. A seriously overcharged A/C system will eventually freeze the condensed moisture on the evaporator, blocking airflow into the cabin. Do you have access to a simple do-ityourself recharging kit from an auto parts store or via the Internet? The newer ones have a pressure gauge incorporated into the recharging hose. This would allow you to check the charge with the compressor engaged to see whether it's overor undercharged.

If the system is properly charged, the compressor is engaging but no cold air is blowing, the expansion valve or tube that meters refrigerant into the evaporator may be clogged.

Q: Is the widespread practice of letting the motor run while waiting or parking simply a waste of fuel? Or is there some advantage in not having to restart the engine?

B.H.

A: The only reason to leave a car engine idling for more than perhaps 30 seconds is for the comfort and convenience of the occupants. Age, medical issues and extreme hot or cold temperatures can be valid reasons, but anything else is a waste of fuel.

Paul Brand, author of How to Repair Your Car, is an automotive troubleshooter, driving instructor and former race-car driver. Readers may write to him at paulbrand@startribune.com. Please explain the problem in detail and include a daytime phone number.

Because of the volume of mail, he cannot provide personal replies.