Lately when starting up in the morning the engine fires only on 5 cylinders for about 10 seconds before all 6 are firing.

Is it possible that if a GP is not heating, the cylinder it serves will not fire for the approximate 10 seconds I mentioned above, any ideas?

Yes, the block heater is plugged in for 2 or 3 hours before starting, but I think lately the block heater has deminished ability and probably need new and doubt this has anything to do with missing one cylinder! BB

common problem on 603s.check each glowplug for 1 ohm resistance at the connector plug to the glowplug relay.the case of the relay shows the plug layout for easy reference.any plug with high resistance is suspect and should be replaced.if all plugs pass there could be other issues such as IP timing,chain stretch or lower compression on one or more cylinders.

Does it miss if you run two GP cycles? I find mine runs without the miss if I run one full cycle and at least an additional 1/2 cycle or more. I don't use the block heater, and temps are usually in the teens in the morning around here.

DS
87 300D
Northern Iowa - Snowing like crazy here - parking lot donuts planned for this evening...

Like Michael said, check the resistance of your glow plugs. It is fairly normal for a very cold diesel to stumble for a little while. That said you can minimize it by doing a full glow cycle (30 seconds or until you hear the relay click). Doing more than one glow cycle is pointless unless the first one is a full cycle.

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Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL

Yes, I always wait the 30 seconds before hitting the starter. Only in the summer do I only wait for the glow light to go out before starting. Basically I just wanted to find out before testing GP's if this scenario was typical as I have never had trouble before with GPs.

Thanks for the input BB

All 603's seem to do it at some point. How cold is it? Usually if you count to 10 after the light goes out they shouldn't skip much unless it is really cold out, say below freezing.


But they all skip, 603's just do it.

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1999 SL500
1969 280SE
2023 Ram 1500
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Are you quite sure of this? If one cylinder was not firing and the other five were firing, the engine would be down on power but would be somewhat smooth. This is provided that all of the other five are firing properly and the last one is not firing at all.

I'm more inclined to believe that one or more than one will not fire properly and you feel a miss in the engine for the first 15 seconds or so. If you let it idle, with no additional fuel, it will miss randomly with some kicking and banging which smooths out with time.

I have had the situation where it can continue to randomly kick for nearly 45 seconds after lighting off, if I give the plugs a very short cycle (10 seconds).

It appears that the longer that you run the plugs, the less likely the irregular firing will occur, and, if it does occur, the faster it will clean up.

(Analogy) It is as though one of six spark plug wires was disconnected, and then after ten seconds reconnected!

Once it hits, it stays and after full warm up, 83c, it has all the power one would expect from the 603 and quiet too!

My temp guage had been reaching 40c + during block heating at around 38F ambient, and with a 30 sec glow it would fire off on all six and run smooth from first light off!

Now at around 38F ambient, the block heater just barely registers anything on the temp guage at all and the 30 second glow yeilds a 10 second miss.

Maybe a combination of bad GP, or GPs, plus the block heater not putting out 100% anymore!

I'll check the resistance in the GPs as soon as the weather breaks. BB

That sounds more like one bad glow plug. I can get the 603 to run perfectly smooth at 40F, without a block heater, with a glow of about 25 seconds.

Mine does the same thing, although it misfires on 2 cylinders if it's freezing outside, 1 cylinder if it's 50°F. (30 - 45 second glow).

I think it's a combination of an extremely stretched timing chain, a bad glowplug, and the need for a can of diesel purge.

Plugging in the block heater all night makes it happy, and it will run on all six with 15 - 20 seconds of preglow. The block heater will make the temp gauge register at 40°C when the outdoor temp is freezing. Outdoor temps of around 50°F will make the temp gauge register 50 - 55°C.

It's time for some maintenance.

I found the #2 glow plug faulty with with ohms test. The remaining 5 GPs showed a resistance of 0.03. ohms.

The spare set of 6 GP's I had on hand show all 6 to have a resistance of 0.05 ohms. What can be known about these ohms numbers and what do they mean? Most say 1.0 is good, but what does 0.3 mean?

