Hot Hydraulic Oil: Causes & Solutions

Hello.

I've been having problems with my Passat 3bg, model year 2001, 1.9 TDI AVF engine, for a year or longer.
Specifically, my steering makes a cracking noise occasionally after about 10,000 miles, and sometimes less frequently.

I've already removed the LWL sensor and replaced the axial joints, checked the steering linkage for secure mounting, changed the power steering fluid (using the original Audi fluid), and lubricated the intermediate shaft, but none of that worked.

It wasn't until I finally caught the culprit, removed the cross joint from the gravel, and weighed it, that I had three months of peace.

Now it's cracking repeatedly from the ball joint, and I've noticed that the steering gear is also getting extremely hot.
The P-line from the pump to the steering gear is extremely hot!

The temperature inside the container is appropriate, I would say.

My question is, how hot can the oil get after the pump, and what should I do next?


Thank you, best regards.

Above 50°C, I would start to worry about the pump during normal operation. However, if the system is stopped at the end of its travel for minutes, it will reliably damage itself.
Is the cooling pipe present in the return line? I know of cases where this seemingly useless loop of tubing was simply bypassed in situations with severe rust.

First of all, thank you for your reply!

I'm sure the pressure line after the pump gets hotter than 50°C, but I haven't actually measured it. If the bellows are so hot that they crack when cooling down and you can't touch them, I would assume it's more than 50°C.

However, under normal operating conditions, i.e., after about 1 hour of operation, and even faster when it's warmer outside.
The pump doesn't struggle even during normal operation; you can hear it humming when it's in the end position.

Is there a pressure limiter somewhere?
Since the pump is cheaper, I would replace that one first.

The cooling tube is present, and I would say the oil in the container is at 50°C.

I've been reading in various forums that the oil should be bubbling inside the container, but in my case, I can only observe a flow.

That's the problem of different subjective perceptions... "bubbling" and "flowing" are very close in meaning. I would personally prefer a "flowing" state, because "bubbling" somehow suggests air, which I don't like in a hydraulic system.

Here's my tip: Everything up to 60 degrees Celsius is still bearable to touch, but anything above that is painful.

"The cooling pipe is there, but is it properly connected, or has anything else unusual been changed in the system? The heat can only be caused by excessive energy input in the area of the pump. Is the original part installed? Could it be that a wrong pulley is installed? Can you compare it to another vehicle?"

Everything is still original, with 167,000 km on the odometer. I'm planning to replace the timing belt in the next few days, and I could replace the water pump at the same time!

The trunnion of the steering gear, which is connected to the universal joint, is difficult to access and can cause burns if touched. Similarly, the P-line is also in a hard-to-reach location.

I also feel that the steering becomes slightly (but really only very slightly) heavier after about 1 hour of operation.

How is the pressure regulated, or is it constantly at 110 bar?

My assumption is that the oil becomes slightly thinner with the operating temperature, which causes it to accumulate in the pump and leads to overheating.

hello

I have now also compared it with other vehicles of the same model.

Apart from the temperature difference, I couldn't find anything else. Everything was hot, from the pump to the pressure hose and the steering rack.

Does anyone have any other advice before I order the pump?

Thank you, best regards.

I don't understand. Before installing a new pump, I would measure the exact temperatures first.
Where does the temperature reading take place? It should actually be quite easy to determine!

Hello everyone!

@LiZ: The "temperature input" can, due to its design and functionality, only be connected at a single location .
...unless he can crank the steering wheel faster than the hydraulic pump .

I would also suspect a pressure that is too high in the system, or something similar, because excessive heat generation due to mechanical friction within the pump would certainly not be sustainable for long.

Best regards from Munich!

dieselbear@

If I understand your interpretations correctly, are you suggesting that the problem is with the steering mechanism?

Could you please be more specific about the nature of the defect? Is it a blockage, a damaged seal, or a deformation?

Thank you, best regards.

Hi Niiinia! ...first of all

No, I can't really be more specific, as I can't tell from a distance where the excessive heating is occurring exactly within the hydraulic system, nor can I know how your seals, deformations, and blockages are doing.

My previous post referred *solely* to LiZ's statement.

In short: I have no idea, sorry.

Best regards from Munich!

A faulty sliding valve can definitely cause significant temperature increases!
Have you already checked the ease of steering without power steering, especially with the wheels raised?

Hello.

Yes, when I installed the axial joints, I moved the rack back and forth, and it moved easily.

I'm not very familiar with hydraulics. How is the pressure regulated in these systems? How does the pump speed affect the system? Is there a pressure relief valve?

Thank you.

For me, this is a fairly simple system. The pump provides a flow rate that is always directed to the rotary valve/torsion spring/whatever valve in the steering gear. If you do nothing, meaning you don't demand any power through steering movement, the oil flows from the pump through the valve and back. The pressure loss is minimal because very little power is required, and therefore very little heat can be generated.

In my opinion, there should be no valve in the pump itself, only in the steering gear.

I can only imagine that something is increasing the pressure loss in your system, which in turn requires more power, ultimately leading to heat... It could be contamination, a problem with the return line... I don't have any other ideas from here.

Hello again.

I have now cleaned the servo oil reservoir and backwashed the filter. (The return line is filtered, which means all the dirt stays in the system, what's the point of that??? Also, there are two restrictors in the line.)

I changed the oil again and flushed the entire system.

Possibly the oil isn't getting hot enough (target temperature unknown), but it definitely can't be considered cold. It's the same temperature as the coolant!

I will be taking measurements with a temperature laser soon and will report the results.

The ball joint is pre-tensioned by turning the screw counter-clockwise and then tightened. My suspicion is that the screw itself in this area might be causing the cracking sound!

Greetings.

Hello.

Today I took a temperature reading. The outside temperature was lower than usual, around 10°C, but the measured temperature seems correct, even though it's quite warm.

So, after 1 hour of driving, the power steering pump measured 47-50°C, and the coolant measured 57-60°C. If the temperature of the power steering fluid also increases by 10°C in the summer, that's still acceptable.

I also noticed the cracking sound today, and it definitely seems to be coming from the ball joint or the connection to the steering gear.

If I ever find a solution, I'll let you know!

Greetings and thank you to everyone!