Glow plugs test OK but hard starting below freezing
 

I would like to get an early start at checking out my glow plug system before cold weather descends on Northeast US, so I have a couple of questions. Like my signature says, we're talking about the '84 300SD with an OM617 turbodiesel.

About 3 years ago, I took her to the dealer to get the valves adjusted and have the glowplug system checked out. They said valves were mostly within spec, just a couple that were a little tight. They tested the glowplug system and replaced 2 glowplugs. They also reamed the glowplug holes.

The car had improved starting that winter after the repair but has deteriorated since (as has the fuel mileage, too!). It's possible that mileage deteriorated because I need to move my car every couple of hours where I work because of a stupid parking situation. Anyhow, she'll do ok down to 28-30F ambient maybe. At 24-25F, I may be able to get her to start, but she struggles and has to crank a lot. Below that, I absolutely must use the block heater if I want her to start in the morning.

Couple times when I couldn't get her to start cold after much cranking, there was just a bunch of smoke out of the tailpipe, like ashy-gray colored smoke. Also, when I start hot, there's often a small puff of gray smoke.

Anyway, short of messing with the injectors or IP timing right now, I'd like to get the "easier" stuff out of the way.

Cold battery measures 12.75V post-to-post. Voltage at the glow plug relay fuse read 12.25V against battery "-" terminal, and voltage at each relay pin when relay was on read also 12.25V against battery "-" terminal. Weird, I thought, why the 0.5V drop? So, I measured voltage drop between relay fuse and battery "+" terminal, and it measured 0V... Maybe some contact points I was measuring were slightly oxidized or something. I'll have to measure some more later. Each glow plug tested about 1 Ohm.

Eventually, I want to pull out each glowplug and see how it glows when I turn the key, to make sure it's hottest at the tip, but in order for me to do that, I need to have a reamer on hand, which brings me to the next point:

I would like to ream out those glowplug seats, as I'm sure I got a lot of carbon in there, but don't have the money to invest into a proper reamer tool for this engine. What size is the bore (mine should be pencil-type plugs, right?), so I could get a plain reamer from a tool supplier?

You say three years since the valve adjustment, but not how many miles. Many people do their valves every two years, some each year, especially in areas where winter starts happen at very cold temps.

Blowing greyish smoke out the back means unburned fuel from cranking, meaning either glowplugs nonfunctional or subpar, bad valve adjustment, or compression issues, or some combo of those.

You don't say how old the glowplugs are. That probably means you can skip the testing part and just go ahead and replace them. Not doing so is false economy (set of plugs about half the price of one tow).

Another possible issue: your starter is tired it may not be turning the engine over fast enough to facilitate starts in the cold.

Anyone worried about starting in cold temps should be running synthetic motor oil.

Agreed on the above!

Valve adjustment....

All new glow plugs....Bosch brand....

Look at replacing the starter.....

Caution using synthetic....it will leak from everywhere....


May also be the glow plug relay is weak too......and remember you can also twice glow or triple glow..

Check your starter cranking
sound against this sound file to see if rpm is fast enough.

www.peachparts.com/shopforum/diesel-discussion/288677-starter-cranking-rpm-sound-files.html

Valve adjustments twice a year

Go to a lighter oil for the winter, like a 5-40 synthetic

What kind of glowplugs and how old?

My odometer doesn't work, but I don't drive a LOT, so ballpark guess would be around 20,000 miles.

Well, two of them were replaced by the dealer with OE (Bosch?) plugs around 20,000 miles ago. The other 3 -- who knows how old they are, but the MB dealer must have been satisfied with their performance back then. They could be as old as the hills...

I did run synthetic for a while, but that made some of the old seals bleed, so I switched back to standard oil and don't want to go back to synthetic.

I listened to those files, and I think my starter is fine so far. It's been a tough cranker even in cold weather, so I'm not particularly worried about it cranking too slow. I'm more worried that my glowplugs may not be doing their job as efficiently as they should.

When I had mine done at the dealer, they told me only one or two valves were slightly tight and were adjusted, and before that I didn't have any valve adjustments done for like 50,000+ miles. I'd like to avoid doing valve adjustment because I don't have a garage and have to work outside, and I'm just really nervous about having the top end open only to have some crud blown in by the wind...

Here's an electrical question -- as a glowplug is glowing, should the voltage at the plug relative to ground remain constant or decrease?

I believe the voltage drops so that it wont burn out the glow plug.

The glow plugs draw 20 amps ea. x5 = 100 amps. That amount of current will pull the battery voltage down. As the glowplugs heat up, the current drops to around 10 amps ea.

The decreasing current as the glow plugs heat up will result in the battery voltage slowly rising, but not by a lot. Whent they go off, that's when you'll see the biggest rise.

Ah, that makes sense. Because I measured voltage drop between the + terminal of the battery and the glowplug, and the voltage drop started around 1.5V (i.e., around 11V at the plug) and descreased to around 1.0V drop (i.e., around 11.5V at the plug) as the relay kept burning the glowplug. In other words, I did observe the voltage recover, and that puzzled me a little first.

Now, what can I use to ream the glowplug holes? What's the diameter?

Go to a lighter weight oil like 5w-20 or 30. You will be amazed how much difference it makes in starting in cold weather.

I think it's 7mm drill... it's on here, search glow reamer and you'll find it.

11.5 is not much drop really... volts should drop to around 10v with the glow plugs on, depending on how tip top shape your battery is. you need to measure ohms of resistance at each of the glow plug tips to ground with the harness disconnected. you can measure right from the harness socket, and the sockets are numbered in order of which glow plug you are measuring.
if you have an amp meeter that reaches 20 amps, you could measure the amp draw of each plug to see if any are bad also.

Here's a good pictorial on checking the glow plugs.....but what I think the majority on here say....is that in order to full test a plug, you need to remove it and see how it heats up....a lot of the time the tip won't glow...but the body will....

Mercedes Diesel Glow Plug Repair

Or do what learned on the forum today and just pull 'em out and hook 'em up to battery. I read a ton of posts put together in one place on glowplugs by the very helpful whunter.

2 of my plugs glowed, but at the 'back' near the threads, not at the tip. 2 glowed at the tip, and one didn't spark and didn't get warm at all.

Didn't take long at all to remove once I read the posts that said to just pull the hard lines for fuel and don't dick with trying to work around them.

When you use the Glow Plugs the Voltage at the Battery may drop to 11.5 Volts. But, that is not at Freezing Temps. I would expect the Voltage to Drop lower when it is Cold.
It is going to depend on your Battery.

When I tested a Glow Plug with Dashboard Type Amp Meter (went to 60 amps); it pegged the meter at 60 amps for about 1 second and than as the Glow Plug got hot the amperage dropped down to about 16 amps and held steady.

When you test the Glow Plugs on the Battery you want to see them get hot at the tip first. If a Glow Plug gets hottest in the center of the heating Element first that Glow Plug is no good.

If you remove the Plastic Cover from your Glow Plug relay you will see a Metal Strip Fuse. If that Fuse is not burned look real close and see if it is cracked.
Also check and see if the Screws holding it are tight.

If you have someplace to plug one in see if your Block Heater is working.
I tried to see if I had a Block Heater as everyone said they came with the Car. But, I could never see it on the Engine Block.
On day while I was doing something else to My surprise I found the Block Heater Cord tied up and shoved under something above My Bumper.
I tested it and it worked fine.

A compression test may be in order.