V6 3.8L Compression Rate

[longstand]

I was talking to a mechanic. and i told him.

" man i did a compression test on just one of my Cylinder. and it show 120PSI. he told me thats about right for that kind of engine. because its a Marine engine. its always pushing.

is that true?

 

[Bondo]

Re: V6 3.8L Compression Rate

" man i did a compression test on just one of my Cylinder.

Ayuh,... Why would you do only 1 cylinder,..??

Well,... Atleast that 1 cylinder is within spec...

 

[longstand]

Re: V6 3.8L Compression Rate

Ayuh,... Why would you do only 1 cylinder,..??

Well,... Atleast that 1 cylinder is within spec...

Sorry to tell you. but damm it.. iam kinda scared...

Something inside me dont want to check.. weird felling.. i guest i dont want to know i just wasted all my money and the engine is no good..

Dont Jugde me guys. i know i should i have the compression before i even purchase the parts.. my fault. but its to late.. i will just put the boat on the water and tune everything up. and it works great. then it work great.


also. when you said Within Spec. .. 120PSI. is good?? both engine has 600HR

 

[krisnowicki]

Re: V6 3.8L Compression Rate

I do not know much about the 3.8 but, you want all your cylinders within 10 percent of each other so lets say you do the rest of the engine ( which you need to do) now that you are in the mess lets figure out how bigso we can clean it up. so you want all the others to be within the 10 percent of each toehr so if 120 was the hgihest you dont want any lower then 108 or so. But 120 seems alittle low to me. My new 350 has about 155-160 on all but like I said I dont know much about the 3.8 and that may be normal.

 

[cr2k]

Re: V6 3.8L Compression Rate

Your 350 has a larger bore so it will show more compression.
120 is (as stated) in the ball park.

Compression check should be done with throttle wide open, ign off or disconnected and all spark plugs out, oh yea..and a fully charged battery.

If there are any low cyls. squirt a little oil in them and see if it makes a diff.

 

[krisnowicki]

Re: V6 3.8L Compression Rate

shouldn't it also be done with the engine warmed up

 

[intrepidvoyager]

Re: V6 3.8L Compression Rate

Not neccessarily ......if you read thru the following article you'll see why ....as long as the engine has been recently run ..... in practise most shops won't do a compression test on a hot engine ...

"As a rule, most engines should have 140 to 160 lbs. of cranking compression with no more than 10% difference between any of the cylinders.

Low compression in one cylinder usually indicates a bad exhaust valve. Low compression in two adjacent cylinders typically means you have a bad head gasket. Low compression in all cylinders would tell you the rings and cylinders are worn and the engine needs to be overhauled.

Checking Compression
Compression can be checked two ways: manually with a compression gauge or electronically with an engine analyzer. The manual gauge method is the only one available to most do-it-yourselfers.

To check compression, all the spark plugs are removed. The ignition coil is then disabled or the high tension lead is grounded. The throttle is also held open. The engine is then cranked for a few seconds using a remote starter switch or a helper while a compression gauge is held in a spark plug hole. The maximum compression reading is noted, then the process is repeated for each of the remaining cylinders. The individual cylinder readings are then compared to see if the results are within specs (always refer to a manual for the exact compression specs for your engine because they do vary from the ballpark figures we quoted earlier).

If compression is low in one or more cylinders, you can isolate the problem to the valves or rings by squirting a little 30 weight motor oil into the cylinder through the spark plug hole and repeating the compression test. The oil temporarily seals the rings. If the readings are higher the second time around, it means the rings and/or cylinder is worn. No change in the compression readings tells you the cylinder has a bad valve.

With electronic testing, a computer analyzer "estimates" compression in each of the engine's cylinders by measuring slight variations in engine cranking speed. The results correlate well with actual gauge readings and can be completed in a matter of minutes without having to remove any spark plugs. What's more, the analyzer prints out the results of the compression test making it easy to see and compare the actual numbers."

 

[Alpheus]

Re: V6 3.8L Compression Rate

^^^

I dont have the time or the patience to explain all of the wrongs in your post, but Ill give it a try anyway.

Im sorry but I dont know where you got your info from but that is 95% wrong.

Checking Compression
Compression can be checked two ways: manually with a compression gauge or electronically with an engine analyzer. The manual gauge method is the only one available to most do-it-yourselfers.

You have never heard of a leak down test???
Engine analyzer wont tell you squat about compression or where its leaking.

Low compression in one cylinder usually indicates a bad exhaust valve. Low compression in two adjacent cylinders typically means you have a bad head gasket. Low compression in all cylinders would tell you the rings and cylinders are worn and the engine needs to be overhauled.

Dont even get me started on this.
How on gods green earth can you say that one low cylinder is an exhaust valve? Ever heard of the valve right next to it called an intake? heck maybe the ring gaps are lined up. I wish it were that easy to deduce from your post that you could diagnose a compression leak from what you just stated.

The engine is then cranked for a few seconds using a remote starter switch or a helper while a compression gauge is held in a spark plug hole.

Dont use the kind of compression tester that you have to "hold in the hole".
use one that threads into the plug hole.

Read the manual. It has a section in it that tells you how to do a "correct" way to do a compression test...

My preferred method is a leak down test. Its a bit more time consuming but it produces much more accurate results.

