BMW e34 525tdS: Cold Start & Running Issues

Hello everyone,

My problems with my car have been keeping me up at night. As a silent reader, I've already been convinced of the good quality of this forum, so I'm going to take a chance and try to describe my issues as best as possible, hopefully without stepping in it on my first post!


The vehicle is a BMW E34 525tdS with the M51D25 engine, first registered in July 1995, with a mileage of 235,000 km.

Due to fewer cold start problems (requiring only two pre-glow cycles instead of the usual amount, resulting in a cleaner cold start), I decided to install six new glow plugs. However, this was likely an example of 'fixing what wasn't broken,' as the following changes occurred:

- Poor cold start performance, requiring 2 to 5 attempts of pre-glow (and this was in a relatively warm underground garage at 6°C!!!)
- When it starts, you can already hear that it's not running smoothly on all cylinders.
- The engine speed was briefly increased (for 10 seconds), but then it dropped to the level of a warm start, and it's definitely running too slowly, I'd say.
- The exhaust smells strangely smoky, it's quite dark, and black particles are being ejected. During short bursts of acceleration, there's a bright blue exhaust plume.
- After the engine warms up, it runs smoothly, but the exhaust smell is still strange, hard to describe, not like usual. By the way, the car has been retrofitted with Euro 2 emissions standards, and it used to smell 'cleaner.'

What have I already done so far?

- Replaced all oil feed lines.
- Glow plugs checked again.
- Fuel filter replaced.
- Replaced the pre-charge pump relay (was not triggering on the diagnostic system).
- Pre-filter pump tested.
- A check valve is installed in the return line (BMW has always secretly installed it during inspections, but more often when there are complaints about overheating).
- Thoroughly checked for potential air leaks (as far as I could tell).

Result: NULL!

I had the error memory read out yesterday at a small, independent workshop.
25. GLOW TIME CONTROL.
Does that mean anything to anyone?

Further plans:
- Glow plug relay (expensive, so not yet replaced as a precaution), but would it be consistent with the reported fault?

If nothing else works, I might have to give up and take the car to a workshop. But is there still a skilled mechanic there who knows the E34 and can fix it?
Perhaps the start of the subsidy period and all tax periods should be reviewed. Would it be better to go to a Bosch service center? Does anyone know a reliable specialist in and around Munich?
These things are beyond my knowledge and abilities.

If this isn't the car brand that's frequently discussed in this forum, there's still a chance that someone here is a BMW expert.
I am, of course, grateful for even the smallest hint, and if it only helps to prevent being overcharged for troubleshooting by WS!

I would appreciate any help I can get, thank you in advance to anyone who takes the time to read this. I'm quite attached to this car, so please don't let me down!


Best regards,
The diesel deer.

Okay, the glow plugs are new and working. Are they being supplied with power during starting? Have you measured that? It sounds very much like there's a problem with the power supply or control of the glow plugs. That would also explain the rough cold start. Try to determine during the preheating phase whether the engine control unit is correctly activating the glow plugs. Then you can check whether the problem lies in the wiring or the relay. The fact that it started better before the glow plugs were replaced suggests that the problem might be in the wiring.
How about the air filter? Is it possibly oily? And what are the driving characteristics when the engine is warm?
If it has a Bosch fuel injection pump, then after thoroughly checking the glow plug system, the air filter, and the diesel filter, it's probably worth visiting a Bosch service center.
It's possible that the injectors aren't in the best condition due to the mileage. It's impossible to assess that from here.

Hello Dieselhirsch,

Did you find my thread about my brother's 525tds starting problems using the search function?

If not, then please find the translation below:
/viewtopic.php?p=44083&highlight=#44083

Even in this forum, there are BMW enthusiasts or people who know them, although BMW is not the primary focus here.

Best regards, Georg.

Hi,

It's likely that your BMW's glow plug system has a fuse. So, definitely check the glow plugs to see if there's voltage present during the preheating process.

Best regards, Rainer.

Evening, everyone!

Thank you for addressing my issue! *Thumbs up*

Did you find my thread about my brother's 525td's starting problems using our search function?


Sure, I found it and read it. It's just a bit different in this case, and I couldn't draw any specific conclusions related to my topic. Still, thanks!

So, the glow plugs are new and working. Are they being supplied with power when starting? Have you measured that?

The glow plug system in your BMW has a fuse... so, definitely check the glow plugs to see if there is voltage present during the preheating process.


