Compression results: 320 - 220 psi, is it terminal?
 

Hi all, can you give me an assessment of these compression numbers?

1 - 300
2 - 290
3 - 310
4 - 320
5 - 220 :eek:

Check done on a 1983 300SD with 350k, cold (70F) engine, US General tester via injector. Valve clearances all checked & adjusted before. Fully charged battery with assist from charger.

I ran the check twice after I saw #5. First time I went 1->5, then charged the battery for another 3 hrs, and ran it again 5->1.

I added oil to #5, but couldn't get it to read above 220 no matter what. Pulled the valve cover and re-checked clearances, no dice. Couldn't spot any leaks, either.

Car runs well, but mileage has dropped since the last valve adjustment (~10-15k ago). Smoke has gotten worse.

All injectors tested good (Metro Diesel in Santa Fe using Diesel Kiki tester), so I think the smoke is coming from #5's low compression.

So here's the question.. is it likely repairable, and if not, think it would run another 50k miles?

#5 is a worry.
Is there much blow by?
It could be stuck rings.
Has the car been run on WVO?
What was the reading on #5 before the oil was added?
Is the car idling roughly?
The test should have been done on a warm motor.
Does it use much oil?

The change in mileage since adjusting the valves points to a mistake in the #5 valves. I would recheck that first.

X2

Unfortunately, if the exhaust valve was set to intake specs, it's likely to have hung open at certain operating points and has burned.

Oil consumption is low.

Adding oil to #5 made almost no difference (200 psi vs 220 psi).

Car has never seen WVO.

Blowby could be better, but I've seen much worse.

I should restate myself - The mileage decrease wasn't immediately after the valve adjustment (May - 10k ago). It ran fine then started smoking more in the last 2-3 months. I'm pretty sure I adjusted the valves right.. I've done it about a dozen times over the last 10 yrs. One thing I noticed is that #5 always needed adjustment.
I was thinking burned valve seat, since #5 had about .2mm clearance on the exhaust valve when I adjusted it before the compression check.

Yep.

I'm thinking about buying a rebuilt head.
What are the chances of me pulling the head and finding the bottom end is toast or something else terminal? Does the information so far point pretty solidly at the valve seat?

Hi
Seems to me a leak down test might be in order.
Joseph

Why?

Once you have excluded the obvious three like leaking injector seal, leaking glowplug seal, and too tight tappets....

NO MATTER WHAT THE PROBLEM IS, the head has to come off to do any kind of remedial work.

Once the head is off, even if you discover the problem is stuck / cracked rings and the head is perfect, you have lost PRECISELY NOTHING. In either time or money.

What is it with this forum that people keep suggesting successive tests, all looking for a specific result that gives a cheap / fast / easy cure?

The guy already did a cold dry and cold wet (add oil) test, it made bugger all difference so as much as it is possible to know, he doesn't have a ring problem.

Take the bloody head off and let the dog see the rabbit.

Have you ever pulled a head on a mercedes diesel motor?

Hi
Well, to each his own I guess. A leak down test is easy, fast & cheap. A broken piston/ring or even a scored cylinder wall might well mean the engine isn't worth repairing. On the other hand, leaking valves or failed head gasket might mean the expense wont be that great.

Being on the lazy side myself, I wouldn't pull a diesel head just for the fun of a look see.

Cheers,
Joseph

x2 leak down test first

And that, folks, is everything that is wrong with this world.

The addition of oil down nos. 5 pot did not much increase the reading.

This points to Valve/s leaking....

Pull the head, re-guide and re-valve, re-cut seats and lap in new valves, fit new valve seals.
Check closely valve-springs and compare to FSM spec, or replace.

Replace the Lock-Nuts on valve-stems, as the high milage will probably mean that these have worn down below min thickness.....

Nice easy weekend job....:)

Lacking a real leakdown tester, I rigged up a 20 psi regulated source to the compression test fitting (See picture, sorry about the quality)

http://lh5.ggpht.com/_o_3o6xcLap8/TM...0/100_3273.JPG

It's pressurizing the crankcase.. If I hold my hand over the crank breather, I can feel the pressure. If I hold it over the compressor inlet, nothing. Sounds like it's blowing into the crank, can't hear anything in the exhaust.

