Muffler valve could cause your Maxima to say boo

Hey there, time traveller!
This article was published 15/04/2006 (7148 days ago), so information in it may no longer be current.

QUESTION — I drive a 2003 Nissan Maxima with approximately 29,000 kilometres. My Maxima is otherwise, as somebody lauded, “a classy catapult, capable of sprinting to 100 km/h from zero in less than seven seconds…” There is, or at least seems to me, however, one problem: It starts making a “booing” noise when the acceleration pedal is pressed between 35-70 km/h, once the ambient temperature drops below -5 Celsius. It is quite remarkable, and even a minor vibration can be felt through the foot on the pedal. I had a hunch that there is some hesitation in transmission from the first to second gears.

Although the service manager at the dealership denied the existence of an abnormal condition, he also admitted that this situation does occur under certain conditions. One of the important factors here is its temperature-dependency, accompanied by increased gas consumption (10-15 per cent). I am sure that the Nissan Motor Co. has not built these automobiles with the Manitoba winter in mind. Your comments on this matter are greatly appreciated.

ANSWER — Actually, Nissan did build the Maxima with typical Manitoba winters in mind. The vehicle had undergone cold-weather winter testing and evaluation so it will perform well in cold winter weather. Although the problem with your Maxima may be more noticeable at colder temperatures, I suspect it may also occur to a lesser degree at warmer temperatures too. It sounds like the problem is with the exhaust system.

In 2002 Nissan started using a “variable capacity” muffler on the Maxima to increase performance. This technology was adopted from their Skyline GT-R racing cars built in Japan. The muffler has a spring-loaded flapper valve inside the muffler that sits in the closed position at idle and low-speed situations. The exhaust gases flow through several passages in the muffler to keep the exhaust note quiet.

When exhaust pressures increase during higher-speed operation, the pressure forces the valve open and the exhaust flows through the muffler with less restriction. You hear a louder exhaust note from the tail pipe.

If this spring-loaded flapper valve were to stick open, you would likely hear a moan or “booing” sound. This is similar to the sound you might hear with a low-restriction aftermarket muffler. The change can also cause a resonant vibration in the exhaust system that could transfer to the rest of the vehicle. That may be the vibration you feel.

As for the decrease in fuel economy, cold weather will cause a significant increase in fuel consumption. However, if the muffler valve was sticking closed at times, this would also decrease fuel economy.

Finally, the transmission likely doesn’t have anything wrong with it. Transmission shifting is directly affected by engine performance. A major change in exhaust back pressure will usually affect transmission shift points.

QUESTION — I have a 95 Chev 1500 V8 5.7 L short box (not 4×4). After a short warmup, the vehicle starts missing and has poor pickup. The missing only seems noticeable at a stop sign or sitting. I had the truck checked out. A monitor check shows no codes and the 02 sensor is switching OK. A scope check shows the plugs and wires OK, compression is 140 across all cylinders and the cap and rotor are good.

The fuel pump checked OK but EGR (exhaust gas recirculation) operation was questionable. The EGR was replaced and the truck ran great with no problems for two months (500-700 km). Now it is back to the same thing. The truck has 270,000 kms. I would appreciate any advice you can give.

ANSWER — I think your truck’s symptoms indicate another problem with the EGR valve sticking open. The EGR valve opens when the engine is warm and under light-load cruise conditions. It returns some of the exhaust gases into the intake manifold to reduce cylinder temperatures and emissions. During hard acceleration and idle, the EGR remains closed. If the valve is sticking open, then power is low and engine idle is rough.

Although you have replaced the EGR valve, this new valve is likely held open by carbon deposits. I have seen this occur on many higher-mileage engines. Carbon forms small balls or pellets in the intake manifold EGR passages. These balls float around and sometimes get stuck in the EGR valve. If you remove the valve, you will likely find deposits holding it open. This can be cleaned out easily with a small screwdriver, but to clean the manifold and exhaust passages, the intake manifold will have to come off. You can try blowing out the passage with the manifold still in place. It works sometimes and it doesn’t take long to remove and replace the EGR valve.

Jim Kerr is an experienced mechanic, instructor of automotive technology and member of the Automobile Journalists’ Association of Canada. You can e-mail questions to Jim at the address below.

kerr.jim@sasktel.net