300sdl eratic temperature. Any suggestions?

[Jeff Petty]

I just finished R&R on my 86 300sdl. It is running great, but after warming up, the temp. jumps from 75 to 110 then down to 75 - real eratic. The car has a new thermostat and radiator. I burped the radiator when refilling(if anyone has the exact steps on this, I'd appreciate it, just to make sure I did everything right) Any suggestions as to what to check first and what might be going on? Thanks! Jeff

[Diesel911]

Is there any evidence of a bad Head Gasket?
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PeachPartsWiki: Do It Yourself Articles - Mercedes Vehicles

[Jeff Petty]

Originally Posted by Diesel911

Is there any evidence of a bad Head Gasket?
DIY Repair Links
DIY Links by Parts Category - PeachParts Mercedes ShopForum

PeachPartsWiki: Do It Yourself Articles - Mercedes Vehicles

I just replaced the gasket and several other parts related to that procedure. We had just bought the car...drove it for 2 weeks before the head gasket blew. We were told the car had never been hot and it shows no evidence of being...extremely clean and well maintained. We are thinking that due to the fact of being garaged for a couple of years and not driven that the gasket dried out and then when it went back into use, it blew...our best guess. It is an awesome car, just don't want to drive it until the temp. problem is figured out and fixed.

[jay_bob]

Check actual running temp with a non-contact thermometer, I would suspect the gauge might be telling lies first especially if it is jumpy. If it is jumping instantly, that's a sensor problem, you can't change the actual temperature of the engine that fast.

Bad ground between engine and body, bad sensor, bad gauge, bad connection at gauge all suspects.

The gauge measures temperature as a change of resistance. Extra resistance in the circuit causes false readings.

86 300SDL, that's an OM603, right? Those are really tricky, you have to raise up the front of the car on ramps, and detach the upper radiator hose from the radiator and fill from the end of the hose with a funnel. I also removed the heater hose at the left rear from the block to verify the block was completely full. Then I hooked the heater hose back up and jumped 12 V to the aux pump (+ to pin 1) and used that to ensure the heater core was full.

On the OM603 the main electrical bond between the engine and body is below the car about where the driver's feet would be.

[Skippy]

I second the idea of the gauge. Mine's been doing that for about five years. One of these days I'll get around to fixing it.

[jay_bob]

@skippy

Check your bonding strap below the car, it actually connects at the juncture of the engine and transmission. If this is loose your gauge will act weird. When I saw your sig with auto to manual conversion it sure made me think of the bonding strap being disturbed and possibly loose.

[Diesel911]

Stick Thermometer into the Coolant when the temp is too high and see if it really is overheating or not.

I would still be looking for evidence of a Head Gasket Failure. Bubbles in the Coolant, Oil in the Coolant or the Coolant Hoses hard as if they were inflated several hours after the Engine has cooled down.

Replacing the Head Gasket does not rule out cracks and you did not mention if the precision flat surfaces were with in spec on the Head and the Cylinder Block.

[Jeff Petty]

Originally Posted by Diesel911

Stick Thermometer into the Coolant when the temp is too high and see if it really is overheating or not.

I would still be looking for evidence of a Head Gasket Failure. Bubbles in the Coolant, Oil in the Coolant or the Coolant Hoses hard as if they were inflated several hours after the Engine has cooled down.

Replacing the Head Gasket does not rule out cracks and you did not mention if the precision flat surfaces were with in spec on the Head and the Cylinder Block.

This evening I went through the burping process again, doing it as recommended. I cleaned all terminals associated with temp gauge. I had the head professional reworked and checked and rechecked all clearances befpre replacing the head gasket. Everything checked out and looked really good. It ran great Sat. After working on it tonight, I let it run for about 40 minutes with ambient temps in the 60s. Took it for about a 10 mile drive. About a mile from home, it started running real rough and lots of white smoke out of tailpipe. I am having a bad flashback to Sept. when the head blew the first time. 4 months of work and I fear I may be back to square one. I feel physically sick. Is this by any great miracle something besides what I am fearing the most???
Thanks, Jeff

[big d]

Cracked head ?Did you check for low coolant?

[Diesel911]

Is the White smoke Coolant or Atomized Unburned Fuel.

If it is unburned Feul the cause could be something besides a Head Gasket issue. Something lke the EGR being open when it is not supposed to.

[Jeff Petty]

[QUOTE=Diesel911;3091987]Is the White smoke Coolant or Atomized Unburned Fuel.