Here is a video showing the basics of the procedure. I dont like the cheap tester he is using but you will get the jist...

http://www.youtube.com/watch?v=UOHY...5C167254&playnext=1&playnext_from=PL&index=46

 

[intrepidvoyager]

Re: V6 3.8L Compression Rate

I don't know why some people on this forum seem to want to jump all over a third party article as though their way is the only way ....i don't have time to respond to all your coments but the one about a leak down test is silly ..a leak down test has nothing to do with a compression test.

""""""In the dark days of engine-building, car crafters had to rely on a compression tester to tell them if they had a weak cylinder by comparing the cranking pressures of the cylinders against each other. But this was crude and a less-than-accurate way to evaluate how well the engine was sealed up.

Today, it's common practice to use a cylinder leakdown gauge to evaluate how well each cylinder performs in retaining pressure. The process is simple enough--pump a measured amount of air pressure into the cylinder, and the gauge will tell you the percentage of leakage. But there's a whole bunch more to this story than just pumping air into a cylinder. Let's take a look at how to perform a cylinder leakdown test properly and what you can learn from the results."""""""

If you read the post I responded to you will see the question was about whether or not the engine needed to be warmed up .

by the way ...for a leakdown the engine does have to be warmed up ...

 

[ErieRon]

Re: V6 3.8L Compression Rate

Jesus, you guys...as if longstand doesn't have enough problems.

We need a "rants" thread..........

 

[Silvertip]

Re: V6 3.8L Compression Rate

longstand - why do feel the engine is bad based on one cylinder reading 120 PSI. This is not bad!

 

[Alpheus]

Re: V6 3.8L Compression Rate

I was talking about using a cylinder leak down after finding a low compression cylinder. I also like to do a leak down test even if I find the compression test within normal limits just because carbon buildup inside the combustion chamber can give false positives in the health of your engine...

While a compression test is a dynamic test (engine moving), a cylinder leak down test is a static test (engine at rest). The compression test measures how much pressure the engine can produce while cranking; in contrast to the cylinder leak down test, which measures how much pressure is lost in the engine. In a cylinder leak down test the engine is placed on top dead center of the cylinder in question and using a similar type of connector as the compression test, you fill the cylinder with pressure. The tester then measures the volume of air needed to maintain a predetermined pressure in the cylinder. This reading is expressed in a percentage. Good cylinder leak down readings should be below 5-8%.

The great thing about cylinder leak down is that it deals with how well the cylinder is sealing and nothing else. The readings are not affected by carbon deposits, cam timing, or even engine cranking speed.

Another great feature of the cylinder leak down is the fact that you can hear where the air is leaking out of the cylinder. When a cylinder has high percentage of leakage, first check the oil filler cap. Do you hear a hissing sound? If so, you may have pressure leaking by the rings. Is there air escaping out the exhaust? Is it escaping out the intake system? Then a burned valve may be the problem. If two adjoining cylinders have similar low readings and you hear leakage out the other cylinder, then a failed head gasket may be the problem.

Being able to pinpoint the exact source of the compression loss will tell you where the problem is; and not just that you have one. This knowledge will greatly assist you in the next step? the repair.

 

[fishinpa]

Re: V6 3.8L Compression Rate

Compression test is fine for most .

Longstand if you could just check the rest of your cylinders and if they are all right about 120 then your engine should be tuneable, and no need to be elaborate about testing methods.

For the better part a compression test is fine and will aid in diagnosing the majority of the problems you will find related to your (engine.)

Most here do not have and do not need leak down testers and a whole lot of extra methods to diagnose condition of their equipment.
Keep it simple and go step by step .


fish

 

[JustJason]

Re: V6 3.8L Compression Rate

manually with a compression gauge or electronically with an engine analyzer

Engine analyzer wont tell you squat about compression or where its leaking.

There are actually a couple of digital compression/carb balancers on the market that utilize a pressure tranducers that plumb into the intake that tell you compression and cylinder leakage. I used them in school but didn't care much for them. Personally I trust an old fashiond gauge instead of a digital reader. They claim to work well... but you can get a quality compression tester plus a quality leak down set for more than less than the cost of one of the digital readers.

Longstand... you need to do a comp test on all cylinders. I understand your fighting a percieved carb problem. If you have a compression issue because of a leaking intake valve you will never get the engine to idle correctly....

And that's the biggest problem with low compression engines that are even across the board (worn rings) but under 100 psi. Engines that show under 100psi across then board will still make pretty good power and you'll get close to full RPMS but it will never hold a proper idle and they are a bear to get to idle under 1000RPMS let along the 650 you want.

 

[intrepidvoyager]

Re: V6 3.8L Compression Rate

Longstand... you need to do a comp test on all cylinders. I understand your fighting a percieved carb problem. If you have a compression issue because of a leaking intake valve you will never get the engine to idle correctly....

JUSTJASON has it just right .......... just for now forget all the stuff about leak down tests etc .........warm up the engine and do the compression test just like he said on all cylinders and let us know the results by cylinder number.

as a aside I agree with ERIERON and FISHINPA ....when the question of whether the engine needed to be warmed up or not arose I could have given a simple answer instead of initiating a whole debate over compression testing.