Hole in the piston?

hmm if i were you i would try soaking number 5 with marvel mystery oil for a few days. Over that time continue to fill the cylinder up as it leaks down, keep it full.... Some people have had good luck with this method and some have not. But its worth a shot imo.

You could do that on any engine ever built and pressurise the crankcase.... piston rings make a lousy seal stationary, even on a brand new engine.

You do realise a head gasket blown across to an oil gallery will also pressurise a crank case....

This is why I am anti all these 99 different bull**** tests in a row.

remove the exhaust and intake manifolds, make up blanking plates, then pressurise the tract and that will tell you something, but who cares, you can't fix anything without taking the head off, so take the head off and let the dog see that rabbit.

Then, at least, you have a HOPE IN HELL of doing something none of these endless tests do, and detecting that you actually have two separate problems, fixing them both, and spending the next ten years driving your car without ever stripping the engine again.

Or you can just do a load more tests.... without really understanding what you are doing, and just pick the cheapest problem and go with that.... you never know, you may be lucky.

Hey, that sounds like a good idea, I was thinking of balancing an egg on the camshaft to see if the lobes were worn. :D

Ok, in seriousness, I think I'm done with the improvised diagnostic testing.

The last thing I did before I cleaned up for the week was fill up #5 with some 15W-40 and see how long it takes to drain. It doesn't. I interpret that as an intact piston.

Injectors are going back in for now, and I'm going to keep driving it till I can order parts for a head job and plan out a weekend to pull the head.

Thanks everyone for the help, I'll post pictures of the insides when I get the head off.


..one other thing.. what the heck does "let the dog see the rabbit" mean? I mean.. I had a Labrador, I trained him for birds, and every time he saw a rabbit he'd watch it run away before he got the notion in his head to chase it. Then he'd look at me askance as if to say "I don't think that was a bird, but whatever it was, weren't you supposed to shoot it so I could bring it back?"

Whats the big deal here? You have some kind of agression problem with pretty standard diagnostics. And for the record, the strategy of diagnostics presented by other posters is pretty darn logical, so far consisting of three tests in a page and a half of thread. Compression test, check. Verify valve clearances, check. leakdown test, check. Im looking around for the other 96 tests, but havent found them. :rolleyes:

A leak down test isnt exactly a difficult thing to do, way easier than just pulling the head and rebuilding it. It also has the utility of telling you what might be wrong in the first place, there is plenty of value to knowing that before embarking on a major and costly rebuild of the entire top end just cause "you have to do it anyway to fix anything".

people drive and maintain cars on a budget. Inexpensive easy testing procedures are extremely informative, and most importantly, don't break the bank.

If you go rabbiting with dogs... etc...

Esp a lurcher, so named because while most dogs will follow the rabbit track when chasing it down, a lurcher will always aim straight at the rabbit, "lurching" the corners.

That's why everyone likes them, cos they are easy and cheap to do.... that's it... end of.

An internet forum or a magic 8 ball has the utility of telling you what MIGHT be wrong.

The whole point of a test is it identifies if you have a problem or not, if you do (and it isn't a leaky injector, heater, or tight valve) then it's head off, no other possible way, not in any universe, not on any planet, to fix it, even if the head is perfect and it is a piston / ring problem, it's still head off.

People DO NOT maintain and drive cars on a budget, people trade on the remaining initial quality of a vehicle so that they can continue to cut corners and do shoddy maintenance and get away with it as long as possible.

For expletive deleted's sake, there are whole threads on here, a MERCEDES BENZ forum, about buying crap oil filters and saving a whole 5 bucks.

What do you call a guy who uses generic rubbers?

A cheap ****er.

Too many cheap ****ers buy old MB diesels and proceed to drive perfectly good cars into an early grave.

This isn't aggression Jack, I just hate to see perfectly good equipment trashed and driven into an early grave by cheap ****ers... THAT is aggression.

And I ask again, Mr. know it all, have you pulled the head on a mb diesel?

Yes, but never the same engine twice.... do it right the first time.

