I have a 1980 300CD with 132K miles that has a irregular missfire when cold. It does not go away with increased rpm. However, it is completely gone once the engine is warmed. This only takes a few minutes unless the ambient temperature is below 30 F or so. Valves have been adjusted recently, one can of straight Diesel Purge has been run through it, and I've attempted many Italian tune-ups. Oil usage runs at approximately 1 qt. per 1000 miles. Does anyone have any suggestions where I should start my search?

Garth Frazier

Compression check. The compression testers that fit most automotive diesels (VW, Benz, International, GMC, etc..) used to cost about $165 from MAC or Snappy if memory serves correctly. Borrow a tester kit if you can or just have a properly equipped shop run a compression check - shouldn't cost more than 1 to 1.5 hours shop labor. I have always found this to be the best place to start. Placing a small amount of oil in the cylinder will show a significant 'bump' in pressure when rings are worn - this is a very old time tested approach. If no noticeable change results, low compression is likely from cracks in heads, blown gaskets, or poor valve sealing/adjustment, bent rod, wholed piston, etc.... Given the robust constuction and conservative operational settings of most Benz smokers, the last two items in that list are rather unlikely but are mentioned here as generic causes. You can also examine glow plug performance in most pre-chamber fitted diesels when the injectors are out during the compression test. Depending on how long the engine 'misses' and how cold ambient starting temperature is, a dud glow plug(s) can be the culprit.

Compression checks might not be the quickest or easiest test of suspected problems in a diesel, but they provide more reliable information than just about any other test available. You can pretty much 'bank' on the information derived from them to give you the most complete picture of your diesel's 'health'.

If compression checks out ok, your next best test point for the symptoms described would be the fuel delivery system (injectors, pressure lines, unions, return lines, pump, etc...) Although problems with fuel delivery are not often associated with changes in operating temperature (unless temperatures are extreme enough to affect fuel properties), it would not be unusual for a marginally occurring problem to be aggravated or become more pronounced at one operating temperature versus another. However, if your engine 'misses' when cold due to improper or inadequate fuel delivery and/or atomization, it is also likely to 'miss' - possibly to a slightly lesser extent - when warm.

courtneyg

Thanks for the thorough reply. I wish my initial post had been as complete. I failed to mention that I've checked compression, and all cylinders were 410 psi +-10 psi on a cold engine. In addition, I've checked all the glow plugs and they glowed quite red. However, I did notice that they were quite pitted, and that one or two seemed to require quite a bit more time to heat up than the others. Do you think this is significant enough to warrent replacement? One other point that I should make is that on many occasions I get blue smoke on start up, but not always. Can the presence of the oil that is creating this smoke influence combustion?

Garth

[QUOTE]Originally posted by wgfrazier
[B]courtneyg

Thanks for the thorough reply. I wish my initial post had been as complete. I failed to mention that I've checked compression, and all cylinders were 410 psi +-10 psi on a cold engine. In addition, I've checked all the glow plugs and they glowed quite red. However, I did notice that they were quite pitted, and that one or two seemed to require quite a bit more time to heat up than the others. Do you think this is significant enough to warrent replacement? One other point that I should make is that on many occasions I get blue smoke on start up, but not always. Can the presence of the oil that is creating this smoke influence combustion?

I would try new plugs first. From the other info it sounds like you might have a slight stem seal/guide issue but I have never heard of a slight amount of oil in the combustion chamber fouling a diesel's air/fuel mix sufficient to cause a vibration or miss. As you know, the glow plug's performance is critical during cold weather start ups. Any inconsistencies in the set are very likely to cause imbalances or percieved 'missfires', especially if the relay cycles off before the plug reaches its specified operating temperature. Be sure to check resistance of each plug. If you have a current measuring clamp, it would be helpful to see if one plug is 'hogging' all the current or if the 'under performing' plugs just have too high a resistance. I like using the current clamp because it takes into account the possibility of varying resistance with temperature and most closely approximates actual conditions of use over the DVM resistance check.

Garth,

My '85 300CD with excellent engine began to missfire and act up when cold a few months ago. Started pretty easily but then stumbled for awhile. I replaced all glow plugs and problem fixed.

Have you replaced them before? There are explanations here on how to do it and what to be careful of. Not too difficult.

Nic in SC

Courtney and Nic,

Thanks for the additional comments. Based on what you guys are saying, it sounds like I will pursue the glow plug route. I'm sure that the ones in there now are originals.

Nic, I see your from Columbia, SC. I was at the Ole Miss-USC game last Saturday in Oxford, MS. The Gamecocks really didn't put up much of a defense.

Garth

Garth,

Yep, not much of anything at that game. Much better against Georgia this week. USC does not have a great team this year but then our sports history is kinda like that.

Good luck with changing plugs. You'll pretty much have to remove the metal fuel lines first or the lack of access will drive you crazy. With lines gone (put'm in a clean plastic garbage bag while out), the job is straightforward, if still a bit frustrating. Hope it solves your problem.


Nic

New glow plugs will certainly fix your problem. Replace them all, they don't cost that much.

My Volvo was getting nearly impossible to start, so I replaced 5 out of the six (the rear one is only accessible from below and behind the engine -- will wait until I get it on a hoist or get to the diesel service).

I'd put Champion plugs in it -- bad move. Four of the five appear to have had the heat filament break the weld at the tip, leaving a neat little hole. No heat, no start.

I now get easy (1 sec) starts at below freezing temps and much smoother running cold -- will probably be fine when I get the last b..... out.

Now, if I could just get vacuum out of the vacuum pump cold....!

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!