L300 compression test

N

Not_Todd

Member
Hi everyone.

1991 l300 4d56

I'm going to go a compression test in the next few days and I'm wondering how you tell when the engine is in the correct temperature range.

Is it basically the standard temperature if I let it heat up by idling?

Or, could it be that a "new" thermostat should open at the proper temp for testing and I'll know it's good when warm air starts coming from the vents?

Thanks in advance!
 
For the most accurate test you want it as close to operating temperature as possible, however doing a test on a hot engine isn't the easiest or safest thing to do. If your goal is just to see if there is an irregularity, you can do that with a pretty cold engine. If you do want to do a hot test, go for a quick drive to get it up to operating temperature (say 10 minutes) and then basically park and do the test as soon as you handle the temperature. Honestly, I'd do a cold/warm test first which will yield slightly lower numbers than a hot test. If you are happy with the results I'd skip the hot test.
 
Delica_Dad said:
For the most accurate test you want it as close to operating temperature as possible, however doing a test on a hot engine isn't the easiest or safest thing to do. If your goal is just to see if there is an irregularity, you can do that with a pretty cold engine. If you do want to do a hot test, go for a quick drive to get it up to operating temperature (say 10 minutes) and then basically park and do the test as soon as you handle the temperature. Honestly, I'd do a cold/warm test first which will yield slightly lower numbers than a hot test. If you are happy with the results I'd skip the hot test.
Click to expand...
Perfect, cheers!
 
Delica_Dad said:
For the most accurate test you want it as close to operating temperature as possible, however doing a test on a hot engine isn't the easiest or safest thing to do. If your goal is just to see if there is an irregularity, you can do that with a pretty cold engine. If you do want to do a hot test, go for a quick drive to get it up to operating temperature (say 10 minutes) and then basically park and do the test as soon as you handle the temperature. Honestly, I'd do a cold/warm test first which will yield slightly lower numbers than a hot test. If you are happy with the results I'd skip the hot test.
Click to expand...


I've tried 3 different Amazon compression test gauges and they've all broken, or were leaking pretty badly. I'm currently looking to have a mechanic do the test though I'm hesitant to spend money on it.

Any idea if there's any general rule of thumb on when a rebuild is needed? Like, if you can get a bunch of oil coming out your dipstick hole you know it's time?
 
Kyle_GP said:
I've tried 3 different Amazon compression test gauges and they've all broken, or were leaking pretty badly. I'm currently looking to have a mechanic do the test though I'm hesitant to spend money on it.

Any idea if there's any general rule of thumb on when a rebuild is needed? Like, if you can get a bunch of oil coming out your dipstick hole you know it's time?
Click to expand...
You really shouldn't get any out of the dipstick, and not a lot out of the filler cap. This video shows 4D56T before and after rebuild... my van falls somehwere in-between.

 
I don't think there is a general rule of thumb, but it's really when your engine is to the point it's not reliable anymore and something catastrophic is likely around the corner. Oil consumption can be an indicator of things worn out, if oil is coming out of your dipstick than you are getting some excessive blow by it sounds like. Things that would mean rebuild time to me on a running engine:

Knocking, other sounds indicating internal wear or damaged rotating assembly
Oil consumption where you are at risk of running out of oil or it's making a mess from blow by, cracks, etc., smoking badly
Poor performance as a result of low compression

There are also many levels of rebuild but that usually can't be determined until you take it apart. A new engine might be cheaper in the long run rather than rebuilding your existing one. They are getting more hard to find though.

Good luck!
 
Delica_Dad said:
I don't think there is a general rule of thumb, but it's really when your engine is to the point it's not reliable anymore and something catastrophic is likely around the corner. Oil consumption can be an indicator of things worn out, if oil is coming out of your dipstick than you are getting some excessive blow by it sounds like. Things that would mean rebuild time to me on a running engine:

Knocking, other sounds indicating internal wear or damaged rotating assembly
Oil consumption where you are at risk of running out of oil or it's making a mess from blow by, cracks, etc., smoking badly
Poor performance as a result of low compression

There are also many levels of rebuild but that usually can't be determined until you take it apart. A new engine might be cheaper in the long run rather than rebuilding your existing one. They are getting more hard to find though.

Good luck!
Click to expand...
sk66 said:
You really shouldn't get any out of the dipstick, and not a lot out of the filler cap. This video shows 4D56T before and after rebuild... my van falls somehwere in-between.

Click to expand...
Thanks!

I did an oil change a few weeks ago and haven't actually noticed the oil dropping yet. It does have a bit of sputtering from the dipstick hole though.

Guessing I'm still ok for a little bit.
 
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