P38 bmw diesel torque converter

[Team Idris]

We have bought a P38 with the six cylinder bmw diesel and the HP24 gearbox in it. We feel it revs way too much in top gear and we wondered if the torque converter is old and weak?

We were both plant fitters and I have to say it feels like a really high-stall unit. It actually revs more than the HP22 behind my 3.5 RV8. So we figure it’s either worn or it’s bad inside (maybe bent).

any thoughts, or are we way off the mark?     I mean, it’s 26 years old, so it is entitled to have a bad torque converter

[elbekko]

That's just how they are, I'm afraid. Gutless engine that needs its nuts revved off to get moving. It's mildly better with a manual.

Revs in top gear shouldn't be dependent on a bad TC, as it should be in lockup then. Do you have any diagnostic tools with which you can verify it's going into lockup?

[Escape]

As Elbekko says, the BMW engine needs to rev to get the Range moving. But if you're easy on the throttle, you can get it to shift at 2500-3000rpm. And lock-up around 50mph. You have checked or replaced the ATF? That would be my first point of attention.

Edited March 8, 2018 by Escape

[Tanuki]

Remember that it's a BMW-designed engine so it's going to be happier to rev sportily rather than stagger along at low-RPM like a truck or tractor.

Provided the torque-converter and transmission work to generally keep it spinning in the RPM-range between the peak-torque (2200RPM) and peak-power (4800RPM) points, everything's fine.

[Team Idris]

Thanks

we didn’t want to rev it to death with it being so old. It just seemed wrong?

all the gears seem fine, which was what confused us.

although, I’m not saying a lower stall torque would hurt the truck. Especially with a turbo change to something that spooled up a bit faster.

I’m still fascinated that it gear changes higher than my RV8.

[David Sparkes]

I can tell you how to get more boost at lower rpm (my interpretation of your comment 'something that spooled up a bit faster'), by taking better control of the turbocharger bypass valve actuator.
The initial point is to realise that, as standard, the actuator starts opening the bypass valve as soon as tickover rpm is exceeded. The slightly open bypass valve allows exhaust gas to 'avoid' the turbo, so the turbo doesn't 'spool up' as quickly as it would if it experienced full gas flow.
Basically you add air control valves that prevent any boost pressure reaching the actuator until boost reaches about threequarters of its maximum. The first additional valve then opens, the actuator opens the bypass valve, and hopefully the inertia of the accelerating turbo doesn't surge the boost pressure past its maximum level. Left on it's own, with the bypass fully open, the turbo will slow down, and the pressure will fall to the point the 'first additional valve' closes, shutting the actuator completely, and the cycle starts again, producing a cyclic surging which is both unpleasant and potentially damaging.

The second additional valve prevents this, by allowing just enough pressure to reach the actuator so that it holds position, limiting the opening of the bypass valve, thus allowing the turbocharger to have enough power to maintain the desired maximum boost pressure.

Note this system is entirely under your control, and its 'mis-management' is an excellent way of sowing the seeds of your own destruction, along with the destruction of the engine.
You can keep the standard maximum boost pressure, and simply reap the benefits of the improved spool-up response.
You can have both the improved spool-up response, and an increase in maximum pressure, enjoying the benefits of additional power throughout the rev range.

IIRC, the maximum boost that can be achieved before the Engine Management calls foul is 22psi. At this point the engine goes into limp home mode, but will reset itself when the engine is switched off and back on again. The 'competion' answer is to fit a different pressure sensor so the ECU doesn't see the excessive boost. I didn't go that far, I recall setting a maximum boost of 18 to 20 psi. This recollection might be incorrect.

Where I sewed the seeds of my engines destruction was holding higher gears in the manual gearbox, while using full throttle and high boost at 'low' rpm, relishing seeing the maintained high boost on the gauge when climbing hills, boost which exceeded the factory maximum at much higher rpm.

I think you would be protected from this folly, of holding high gears, by the automatic gearbox, although I've never experienced driving the diesel modified as described, and backed with an auto gearbox.
It should go without saying that if you went for higher maximum boost you should fit a larger intercooler, accepting the negatives of reduced airflow through the coolant radiator, and the difficulty of cleaning the radiator fins should the car ever go through muddy water.

One modification I did do, which helped dampen the temperature spike when slowing down after arduous use, or even normal motorway cruising, was to introduce a manual electric fan control, switching the fans on low speed, to run continuously after the engine had reached operating temperature. You won't see the difference using the standard, heavily damped, temperature gauge, but I was monitoring the engine with my Rovacom, so saw more detail. This not only gave better control of the engine temperature, but also that of the intercooler (inlet air temperature). I naturally kept the standard viscous fan.

Regards.

[Team Idris]

It might like a charge cooler then, to reduce the cooling side air volume.

[Bowie69]

On 10/03/2018 at 6:28 AM, Team Idris said:

still fascinated that it gear changes higher than my RV8.

Perhaps you need to adjust your kickdown cable then

[Team Idris]

It’s having the transmission pulled and the converter sent to ATP for a look see.

[Tanis8472]

Quote

On 11/03/2018 at 4:01 PM, Bowie69 said:

Perhaps you need to adjust your kickdown cable then

Its an electronic throttle. 

On 07/03/2018 at 7:29 PM, Team Idris said:

We have bought a P38 with the six cylinder bmw diesel and the HP24 gearbox in it. We feel it revs way too much in top gear and we wondered if the torque converter is old and weak?

We were both plant fitters and I have to say it feels like a really high-stall unit. It actually revs more than the HP22 behind my 3.5 RV8. So we figure it’s either worn or it’s bad inside (maybe bent).

any thoughts, or are we way off the mark?     I mean, it’s 26 years old, so it is entitled to have a bad torque converter

The diesel should have the 4HP22 not 4HP24 

 

[Bowie69]

I meant on the RV8.....

[Team Idris]

To be honest it’s on now, but I ran without it for a bit. It launches like an idiot now

although, it was mad without it? 

Anyhoo, rebuilt box, torque checked and the bmw has been chipped. It now drags the trailer well, but is prone to smoking. (Which it would I guess) I think it needs turbo seals.