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Car Tools Leakdown
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Several years ago, I was chasing an elusive miss at idle on a Chrysler minivan. The miss was just barely noticeable at idle, and cleared up as soon as the throttle began to open. I checked the fuel system, I checked the ignition system, and finally started looking at the engine's mechanical health.
A vacuum gauge reading showed me nothing, so it was time to test compression on this transverse V6. Sure enough, the rear bank, front cylinder tested a little low. My next step was a cylinder leakdown test, and I then confirmed that the exhaust valve on that cylinder was leaking... just enough to cause the idle misfire.
The problem was twofold. First, I didn't test that possibility early in my diagnostic process and wasted a lot of precious time, hoping I'd find the problem in something easier. Second, the compression and leakdown tests were time consuming...many transverse engine designs aren't exactly the easiest when it comes to accessing that rear bank. Luckily, this vehicle was low mileage and still under factory warranty, so the customer didn't have to pay for it.
But what if he did? Could I justify the 2 hours plus it takes to perform these tests if they had proven to be OK? Then I learned I could quickly perform a relative compression test with digital storage oscilloscope (DSO), and in a matter of minutes, not hours.
Now, this is one of the first tests I perform when diagnosing driveability issues. I want to quickly eliminate the possiblity that an engne mechanical failure is the cause of the problem.
Relative Compression and Starter Draw
The first method I ever learned involves starter draw. Think about it a minute...it takes current to turn over the starter and the more resistance to turning the starter has to deal with, the higher the current draw will be. We all learned as young technicians how to measure starter draw when testing that system, but when this test is powered by a scope, with its ability to graph the current while adjusting the time base, it allows us to see the impact of each cylinder on that current draw.
To perform this test, use your high amp clamp around the positive battery cable leading to the starter. Fire up your scope, and adjust the time base to 100ms per division. (You can make finer adjustments to clarify the pattern). Make sure the battery is strong and disable the fuel and/or ignition system to prevent the engine from starting, just as you would if you were performing a regular compression test.
My scope of choice is a PICO 3443. With it's ability to record, I can go back and review the pattern starting from the beginning. On my older UEI scope, the pattern wasn't as clear and would disappear from the screen if I tried to adjust the voltage divisions to make it clearer. That, however, is easily corrected by using the AC coupling feature on your scope. By removing the DC component, only the pattern remains...centered in your screen.
Remember, I'm looking at the relative amount of current that each cylinder is requiring to overcome the compression pressure and turn over. I don't really care about how much the current is...what I'm looking for is any "peaks" that are lower than the others. If I find one, I can then use my synch to trigger off of cylinder #1, then use the firing order to identify exactly which cylinder is at fault. In my experience, I can easily pick up on a compression variation of as little as 10% with this method.
But it is relative to all the cylinders. If there is a mechanical issue resulting in an equal loss to all, the pattern will look normal. However, this is a rare occurrence and the vacuum test would show this as a lowered intake manifold vacuum.
Peter F. Meier is an ASE certified Master Technician with over 35 years in the field. He also writes for several industry publications and is the author of "Automotive Driveabiity: A Guide to Engine Performance Diagnosis". You can find Pete at http://www.autoservicetech.com
What To Do If Your Northstar Engine Fails
The Northstar engine is top of the line and has previously been hailed as one of the top ten engines by Ward's Auto World. In 1993, cars such as the Cadillac Allante, Eldorado and Seville STS were among the first to be made with the Northstar engine. Manufacturers are working on an updated version of this engine which is expected to produce up to 750 horsepower. Even though the engine has few problems, requires less maintenance and is fuel efficient there are some concerns you should be aware of if you own a new Cadillac or used Cadillac engine.
There are particular problems you may run into with your Cadillac if it runs with the Northstar engine. If your used Cadillac engine fails to run, the first thing you should do is check to see if it is overheating. Coolant may be leaking into the engine. Look for white smoke coming from the tail pipes, the smell of coolant odor coming out of the exhaust pipe, the engine temperature increasing with acceleration, oil that is either creamy white or light brown or massive oil leaks coming out of the rear main seal area.
Finding a damaged head gasket in a used Cadillac engine is common when it fails. A compression test, leakdown test and coolant pressure test can determine whether or not the head gasket is blown. The Northstar engine becomes too hot when coolant is not maintained at ideal levels and this causes the threads of the gasket to dissolve over time. Once the gasket is damaged typically the engine is discarded because the entire used Cadillac engine must be taken out which can be costly, time consuming and with poor results. Few repair shops even have the parts available to repair the Northstar engine. A better solution is to replace the damaged engine with a fully functioning used Cadillac engine that has reinforced bolts.
The Northstar V8 engine is an interference engine which means the timing belt is part of the internal combustion engine that controls the timing of the valves. This means that if the chains fail or disconnect the valves could hit the pistons and cause engine failure.
Another common area to check when your Northstar engine fails is whether or not there is oil leaking due to the high pressure casting process. If you have oil burning then do a ring cleaning procedure using GM cleaning kit and Kent-Moore induction/evacuation tool then change the oil.
If the engine has indeed failed then Cadillac suggests to replace it with a new engine because rebuilders don't have access to some parts needed for the repairs. It is actually cheaper to purchase an already used Cadillac engine than to repair the old one or buy a completely new one.
About the Author
Even thought the Northstar engine is high horsepower and is fuel efficient, a knowledge of possible issues with the engine can prevent future headaches and in most cases it is more cost efficient to replace a faulty engine with a used Cadillac engine.
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