Compression tests on modern vehicles

[Red'n'Black]

By 'modern' I mean pretty much anything built this century, with an electronic throttle body.

With old cars it was simple, screw adaptor in, foot to the floor and turn the key. But with modern stuff, every time I've looked at live data the throttle position only seems to be vaguely related to the pedal, running or stopped. Plus I'm never sure if I'm going to damage anything by cranking it with the ECU powered up but the coil packs disconnected.

My last car, a 2006 Saab, I used to do the test with the key out and everything else powered down. The return spring was set up so that it was about a quarter open at rest, so I just bridged the starter relay with a wire and I was away.

Everything I've owned since, including the current gen 4 Legacy, the throttle is sprung towards fully closed. I expect professional-grade scan tools have a 'WOT mode' or similar for this situation, but is there an alternative method a bit more in the DIY-er's budget?

One idea I had was to unplug the throttle from the loom and feed a small current into the motor terminals, just enough to hold it fully open against the spring.

Any other suggestions welcome.

 

[Bullet2012]

with the stuff i work on, the diagnostic system has a compression test mode, this does 3 things
1) opens throttle butterfly, 2) shuts off fuel injectors and 3) shuts off the spark.

 

[fizzy]

with the stuff i work on, the diagnostic system has a compression test mode, this does 3 things
1) opens throttle butterfly, 2) shuts off fuel injectors and 3) shuts off the spark.

How accurate are they though? And how would it cope with only 1 bad cylinder on a v8?

 

[sako243]

How accurate are they though? And how would it cope with only 1 bad cylinder on a v8?

I think you've misunderstood the post. You still need a compression tester, that's just a mode to turn things off that aren't needed and let the ecu know you're doing a compression test so don't scream when you don't detect any bang bang explosions.

 

[fizzy]

I think you've misunderstood the post. You still need a compression tester, that's just a mode to turn things off that aren't needed and let the ecu know you're doing a compression test so don't scream when you don't detect any bang bang explosions.

Ahh ok. Only I have seen plenty of garages using their diagnostic machines to run a "compression test" - I assume measuring the resistance to cranking?

 

[Guineafowl]

Ahh ok. Only I have seen plenty of garages using their diagnostic machines to run a "compression test" - I assume measuring the resistance to cranking?

Current clamp round the starter wire? You’ll see compression humps in the scope reading, which should all be the same. Sometimes there’s a second trace watching cyl 1 spark, so you can identify the individual cylinders, knowing the firing order.

 

[hotponyshoes]

My vw let's you do an electronic compression test.
No idea how accurate it is (or how it works!)

 

[JOEPRO]

Ahh ok. Only I have seen plenty of garages using their diagnostic machines to run a "compression test" - I assume measuring the resistance to cranking?

Current clamp round the starter wire? You’ll see compression humps in the scope reading, which should all be the same. Sometimes there’s a second trace watching cyl 1 spark, so you can identify the individual cylinders, knowing the firing order.

Its a dynamic compression test I think its called. MB etc have it. The software cranks the engine and compares the cranking speed VS the firing order to tell if one or more cylinders deviate from the rest in speed indicating a potential mechanical problem.

But as said, its only one test and in compression tests and or even better, leak down tests are more accurate and to be used in addition to a dynamic test where available.

 

[Bullet2012]

as @sako243 says, we can do cylinder drop out tests with by turning off individual injectors or sparks. a very handy feature.

 

[Red'n'Black]

I have a leakdown tester as well, maybe that's easier all round without any test software.

 

[finyuk2005]

My vw let's you do an electronic compression test.
No idea how accurate it is (or how it works!)

think thats only on diesels, somehow the injectors can do it i think

 

[m_c]

Its a dynamic compression test I think its called. MB etc have it. The software cranks the engine and compares the cranking speed VS the firing order to tell if one or more cylinders deviate from the rest in speed indicating a potential mechanical problem.

But as said, its only one test and in compression tests and or even better, leak down tests are more accurate and to be used in addition to a dynamic test where available.

This.
It's essentially a basic test to identify a problem, which gives a basis for further diagnostics. Far quicker to do than having to fit a compression tester.

As for the original question, provided all cylinders are tested under the same conditions, the usual point of a compression test is to compare cylinders.
If you're at the point of testing for overall lack of compression, then chances are a compression test isn't going to confirm much, as there are usually far more definitive symptoms.

 

[Red'n'Black]

This.
It's essentially a basic test to identify a problem, which gives a basis for further diagnostics. Far quicker to do than having to fit a compression tester.

As for the original question, provided all cylinders are tested under the same conditions, the usual point of a compression test is to compare cylinders.
If you're at the point of testing for overall lack of compression, then chances are a compression test isn't going to confirm much, as there are usually far more definitive symptoms.

Sure. I used to routinely do a compression test every time I had the plugs out, to see if anything was deteriorating over time. I caught a leaky valve that way once before it got too bad.

Sounds like a tool that's been mostly superseded by built-in diagnostics then, if you can get access to them.

 

[oldbikedavey]

Leakdown testers are very handy things., and MityVacs aren't so shabby either....

 

[keithski122]

I'm sure we used to be able to do dynamic compression test with this beast back in the late 1980's.Shows that nothings really new.