compression test

[Dirk]

Lazy question.

 

What are thye results likely to be If I want to check the compression of one cylinder but can't be bothered to remove the other 3 spark plugs?

 

Rick, I know you will say that this is pointless but Im just curious to know if the reading will indicate anywhere near the truth or be wildly inacurate.

 

Cheers

[Petersubaru]

..if the motor is running smooth leave it alone or if that one plug has no oil on it, it's good to go

[GeneralDisorder]

Well - the other plugs are removed in order to get the enging to turn over easier.... and therefore faster - causing a more accurate reading as compression takes place at RPM's far higher than cranking RPM's (which is as low as 100 RPM or less). The faster you can crank it the more accurate your test will be since even at idle you are turning 700 RPM. Basically that means that small leaks in valve seats and rings don't matter as much so you will get a higher, and therefore more correct, reading with all the plugs removed.

 

Think of it this way - if there were NO leaks in an engine it simply wouldn't turn over at all by hand and you would need some kind of compression release mechanism to even start them. So even new engines have *some* blow-by and such - allowing them to be turned over. Those leaks will contribute to a lower compression test - they will contribute *more* and become a bigger problem the slower you crank it durring the test.

 

It also takes fewer amps from the battery to crank it when the plugs are out - leading to less testing innacuracy due to the battery draining as you move from cylinder to cylinder.

 

Then there's the concern with carbed engines that if you have three plugs in it's going to suck fuel into the engine and wash the lubricating oil off the cylinders leading to more false low readings. Not a concern on FI motors if you pull the fuel pump fuse and crank them over till they run out pressure in the injector supply.

 

So there's a lot of reasons to not want to do it that way.

 

GD

[Dirk]

Nice one.

 

Thanks man.

[Txakura]

 

Not a concern on FI motors if you pull the fuel pump fuse and crank them over till they run out pressure in the injector supply.

 

 

GD

 

what about a remote starter switch? not energizing the pump or ignition, 5 seconds to clip on

[GeneralDisorder]

what about a remote starter switch? not energizing the pump or ignition, 5 seconds to clip on

 

Won't matter on a carbed engine - still have to pull the plugs or run the float bowl dry.

 

But yeah - remote crank button helps for reading the gauge, etc.

 

GD

[asis]

I agree with GD's assessment as far as being accurate, however I think if you are just trying to locate a bad cylinder, it can be done with the other plugs installed as long as your battery is up to snuff. If you only get 60psi on one cylinder with the other plugs in, I doubt you will get 120psi by removing the other plugs.

Once you locate a low cylinder, then doing a leak down test on that cylinder can help identify the problem as to rings or valves.

 

You will want to pull the coil wire while testing regardless

Edited April 18, 2010 by asis

[GeneralDisorder]

I agree - if you are looking for a serious problem then it would be ok to go with only a single plug out. But really - for the type of problems that could actually be found using a leak-down test - wouldn't you want the most accurate readings possible so you have an idea what you are getting into? Most of the problems located with these types of tests will involve tearing down the motor at least partially to make an effective repair..... though I did lose a rocker arm on my 350 SBC that I located with a leak down test and *that* was a relatively easy fix - replaced it and did a valve adjustment .

 

But if you are just looking for a general pass/fail from the compression test then it will work. Don't bother writing down any of the numbers though.

 

GD

[Txakura]

Won't matter on a carbed engine - still have to pull the plugs or run the float bowl dry.

 

But yeah - remote crank button helps for reading the gauge, etc.

 

GD

 

roger that, why I clipped carbbed off of quote, yes, pull all plugs on any engine

[Dirk]

Alright, Ill come clean and tell you why I asked.

 

I was changing the oil on the wifes car and thought I would do a compression test while I was at it. Two cylinders came up really low. About 60psi.

 

Then as I was putting everything back together (I had to remove the air cleaner to access N.O. 2 and 3 plugs) it occured to me that I had done the test with out any oil in the engine. (yes she has a non soob car but I wanted to post here so I omitted that detail)

 

It wouldn't have taken long to re-do the test but I had other things to do that day so I just pulled the most accessible plug and tested it (with oil run back through the engine). It came back up to about 160psi.

 

Interestingly even with the HT leads dissconected from the other three plugs it felt like me that the engine turned over much faster with the plugs in. Which is why I wondered if the reading was indicative or not.

 

P.s. curiosity finally got the better of me and I tested again yesterday with all the plugs out and all cylinders were within 3 pecent of each other. so all ends well.

 

Cheers

 

Dirk.

Edited April 18, 2010 by Dirk

[asis]

Absolutely Rick-If you find a low cylinder, I would then pull the other plugs and note the change in readings before doing a leak down. Otherwise there "is" the chance of doing a leak down and not finding much if any thing.

Actually if you have the setup for a leakdown test, it is the most accurate indication of the cylinder.

 

Oil is IMPORTANT!!! It does help the rings seal

 

Perhaps I am applying my knowledge of aviation as well as small engines for Outdoor Power Equipment, to automotive, and I do realize that auto has many variations, smog, vacuum, carbs,....etc. so I don't intend my thoughts to be 100% accurate as far as subie engines. Just chiming in to get some feedback myself....hope that is ok...?

 

I have over 25yrs as a mechanic but (thankfully) only about 1 yr as an auto mech...cuz I hate bending over fenders AND on today's auto's, most Certified Mechs only need to plug in a machine and find out what part to replace, many actually have no idea how an internal combustion engine actually works...least the ones that bring their lawn mowers to me to fix a leaking needle valve/seat

[GeneralDisorder]

Oil is IMPORTANT!!! It does help the rings seal

 

Some engines more so than other's. There are engines out there (*cough* Jaguar *cough, cough*) that will lose all their compression if the oil is washed out of the cylinders.

 

Perhaps I am applying my knowledge of aviation as well as small engines for Outdoor Power Equipment, to automotive, and I do realize that auto has many variations, smog, vacuum, carbs,....etc. so I don't intend my thoughts to be 100% accurate as far as subie engines. Just chiming in to get some feedback myself....hope that is ok...?

 

You're fine - not to worry.

 

AND on today's auto's, most Certified Mechs only need to plug in a machine and find out what part to replace, many actually have no idea how an internal combustion engine actually works...least the ones that bring their lawn mowers to me to fix a leaking needle valve/seat

 

Most of them are entirely clueless when presented with a carb of any sort. It's not in their training anymore.

 

GD

[asis]

 

You're fine - not to worry.

 

GD

 

Thanks..I think

 

Grill is gettin fired up around 4

[GeneralDisorder]

Wish I could make it - busy here .

 

GD