Cylinder leakdown test better on older engines

Hey there, time traveller!
This article was published 29/11/2002 (8346 days ago), so information in it may no longer be current.

QUESTION: I own a 1990 Ford 150 pickup truck with a 302-cubic-inch motor. The motor has an annoying engine back-pressure leak.

The motor has 161,000 kilometres on it and runs like a charm. The compression is good — the PC valve has been changed and the motor uses very little oil. The problem seems to be a slight moisture leak into the air cleaner and drip on the garage floor. Is my oxygen sensor to blame, is this an early warning sign the motor wants to die, or is this normal for this engine. Any advice would be welcome.

ANSWER: The oxygen sensor wouldn’t cause any of these problems. Assuming that crankcase pressure is causing blow-by in your engine, I would first check the PCV valve. There are many look-alike valves and the wrong flow rate can cause blow-by. Use a Ford original equipment one to be sure.

Because your engine uses very little oil, the rings and cylinder walls are likely fine. Compression testing can be inaccurate on badly worn engines, because the excess oil on the cylinder wall seals the rings at cranking speed. A cylinder leakdown test is better. This test forces air into the cylinder and measures the percentage of leakage.

The problem may be worn exhaust-valve guides. This can cause excessive crankcase pressure without any oil consumption. Valve- guide wear can be checked by removing a valve cover and physically moving the valves sideways.

Finally, there may be no problem at all other than a higher-mileage engine with a leaking oil seal. Moisture in the air cleaner can be caused by short-trip driving in cold weather.

QUESTION: I’m driving a 2001 Jetta GLX with sports package. VW claims that the shocks from the sports package are 15 per cent stiffer than the non-sport one. The springs are the same, while the size of the anti-roll bar is larger. In order to improve the handling, I am thinking of getting high-performance shocks and springs.

Here is the problem. Some people said there is no need to get new shocks since I’m going to install the sport lowering spring (instead of race) provided by VW. The reason is that the shocks are already 15 per cent stiffer. However, some people said I have to get the new shocks too. So, in your opinion, should I get both springs and shocks or are just the spring good enough?

ANSWER: For optimum handling, the springs, shocks, sway bars, and tire package all have to work together. Change any one of these items without considering the total package and the vehicle handling will suffer. Before changing anything, you need to ask a few questions. Is the spring rate of the sport lowering spring the same as the sports suspension package spring? What do you really want to do with the car? Are you going to be changing tire profiles some time in the future? Can you live with a firm ride or do you want a softer ride for highway travel?

Personally, I would install the lowering springs and keep the shock package (struts) that you already have. VW sports handling packages offer a very good compromise between handling performance and a decent ride. From what I have been able to determine, the springs will lower the car, but not stiffen the suspension significantly. This may provide the performance image you seek and still be a comfortable every day vehicle with good handling.

Many performance-seeking drivers will install stiffer springs and shocks, and find that ride suffers. While high-performance race vehicles do use stiffer springs and shocks, they try to keep them as soft as possible and use sway bars to help control body roll. Stiff springs and shocks prevent the tire from lifting over bumps, so the whole car is moved and handling suffers.

I would start with the springs, move on to better tires if handling is really important, and then upgrade with a matched set of high performance front and rear sway bars. When it comes to ultimate handling, the set-up changes all the time for professional racers, so street driven vehicles are always a compromise. Need even more handling performance? Then install a complete suspension and tire package available from one of the aftermarket specialty tuners.

Jim Kerr is an experienced mechanic, instructor of automotive technology and freelance journalist. You can e-mail questions to Jim at the address below.

kerr.jim@sk.sympatico.ca