If it is unburned Feul the cause could be something besides a Head Gasket issue. Something lke the EGR being open when it is not supposed to.[/QUOTE

Not sure how to tell what the smoke is. It does not have a sweet smell at all..how can I test to know for sure? I am copying my whole experience with the car so you can see the whole picture...maybe that will help. Thanks for your time, man.
Had blown head gasket in Sept., supposedly caused from sitting in a garage for 2 years and then going into daily use? Tore down, had head resurfaced (wasn't cracked, just had some inperfections), purchased all recommended new parts for putting back together. Finally after 4 months got up and running Sat. Ran great. Watched temp gauge. It would go from 70 to between 100 and 110 and then back down within seconds. Parked it until last night. Reburped cooling system. Started and let run for about 30 minutes, drove about 10 miles and about a mile from home started running rough and blowing white smoke. I got a real sick feeling that something really bad had just happened, like another blown head gasket, even though the temp guage showed more leveling out and stayed between 90 and 100. My father in law, a 50 yr plus diesel mechanic (although not familiar with Mercedes) came by this morning. The car starts and initially ran a little rough, but in a few seconds leveled out and is running really smooth right now. There is still some white tailpipe smoke, not a lot. It smells like fuel, not sweet smelling at all. He observed that there is exhaust coming out of the cooling system. With the cap off, it pours white smoke as soon as the car starts. There seems to be little water in the radiator hose even after reburping system and adding another 1/2 gal of coolant last night. It seems like compression in the engine is pushing the water out of the block. Do I have ANY options or any suggestions other than to tear this whole thing down again? There are so many variables, I am confused and don't know where to start because I don't know what went wrong and what to do different. We did everything by the manual, and checked and rechecked every move and part and procedure. (although the head was resurfaced and checked, there was a fracture in the valve area, not in the water jacket - would this be a potential problem?) If it were you, would you tear it back down and just try to find a #17 head and start over?...not knowing where exactly to go next here.

[JPUTT]

Had this same problem several years back on the '87. Replaced the head gasket and new bolts and the same white smoke out the tail pipe with erratic temps and cylinder pressure in the coolant system. Pulled the head again and found a slight crack (very slight) at the #5 exhaust valve.

Purchased a new #22 head , had the valves renewed and good as gold now with 335,000 miles.

John

[Jeff Petty]

Originally Posted by JPUTT

Had this same problem several years back on the '87. Replaced the head gasket and new bolts and the same white smoke out the tail pipe with erratic temps and cylinder pressure in the coolant system. Pulled the head again and found a slight crack (very slight) at the #5 exhaust valve.

Purchased a new #22 head , had the valves renewed and good as gold now with 335,000 miles.

John

Thanks for posting. Where do you begin to search for a head? I am going to look at all that is compatible. Anyone got any suggestions as to what is preferable? I know there will be a great price variation as far as what all will have to be changed out if I go with a #22. Is that the case with a #17 also. I am open to sticking with a #14 if I can get a guarantee that it is not warped or cracked. Also, just curious....hindsight maybe...If the head was shaved within tolerances, should I have put a thicker head gasket on?

[JPUTT]

Originally Posted by Jeff Petty

Thanks for posting. Where do you begin to search for a head? I am going to look at all that is compatible. Anyone got any suggestions as to what is preferable? I know there will be a great price variation as far as what all will have to be changed out if I go with a #22. Is that the case with a #17 also. I am open to sticking with a #14 if I can get a guarantee that it is not warped or cracked. Also, just curious....hindsight maybe...If the head was shaved within tolerances, should I have put a thicker head gasket on?

I ordered the #22 head through the MB Classic Center in CA. They gave me a good deal. I was a MB Club member at the time. After I got the head I took it to a local MB Indy shop with a great reputation and they did the switchover of the valves and pre-chambers for about $250 which included reconditioning the valves. I installed the new head and gasket myself along with 12 new hydraulic lifters ( 1 had a tick in it for sometime) and when done I had approx $2500 in the job.

My thoughts were that I could mess around with used or reconditioned older heads and still end up where I started so I went for the new head and am glad I did. The #22 head is a much better designed head and basically eliminates the head gasket problem that many older 603's experience between the #1 cylinder and the timing chain area. I did notice that the new head runs a little warmer than the old.

The only problem I had when installing is that one of the pre chambers was not sealed good and leaked but the shop took the car in and fixed that for free since they prepped the head for me.

PS. The head I did replace with the #22 was a #17 that was installed before I owned the car. I have put approx 135,000 miles on it since owning the car.