I ran a compression test this evening on my 78 240d. I did this because I mysteriously leaked a bunch of oil last week and I thought my engine was producing a lot of blowby. I decided to invest in a compression tester and find out how healthy the compression is before I put more time into the car.

Anyhow, this is what I came up with (first number is how high compression jumped after first crank, second number is where it topped out after 10 cranks)

1. 175/305
2. 200/375
3. 200/380
4. 200/375

My understanding is that these numbers are not horrible, however I am concerned that cylinder 1 is markedly lower than the rest.

Can anyone offer feedback on what I should think about these results? Any advice on what I should do about cylinder one being lower than the rest.

Also, I should mention that when pulling the injectors cylinder one was very dry while cylinder 4's pre-chamber seemed wet and oily. I would have expected the opposite considering the test results.

Thanks.

The compression numbers are far from horrible. First have a look around the back of the head for an oil leak caused by a failing head gasket. It is worth having a look.

When you get the oil leak located and eliminated. Remember to check the air filter as well to make sure the oil is not being blown into the intake with excess blowby as well. The filter would be oil soaked. Also make sure the crankcase vent system is clear. Try the loose oil cap test to estimate the blowby. If it goes into orbit there is too much blowby.

In my opinion with the number one cylinder results you want to make sure there is proper fuel supply pressure in the base of the injection pump. At this point it is just an opinion by the way. I have harboured a suspicion that low base injection pump pressure over extended time makes the number one cylinder carry an unusual percentage of the load.

The car will run better and stronger including a smoother idle with the proper pressure anyways and fuel milage may be better . Cheap area to learn in and possibly improve. Once again it has been a suspicion of mine that this may be the cause of the number one rod failure on the 616 engines. Just no real proof.

Now if blowby is excessive and the engine has seen wvo then a treatment of the rings is over due. Also before doing a compresion check the valve clearances should be checked for any that the clearances have closed up on. If no gap is there the valve will not be totally seated. Should be checked at least every fifteen thousand miles. I also suspect there are many that have not even been checked mind adjusted since Washington was president.

Interesting. Would this apply to the 617 engine as well? I've been wanting to do the fuel pressure test on mine but never got around to it. Please link me to a good thread with pics how to set it up.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

The 617 is not as failure prone because there are more power strokes in my opinion. Yet if a rod is destroyed it will almost always be the number one even on the 617. Look for the relief valve spring thread started by carven I believe.

The gentleman posting is showing compression numbers that may indicate more wear has occured in the number one cylinder. This could be consistant with the cylinder working harder than the others. Still all this is just subjective and came about when a poster a long time ago asked on site. Why does the rod failure almost seem to be the number one on the 616?

I could never buy the length of the oil feed to the number one rod bearing as the cause. The distance is shorter than the 617s oil feed for starters. The most likely cause of low base fuel injection pump pressure is running until a fuel filter totally obstructs. The time the filter is so restrictive that it has lowered the operating pressure may be a long time before the filter chokes right up.

So without a pressure gauge in the circuit you should change the fuel filters pretty often. The gauge setup just saves money by telling you when you should. Also if pressure is low you have lost the cooling effect of the fuel oil carrying the excess heat back through the return lowering the injection pumps lifespan perhaps. There is no flow through the return relief valve at substandard pressure. That is unless the spring in the relief valve or the sealing of the valve seat in there is defective.

To be sure I was pleasantly surprised that compression came back as well as it did. Based on the results my plan is to keep the car and continue enjoying it.

In the meantime though I need to figure out why it suddenly leaked about a quart of oil last week. The oil seems to have come from the top of the engine as there is oil puddled near the fuel filter and spilled down the injector side of the engine. Can a quart of oil come out of the crankcase if the oil cap seal is worn?

Also, are there any procedures/additives/tricks that would be worth trying to bring up the compression on cylinder 1?

If the car has not been using waste vegatable oil it is unlikely any additive will help.Checking the valve clearances might though.

The evidence of the number one cylinder working harder is also indicated in the appearance of the number one injector tip. They sometimes appear far drier or they appear to have run hotter than the others.

Just yesterday I noticed when someone was asking why the differance between the appearance of his injectors. He posted pictures of them all. Number one injector was much cleaner. By the time he got to the end of the injectors they were progressivly oilier in comparison.Now to be fair it could easily be coincidence that that individual number one injector was in better condition. I just suspect if he checks he will find low injection pump base pressure. It was a 617 by the way.

Anytime the glow plugs or injectors when removed show a running temperature difference progressivly from front to back the base injection pump pressure should be checked.

I should add that adding a quart of miracle mystery oil to the crankcase oil to top it up when down a quart should do no harm. It has a slow solvent action on the rings. Over many miles it might clean the ring lands up a little. Perhaps snake oil but at least very cheap snake oil.

x2 on the Valve Adjustment. Run it a while after the adjustment and retake the Compression on that Cylinder to see if it goes up.

