My '85 300CD began to have cold start problems a few months ago. Missing and smoking. Then glow plug light would not come on at all and I rightly figured that was the problem. Bought five plugs but only did first four before daylight ran out. That made glow plug light come on and car started better but still had a little fit when cold. I began to think I had air getting into fuel line somewhere.

I put off doing last plug (near firewall) because I have read several times that the number 1 glow plug could be cause of problem and thought that four out of five (including number 1)should do the trick.

Finally got last one replaced last week and that completely fixed my cold start problems. No missing and no smoke to speak of.

I don't know about other 300's but mine wanted ALL the glow plugs replaced before being happy. Number 5 was causing big trouble. I assume the numbering is 1 to 5 from front of engine.

Nic
Columbia, SC

Number one refers to number one cylinder, not number one in the chain. The first in the chain being on the back would be the early series glow plugs which rarely need replacing, just cleaning.

I learned from experience a number of years ago to replace all the pin type glow plugs and be done with it. It will usually make a big difference. Of course, in those days the plugs were only three bucks each.

Number one, the front cylinder, current is measured by the relay to decide when it is ready to start.

Good luck,

Instead of replacing all of the glow plugs to try to correct your problem I suggest that you do some simple trouble shooting first.

Note : The glow plugs are connected in parallel within the glow plug control box.

This is for a 82 300SD, it should be similar for other years.

On the drivers side wheel well, disconnect the connector which connects the control module to the glow plugs. Do a resistance test between each of the 5 contacts of the connector which go to the glow plugs and the block of the engine. The resistance of new glow plugs is 0.4 and 0.5 ohms. If any are greater than 1 ohm or so I replace them.
At this point, you will know if you have any bad plugs and how many. Do NOT reconnect the glow plug connector. If you have any bad ones, do a resistance measurement from the power connector of each glow plug to the engine block to determine which plugs are bad.
Reconnect the glow plug connector.

I agree with Sweenewl to check for the bad Glow plug (GP) instead of replacing them all. A resistance check will detect a bad GP if it is open but not always if the GP is marginal and a small difference in resistance may not be detectable. A current test is better because it checks the GP hot compared to the resistance check which tests the GP cold. You need a 20 Amp DC meter. Any GP that reads a lower current than the others should be replaced.

P E H