Case Study - E38 - Juddering and Droning

Date of manufacture	Oct 2000
Mileage	110,000
Model	740i
Engine	Vanos M62

Symptoms

This first case study is a strange one and took quite a while to sort out. The car had near-normal power levels, drove well most of the time, good gear changes but would occasionally seem to misfire and judder, especially after a long motorway run when leaving at an intersection. The other problem that slowly got worse was a regular drop in revs when cruising, especially in 5th gear at 60 MPH. The drop in revs was barely noticeable but could just be seen on the tachometer and was very regularly occurring every two seconds. Also noticeable was a distinct droning when accelerating with the revs below 2000 RPM. The only way to stop the droning was to change to Sports mode or give the throttle a poke which raised the revs above 2000 RPM.

My first thought (and I had many) was that it must be something to do with the closed-loop fuel injection system as that is the only engine function that cycled at that rate. Possible faults could be with the MAF or Lambda sensors, possibly an air-leak or failed PCV valve. After quite a while the E-OBD warning illuminated on the instrument cluster (Service Engine Soon indicator in the US). Carsoft diagnostics showed two errors - Bank A and Bank B adaptations reached limit. This means that the fuel/air mixture is wrong, more evidence for a sensor failure.

Repairs made

The first repair was to replace the thermostat as the OBC showed that the engine was running at the wrong temperature. The temperature dropped as low as 66 Degrees Celsius when cruising and that would surely affect the fuel mixture. After replacement the car ran at the right temperature, managed better MPG but the original faults were still there. In fact, replacing the thermostat made things slightly worse....surely this was a fuel mixture fault?

The next repair made was to replace the PCV and the MAF. The MAF had been previously replaced with an aftermarket (Ebay) unit, this was tested during the repair and was found to be absolutely miles out. Be warned, aftermarket MAF copies don't work!

The result of this work was that the car accelerated a lot better, the E-OBD warning did not come back - but - it made absolutely no difference to the droning, misfiring and juddering! At around this time I heard of another member on The 7 Series Register who had the same problem and was having similar success in fixing it. He had already changed the fuel filter and the fuel-pressure regulator (which was my next plan). Fortunately one thing that was mentioned is that his faults started after a transmission refill.

The fix

Quite simple really, drop the transmission sump, replace the screen-filter, refill with fresh fluid and that fixed it! Even though the transmission in the E38 is 'sealed for life' the fluid and filter should be changed after 100,000 miles. The problem wasn't the gearbox itself but with the torque converter.

What was thought of as a misfire was in fact the torque converter rapidly juddering, probably because the lock-up clutch was engaging incorrectly. This was also the cause of the droning, the lock-up clutch was prematurely engaging keeping the revs too low. In normal operation a slight acceleration should cause a rise in revs immediately, this was not happening before the fix. The slight drop in revs that happened every two seconds was also the torque converter, it was failing to engage when it should have been.

All these faults can be attributed to the quality of the ATF, below 2000RPM the viscosity of the fluid could not provide the energy to operate the torque converter correctly. The hotter the fluid was, the worse it got which is why the problem was more noticeable after a motorway run.

I was absolutely sure this was an engine fault, the 'misfire' felt just like a couple of pots giving up and the transmission was the last thing I suspected.......just goes to show, sometimes it is the least obvious thing that fixes the problem. Why did the other member's problems get worse after an ATF change? It was probably under filled!

Update +15,000 Miles

The car is still driving well with no cyclic drop in revs when cruising and no juddering, however the droning is back in certain circumstances. This is due to the torque-converter failing to break lock during very mild acceleration. My diagnosis is that the lock-up-clutch (part of the torque converter) is locking-up prematurely. This facet of the original fault started re-appearing slowly during the next 15,000 miles, especially when the car had been given a good thrashing. It is very easy to drive around as adding a little more accelerator or using Sports mode solves the fault entirely.

Torque-converters are simple devices but quite complex in their operation. It could be a fault with the stator which should spin in certain circumstances (when the pump and turbine are rotating at similar speeds). It could be that the stator fails to rotate in these circumstances, it would certainly explain the droning noises as the turbulence would be excessive. At the present time the original faults are, in the main, fixed and I am happy with the performance even if I have to 'drive around the problem' to some extent. In conclusion, if the performance eventually returns to the original faults I will probably exchange the torque-converter, presently, the car is infinitely better and I will continue to occasionally 'drive around' the droning.

Update +25,000 Miles

The problem is still getting worse but not anywhere near as bad as it was. It is definitely the torque-converter lockup clutch that is the problem. Whenever the EGS (gearbox control unit) demands that the torque-converter locks up the droning starts. This can be driven around quite easily as the lockup clutch is controlled differently depending on which mode the gearbox is in, for instance, although selecting Sports Mode will put the gearbox into 4th gear at 60MPH this is a different mode than selecting 4th gear manually (by using the + and - in Steptronic mode). When in Sports Mode the EGS will enable the lockup clutch at a much lower speed than manually selecting 4th gear. This means that the droning can be stopped by manually selecting 4th gear rather than using Sports mode.

As the lockup clutch is not used in any mode when travelling under 40MPH there is not a problem at these speeds. The speeds between 40MPH and 70 MPH are the problem areas and these can be driven through by using the gears manually without any droning. Only one speed causes any problems and that is at 50MPH, it is a bit too fast for 3rd gear and causes droning in 4th on very low throttle settings.

I think there are two ways of curing this problem, one is obvious, replace the torque converter. The other is to remove the connection to the lockup clutch solenoid. The WDS (wiring diagrams) do not show which solenoid is responsible for the lockup clutch but it should be pretty easy to work it out. This would stop the operation of the lockup clutch entirely. This isn't such a bad thing, it slightly reduces fuel economy but has few other side effects. I was driving a 740i with a failed lockup clutch recently and the only noticeable difference was that at 80MPH the revs rose and fell with any slight movement of the throttle.

Update + 37,000 Miles

The problem was the same a year or so later, I heard reports about Lubegard's 'Dr Tranny Instant Judder Fixx' helping the problem on a few Forum's so I found a seller on Ebay with stock and had some imported from the States - and then ignored the stuff for a few months. At MOT time I asked the garage to squirt the stuff in the gearbox (by removing the upper filler plug and introducing the liquid from there). I wasn't expecting much - not from a gearbox that has had the problem for 50,000 miles. But, it works, and it works perfectly! For the first time in my 740i, I could just leave the box in 'D' and drive it without any shuddering, droning or listening to the revs hop up and down. It feels like a different car - perfect. Lubegard's 'Dr Tranny Instant Judder Fixx' is a friction modifier, it solves this problem by stopping the torque-converters lock-up clutch fully locking when it should only be partially engaged.

I will update this thread if the friction-modifier causes any side-effects or damages the gearbox. But, seeing as the torque-converter is not shuddering and prematurely engaging, it should last a lot longer.

An Email from a fellow sufferer

Hi Tim,

Thank you very much for posting this case study...after suffering the symptoms reported for several months I was just about to give up (after spending about 1000 pounds on dealer and independent garage diagnostics)...Once I read this case study I realised what the problem was straight away...(I had already tried changing the transmission fluid recommendation) and had the Torque Converter changed (at a price of 730 pounds though I might add) and now the problem is solved and I love my car again...Thanks! If you are interested I got the work done at Devon and Cornwall Transmission in Ivybridge...job done in 1 day...they were really good......Mike