What does Marvel M Oil do to multiviscosity type Oils concerning their ability to be multiviscosity?

If He wants to free some stuff up some members have said that using Synthetic Oil will do the trick.

He could soak the #1 Cylinder in Maver M Oil; that way there would only be about 1/4 cup to to dilute the crankcase Oil.

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84 300D, 82 Volvo 244Gl Diesel

I did do a valve adjustment before running the test, although I did not drive it following the adjustment.

Is there a write up anywhere on "soaking" the cylinder?

I can confirm that this was the same on my car; the 1 cylinder was very dry and while the 4 cylinder was very wet.

Two Four Zero Quad D,

MMM (Or anything else) in place of DIESEL Rated Oil WILL Exacerbate/Increase Bearing surface WEAR.
"Lubrication Dilution" is the name of that engine failure.

Howeveah,

Your Mercedes ,Your Pleasure...

Now, filling #1 (through the Injector Orifice) with MMM and allowing the engine to "SIT" for a couple of days
WITHOUT OPERATING,before Removing the MMM
by using the Starter
(WITH THE I.P. "STOP" LEVER LOCKED DOWN,and the Injector out of the Engine)
couldn't hurt.
(Lots if Rags,Blowing out all that MMM will make a horrid mess!)

Me,I'd be filling my cylinder with KROIL (Instead of MMM) ,If I suspected
and CONFIRMED "Stuck Rings".

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'84 300SD sold
124.128

I have been looking for a possible simple test to prove that unequal loading of the injection pump elements is actually occuring. Suspicion is fine but I preffer better. This hopefully being a simple enough test anyone can manage it.

In the last week there have been two instances of injectors having different appearances front to back on the engine. Both engines had injectors in the number one cylinder that apeared to have run hotter. This in my mind is simular to comparing spark plugs. In theory with consistant equal combustion the injectors or glow plugs build up etc should appear simular.

Now if these people would check out their operational base injection pump fuel pressure we might semi validate the test over time. Plus learn to have a critical look at them anytime we remove glow plugs or injectors during any proccedure.

If I owned your 616 I would check the fuel pressure. I would normally want to check it anyways being a 616 but with the injectors apearance you mention it becomes manditory in my mind. Again the pressure just might be fine as there are other conditions that may cause what you have seen.

On the otherhand there is a possible indication there that the number one cylinder with less compression is running hotter than the others. This should not be. The less than 15.00 expendature to install a permanent fuel pressure gauge is starting to look more and more like money well spent.

If a higher percentage of the load is being serviced by the number one cylinder eventually you will hear about it. It may come knocking on your door so to speak.

Another test to verify the issue is to read the milli volts on the number one and four glow plugs. Swap the glow plugs and read again. If the test indicates the lower compression cylinder is runing hotter with it's lower compression. It is pretty safe to asssume it is working harder or taking on a heavier percentage of the load than normal or designed for. With it's lower compression it should in reality be running a shade cooler than the other cylinders.

This I suspect would be consistant with unequal injection pump elements outputs both in volume and timing or questionable injectors. There are already pretty solid indications that this is exactly what occurs when the base injection pump fuel pressure is low. You want to see about 16-19 pounds pressure in there.

Even to myself I feel like a dog with a bone at times. I just still feel this is just too important an item to let die out. If we were able to save even a hundred engines from damaging their number one bearings and excess wear on the number one cylinder It would have been worth the effort. If on the otherhand I am wrong no harm is done.

The other things the fuel pressure gauge can indicate in themselves are worth the expendature. As a time to change the fuel filter for example would be positivly indicated well before it was a major problem. Condition of the antique lift pump etc. can be monitored as well.

There at this point is no doubt that you will get better performance as well with the correct fuel pressure in the base of the injection pump. This is nothing to ignore on a 616 especially. Where else in this world can you get a signifigant improvement for less than 15.00? Defects in the fuel supply system in general can be very cheap to rectify if found to exist is a major bonus as well.

I will even take this a little farther. If I found the operational pressure to be very low. I would rectify that first properly. Then since it is easy to get to the number one rod bearing I would plastic gauge it. This is just in case it has worn more than it should. This does not have to be done immediatly. Although since it is the primary failure mode of the 616 engine I would like to know if it is still good enough to keep in service. Call it preventative maintenance. At this point the crank journal is probably really good. Not so when the bearing lets go if excessivly worn.

I'm game. Any info on adding a fuel pressure gauge, i.e. best place to put it, best place to buy one, etc...

I did the soak on my Volvo. I had not relized until Summer came I had been driving with a Thermostat that was opening too soon and engine temp as too cold. The result was I had light Gray Smoke most of the time.

