Parts Catalog Accessories Catalog How To Articles Tech Forums
Call Pelican Parts at 888-280-7799
Shopping Cart Cart | Project List | Order Status | Help



Go Back   PeachParts Mercedes-Benz Forum > Mercedes-Benz Tech Information and Support > Vintage Mercedes Forum

Reply
 
LinkBack Thread Tools Display Modes
  #1  
Old 01-14-2007, 03:55 PM
Registered User
 
Join Date: Oct 2006
Location: Indianola WA
Posts: 65
Compression testing

Hi All'

I'm rusty on compression checks. Can someone refresh my memory on two points: should the engine be all warmed up? ( it seems like it should) and; I can't remember how to do a leak down test but I seem to recall this as a good time for it. (or maybe I'm thinking of something I do to check the integrity of hydraulic seals....)
Thanks, I'm going to play frisbee with our dog for a few minutes and come back and check shortly because it seems like I usually get a rapid response on this forum!

Basil

Reply With Quote
  #2  
Old 01-14-2007, 05:15 PM
t walgamuth's Avatar
dieselarchitect
 
Join Date: Apr 2005
Location: Lafayette Indiana
Posts: 38,613
my old chiltons lists cold tests.

many of the dieselheads here prefer the hot test which gives a much higher number.

take your pick.

tom w
__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.
Reply With Quote
  #3  
Old 01-14-2007, 07:19 PM
Registered User
 
Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,804
Generally speaking, a leakdown test is performed after one does a compression test and finds low compression in any/all cylinders. The leak testdown test is then used to pinpoint the cause of the low compression, be it valves,rings, or gasket.
If one has good compression, a leak test is not needed..

Rather than hot or cold , the main concern on a compression test is to have all the plugs out when cranking with a fully charged battery to assure rapid starter rpm and also have intake air available by opening the throttle air source. It is also a good idea to disconnect the fuel power/line so no fuel is fed to the engine when the test is performed [ this usually means simply remove the pump fuse , if electric pump]
It is also wise to number the plugs as they are removed so you can get a read from them , knowing which cylinder they came from. [ ie .. an oily plug from a low compression cylinder could indicate a bad ring in that cylider]

Last edited by Arthur Dalton; 01-14-2007 at 07:27 PM.
Reply With Quote
  #4  
Old 01-15-2007, 03:46 AM
Registered User
 
Join Date: Oct 2006
Location: Indianola WA
Posts: 65
one cylinder has no compression

Yes, one cylinder has no compression. As I feared. Here are the circumstances:
'68 SE (gas w/ mech FI) purchased last October.
Burned a quart of oil every several tanks of gas; no drips under parked car; slightly smoky exhaust with blue puff every now and then.
Plugs 5 and 6 particularly cruddy, especially 6.

Ran great two weeks ago. We drove 1200 miles South to southern California, via Interstate 5, for the holidays. Maximum speed was 95 MPH on flat boring stretches mostly 82-87 MPH through mountains and cities. (Pleasant Driving)
By the time of our arrival, oil consumption increased to about 1 quart per tank of gas and has remained at that level.

A side trip to the mojave desert (about halfway to Las Vegas from LA) was taken and there were some stretches where I drove about 100 MPH for several minutes, never over 103 usually 92 or so, although if I had to pick a favorite all round speed, 87 MPH seems to be where I find the smoothness to overall noise the most pleasant.

A short foray on sandy, bumpy dirt roads and the engine sounded like it needed a tune up. Sounded fine at high speeds so we drove it back to where we were staying.

The next day we left for home. There was a strong gasoline smell and I scraped the deposits off of number 6 spark plug, which was completely fouled. and it seemed to run better and was fine at higher speeds. the gasoline smell dropped way off. By the time we hit northern California I noticed the temperature climbing and replaced about 3 quarts of coolant.

Well up into the Siskyous (mountains) we stopped in Weed for the great Barbeque they have there and put in gas and oil and more coolant (even though it was snowing it got too warm as coolant was disappearing). Replaced #5 plug with a new one. The electrode arm was gone! We spent the night in southern Oregon. I didn't sleep well.

In the morning I decided to limp it home. We put in all fluids plus some radiator stop leak pellets (which stopped the coolant from disappearing). No water in oil or bubbles in radiator. #5 plug seemed to be firing and getting gas but not contributing (pulling off plug wire at idle produced no discernable change in rpm).

Horrendous rain most of the way home. at higher speeds you'd hardly think anything was wrong. Dramatic scenarios of destroyed heads and valve trains haunted my imagination.

Replaced the plug wire with a good one when we got home. No change.

Took a compression reading today. All cylinders had a normal build up pattern and close readings ( 166- 172 with a worn out O-ring on the compression guage so they might be closer and higher) except for #5 which had about 5 lbs. My biggest fear was adjacent cylinders might be firing into each other. Glad that's not the case! All plugs looked normal. I gave it a pretty thorough tune up with lots of new ignition parts about 6-8 weeks ago (see thread on ignition/injection mystery).

