Here is an odd one I'd love to gather thoughts on. Thanks everyone so much in advance:


My '85 300TD (picture attached), chassis with 85K miles, less than 500miles on a Metric motor rebuild, recently rebuilt A/C system (now compressor, parallel flow condenser) has developed two odd habits.

One is the coolant temps is wavering with occasional spikes to the 110C range. This I associated with fluctuating heat. In general when the coolant temp goes up the cabin heat goes off.

At the same time the idle, always around 800 rpm is now 1200 rpm and will occasionally climb to 3000 rpm in park or neutral.

I've searched the forums but don't really have a clear diagnosis on these problems. With respect to the overheating and heat supply could an air lock in the system be the cause? Would a bad auxiliary water pump do this?

As for the sudden onset high rpms and odd climbing with the transmission disconnected could part of the vacuum system be to blame?

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Fluctuating engine temps could be air in the system, if not air a defective/sticky thermostat. Use an infrared thermometer when the gauge is normal and is hot to see if it is running hot or the gauge isn't accurate.

The climate control issue could be air in the system or a failing monovalve.

Park the car nose up with the radiator cap off overnight, it may self bleed the air.

For a diesel engine to increase RPM's it needs additional fuel (diesel or crankcase oil) to burn. Is this the same injection pump as before? What was wrong with the old engine.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Thank you so much for those thoughts.

With respect to a sticking thermostat, if the thermostat was sticking closed wouldn't I expect to see constant high temperature coolant going to the heater core? Let me know if I'm not understanding this correctly.

Also I think a failing mono valve with mono valve stuck either open or closed should have not impact on coolant temperature.

Air in the system seems is a good thought, I can try the parking on a slope trick.

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Engine was rebuilt just to freshen it up, it was a bit tired.

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I didn't mention but car had a period of high temps, over 120 for a few minutes last week, it's only after that that the hight rpm problem started.

I re-read your post, it may have an air pocket. Thermostats can stick completely closed, completely open or get quirky anywhere in between. On a new engine I'd definitely have a new high quality thermostat, perhaps you do already.

Is there excessive oil in the breather tube from the top of the valve cover to the air cleaner.

How was the remanufactured engine supplied with or without the injection pump? Random rpm increases is concerning.

Monovalve failure is usually no heat, by itself I wouldn't link it to the engine running warm.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Thanks for your thoughts, I do like the idea of an air pocket.

Engine was rebuild by Metric Motors our in LA. Injector pump was done as well.

After the install I had an occasional coolant leak, now wondering if it was the auxiliary cooolant pump and if that could be related to the and air lock.

Car overheated. Now running normally except for the new high rpms with occasional rpm surge when in park or neutral. I'd be very interested in the what could be behind this new finding.

Any thoughts on if it might make sense to reset the idle screw?

If the idle speed is normal when it isn't surging, then no to resetting the screw.

How much oil is in the hose on top of the air cleaner? How much is on the dipstick, has it gone down?

Uncontrolled RPM's can be from crankcase oil getting into the air intake and getting burned as fuel. This is called a "runaway" diesel and can be dangerous. If the breather hose is directed into a jar does it catch oil and does the rpm surge stop?

If the engine RPM's get really high turn off the key and get away from the car. I'm not trying to scare or alarm but want you to understand what you could be dealing with.

Good luck and check that breather hose and the air cleaner element too for excessive oil.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

The base idle setting in the IP was okay prior to the engine rebuild.
Now the base idle is a bit too high for no load conditions, allowing idle hunting that can reach breakaway idle speed that the governor can no longer control.

I'd try adusting the external base idle screw as outlined here:
http://www.peachparts.com/shopforum/diesel-discussion/305016-diy-adjust-base-idle-om615-616-617-a.html

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Good morning and thanks so much for that information on "runaway" diesel and the consequences of crankcase oil burn. Checking the breather hose and air filter seems very simple and I'll do that this morning.

Idle speed, however, is now around 1200. Previously, before the car had its overheating episode, it was around 750.

If I don't see excess oil coming out of the breather tube it would be my inclination to next adjust the idle screw.

I read through that very helpful and detail post on how to set the idle from scratch, which involves removing linkage rods and dealing with the vacuum system. This is beyond my current skill set. However, could I just loosen that 14mm nut and turn the idle control screw counterclockwise until I reach an idle speed back around 750 rpms?

Thank you all so much again.

Alec has a good idea/suggestion and I hope it works.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Yes, you can get away with just turning the idle control directly. I've done plenty of small adjustments that way with no issues. Big adjustments, however, should probably be done the "proper" way.

Gorgeous wagon by the way! A green wagon is my dream W123

Thanks! Green is my favorite color too.

Appreciate the help everyone, will let you know how things turn out.

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You mentioned heat from the heater stops when the temperature went high. For the time it takes after one of those episodes. The next morning check for a hard coolant hose.

Thermostat replacement sounds like a good start. They probably installed a new one at the time of re building. It could be easily be flaky.

You also mentioned coolant leak this could be from higher coolant pressure. Than normal. If a new thermostat tested in hot water before installation does not fix the heating problem. A check for the by products of combustion might be checked in the coolant.

I suspect the temperature gauge was operating normally. As your heater output stopped when the gauge indicated hot. That sounds to me like circulation of the coolant was either lost or substantially reduced.

My understanding, (and please correct me if I’ve got this wrong) is that the inverse relationship between heater output at the vents with heat set to full high and coolant temperature essentially rules out a bad thermostat being the cause.

My understanding is that a closed thermostat cuts off coolant flow to the radiator while and open one allows flow to the radiator.

If the thermostat was sticking open I’d have coolant at correct temperature, about 90C, and warm air out of the heater. If is stuck closed I’d have supper hot air out of the vents and coolant temps spiking to 120C.

Observations are the opposite of this. Cool air out the vents (with heater set to full hot) when the coolant temps are headed to 120 and hot air out the vents as the coolant temps are back at 90.

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My knowledge base and experience is limited but right now differential diagnosis of this problem has air lock at the top. And could a failing auxillary water pump be related to this, allowing both coolant out and air into the system? I have ordered a new pump, as far as I know the old one is 37 years old, I'll see what happens and let everyone know. Thanks again.

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Also my plan is to adjust the idle down. Any hints on how to loosen that 14mm nut - it looks tricky to get to.

A properly operating thermostat only controls the minimum operating temperature. A properly operating cooling system other than the thermostat sheds any heat over that minimum. A defective thermostat can completely stick closed or open but commonly get "wonky and sticky".

Anytime coolant temp control gets erratic I replace the thermostat with a high quality part.

Good luck!!!

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Interesting findings with respect to the idle.

1) turning the idle adjustment screw seems to have no effect on idle speed (still at about 1200 rpms)

2) the manual stop function does not work. If I push the stop lever forward nothing happens.

Any thoughts on why that might be, and is it possible to connect these observations to recent episode of overheating?