I was skeptical about it working but the Marel M Oil worked. I just removed the Glow Plugs, used a Funnel with a Hose on it and dumped in about 1/4 cup. Let it sit 5 days; rotated the Engine and soaked it again for 3 Days.
Change the Oil and tool it for a hard Drive on the Freeway.
Remember it will only work if you have Stuck Rings due to A Carbon Build up or something that the Marel M Oil can work on.
But it worked for me.

Below is what I have in my notes.
Marvel MM oil results sticky rings
Origional thread also has + response
http://www.peachparts.com/shopforum/showthread.php?t=258757&highlight=Marvel
Well it loosened the carbon as the compression has increased in the two low cylinders! # 3 was 320 is now 380 and #5 was 160 and is now 340! Drove it about 20 miles hard and fast, there was some carbon coming out the exhaust! Will be taking iton two longer trips this next week: Monday I'll be going over to the coast (SLO) and the SoCa gtg on next Saturday. The trip over the Grapevine should help!
Email response about if it reduced blow-by
The blow by was reduced and also with the nice drive to San Luis Obispo my rpms are lower than they were before! Before 65@3,250 now 2,850, before 70@3,500 now 3,150? and boy does it have power... but the glow plug light is now out (starts ok but the weather is warm. The relay works, so I'll be checking the plugs) and the fuel gauge is dancing (ground?). Oh, well gives me something to do...

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84 300D, 82 Volvo 244Gl Diesel

The only hold up to installing a Fuel Pressure Gauge is that the needle on an undampened Gauge vibrates around so much that you cannot tell what pressure it is reading.

I have only read of the below done for hooking up a test Gauge.
Somewhere there is a thread where one of the Members made a PVC aparatus that dampened the pressure. Not unlike they use in Plumbing to stop Water Hammering. It is a Vertical Pipe with a quanity of Air Trapped at the top.

The other way is to use a Liquid Filled Gauge.

I made the adapter (to connect a test Gauge) in the Pic from one of the Banjo Bolts on the Filter.

But, even with a Liquid filled Gauge the needle danced a little. I believe it would work better if there was a Valve that I could partially closed or a restriction in the fitting to help dampen out the pulses of fuel.

It is another one of those projects I have never done anything more with.

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84 300D, 82 Volvo 244Gl Diesel

The liquid filled thirty pound pressure gauge is about ten dollars or less from outlets like harbour freight. Our Canadian equivelent is princess auto where I got mine. They are genuine chinese units. Although they do look very good for the price in my opinion.. The gauges with no fluid to dampen them in the gauge are unusable. The needle just slams back and forth in response to the elements loading pulses in the injector pump I think.

I believe diesel 911 has reached the right conclusion. A major adjustable restriction in the line feeding the gauge should provide even further stability to the liquid filled gauge. This restriction should be located closer to the take off point than the gauge is I believe. The liquid filled gauge is still readable with no restriction as well just less stable. Any air trapped in that line will be benificial as well to help average the needle readings.

An accumulator type device will only work if it has an air cushion above the fuel. Fluid like the fuel for common purposes does not change volume much if any as it is compressed like air does. A simple restrictor should work well. There even was a modification to mercedes for a dancing oil pressure gauge that was just adding a restriction to it's feed line as well.

The only logical and cheap way to feed it is to insert a barbed tee into the feed line between the secondary fuel filter and the injection pump.Since that piece of plastic fuel line is cheap and old plastic tends to harden and become brittle with age. I would buy a new piece of clear plastic fuel line.

This gauge if possible should be installed in the passenger cabin in a permanent fashion. I do not think anyone including myself has decided on the best location for it. Perhaps you can come up with a good one.

I thought in desperation it could be mounted in the back panel of the glove compartment for example. That way a passenger could read it for you if cruising on the highway for example. You do not have to watch it all the time like an oil pressure gauge.

Once you have the gauge installed every six months or so you can close the return line off. This allows you to observe the maximum pressure the lift pump is producing. Overall since most engine difficulties experienced are fuel supply related this is also a step that improves reliability as it can provide early warning that your pressure is slowly falling off.

I suspect the lift pumps slowly fail to substandard condition as the internal valves age or their seats get dirty. A good lift pump should give you thirty plus pounds pressure with the return blocked off and the tank screen fairly clean.

Another reason for the gauge in the passenger compartment is it alows you to monitor your fuel pressure under load conditions on the highway. You for example may have good pressure at idle or by reving the engine a little while parked.

When the injection pump delivers a lot more fuel under load you want to know the pressure has not sagged down. The injection pump is working hard and generating heat in the core area. Adaquate pressure allows the return valve to stay open and remove the excess heat back to the fuel tank through the return line. This was an intentional design by the injection pump manufacturer. Low pressure defeats it.

Diesel 911s ideal of drilling and tapping the banjo bolt for an output fitting is very good .