I disected the faulty GP to see what makes them tick and basically are a coil of wire with insulation encapsulating. I suppose the wire parted somewhere inside!

Found this post (below) on how to test GP's and thought it may be useful to post again. BB
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Hello again.

Glow plugs last until they fail. Whether to replace them all before they blow, only those that blow, etc. has been discussed at length elsewhere, so I won't go into that now...

GPs can be tested in place. You will have to locate the glow plug relay.

Disclaimer: My experience is with a different body (W126) but the W210 (yours) will have similar components. I also assume you have a bit of basic knowledge of electricity and various underhood components...

Find the GP relay

The GP relay is a black box, likely to be in the area in front of the left front fender. Mine has markings for the 6 glow plugs "G1" "G2" etc. on the right side, where the GP wires enter the relay, which serves as a connector reference.

Have a look at the GP fuse

If the GPs draw too much current, or one fails 'shorted' the GP system will draw too much current, and the fuse will open. You can have 6 good GPs and they won't be able to function. You may still be able to start the engine, but without GP assistance.

The fuse is a strip of metal between two screws, visible when you remove the relay cover. Have a close look for any gaps or cracks.

Test this with the ohmeter, as looks may be deceiving. Touch a test lead to each end, such as the two screws that hold it. There should be zero, or nearly zero, resistance. "Infinity" or blinking digits, etc. means it is open, or blown.

It's a cheap replacement part if it's gone.

NOTE: you must continue to test to find the short or the new fuse may blow again.

Unplug the GP harness

The top cover pops up and off, exposing two plugs. The smaller plug is the control, the larger 6 wire plug connects to the glow plugs.

With a bit of coaxint, you can unplug the 6-wire plug. You will see 6 prongs on the relay box and 6 socket holes in the plug.

Test each GP

Get (buy or borrow) a multimeter and set it to measure resistance (Ohms) in the smallest measurement number. Good glow plugs will measure 1 Ohm or less, but NOT ZERO.

Connect one lead (I use Black, but it does not matter) to Ground. Any bare metal on the chassis, or better yet, bare metal on the motor, should do nicely. (I clipped the Black lead to a metal fuel injector line.)

With the other lead in the air, the meter will read 'infininity.' This may be a a large number that blinks on a digital meter, or the needle is full scale on an analog meter. Use this to become familiar with how your meter displays Infinite Resistance or Open.

Now touch the other lead to bare metal. The meter now should read zero resistance. (My Radio Shack cheapie has a Continuity setting, which means that it beeps when there is little or no resistance. This was very handy!)

Now you are ready to read glow plug resistance. Touch the 'other' lead to one of the socket holes in the 6-pin plug. If it is connected to a good glow plug, you will read 1.00, or less. (My meter beeps so I didn't have to take my eyes off of the connector.)

Touch each socket in the plug in turn. If all glow plugs are good, each will read 1.00 or less. If any are burned out, you will read "Infinty" by flashing number, no beep, etc.

If you find one (or more) that test 'open' read the number next to the plug socket, and confirm it with the corresponding number printed on the relay cover. I my recent experience, #4 and #6 both tested open, all others were good.

Test the GP wire

See if you can locate the GP itself, and if you can touch one meter test probe to the nut or wire terminal on the end of the GP. If so, touch one probe to the GP and the other to the corresponding socket in the plug. A helper may be necessary. (My test leads have a slip-on alligator clip, works perfectly for this.)

A zero reading here says that the wire itself from the GP relay to the GP is OK. An open reading mean a bad wire. Take your time and be sure that you have contact on the GP end of the wire, or you will continue to read Open when you in fact haven't made the connection.

This test is a bit frustrating, but I recommend it since a 20 minute struggle to change a good GP, only to discover a bad wire, can be more frustrating!

Sorry for being long-winded, and I hope this makes sense.

Drop a line if you need more detailed help.

Best Regards,
Jim
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'86 300SDL 251k+, part of a fleet