Author Topic: No-starter compression test? (Read 5598 times)

Turbinepowered · « **on:** April 06, 2007, 07:55:45 pm »

So, I have a diesel. I have a diesel that is not presently in a car and has a dead starter. The gas starter I have doesn't fit the engine properly and won't engage, but I'm working around that and having the starter rebuilt.

The question is, can I do a compression test without the starter? Will turning the engine over by hand generate the compression necessary to test it, or maybe turning it over by some other method? I'm trying to determine if I need to go ahead and get the engine re-ringed while it's out of the car, or if it's good to go and I can change the belts and drop it right in.

Wingaman · « **Reply #1 on:** April 06, 2007, 08:02:41 pm »

No, you need about 200-250rpm to do a right compression test. So you need a starter.

If you can do 250 rotation per minutes with your hand I want to see that hehhehe

jimfoo · « **Reply #2 on:** April 06, 2007, 09:44:39 pm »

Get a really strong drill and a 3/4" socket you can chuck in it. Remove injectors or glowplugs in all cylinders and install your tester.

Vincent Waldon · « **Reply #3 on:** April 07, 2007, 01:23:12 am »

The only other thing to remember is that you'll be measuring "cold" compression as opposed to compression with everything all warmed up.

Likely the numbers will be a bit lower that what you can expect when the engine was at normal temps so you might not find something that's on the edge, but you'll certainly find anything bad. And you could always pre-warm the oil if you wanted to be really really close ?!!

Turbinepowered · « **Reply #4 on:** April 07, 2007, 08:45:52 am »

Quote from: "Wingaman"

No, you need about 200-250rpm to do a right compression test. So you need a starter.

If you can do 250 rotation per minutes with your hand I want to see that hehhehe

Thanks for the numbers, that's the kind of thing I was looking for.

How about hooking it up to a bike by chain and pedalling it up to 250?

Quote from: "jimfoo"

Get a really strong drill and a 3/4" socket you can chuck in it. Remove injectors or glowplugs in all cylinders and install your tester.

How strong a drill are we talking? I have some hammer drills (Probably a BAD IDEA there), a nice torquey angle drill that chews up masonry with ease, and the usual plug-in drywall screwguns. Dad's a contractor, what can I say? I get to play with all the cool toys.

Quote from: "Vincent Waldon"

Likely the numbers will be a bit lower that what you can expect when the engine was at normal temps so you might not find something that's on the edge, but you'll certainly find anything bad.

Right, thanks for the heads up. My thought was if I found one that was out of whack with the others, off by more than a little bit, I'd go ahead and assume a problem and rering and hone. Along with, of course, the attendant replacement of headgasket and head bolts that comes with pulling the head for a honing job.

If they were all pretty even, even if they were all evenly "low," I'd operate under the assumption that were this a real, true compression test with the starter and proper conditions then they wouldn't be low. I have yet to see an engine where all the cylinders were uniformly low compression, so it seems to me to be a decent gauge of problems.

jtanguay · « **Reply #5 on:** April 07, 2007, 09:00:05 am »

if you had popey arms i bet you could do 250 rpm... thats only like 4 revolutions per second :lol:

Vincent Waldon · « **Reply #6 on:** April 07, 2007, 05:46:41 pm »

I'm in the middle of re-ringing my gasser... with the engine out the head falls right off in about 10 minutes and I figured... hey... half a day's work and I know my engine is going to stay back in there for a good long time. It's actually kinda fun !!

Perfect time to change the belts anyways... and with the belts out the head pops off even quicker... if you've got the time and a bit of cash I'd say do her up right while everything is accessible... saves having to pull it a year from now due to low compression.... or an old headgasket... or worn valve guides... or....

SMOKEYDUB · « **Reply #7 on:** April 07, 2007, 07:10:06 pm »

anyone ever see the family guy episode were they pan off to a popeye segment and it has him on a doctors bench and he says gagaga gagaga and the doctor says you know those monsterous forearms are over grown tumors and i cant believe your still living and also that thing you do with your eye and your speech impediment is from a stroke you suffered from 4 years ago. I howl everytime i see that episode. (impediment ..hopefully i spelt that right).

Turbinepowered · « **Reply #8 on:** April 07, 2007, 09:37:22 pm »

Quote from: "Vincent Waldon"

Perfect time to change the belts anyways... and with the belts out the head pops off even quicker... if you've got the time and a bit of cash I'd say do her up right while everything is accessible... saves having to pull it a year from now due to low compression.... or an old headgasket... or worn valve guides... or....

The belts I know I'm changing, as well as the water pump and the idler/tensioner pulley (That's a spendy little piece right there, more expensive than the rest of the parts put together!), and if it lasts a year then that's fine.

I need to have it in and running in ~ a month for the Chicago trip, then just to serve as an infrequent "daily driver" until I get my project Dasher finished, the majority of which work will likely be taking place over the summer and it's a project that has a year or so timeline to completion.

I just really, really don't want to be buying gas for this trip, you know? That's expensive...

tylernt · « **Reply #9 on:** April 07, 2007, 10:53:31 pm »

Why not do a leakdown test? Charge the cylinder with compressed air with an adapter in the injector hole and see how long it takes to leak out. I understand this is they way they test aircraft engines, only problem is we're missing the specification data on how long it should take to leak out. Maybe there is someone here who can to try it on an engine that has known good compression? In the name of science.

Black Smokin' Diesel · « **Reply #10 on:** April 07, 2007, 11:11:13 pm »

Quote

anyone ever see the family guy episode were they pan off to a popeye segment and it has him on a doctors bench and he says gagaga gagaga and the doctor says you know those monsterous forearms are over grown tumors and i cant believe your still living and also that thing you do with your eye and your speech impediment is from a stroke you suffered from 4 years ago. I howl everytime i see that episode. (impediment ..hopefully i spelt that right).

LOLOL I do remember that!!! Great episode :lol: :lol:

fatmobile · « **Reply #11 on:** April 12, 2007, 01:27:11 am »

With the T-belt on the engine, turn the crank with a wrench on the crank bolt.
If it springs back when near TDC, your compression is near or over 400 psi... maybe 375 but that's still good.
If it doesn't spring back but turns hard it can be around 350 psi, maybe a little more.
If it spins past the TDC spot with little trouble it's around 325 or less.
Below 300 and you'll have a hard time starting even in the summer.

Turbinepowered · « **Reply #12 on:** April 12, 2007, 08:09:35 am »

Quote from: "fatmobile"

With the T-belt on the engine, turn the crank with a wrench on the crank bolt.
If it springs back when near TDC, your compression is near or over 400 psi... maybe 375 but that's still good.
If it doesn't spring back but turns hard it can be around 350 psi, maybe a little more.
If it spins past the TDC spot with little trouble it's around 325 or less.
Below 300 and you'll have a hard time starting even in the summer.

I did this when I first got it; hard as all hell to turn it all the way over, but I wrestled it through two complete rotations with some brute force and ignorance applied through an 18" breaker bar.

What does that tell you?

fatmobile · « **Reply #13 on:** April 13, 2007, 05:12:20 am »

Sounds like it's pretty high and should start well.
Did it ever spring back when you let go near TDC?
That's a great sign.

Turbinepowered · « **Reply #14 on:** April 13, 2007, 07:16:01 am »

Yeah, it had quite a bit of spring-back as soon as I pushed it past the hardest point; on the first half of the "downstroke" of each rotation I barely had to apply any force at all.

I take it this is an even better sign?