High Compression

Nenw

Nenw

Well-Known Member
***Admin, feel free to shift to another thread if you need.***

Sorry crew, posting this in the L300 thread bc I honestly don't think it will get seen in the non-delica thread. Also, posting this for a friend as I'm currently down with covid, f***!, so not getting my own stuff done.

He's got a 92' Pajero 4D56T. We were walking through a compression test over video chat and he was getting a super high PSI reading, between 590 and 670PSI! Felt like he was doing something incorrectly, but there is not much to do wrong. Fuel was disconnected, all plugs were removed, and hot engine.

Not finding too much good info out there on what would be causing this. The manual only calls out low PSI reading. Some websites say that it's a breathing issue. Worn cams, clogged exhaust system, carbon buildup on pistons, bent rockers.

What we did notice was that he did have a significant amount of exhaust puffing from the glow plug holes while we were running the test. This is not something I recall occurring when I ran my test two weeks ago.

This is across all cylinders so I don't suspect he has 4 bent rockers. Any thoughts from the brain trust on where to start looking? What other issues would too high of compression lead to, leaky seals perhaps?

Thanks.
 
Only a few things can cause high compression readings... excessive carbon buildup in the combustion chamber, rebuilt with undersized head gasket, deck over-shaved, etc; it's not "breathing" related.

Too high of compression can destroy the engine, primarily by causing early detonation. Other than having a MotorVac type cylinder cleaning done IDK what to suggest.
 
Have you used the compression tester on another vehicle? If not, try it. It may be the gauge…?(I like to rule out the easy diagnosis stuff first…)
 
It hasn’t been tested on another rig. Its brand new, but Amazon no name. I will recommend he returns it and gets a new one.

I think the engine was refurbished when he got the truck, it is entirely possible the head was decked too much or the head gasket is too thin.

I know he has a spare head, May be worth swapping that on.

I also saw on epc-data there are different thickness gaskets.

Once the machine work is done, is there any way to work backwards to know what was removed? This way he could know what size gasket to use?
 
I will second the idea of questioning the compression tool. Frankly it's the most likely answer, especially since it's a no-name Amazon tool.
 
Nenw said:
This way he could know what size gasket to use?
Click to expand...
For our vans, prior to April 1993 all 4D56T engines used the 1.5mm head gasket (4 dot/hole marking). After that (conversion to recessed valves/pistons) they went to the variable thicknesses of head gasket.
I think I would measure the existing gasket thickness and go up one (or install standard thickness); I guess this begs the question as to why the compression was being tested to start with...

It should be easy to check the compression tester to get a ballpark idea of accuracy. But I highly doubt it could zero out w/o pressure applied, and then read almost 2x high while seeming to function normally... that kind of error is more than just poor tolerance/inaccuracy.
With how much blow by these engines generate, along with turbo weep and the EGR, I would suspect carbon buildup as the most likely issue if the engine wasn't refurbished; followed by the wrong thickness of head gasket if it was. It took me quite a bit to figure out what is "right" for which engines, and a lot of the third party replacement gaskets come in various thicknesses (or don't state it) while still saying they are suitable part # matches for the earlier engines.
 
he was commenting that he was going through a bit of oil on a trip a few weeks ago. Not recalling if it was a quart every 1000km or miles. Regardless fairly significant.

He brought it to a shop that some others in the Seattle area had sworn by and after doing whatever it was that they did, said that it was likely a worn cylinder wall, as this was a common fail on the D4BF according to some web search they pulled up, and said it would need a rebuild.

they refused to test compression, they referenced the wrong engine, just in all a bad experience.

He has a leaky rear main seal but i would think with that much oil loss, his under carriage would be a mess.

So i finally convinced him to dig into this on his own to get some answers. So here we are with a 600+psi compression reading starting to trouble shoot.
 
Good thing he didn't listen to that shop.

The only way I can see that high compression would be related to oil consumption is excessive blow-by leading to carbon buildup I guess... but that kind of consumption isn't just blow-by. It can be surprising how much oil can sneak away with a leaking seal... how much it's leaking is often more evident on the ground where parked than on the undercarriage.

