OM647 Compression test quick question

In an effort to rid myself of some glow plug MIL codes, I'm getting ready to change out some glow plugs. While I have them out, I was planning to do a manual compression test since I recently acquired my sprinter with 223K miles and no record or maintenance or anything for engine health other than it runs. I don't have a DRBIII or other means of doing it electronically, so that's why I'm going to do it manually while changing the glow plugs.

I have the service manual and the process seems very simple (paraphrasing: get engine to temperature, pull plugs, install tool, crank engine with starter, and record results for each cylinder). My question is in my experience with doing compression tests on gas engines, disabling the fuel system or injectors is always an important part of the compression test. The sprinter manual makes no mention of disabling the injectors or fuel system while performing the compression test. Is there a reason this doesn't have to be done on diesels or sprinters specifically? Should I do it anyway to be safe? If so, I'm guessing that disconnecting the camshaft position sensor would be the easiest means of disabling the injectors?
 

autostaretx

Erratic Member
Remember that a Sprinter has a nominal compression ratio of 18 to one.
That's almost 300 psi, and a good engine can be a lot higher.

Choose your gauge, hoses and screw-in head attachment to handle that pressure.
-------

The fuel injected per revolution at 993 rpm in my OM647 is 0.000732 cubic inches per squirt in neutral.
Yes, 7 ten-thousandths of a cu inch.
12 cubic millimeters (1 mm-3 = one one-thousandth of a cc, so 12 thousandths of a cc (or milliliter))

I would think that disabling the camshaft sensor would kill the squirts, but i haven't tried to find out.
(since the "timing" of the engine is *when* the squirts happen, it darn well should kill them)

--dick
 
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lindenengineering

Well-known member
Better to disconnect the crank sensor.
Read C/R for a turbo engine is 400/425-psi
Minimum C/R is 325.
Allowed 10% variation between cylinders and 25% across the whole 5 pots.

Any C/R at/or below 300 and you will get bad starting especially on transition between glow plug operation and a partially warm engine. Gobs of smoke emit once it catches!
Hope this is of use.
Dennis
 

MillionMileSprinter

Formerly Type2Teach
Dennis suggests disconnecting the crank sensor, but I think he may mean the camshaft sensor. Both will keep the engine from starting, but the crankshaft sensor is located under the van, while the camshaft sensor is located right there in the head, to the left (passenger side in the US) of injector #5. I've always had the injector cover off when I did glow plugs, so it's very easy to get to .
 
Thanks everyone for the prompt responses and guidance. Sounds like I'll pull the cam sensor due to its easier access. Hopefully the glow plugs come out without being too stubborn.
 

lindenengineering

Well-known member
Thanks everyone for the prompt responses and guidance. Sounds like I'll pull the cam sensor due to its easier access. Hopefully the glow plugs come out without being too stubborn.


Oh you wish!:bounce:

In similarity not not quite the same if you get my drift!
The Ford 5.4 Triton engine has some funky style spark plugs that break off in the head!
Renown for it and the subject of a class action law suit !----Sound familiar.
The technique to getting those nasty buggers out is some engine heat, kroil, a deep special guide socket AND a 3/8th/1/2" airgun no less!
The other day I had a similar bash experiment with a 1/4 drive air gun and some glows in the 647. Spun out with no breakage!
Maybe the windy gun and scaring them out its the real answer!--If you can stomach it!
Best of luck
Dennis
ps
For those interested in Ford--A Triton spark plug:-
http://www.summitracing.com/parts/mof-sp515?seid=srese1&gclid=CI7W8erkn8sCFVc1aQodRWAAfw
 
This project quickly became more complex and postponed. #1 plug came out so easy, it was like it wasn't even torqued. #2 plug did about 1 full turn then started to tighten up like it was galling, so I stopped. #3 plug was the best: Upon pulling the connector off, a whole rusted piece of electrode or whatever the internal part of the plug is came out with it. This was clearly the plug causing my engine light to be on, but what was worse: no hex present to even put a wrench on. Evidently the previous owner tried to remove this plug and broke it off. I just stopped there and put #1 back in, tightened #2 back up. Will revisit this when the engine is out in another month.

Can anyone advise what special tools I may need for extracting broken glow plugs? I plan to do it after the engine is out and cylinder head removed. I have access to a machine shop with drills and mills.
 

lindenengineering

Well-known member
With the head off, seek a machine shop with spark erosion machine to erode them out!
That's what I/We use on these 5 banger engines.

On the Vee 6 we use a different technique with aircraft precison drills, short jobber lengths won't do!------She said!~:laughing:
Dennis
 
Piggy backing off my own thread since the subject line still applies. Anyone have recommendations on good adapters to screw in to the glow plug ports for doing a compression test? The harbor freight set sits so low that you can't get the guage quick connect on. I bought another "sprinter specific" adapter that was a nice hose extension setup, but it had the wrong threads. I can't seem to find the manual's recommended special tool #8927 for sale anywhere, so I'm hoping others might know of alternatives. My guage has the 1/8" quick connect like the harbor freight kit does, but it's a better quality one that shouldn't break after 1 use.
 

