Corvette_Ed

For the first time since I bought my first car over 40 years ago I'm having to pay someone to change out my plugs and wires, and the shop mentioned that when they do "tune ups" their standard practice is to run a compression test. Since I've never had a shop do this work for me before this is the first I've heard of this being standard. I don't think it would hurt to do the test, and they're only charging $60 for it since I'm already paying for the labor to remove the plugs, but thought I'd check here to get opinions on this.

StayinStock

Eh $60 bucks isnt bad to know the numbers... if the cars running fine i wouldnt worry to much about it though.

GS-2013

While $60 isn't bad it's probably unnecessary in a car that's running fine. This is an income-generator for the shop. Not just for the test but for work that they determine that the engine needs. If it's standard practice then why do they charge for it? I've seen plenty of compression test results through the years. The results are always subject to interpretation. Sure it's not bad to know the numbers. On a good running car, I'd rather put the $60 into a gas fill or dinner.

Dave S

Somewhat unusual in my opinion but not harmful unless the shop uses the compression test as basis for making bogus claims of unneeded repairs.

PMPerformance

Not harmful, but I wouldn’t bother with it if there is no reason to
It’s a waste of money

Corvette_Ed

My car does have 110k miles on it, but it still runs like a brand new car. I'd actually intended to ask the shop to save the spark plugs and mark which cylinder each came out of so I can inspect them and see what's happening inside the cylinder. The compression test is sounding like something I really don't need. I think inspecting the spark plugs would tell me more than a compression test would as far as the current shape of my engine.

1Hotrodz

The shop is fishing for work, period. Unless there is a problem, never ever heard of that, someone is blowing smoke up your A$$. A compression test is a diagnostic test, meaning there is an initial problem and you are trying to narrow it down by process of elimination. No problems , no diagnostics.

FAUEE

Agreed. I wouldn't waste the money if the car is running fine

FAUEE

I'm a fan of them as a general health check. Iike to do them on a cheap car purchase just to make sure everything looks right. That said, I probably wouldn't bother on a car I already own that has no issues.

PMPerformance

this is a perfect excuse for them to fudge a problem to drum up business. If he is paying for spark plug removal, they think they got a whale on the hook.

think about it, when has a complimentary brake pad check or alignment check NOT resulted in a suggested service? We all know they lean on a wheel to bump it out .002 to fudge the numbers. Or they will state your pads are 6/32 and they suggest replacing them even though they will pass until like 2/32

FAUEE

Agreed, really easy to say "oh your compression is a bit out, we recommend a top end cleaning, if that doesn't work we might need to talk about rebuilding your motor".

C5 Diag

A compression test tests only the ability of the cylinder to seal…something like an issue with a valve not opening or a wiped cam lobe will not be revealed in the old school compression check…I do a “relative compression” test with a lab scope that takes 2 minutes and no removing of the spark plugs !!

grinder11

Not harmful.....As long as they don't knock a piece of crud into the cylinder. IMHO, they're answering a question that wasn't asked. If the car uses no oil, your compression is fine.......

Mr D.

Had to go google this one, never heard of that term before.

Relative Compression Test
The theory behind the relative compression test is that the amount of current required to turn over the engine while a cylinder is on its compression stroke will indicate how much the cylinder is compressing or leaking. This will information makes sense when you compare all the cylinders, as you can easily see if one is weaker than the others.

C5 Diag

That is correct !!…I can basically do an RC test without even opening the hood by accessing pin 16 of the DLC.

PDsVettes

Will a 400a amp clamp be sufficient for a relative compression test or do you have to use a 600a amp clamp?

Vito.A

Actually, a leak down test would be the best way to check each cylinder. Keep a record and compare the leak down at the next test.

C5 Diag

I use a Pico 200/2000 amp clamp set at the 2000 amp setting but 400 amps should be good…if you are using a labscope you can use your AC setting set on a 1 or 2 amp scale and connect to your battery and read relative compression.

PDsVettes

Thanks, Rob.

Corvette_Ed

I had the work done yesterday, and told them not to do the compression test. I had them save the plugs for me so I could inspect them, and every last one was in perfect condition for 110k mile plugs. They were all clean, no oil to be seen anywhere, and they all showed even signs of burning with no particulate stuck to the plugs. I'd say that's the best indicator as to what's going on inside my engine at this point.