He should probably get someone to follow him and see if there is a bit of blue/grey oil smoke when he's driving (not the black fuel smoke from hard acceleration)... if there is then it's probably burning a good bit of oil, and I would proceed down the head/compression direction first.
If not, and it seems to run perfectly fine, then I would be looking at doing the seals... if the high compression was going to be problematic it would probably have already shown and done whatever damage it's going to (and it would likely be pretty obvious).
 
So I was theorizing to him the other day that if the compression is truly as high as it is, then that additional pressure is trying to get out every place that it can assuming the exhaust valves can only move so much out.

Some of that additional pressure may be working its way past the rings into the oil pan and perhaps out the rear seal or any oil seal for that matter.

But theorizing aside. I think the course of action is to begin with:
1. a compression test using a different kit. 2. Driving and checking for blow by-as you mention SK,
3. pulling the injectors and looking at carbon build-up on the cylinders and valves and while he is in there ensuring the proper head is on the block.
4.adjusting the valves making sure they are allowing things to breathe. And then just taking it from there.

Will let you all know how this turns out! Thanks again for the ideas.
 
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Nenw said:
So I was theorizing to him the other day that if the compression is truly as high as it is, then that additional pressure is trying to get out every place that it can assuming the exhaust valves can only move so much out.

Some of that additional pressure may be working its way past the rings into the oil pan and perhaps out the rear seal or any oil seal for that matter.

But theorizing aside. I think the course of action is to begin with:
1. a compression test using a different kit. 2. Driving and checking for blow by-as you mention SK,
3. pulling the injectors and looking at carbon build-up on the cylinders and valves and while he is in there ensuring the proper head is on the block.
4.adjusting the valves making sure they are allowing things to breathe. And then just taking it from there.

Will let you all know how this turns out! Thanks again for the ideas.
Click to expand...
Pulling the injectors will not allow you to see the cylinders. There is a combustion chamber so you cannot see down to the pistons. Just saving you the hassle of finding out on your own.
 
Dang, I guess maybe run some heavy duty cleaners on the next fill up, retest and see if there is any change. Right On..I will work it out with him. This will be a fun one. Glad my stuff is a little more straight forward right now, buuut kind of wishing I was there to physically help him dig in.
 
Nenw said:
So I was theorizing to him the other day that if the compression is truly as high as it is, then that additional pressure is trying to get out every place that it can assuming the exhaust valves can only move so much out
Click to expand...
Yeah, I thought of that for a second... but that extra pressure is nothing compared to what happens when the fuel combusts in the chamber. The extra pressure isn't from more volume; it's due to the same volume being compressed into less space. If the exhaust valve didn't evacuate the extra pressure it would just cause the cylinder to pull in less air on the next intake stroke, because the cylinder is already partially full/pressurized when the intake valve opens; that's why I said it's not "breathing related", it really can't be.
That's essentially what happens during a compression test, the first exhaust "puff" doesn't release much air because it's not at as high of a pressure, second intake stroke pulls in almost as much as the first and the second compression stroke generates a little higher pressure; the second exhaust "puff" lets a little more out and it pulls a little less in on the next intake stroke; until it reaches an equilibrium point w/in about 10 revolutions and reaches the max compression reading. If there was a related restricted intake/exhaust issue it would just take more/less revolutions to max out.

I could see it causing head gasket failure into an oil passageway; but then that cylinder wouldn't have a high reading. And I don't really see it contributing to blow-by notably as that is air past the piston rings during combustion; and the rings seem to be sealing quite well.
Basically, I don't really see the oil loss being related; unless it's the cause.
 
Ok so my boy got his hands on a new compression tool and it tested much more normal. 430 - 440 across the cylinders.

He can be slow to come around to things sometimes. He felt like it was a better use of his time and resources to try to verify that the tool was working properly rather than just return it to uncle Bezos and family and get a new one.

Next steps is to have him clean off the engine and track down any oil leaks and fix them before digging into the rear main seal. Also he needs to check for blow-by, but I would think with the numbers like that he won’t have any.

Thanks in advance. Will continue to update as we learn more.
 
With numbers like that there won't be any blow by he can do anything about... maybe add a catch can just-because (even new these motors are heavy breathers).
 
Cool, I just let him know. So this thing that that the shop said to him about the cylinder wall potentially being worn sounds like a bunch of crap to me. Wouldn't that yield a result similar to a worn piston ring when testing compression?
 
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