I don't claim to know much, but my guess is I need a new head gasket and some valve seals. It's difficult to imagine where all the oil is going. I don't think she's seen those higher RPMs for many years and some of the old gaskets and seals are not up to it.

Any advice on who to buy a head gasket from? This might be a good chance for you Mercedes Detectives to guess what I'll find when I pull the head and certainly I'd appreciate any advice concerning the operation as I am not too experienced. I've read enough to feel like I've done more than I have!

I don't feel like this is a serious problem. I hope that's true!

Thanks and of course I'll let you know how it goes. I'll be looking for a headgasket tomorrow.

Basil
Reply With Quote
  #5  
Old 01-15-2007, 03:48 AM
Registered User
 
Join Date: Oct 2006
Location: Indianola WA
Posts: 65
Darn! Forgot to add the blowby seems low so I think the rings are OK.
-Basil
Reply With Quote
  #6  
Old 01-15-2007, 03:55 AM
Registered User
 
Join Date: Oct 2006
Location: Indianola WA
Posts: 65
Agh! I also forgot to add I hear what sounds like a head gasket leak on the injector side of the head, although I couldn't feel it. It's under the exhaust manifold which heats up pretty quick, limiting my exploration! -Basil
Reply With Quote
  #7  
Old 01-15-2007, 06:01 AM
Registered User
 
Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,804
Well, getting back to diagnostics.........now that you know you have a dead cylinder due to low compression, first check is to pull the valve cover and see if you have too tight of a valve adjustment on that cylinder. Then you do the leakdown test to see why that cylinder has bad compression. That is what the leak test is for.
Reply With Quote
  #8  
Old 01-15-2007, 11:12 AM
DIY or Die
 
Join Date: Apr 2006
Location: Los Angeles, California
Posts: 298
I have to jump in and say that you're a brave, brave man to take a 39yo car the places you do! Dirt roads in the Mojave???
__________________
1973 280 - Current Project Car
1979 240d - 100% Stock
1982 380sl - 100% Stock
1985 190e 2.3 - Heavily Modified
Reply With Quote
  #9  
Old 01-17-2007, 08:24 AM
Registered User
 
Join Date: Nov 2004
Location: Nova Scotia, Canada.
Posts: 6,510
If you did not adjust the valves every ten thousand or at least check the clearances a lot of what you describe is normal. Can result in a burnt exhaust valve or two. Your guides and valve seals may be a little tired as well. Inject some air into that bad cylinder. I suspect you will hear it out the tailpipe. It may be too late for a valve adjustment. I hope not but think otherwise. Remember to take a feeler gauge and check those valve clearances every ten thousand miles after you repair the head. Or you will be doing it again. The early adjusters did not seem to have a very strong interference fit until sometime in the late 60s. Therfore the valve clearances tighten up resulting in burnt valves. When putting everything back together keep an eye open for a substandard interference adjuster as well. As you are adjusting the clearance with a crowsfoot wrench you will feel a poor one if there. Remember the essentials like marking each rocker arm. They must go back to the same exact place. The upside is that after adjusting the valves a couple of times it only takes five minutes to check them the next time unless adjustment is needed. Even then it is quite fast. The bottom line is never neglect to check valve clearances on these engines. You will pay for it if you do in my opinion. Been there done than. Never again. Try to pick a good shop to do the head. They are not rocket science to do but the lesser valve stem clearances to guides on these old six cylinder mercedes are important and some shops just generalise. When you have the head off you might be suprised by the total lack of a top cylinder ridge as well. It has been awhile but these engines were pretty easy to do a head removal and install.
Reply With Quote
  #10  
Old 01-22-2007, 10:16 PM
Registered User
 
Join Date: Oct 2006
Location: Indianola WA
Posts: 65
Wow,

That's a bunch of good information!

I went out and adjusted as many valve clearances as my wrench would fit and checked the rest. Tomorrow I have to carefully grind my crowsfoot so I can fit it on the ones the wrench wouldn't get (too tall) Cyl #5 , the one without compression had the intake valve clearance too tight but the exhaust was OK. I think it could still be the headgasket but a burnt valve seems likely. I would be happy if it was just an overtight intake valve, but I don't think the odds are in my favor...
  • I'll try the compressed air. I still don't know what a leakdown test is.
  • It looks like I'll be taking the head off, barring a miracle.
  • I could use some advice picking out a valve spring compressor. I imagine I should make some marks and leave the crankshaft in a good position to make the timing chain end up where it's supposed to be and a little easier. I could use some advice on that topic also.
  • Anyone know a good place to buy a headgasket and some valve seals?
  • What haven't I thought of?
Thanks,