So, stating the painfully bloody obvious, that you HAVE to pull a head to do any internal valve / piston work, is all it takes to earn the label Mr Know It All is it?

What does that make your average diesel fitter, Brain of America?

I, for one, am looking forward to seeing what you find when you remove the head. BTW, did you study the valve action on number 5 while you were adjusting the valves?

I'm glad my doctor doesn't take your approach.

Valve action? :confused:
I spend a little time seeing if parts (rockers, valves, etc) have any abnormal play, is there anything else I should do?

Did you ream the glow plug holes before the compression test? What's the chance that debris got in between the valve seats causing the low compression? Could that spec of carbon bend a valve or will it burn off eventually with no remaining symptoms? How likely is debris from reaming causing such issues?

Lol, well, you can't be a guy who works on cars for a living if this is your diagnostic procedure. Also, spare us the "brain of America" BS please.

What do your represent, the "brain of the UK"? :rolleyes: Im not sure basic testing and common sense deserves to become a nationalistic issue.

You are lashing out with a stream of invective and aggression at VERY basic testing procedures which is pretty comical. Ill try your approach on the next possible customer I see.

"I didn't bother to do further tests because its a waste of my time to know anything else about it, and you don't want to be a cheap ****er sir, its not worth testing further because whatever the problem is, it will need 1000+ dollars in work. Also, if you DON'T allow me to pull the head right now at vast expense and time, you ARE a cheap ****er, so get out of my sight and think it over. Remember, here at **** you auto care, the customer is our top priority" :D

I see what you did there.

You had to put a whole string of words that I never said in order to try to make yourself appear to be correct.

What I would actually have said is this;

"I've checked for blowby on the injectors and heaters, and checked valve clearance, and you're 100 psi down on #5."

"There are three possibilities at this point, a valve problem or a bore problem, or a bit of both, though the compression test suggests it's mainly a valve problem, either way, the head has to come off, both for an accurate diagnosis and for repair."

"There is no point me doing any further mickey mouse tests or following any advice you printed out from an internet forum, because time is money, for you and for me, and none of these alleged tests are going to tell us anything useful."

"At this moment in time I know for a fact you'll need to fork out for a top end gasket set. That's it".

"You may need new valve seats cut, or new guides, but these are all fairly cheap jobs individually, the money goes in the labour, so let me work smart and do the job right the first time with minimum hours labour, and that way you both get your car back fastest, and 100% fixed, and the lowest bill in the long run."

Which is what I would actually say.

Which is about a million miles away from what you claim I would say.

And if the punter turned around and said "This guy on the internet reckons that's the wrong way to do it and there are lots of better and cheaper ways of doing it." I'd answer them.

"No problem, go and give your car to the guy on the internet, and when it all goes wrong and you come back here your bill could well be five to ten times as high, because I not only have to fix the existing small problem, but also every problem you create in your tests and attempted cheap repairs, plus the wear and tear of continuing to run an engine with bugger all compression on #5..."

Car owner "So you don't think it is a good idea to listen to the internet guy then?"

"Well, it depends, my bank manager will think it is a great idea..."

There is the right way to do a job, and there is every other way, and the right way is ALWAYS the cheapest way in the long run, even if you value your time at zero.

Those of us who are competent do not value our time at zero.

Daddy?

Here's my take on the situation. I'm in the US but I am a UK citizen. It appears that such things are relevant to this problem somehow.
At 304k miles, the engine is reaching the limits of it's serviceable life. So in my mind, the question is whether it is worth doing anything more than just a head job on it. If you can confirm via a leakdown test that the valve is the culprit, you face a simpler decision. Is the car and engine worth the cost of a head job? The cost of a complete engine rebuild is far higher and involves much more work. If the valves are seating well and the leakdown test points to a ring problem, it might be better to simply continue driving the car while looking for a low mileage replacement engine which would be much cheaper than rebuilding the existing one.

+1. Cost of a head gasket and reworked head is about the same as a serviceable used engine. If you yank the head a find a problem in the bores, you're taking the bus while you find a replacement.