To be honest, I've only used my test lamp to check if there's voltage, and that's the case in 6 places. Lacking a proper measuring device, I could resort to the rather unreliable method described in the technical article, using a 55W bulb to determine if a glow plug has failed. However, due to the engine's construction, I'll have to remove the intake manifold in any case to get close enough to the area.

Can I understand Rainer's suggestion regarding the security measure (sorry, I'm not an electrician) to mean that it might be possible to use a measuring device to check the current flowing through that area briefly during the preheating process? This could potentially provide information about the failure of one or more glow plugs?

How about the air filter? Is it possibly oily? What are the driving characteristics when the engine is at operating temperature?


The Lufi (air filter) is brand new and I replace it fairly often, I'm quite particular about that. There's no oil in that area.
Smooth and quiet operation, with no loss of power or other noticeable issues across the entire RPM range.

due to the mileage, the fuel injectors are no longer in the best condition.

I'll definitely get them checked out soon; that should provide some answers, and it's a possibility.


Well, still questions upon questions, I hope I can figure this out even without the guys from Bosch! Unless someone from there is reading this.


Best regards,
The diesel deer.

Okay, so the testing with the 55W lamp involves screwing the glow plug into the motor opening, and the power-carrying rail (or connector) at the top of the glow plug has been removed. Then you can test the function by connecting it to the positive terminal of the battery. What Rainer and I meant is whether there is any measurable voltage between the connector/rail and the engine block when starting. If there's a blown fuse there, no current will flow at all. You need to check that. Use a multimeter suitable for 12V, place it between the voltage-carrying parts, and preheat the glow plug, observing the reading.

You can also check the fuses beforehand for continuity, possibly in the main fuse box, or somewhere else in the engine compartment. This should be indicated in the owner's manual.

Hi,

I would take a closer look at the glow plug relay, possibly examining the solder joints for any suspicious signs. The error message has to be coming from somewhere.

Here are three more suggestions that are worth checking out:

- Measure the battery voltage during preheating and starting. It's possible that the voltage drops so much that the preheating relay shuts off (prematurely).
- Use the diagnostic tool to check the coolant temperature sensor(s) for the engine control unit (there may be two, as with VW engines) to ensure they are reading plausible values. This could also explain the rough running when the engine is warm, if there is a fault.
- Measure the compression of all cylinders.

Roger wrote:

Check the coolant temperature sensors for the engine control unit (there may be 2, as with VW engines) to ensure they are reading plausible values.

The 525tds even has 3 temperature sensors... I have no idea what they're all for.

Hello everyone,

Thank you again to everyone involved!
However, there have been a few minor misunderstandings, and the reason for this is the design of this engine.

The procedure for checking the spark plugs while they are installed is fairly clear, but unfortunately, you can't really do it quickly on an older Dino. You really have to remove the intake manifold to access and unscrew the power terminals from each spark plug. There are no connectors, unfortunately; you have to remove a nut from everywhere, and you can't even reach them with the hands of a gynecologist.

I'll still give it a try; it doesn't hurt to try, and I understand the principle!

The 525tds even has 3 temperature sensors... I have no idea what they are all for.

Basically, it's even 4 if you also have a radiator shroud, like in my case. Two of them are worth investigating and are likely responsible for the glow control, which I will examine.

explain the uneven running in warm conditions.

He doesn't have that, and the cold start phase is completely unacceptable.

What I haven't mentioned yet:

- Battery is brand new.
- The idle speed was considered very good and sufficient.
- Compression test in September, all cylinders >20 bar, the largest deviation was 0.2 bar, so it's quite good and well-balanced.

I will definitely take your advice and investigate the glow plug relay more closely. I will also continue to report on any developments.

Thank you all very much for now.

Best regards,
The diesel deer.

Hello,

I wouldn't loosen anything there; I would simply use a clamp meter to measure how much current is flowing during the preheating process.

Best regards, Rainer.

Quote:

The exhaust smell is still strange, hard to describe, not like usual

. That's what I meant by unclean.

Okay, after a long period of back and forth, suspicion, and all that.
I have finally decided to seek help from a kind person.
What was decisive was - the master himself drives such an old car!

The following actions were taken:

- DDE diagnosed.
- All glow plugs tested -----> Brand new glow plug on cylinder 4 was defective, replaced.
- Install a check valve in the return line, in addition to the one already installed, and also place one in the supply line, close to the tank.
- Slightly increased LL speed.
- Spraying started after an early adjustment.

All these measures result in the fact that all problems such as cold starting/running and warm-up issues are now practically non-existent!
Positive side effect: It's hardly possible to detect any more soot.

Thank you again for your help, and have a safe journey!

The Diesel Deer.