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#4781417 - 06/08/18 03:40 PM Is thick oil the culprit?
NICAT Offline

Registered: 04/05/18
Posts: 64
Loc: Azerbaijan
Hi, As stated in previous threads, i had manual gearbox replaced.
Previous one had mineral oil of unknown viscosity (Very likely SAE 140 or something even thicker). New one has Motul 300 75W-90 Synthetic in it.

When i was driving with previous gearbox, car was slowing down faster when i shifted into neutral gear. Also when the car was in neutral gear while the engine running, if i was releasing the clutch pedal, rpm dropped a little, and idle became rougher.
One more thing, The car was moving forward even in Neutral gear. (I am not sure if this was happening only at cold starts or even after the gearbox was warmed up. But even when the ambient temperature was 35 C, the car was moving forward)

Now all the problems have been solved after replacing the gearbox and the oil. Car goes very well in neutral gear. (Makes me feel as if it will never stop), there is no rpm change when clutch is released, and car doesn't move forward in neutral gear.
Last but not least, Car's fuel consumption decreased also.
Before it was consuming 10L/100KM in city driving, where heavy traffic was usual. Now it is not more than 8.5L/100 KM. (The results are not based on single test. i have tested it many times and results were virtually same)
I find it hard to belive that oil viscosity can make such difference. But nothing else was changed in the car recently.
I have tested all wheels one by one, to see if there is brake drag. But there was none

Edited by NICAT (06/08/18 03:42 PM)

#4781430 - 06/08/18 03:52 PM Re: Is thick oil the culprit? [Re: NICAT]
BrocLuno Offline

Registered: 09/06/15
Posts: 5700
Loc: Kalifornia Kollective
No, there was something wrong with previous system. Sounds like a cluster shaft bearing was dragging (failing), which the previous owner tried to cover up with heavy oil.

It is physically impossible for transmission (manual) to transmit power in neutral, unless there is an internal issue ...
Formerly in marine engineering. In an earlier life I owned my own petroleum tank truck, and hauled for the majors and independent's.

#4781446 - 06/08/18 04:12 PM Re: Is thick oil the culprit? [Re: NICAT]
Blkstanger Offline

Registered: 08/13/11
Posts: 1995
Loc: Lakeside, CA
You had other issues besides think gear oil going on. I agree with BrocLuno.
99 Jeep WJ 4.7 186,000 Maxlife 10w30, Fram Ultra.

#4781453 - 06/08/18 04:18 PM Re: Is thick oil the culprit? [Re: NICAT]
Smash209 Offline

Registered: 02/28/18
Posts: 34
Loc: Perpetual Winter USA
I would mostly agree but the heavy oil could create a bigger drag in the bearings. I had a similar issue on a Jeep and it turned out to be a bad clutch pressure plate (on a new clutch). New oil in the trans did help the issue but the drag was still there. Mainly could not get into first gear when stopped.

Did you happen to change out the clutch or pilot bearing in the process? That may have been the culprit.

#4781614 - 06/08/18 07:07 PM Re: Is thick oil the culprit? [Re: NICAT]
zeng Offline

Registered: 09/01/15
Posts: 2215
Loc: Malaysia
I suggest OP do further reading on:
a)engine system, ie flywheel and power output;
b)clutch system or mechanism, how engine power is transferred from engine to manual transmission, and
c)manual transmission system, how transmission input shaft receive mechanical power from clutch system.

It's genius associating problems arising from mechanical components deficiencies with oil viscosity grade in use.

Edited by zeng (06/08/18 07:09 PM)