It starts, it pulls, good enough till you find a good one in a deerslayer.

bzzzzt

Devil you know with a fairly minor problem that can be easily repaired, vs Devil you don't know, and A VAST AMOUNT MORE LABOUR.

Seriously, an engine swap being billed as being easier and faster and cheaper than a little head work on a known good motor???

Can I have some of what you people are smoking.

Market's different in LA vs NY (born NY'er here, while we're on the subject of origins..). NY was full of people who change the oil and never wash the car. LA people wash the car weekly and never change the oil. Lots of good chassis out here with bad motors. My engine is actually a NY engine, swapped into a low-rust chassis. This mill has 351k on it right now (I should update my sig..), 200k by me.

In all this testing, what was at question was the bottom end, rings & pistons. Either I can determine with confidence they're bad, or I reach some set of possible faults.

1. Conclusion: bottom end failure. My action: drive it till it quits, enjoy the rest of my time with it.

2. Conclusion: tests inconclusive, possible bottom end, possible head. My action: drive it until I have time to pull the head, enjoy another 10 years with it.

I'm with #2. We'll see what the head removal brings.. give me about a month or so.. I'm not that great at getting time for stuff.

Worst thing you can possibly do to your engine is drive it like that.... at 100 psi low you're running at about 14.5:1 compression, enough for a tuned petrol engine, not enough for a diesel, so low in fact you are probably not even getting ignition, which means diesel fuel running into the bore, acting like a cutting compound on the piston / rings / bore, and eventually going into the sump causing sump dilution and possible eventual runaway diesel...

Despite what you state above, the tests you have done are anything but inconclusive and quite sufficient (assuming you checked injector / heater blowby and tight valves) to say with a great deal of confidence that what you have at the moment is LARGELY a single valve problem, almost certainly the exhaust valve.

I was thinking about the possibility that a worn cam lobe might not be making the #5 intake valve open fully.

At cranking speed almost any inlet valve lift is sufficient to full charge the cylinder. Plus of course inlet valves are always larger so the same amount of lift = greater opening WRT an exhaust valve.

With just short manifolds you can easily hear blowing valves on tickover.

Actually its way easier and cheaper, certainly for this vehicle. A complete engine swap can be accomplished in a few hours by one guy. Lets say half a day to a complete day at most to do the lions share of the labor if its done at a shop.

Additionally, it can be accomplished by a far less technically competent mechanic, someone who knows nothing about the internals, allowing the vehicles owner to find just the right place at the right price, or do it themselves in a driveway over a few weekends, and also requires no special tools of any kind, which is an investment needed to turn the engine into a short block. Engine for a few hundred bucks, labor at a few hundred bucks, you could have a good yard engine in the car in one day for a good price.

Of course, you still have to do some preliminary tests, like a compression test on the donor engine, but heavens! what a waste of time! :rolleyes:

It does not sound like the OP wants to do this, and will be working on this specific motor and weighing his options, but its definitely a viable option, and since people A- need their cars, and B- don't usually have money to burn, its a great option to discuss.

Engines can be had around here for 500 or less. I don't pretend to know what engine prices are in your area, so lets try and be a little more polite to other people Mr. "this guy on the internet", while forgetting you are also "this guy on the internet" to someone else. :D
You sound like you have valuable data to give to the OP, so less obnoxious grandstanding, and more civility please.

not a problem - I found a shop in local to me (Metric Diesel) that charges $550 to rebuilt a 617 head. If I can find a crane or a friend with big guns, I'll have it off this weekend.


Oh how I hope you're right, but I'll believe it when I see it.

Actually this is not necessarily true. If the lobe is worn in a way that changes the duration of it's open position the cylinder will not fill properly and hence will give a low compression. This would show in a leak down test as a cylinder with sound rings and sound valves.;)

Your advice is good to a point but not necessarily for a do it yourselfer.

And a repalcement cam is a lot less work than pulling a head and a good used cam can be had for probably less than a head gasket set.

In this scenario, low compression would only be possible if the valve restricts airflow to such a degree that there is a slight vacuum in the cylinder prior to the compression stroke.

I cannot fathom any scenario where this would occur at cranking speed unless the cam lobe is basically non-existent.

