2 burnt glow plugs
 

Just returned from an epic trip to far northern newfoundland in the Benz—2700 miles in a week. I left knowing I had one dead glow plug from the smoky start.

Halfway through I lost my second glow plug— the dash light went dead. The old girl still started but only with 45 sec of cranking. On three it fired immediately.

Back home I pull the plugs and find two of them are still glowing so I’m wondering why it didnt start more readily. You’d think two plugs would get it to running pretty quickly, when I let it glow until the relay went off. Am I just wrong about that or is something else the matter? I checked the strip fuse and it was fine.

A lot has to do with the outside temperature your working in. In the winter here in New Jersey, my E300D had one bad plug, and still started in temperatures in the high 20s. The engine ran rough for about a minute or two, but evened out quickly once it was running. The actual starting time was longer than it would have been with all the plugs working.

That's why I always change them as a set. My thinking is, if two fail then the others are probably weakened and on their way out as well. It sure saves on a lot of frustration in the long run.

The 190 glow plugs are so hard to change I would never change only 2!

That is true for just about any MB Diesel engine!

I have no explanation in mind. Just have observed over the years that similar engines react differently to glow plug situations.

I even had one that even needed a touch of glow plugs when hot to restart. Yet the engine seemed solid in general otherwise.

Some four bangers show a real reluctance to start with even one plug out was another observation.

About the only rule I would apply. If I drove a diesel over time. Observing that it was always the same glow plug that failed. I would have that injector checked out for spray pattern etc.

Logically it is so easy to make a reamer it should be used with every glow plug change. Carbon can build up. Just the right size drill bit with some grease on it to catch some of the carbon can do it. Both cheap to do and takes little time top accomplish.

I'm wondering why someone would go on a long trip and ignore the fact that glow plugs are bad. I've never owned a 190. Are those plugs exceptionally difficult to access?

Also, once the 1st plug goes (on a 617), the remaining plugs are soon to follow. I have changed 1 plug as a stop gap measure even reusing one from the parts box - but only as a temporary measure. Glow plugs are too cheap and easy to let them cause problems.

NDT is the test method which usually indicated one or more plugs are tested to the fail point. For an example...

1000 glow plugs are run. Two, three, five or 10 of them are glowed repeatedly until they fail. A predetermined limit is either met or not. That limit may be 1000 glow cycles or 100 glow cycles or any other number according to how great the quality of glow plugs the manufacturer want to meet.

You can bet your booty that if a manufacturer sets a low number of GP to burn up in testing and or requires the pass a low number of glow cycles the plugs will be junk and you're making a poor choice if you buy them.

If a company sets a goal of manufacturing glow plugs which are high quality the number of GPs tested and the number of cycles it takes to burn them out will be reflected in the product.

Either way, if one goes bad the others in that lot won't be much better. Anyone who depends on backups from the last GP change better be heading home to change them all when he gets there.

Glow plugs are too cheap to risk burning up a starter and the trouble of having to replace it on a w126. I get a new set once a year.

3 plugs a winner!
 

I just found out that a 3rd burnt out plug on my 6-banger is the straw that broke the camel's back on trying to get started in around 0*C temps. I knew I had one or two gone by rough starts... then one day she no go.

Easiest way to find 'em is pull the glow plug relay connector off and probe each hole in the harness looking for low resistance (0.8 Ohm) for good plugs and infinite resistance are the bad ones.

2,3, and 5 in my case.

Turns out it is possible change each of these holes without cracking and pulling any wet lines or taking the intake manifold off the car, it all stays in place.
Using combination of long wobble socket wrench extension bars to reach in there. A couple extensions give the wiggle room to get around minor obstructions.

Not so sure plugs number 1, 4, and 6 would be as easy to get to this way. For sure not 6, that firewall and oil filter housing are a problem!
When I last did plugs I did all 6 and I recall having the manifold off. That was a bigger set up for the job but did make the actual job of replacing plugs easier.

I did use some used spares I had around the shop this time, because I needed to get going next day.

When they burn out I'll do 'em again, not going to pro-actively replace them again with new plugs.
A little anti-seize compound helps for next time. :)

Step 1: Remove intake. It's a pain in the ass, which you get tired of.

The W211's actually tell you which GP is bad on the reader.

The w124 om603 also tells you via the glow indicator if some plugs are bad using the glow indicator light on the dash. The GP relay monitors plug #1 and #6 individually and powers #2 through #5 as a group. When it detects and imbalance of power draw it does something different with the indicator light. I forget what.

But I find that I can tell well enough when a GP is bad: the car starts funny.

I can find which one in a few minutes: probe the GP relay harness going to plugs with an ohm meter.

No special reader tool required.

Oh yes oh yes. I’m trying to do this without disconnecting the fuel lines: I got the intake out but well see if it goes back!

I took off with one dead plug because Rednecks dont plan— they react.

In the end I got back. If you run old Kubotas its not scary to have to crank for a minute or two.