Basil
  • Most of those desert roads are fine if it's dry and you drive slow, stay out of holes and deep sand, drive around rocks and are prepared to drive in reverse to the last safe turn around (that means you know when go no further which I guess separates the brave from the stuck! )We looked at satellite photos on google earth beforehand and knew where we wanted to go and which roads looked best. I would have enjoyed driving like there's no tomorrow but of course there is. I think for us that's today....
Reply With Quote
  #11  
Old 01-22-2007, 10:49 PM
t walgamuth's Avatar
dieselarchitect
 
Join Date: Apr 2005
Location: Lafayette Indiana
Posts: 38,613
a burnt exhaust will cause it to run hot.

and if you have one burnt a little running at those speeds will "speed" along the burning process.

reminds me of my 62 190c. burnt the exhaust valves like clockwork...cant remember how many miles it took....30K maybe? not sure.

tom w
__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.
Reply With Quote
  #12  
Old 01-23-2007, 07:37 AM
Registered User
 
Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,804
<>

Think of it this very basic way:

On a compression test , you are measuring the pressure of the air that the cranking piston compresses in the jug.
On a leak test, you are putting compressed air into the jug and measuring the amount of air that LEAKS out. [a little always leaks out] . When it leaks out , it has to go out the rings..or the valves ..or the head gasket ..or even cracks in the head,etc.
So , on a leak test, you put about 100 PSI into the jug and then you listen/watch for it to leak out ...if you hear it leak out the tail-pipe, you can assume you have a tight/burnt/bent EXHAUST valve..
Same for an intake valve ..you listen for leaking air at the intake manifold.
And if you take the oil filler cap off or pull out the oil dip-stick and hear leakage in the crankcase, you can assume the rings are leaking by...
And if you see bubbles in ther radiator , you can assume a head gasket or crack into the water jacket. etc.
Do a google search and you will find more info ..this is just the basics I list.
As I mentioned before , if your compression is good , a leakdown test is not warranted..but if you have a jug that came up low psi on a compression test, you can see where a leak test will aid you in determining WHERE/WHAT is leaking before pulling the engine apart..
A leak test can almost be compared to blowing up an inner tube and putting it in a water tank to find where it leaks...
..just a diagnostic tool..
You can buy a leakdown tester that has a gauge on it that will actually measure the psi and rate of the leak, but for all practical purposes, just sticking some compressed air from an air compressor hose into the spark plug hole with a threaded adapter will find your leak cause/location.

Last edited by Arthur Dalton; 01-23-2007 at 07:43 AM.
Reply With Quote
  #13  
Old 01-27-2007, 06:19 PM
Registered User
 
Join Date: Nov 2004
Location: Nova Scotia, Canada.
Posts: 6,510
Basil, at least it was a tight intake valve. If that by some chance restores the compression you are fortunate. It was always an exhaust valve for me. When you change the valve seals they sit on a little fairly thin shoulder. The remaining thickness of that shoulder gives you some ideal of how much the guides are worn. It is not scientific just an indication of how they are. I have even seen the shoulder worn right through.
Things I would watch out for. As you remove each rocker inspect the cam lobe contact area on the rocker. These are surface hardend. If you see it has wore through the hardness replace it. Also inspect all the cam lobes. Especially number one lobe. As for pulling the head if required just follow any manuals instructions. You will need the torque values anyways.
It is possible that the old vave seals called it a day on your trip and increased the oil consumption. They might have been in a deteriorated state before the trip. I remember on some of these engines that I suspected a piston had a hole in it the smoking was so bad. It was just guides and seals when I got into it. Let us know what you find.
Reply With Quote
  #14  
Old 01-29-2007, 02:06 PM
Registered User
 
Join Date: Oct 2006
Location: Indianola WA
Posts: 65
Tomorrow, I take off the head.

My gaskets, seals, and exhaust valves are supposed to arrive tomorrow afternoon. I have a friend with a machine shop who said he will pull his valve grinding equipment out of storage and let me use it if my seat isn't in need of replacement, in which case I'll be sending it out....

Thanks for the advice so far. If anyone knows the size of the head bolt hex (which I forgot to look at) it would save me some time. Also it seems there must be an optimal position to leave the crankshaft in for ease in reconnecting the timing chain to the cam sprocket; if anyone can tell me what it is, it will save my brain for whatever else comes up. In fact, I'm open to any and all hints and helps.

I'll keep on posting,

Basil
Reply With Quote
  #15  
Old 01-29-2007, 02:08 PM
Registered User
 
Join Date: Oct 2006
Location: Indianola WA
Posts: 65
I ordered one exhaust valve. not valves -Basil

Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are On




All times are GMT -4. The time now is 07:24 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2024 Pelican Parts, LLC - Posts may be archived for display on the Peach Parts or Pelican Parts Website -    DMCA Registered Agent Contact Page