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Quick Tip: Relative Compression Test

Applies To: MODIS Edge™, MODIS Ultra™, VANTAGE® Ultra, VERUS® Edge, VERUS® PRO, ZEUS®

Having a hard time diagnosing misfires due to a compression issue? In this Diagnostic Quick Tip, National Field Trainer Jason Gabrenas shows you how to find the problem cylinder using a lab scope and amp probe to perform a Relative Compression Test.



When you have a vehicle with a misfire you might think it could possibly be a compression issue, right?

But what cylinder do you check? How do you speed that up? How do you save some time when doing that?

We have a nice easy test that we can do using a low amps probe. It's called a Relative Compression Test and it'll show the compression in the cylinders, not actual values but relative to each other so you can see which cylinder you might need to check out that may have the low compression.

Let's run through it right now. I have the low amps probe hooked up on the negative battery cable. It's also set to a 60 amp scale. Any low amp probe is going to send out a specific amount of voltage depending on an amp. So this is 10 millivolts per amp that this thing puts out for each amp that it reads.

I actually have this set on a voltage setting, not an amp probe setting. because all we're really looking for is what the pattern looks like.

I also need to know what cylinder is number one, so I can check my firing order. So I'll turn on my channel two here. I have that plugged in on this coil number one and that's going to act as my trigger to show me my pattern.

You want to make sure before you crank over the vehicle, you disable the fuel system somehow. On this we pulled the full fuel pump relay but it could be a fuse or maybe the vehicle has a clear flood mode by pressing down on the gas pedal all the way.

We just pulled the relay on this so let's have Jimmy crank that over and see what we get.

It looks good. Let me scroll back here a little bit and there we go, there's a nice pattern here. There's cylinder one firing right there and there's cylinder one firing there again, so we can see we have four different compression events on the screen there.

Now let's pull a spark plug and see what a bad pattern would look like. I got it recording again so let's have Jimmy crank it over and we'll and take a look.

If we look here it's one, three, four, two is the firing order, and we'll see right around here, there's a whole cylinder missing right there.

We have a peak. I'm missing peak and two other peaks. If you were taking a look at this on a vehicle that even had maybe a little bit lower of a compression you can see relative to each other that one will be lower than the other, or one will be higher than the other.

It's a really quick, easy way to at least narrow down what cylinder you're trying to look at.

Last Updated: January 17, 2020

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