Well said.

X3

The diesel benz crowd seems to have more dedication to detail and general enthusiasm than your average group of motorheads. I appreciate everyone's comments and suggestions, there's lots of experiences out there to hear about.

To answer some questions raised:

Cam looked fine. All lobes smooth, bright, & shiny.

Glow plugs have been serviced several times in that motor's history, including #5.

Motor swap is out.. the local market for motors is poor, I'd be looking at least $2000 before I got a motor in significantly better shape than mine. I watch craigslist for other 300SD's, but if they're good, they're priced as gold, and if they're crap, they're still priced as if they're good. Besides, I think I'm more attached to the motor than the car.

I'm driving this car because I have a high-mileage commute through the pit of zombie road pirates known as the San Fernando Valley. I want something that doesn't depreciate with miles and is tough enough to take a hit from an Escalade with no loss of life. I will keep it alive because it keeps me alive, and because I've developed an irrational emotional attachment to which I'm sure most of you can relate.

How about we all take a deep breath & let this guy have some time to digest all the suggestions that have been made. Its his car & at the end of the day he will make his own mind up!! If he wishes to do more diagnostics he will. If he wishes to pull the head or motor, he will.
I am very interested to follow this problem to the end. No one on here is too old to learn something new!!
Any encouragement that I can give you is on offer mcneil !!
Good luck with your motor!!!

Yup.


Yup.

Okay, you guys let me know if you need anything else. :D

Only real test that would Confirm whats-what is a Cylinder-Leak Down/Cyl. Leak Test....

BUT!--

Most guys dont have the Equipment.....

I have the Equipment, The adaptors, the gauge, the pipe-work, all in a nice plastic tool-case....

BUT NO Compressor to run the darn thing!;)

--Guess I could rent one, or maybe lash summit up, but the basic tests done indicate Valve issues, and by the time I 'wasted' sorting a compressor, chances are I could get the head off and have it half repaired!

Guess what I'm saying is, each to his own, Some guys Love testing/meddling round and potching coming to conclusions--And Thats Fine!:rolleyes:

Others just like to get stuck in with minimum testing or messing round and find out when they are in there.....:)
-Guess I'm the latter....:)

As a former UK'er now here in the states, having worked on numerous different vehicles, and put together a Westfield (Lotus 7 Clone), the biggest thing one has to do is Make Do With What One Has.

And make the best of what one has. If money was falling out of my arse the engine would be rebuilt, I'd have liposuction, be polygamous and married to both Catwoman and Lucy Liu and live in a mansion. All my tools would be Genuine Mercedes, etc. etc.

But many of us do NOT have that luxury. If you depend on the car as your only means of transport, pulling the head (what with the fun of not buggering up the IP timing / dropping the chain - nevermind the sheer weight of the damn thing) is not going to happen casually.

Oh yeah, what if there's no garage and we're doing this in the street?

I'm not sitting here snickering on how I'm deliciously butchering 'proper procedure'. Environment and Situation dictate the envelope; it's how you apply intelligence to approach challenging situations.

My 82 300SD just started running rough at 370,231 miles. I cannot afford an engine rebuild. I just sold my DJ Decks to afford new parts - being a DJ for a living means this has made my life substantially harder. But, I believe in the car and I'm willing to do what I can with what I've got to give it a try.

In a perfect world I'd yank the engine, resleeve the bores, surface/RR the head, etc. I do not have the funds. I do not have another car for the winter. I'd far rather apply myself to solving the problem, even if it's a patch, until the funds arrive.

In many cases, across the board one DOES have to make an educated guess. Gathering information helps eliminate firing shots into the dark. Having worked on everything under the sun, Mercedes engineering responds best when you've sat down and thought about things, first.

It also might save you from jumping the gun and tearing into a healthy engine, only to discover it's a stupid little problem masquerading as a big one.

Well said.;)

Head's off..

I'm happy. Pistons look pretty good, cylinders show no signs of wear. As in, I can still see honing marks in the cylinders.

Intake looked pretty clogged.. lots of crud, particularly at the end of the manifold around #5.

Pictures in the morning, then the head goes to the shop Monday.