Low compression = poor combustion Remember a diesel uses compression to ignite the fuel. If a valve or rings are leaking and are not recieving proper compression it will hamper starting, but once running the leak may be small anuff to run fine and heat my lessen also.

 

Good morning! .... well...it is here. I am new to the post and have have to admit your postings motivated me to join so Good Job-!! and thanks for the motivation...

So, from what I have read you have verified good glow plugs and the relay clicking when you turn the key to the ON position
you have also verified that the bulb is good in the display .. yes?...ok... If assuming that these things have been verified I will need more information... If you have a test light, ground it at the engine and place the probe on the #1 connection point at the glow plug have some one turn the key on and see if the test light luminates and for how long it stays on if no lamination place the ground cable of the test light to a body ground and again the probe to #1 glow plug and cycle the key to the on position once again record you finding and post back we need to know and verify power to the #1 plug.

This will give us a good idea the next direction we will go in the diag of this problem.... here is my method to the madness ..lol..
if the test light luminates in the first test and shuts off very quickly it is possible the engine coolant temp sensor that supplies temp information to the relay is defective (telling the relay very little glow plug activation is needed) this sensor is a different sensor than the one for the temp gauge it is located front lower area cly head...... in addition the relay has a built in sensor also depending on the year there where some changes along the way. In addition here the #1 glow plug was used on some models as a diagnostic tool..... if the light light and then shuts off in apron 15 seconds at 40/0 c system ok, if the light flashed after starting one of the glow plugs are defective. and... if no light on 1 or more glow plugs defective including the #1 plug


So as you can see we have to know more about how this system works before you go to far off track ... and we all have been there. I remember working on the 123 body 617 engine glow plugs system through out is evaluation and Mercedes slipped in a lot of changes along the way in addition to big changes to that system in 85 with turbo. You might want to put a battery charger on and get your battery voltage back up to above 12.5. let us all know your findings and we will go to the next step.

Have a great day! good luck! if you would like a detailed description of the operation of the glow plug system on the 1980/84 drop me a note.

 

GoooD NEws. Problem (mostly) solved. This last post (very good thinking BTW mb4uandme) inspired me to check my work and Start from the beginning. I checked the number 1 plug with test light and it was only staying on for about 5 seconds which I had never seem before. I removed #1 glow plug (and rechecked compression; okay at about 21:1) and found it was totally caked despite having replaced them a few months ago. Autolites. It's funny, the other Plugs tested okay - but initially I had cheated and only removed and tested 3 of them. I knew, or thought I knew from Alldata, that the dash light only stops illuminating completely if 3 or more plugs are bad. Apparently not so - which lends even more credence to MB4u's idea that #1 is used for testing on some models. Replaced #1 and it stays on, when cold, for about 37 seconds.

I had two other plugs on hand and replaced numbers 2 and 3 with them. Those looked pretty ugly for only half a year's use but still tested okay (got redhot with 12v applied power). Starts without hesitation. HooRah!.

I did get under dash previously and checked the GP indicator light: Good. But still not illuninating after these GP changes. I wonder now if this was a bad batch of plugs that, internally, aren't allowing enough juice to pass through which signals the system that everything is not OK. I really know nothing about electronics/wiring though.

I recharged the battery as you recommended too. I plan to bring these crap autolites back to my car store and swap GP #4 and 5 for premiums.

This is probably my 8th 300sd 1984 and none have behaved like this one.


Profuse Thank You's to all who thought hard about his. Even though it was mostly a result of my carelessness your ideas about compression and deliberate step by step thinking will certainly be useful on my future projects.



Next Question: Same year and model but different car; Heat works great at Idle but blows cold while driving. Thermostat or head? I'll research and post in new thread if necessary.

 

The answer to the "next question" is the heater control solenoid thingy on the firewall. That behavior is symptomatic of a tear in the diaphragm. Pull it out and inspect it to verify.

 

Cycleboy - you're exactly right. Assuming this is the part you're talking about it's the Mono Valve/Heater Control Valve/ Water Valve (Slang, according to the MB Techs) and mine does have a tear in the rubber. Excellent help. I'll install the fix kit as soon as I can get it.