Compression Test Questions

I'm looking to do a compression and a leak down test this coming weekend and after scowering the interwebs I have at least one question. I didn't see an official Walkthrough so maybe I will add one to the annals of LGT history- that said...

 

Concerning the Compression Test

 

1) Is there anything else (aside from maf) that i need to disconnect in terms of sensors along with the fuel pump fuse (15a)? Is that fuse sufficent for disconnecting the fuel pump/ignition system together?

 

2) Do i need to remove the wire clips from the coil packs? or is just pulling the packs enough?

 

3) Pedal to the floor while cranking key to maintain an open throttle body?

 

From what I can see online it seems fairly straight forward, remove all the packs/plugs, disconnect fuel pump fuse, thread in the corresponding gauge and let er rip. I've done the plugs on my 06 LGT wagon before but my biggest conern is making sure I disconnect all the proper circuts etc prior to turning over the key. Any tips would be apprecaited.

With the fuse pulled, the pump will not work.

 

You can leave the col packs connected, just leave the plugs out of them.

 

Yes.

Thanks Max - I should have results by Sunday then when I have a chance to get under the hood.

I actually made a video on this, full walkthrough. You don't need to pull the fuse, just have the gas pedal floored when you crank it(sounds like I'm joking but I'm not)

 

Here's the link:

 

 

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Oh yeah, that's right too, by flooring the pedal you put the car into thinking its flooded and it will not pump gas.

Yeah, pulling the fuse is fine, and since it's better to be a 2-person job anyways (watching the gauge build pressure and seeing max, watching for any trends), have the person turning the key to crank it hold the throttle WFO.

 

 

Also, make sure your battery is good to go for this. And take some time between cylinders, just let the starter cool down a tad...

Thanks to Everyone for their tips. I didn't get a chance to get this done this past weekend but should be able to find a few hours early this week. Really appreciate everyone's help and advice.

? what are you doing a compression or leakdown or both?

 

Compression test you need to do all that fuel and cranking stuff. For the leakdown you are just pressurizing the system and don't need to be inside the car or turn the motor. leakdown is better for isolating an issue like piston rings or valves. I prefer the leakdown because you can figure out if your sb or ur heads(intake or exhaust valves) etc. are borked. Here's my logic, if you do a compression test and its not good, what next lol, a leakdown, might as well go straight to meat and potatoes. If the system is holding pressure then I would assume compression is good!

 

Edited August 4, 2020 by Tehnation

sure I see what you're saying Tehnation. You're absolutely right, I was planning to test compression mainly to isolate the cylinder which cylinder(s) need the leak test but I am also just generally curious what my compression numbers are.

gotcha! you can never be to careful with these cars! Compression is probably the easier and quicker test, you don't need air compressor or have to monkey with the crankshaft. When you get bad numbers it usually leads to the same thing, pulling the engine, which at that point, a leakdown is pointless. I guess it doesn't matter, all roads lead to rome! Edited August 4, 2020 by Tehnation

gotcha! you can never be to careful with these cars! Compression is probably the easier and quicker test, you don't need air compressor or have to monkey with the crankshaft. When you get bad numbers it usually leads to the same thing, pulling the engine, which at that point, a leakdown is pointless. I guess it doesn't matter, all roads lead to rome!

 

But you can do a leakdown test with the engine pulled, can't you? And have easier access to the cylinder heads and crankshaft?

Compression and Leakdown are two separate, but related tests.

 

 

Compression tells you what specific cylinder values are, compared to the others. Since you are cranking it for compression test, you get the full stroke and valve actuation. So you get a picture of how the engine actually runs. Getting those numbers is not difficult, except on these cars where spark plug access is a pain... on other engines (such as EJ22) I could knock out a compression test in under 30 minutes.

 

 

Leakdown is similar - you will have values that you have to compare against each other. But since you're keeping a pistons at/near TDC, you only get that picture there.

 

 

The first thing you should do is visually inspect the head gasket mating area thoroughly. If you see any built-up crud, there's a chance it's suspect. The leakdown test will mainly tell you about rings and IN/EX valves.

 

 

If it were me and all the tools were there, doing compression/leakdown sequentially on each cylinder would be the way to go (after trying to verify that the HG appears OK). But again, the compression test is so easy and quick to do (barring wrestling with spark plugs on these cars), I'd focus on that. If the HG looks like it may have a leak near one or more cylinders, and you get odd numbers for those corresponding cylinders - well you probably found it.

 

 

But really for TorqueLegend ..... what is the issue with your car?

But you can do a leakdown test with the engine pulled, can't you? And have easier access to the cylinder heads and crankshaft?

 

 

Yes. And then you'd immediately be able to further diagnose/fix. Right down to the specific valve, if you get a high leak cylinder.

 

 

 

You can do a compression test too, but it's easier to do it in the car and with the starter doing the cranking.

JP the car has a multiple issues. Big'ns are rough idle and it's not pulling like it should on acceleration. Additionally in an unrelated issue, the turbo also seems to not be spooling. I tried to get into the turbo this past weekend but my pipe bolts are so rusted I'm going to need to replace the entire exhaust system if I yank the turbo out because I am going to have to cut those bolts off. I'll end up doing a boost leak test for the turbo sometime in the next few weeks.

 

As for the engine, it's got fresh plugs, new front and rear 02 sensors (still need to check the injectors) - plugs and sensors seemed to help a little but power is still sub par and I have a sneaking suspicion it's the head gasket. I've got a new PCV valve to throw in once I can get the turbo off along with new oil feed lines and a lightly used OEM turbo that's in great shape but I don't want to invest in replacing the down pipe mid pipe etc right now if the engine is shot. At that point I'll just continue to slowly accumulate the parts as I find deals on offer up and CL the forums etc. However if the engine is reading consistent compression I'll go ahead and replace the rusted out exhaust system and turbo and be on my merry way.

 

The other issue is that the clutch is totally shot so that is another repair that will be done once I get some cleaner pipes down under.

 

A/C blows cold tho...

gotta have ac!

Ac is the most important part of any car! I don’t care how much power my cars make, I won’t drive em unless the ac blows cold! On these motors, I almost think the leak down is more important than the compression test.

being that you can't pull the heads of in the car, or mess with the pistons or rods from the bottom... the entire design is a pita. So doing the easier and faster compression test makes the most sense. Bad compression or bad leakdown, any way you slice it means the motors gotta come out. Basically anything wrong and its gotta come out.

Fingers crossed that I've got a healthy motor and don't have to pull it. Buuuut if I do then it looks like papa has an excuse to add to the tool collection.