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Old 05-27-2001, 11:13 PM
RunningTooHot's Avatar
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I know that nothing will suffice as a substitute for an actual compression test. That being said, I’d like to know if the following could be used as an indicator of an engine with good compression.

I looked at an 82 SD that had been sitting for several weeks. It was dead cold, and the owner put the key into the ignition switch and immediately fired up the engine without waiting for the glow plugs for even a second. It fired up immediately after less than 2-3 seconds of cranking. The ambient temperature was about 70 degrees. I asked him about this, and he said that he usually doesn’t need to use the glow plugs except in winter.

I would like to logically assume that the engine has relatively high compression; at least high enough to heat the air to allow combustion to take place without much cranking. Do you experts out there feel that this is a good enough indication of the condition of the engine?

Thanks again everyone!

(the quest continues….)

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Old 05-27-2001, 11:55 PM
Johnson Chan
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Interesting theory.

I just tried it for fun. Its 61 degrees F outside and mine did start but it did take a few seconds (5 sec or so) of cranking. There was also smoke out the back afterwards.

It was weird to hear that as long as a diesel can start, then it has enough compression. I dont know how true that is now, I just read it from previous posts.

Out of curiosity, in the compression test results, mine had 4.5 in #4 cylinder and 9.5-10.0 in the other 4 cylinders, whats the normal or compression numbers supposed to read for a new diesel engine?
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Old 05-28-2001, 12:17 AM
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Running Too Hot,
The first line of your original post said it best. Which ever vehicle you decide on make sure to have a compression test done as well as a complete inspection.
Scott Diener
89 300E
93 300E
92 Volvo 740 Wagon aka "Mutt mover"
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Old 05-29-2001, 12:55 PM
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Maybe the type of diesel fuel also makes a difference to starting, the if any additives have been added
1996 E320
2000 C230 Kompressor
1988 190e 2.3 - 225K miles, owned for 7 years. I cannot say enough good things about this car. Very well built, even at 225 it ran like new.
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Old 05-29-2001, 09:34 PM
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I was told you wait until the plugs are lit and hot to keep from dumping huge amounts of fuel into the cylinders while cranking and waiting for the plugs to get hot enough to ignite (on the assumption that starting your car with a big BANG in every cylinder is typicallly bad for it). The excess would explain the smoke - perhaps this is all about air quality rather than longevity though...
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Old 05-29-2001, 11:50 PM
longston's Avatar
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Location: Mark West, CA
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Miss Conception...

Originally posted by makakio
I was told you wait until the plugs are lit and hot to keep from dumping huge amounts of fuel into the cylinders while cranking and waiting for the plugs to get hot enough to ignite (on the assumption that starting your car with a big BANG in every cylinder is typicallly bad for it). The excess would explain the smoke - perhaps this is all about air quality rather than longevity though...
Makakio San,

Diesel is a compression ignition system. Therefore, it works on the simple concept that when air molecules are compressed, they heat up due to the friction produced by the molecules rubbing against each other under high pressure, thereby producing enough ambient heat to ignite the diesel fuel. If I remember correctly, diesel fuel will not ignite until heated to 148-185°F.

The glow plugs are merely an assist, whereby they pre-warm the air in the prechamber prior to compressing it. They do not ignite the fuel itself, they simply warm the air to assist in the compression-ignition process.

The smoke is, in fact, simply unburned diesel fuel combined with the soot of the inefficiently burning fuel that has finally ignited.

And every internal combustion engine starts, "with a big BANG in every cylinder", everytime you start it, then it continues to do the same thing over and over again, just faster as you add more fuel and air...

Saka sama to ishiro muki...
"We drive into the future using only our rearview mirror."
- Marshall McLuhan -

Scott Longston
Northern California Wine Country...
"Turbos whistle, grapes wine..."
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Old 05-30-2001, 12:13 AM
Wm. Lewallen
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Scott and others,
I was told years ago(in the '60's)that the diesel fuel was ignited when the piston reached the top of its stroke,struck the head creating a spark igniting the fuel. He was from Michigan and knew it all. He had driven diesel farm tractors for years.
Bill Lewallen; Lexington,Ky.
Home of the Rolex Three Day Event
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Old 05-30-2001, 12:27 PM
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Oh! Bill! My leg is a bit longer
1983 300CD 246k sold
1998 C230 52k
1980 450SL 75k needs AC
1999 Subaru Outback Ltd 43k
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Old 05-30-2001, 04:26 PM
Wm. Lewallen
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Some people don't understand how diesels run when you tell them that your engine has no spark plugs. They look more puzzled when you tell them your car will run without a battery/alternator. I know this because it has happened to me twice. Luckly it was daylight, and it wasn't raining...
WE jump-started the car and didn't shut it off till we got home.Over 200 miles one time;125 the other time...
Bill Lewallen:Lexington,Ky.
Owned too many Mercedes to list them all...
Let's just say I've owned a few...
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Old 06-01-2001, 06:45 AM
Joe Mc
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Diesel engine

Bill, Longston is absolutely correct with his discription as to how a diesel engine works. Now as to why you don't need electric (alternator) to keep your diesel engine running, your engine has a mechanical gear driven fuel injection fuel pump. As long as engine is running, this pump is working. Inside this pump is a control rod which either allows fuel in the pump or shuts off the fuel. On some diesel engines this control rod itself is hooked up to a steel cable, electric shutoff solenoid, or a vacuum solenoid. On your MB, it is vacuum controlled. Diesel engines do not make vacuum and if they use vacuum controls, they must have a vacuum pump. With todays diesels, things are controlled by electric. Electric controls, injectors, and computers all in the name of EPA and progress.
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