RileyC

2002 140 shc, 2005 158 sh
I have a Mityvac analog diesel compression tester that I ended up switching the end of the quick connect from the Harbour Freight diesel compression test kit onto. That way you can use the fitting from that kit that works with our vans. I had to use some long curved needle nose pliers to attach and release it because of the tight clearance in the glow plug cavities. It seemed to work well.
 

kite

Member
I'm going to do a compression test today. I built up a ridiculous stackup of air fitting adapters to connect a mityvac glowplug adapter to a harbor freight gauge. An added benefit is it lifts the quick connect above the fuel lines, so will hopefully be easier to connect and disconnect the gauge. Hopefully no leaks...

Quick question - to disable the fuel system, is pulling the fuel pump fuse OK? I did it, the engine cranked but didn't start. This seems like the intended outcome, but I want to make sure I'm not missing something.
 

lindenengineering

Well-known member
Yes you can pull the fuse!
Disconnect the cam or crank sensor.
What is at gauge set to????
Remember you will need at least 400 lbs for a diesel, else the needle will break off the tube.
Dennis
 

kite

Member
Thanks Dennis,

Yea, the gauge is a diesel compression tester that goes to 1000psi.

Since I wasn't able to get to this today, I'll ask a couple more questions.
1) to clarify from your last post. Should I remove the fuel pump fuse OR cam/crank sensor? Or do I need to remove a sensor AND fuse?
2) the harbor freight tester I got has a 10mm glow plug fitting, but I can't use this b/c I can't get the quick connect all the way down. The mityvac adapter I got is 10x1.0, and fits just fine. But it is different in that it has an O-ring near the end of the adapter. do I want this O-ring? I'm kind of worried it could slip off inside a cylinder.

The actual GPs just seal against the head, correct?

pics of each attached. the o-ring is in the red circle on the mityvac. you'll see the adapter stackup as well....
 

Attachments

Aqua Puttana

Poly - Thread Finder

kite

Member
Ah, thx for the pic. I thought that was the seal but good to know for sure. Clearly my adapter is way shorter than a GP. BUT it does have the o-ring. I guess I could just try it and see if it seals.

My concern with it slipping off it is isn't in a groove, it just sits on a smaller diameter nipple at the end of the adapter. If the o-ring is tight enough to seal in the bore, it might roll/slip off during removal...
 

Nautamaran

2004 140” SHC T1N 2500
The adapter needs to match the GP length so it can reach the conical seal at the end of the major bore, otherwise your compressed volume includes the empty GP bore hole and your readings will be artificially low. If your O-ring is sized to the major diameter of the plug stem I wouldn’t worry about losing it down into the cylinder, but if it is designed to seal the clearance around the glowing tip beyond the seal surface I agree - it could get stuck in there, and cranking seems as likely to suck it in as blow it clear?

-dave
 

autostaretx

Erratic Member
You can simply unplug the connector at the cam or crank sensor.

If the ECU cannot see either of those sensors, it will not attempt to squirt fuel into the cylinders (since it cannot determine which stroke is happening where).

--dick
 

kite

Member
The adapter needs to match the GP length so it can reach the conical seal at the end of the major bore, otherwise your compressed volume includes the empty GP bore hole and your readings will be artificially low.
Won't the volume of the gauge also affect the reading? and won't gauges vary?
A quick estimate of the amount of extra volume makes me think the GP bore volume will be in the noise of this measurement. Also, it will be consistent, so at least I'll find any outliers.
Estimate GP major diameter is 6mm
length missing = 40mm
volume = 4.5cm^3
vs 2700/5 = 540cm^3
so it's less than 1% of the volume of the cylinder.

If your O-ring is sized to the major diameter of the plug stem I wouldn’t worry about losing it down into the cylinder, but if it is designed to seal the clearance around the glowing tip beyond the seal surface I agree - it could get stuck in there, and cranking seems as likely to suck it in as blow it clear?
Good point. The O-ring is indeed sized to match the major diameter of the plug stem. So even if it comes off it'll be stuck above the seal surface. I guess then I'd have to do some fishing to get it out, b/c like you said, cranking to clear it sounds risky.
 
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Aqua Puttana

Poly - Thread Finder
...
so it's less than 1% of the volume of the cylinder.

...
Isn't the compressed volume what you should be using for calculation?

Won't the straight, no taper non-sealing threads leak if the seal face doesn't contact?

I do agree that it will be an ok test for comparisons between all cylinders. The absolute data will be suspect.

vic
 
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I ended up using a similar adapter that did NOT seal on the conical face down in the bore like the glow plug normally does. It sealed on the O-ring around the fitting like yours. I too believed this would give me slightly lower readings, but did the test anyway. I ended up with 375 to 400 psi between my 5 cylinders, which is acceptable per the manual. So even if the setup increases volume slightly, it wasn't enough to give the cylinders an unacceptable compression reading. I'd move forward with what you have.

One tip from my troubles even though I realize its compression testing 101. The O-ring is too large to find its way all the way down in to the cylinder bore, BUT make sure you depressurize your gauge before moving to the next cylinder. I was trying to move too quickly while the engine was still warm and pulled the quick connect fitting before depressurizing the gage and it blew the internal O-ring right out of the quick connect fitting. I found it sitting on the head right next to the GP Bore.
 

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