Kangarooing, Surging, Pulsing, Oscillations, Resonance.

grahamaaron · Post by **grahamaaron** » Mon May 08, 2006 2:41 pm

Whatever you want to call it, the symptoms my 1984 NA diesel T25 Vanagon/Transporter is exhibiting has got me totally baffled. At around 20 to 30 mph (any higher\lower and it doesnâ€™t occur) and when the engine is under neutral loading (i.e. when the van is neither accelerating nor decelerating) the engine begins to twist backwards and forwards on its rubber mounts causing the whole van to surge back and forward. It takes around 5 seconds for the pulsing to build up and the frequency is around 5 times a second and boy is it annoying.

Initially I thought it was a diesel supply issue so I have changed the fuel filter and used fuel additives to clean the injectors, etc. I have also changed one of the engine mounts thinking that maybe it wasnâ€™t dampening the engine properly. None of this has made any difference. Iâ€™ve also checked the throttle cable outer shroud mounting and cable tension and these appear okay. Has anyone got any ideas as to what might be wrong?

If you would like to see the fault in action visit my website www.grahamaaron.plus.com to see a short video.

Also visit http://www.thesamba.com/vw/forum/viewtopic.php?t=170780
to see more information on what I have attempted to do and other peoples help.

I have now started to wonder if this is a diesel specific issue, hence bringing it to this forum.
Thanks in advance for your time.

Stoker · Post by **Stoker** » Thu May 11, 2006 5:57 am

I think the resononce is from the throttle cable being too short.
Just like a Beetle when the front trans mount fails completley. In first the trans rairs up, and pulls on the high mounted throttle cable, and then the car really accelerates to the new diled in speed, then the engine falls, due to not being driven up by the wheels, repeat.
I think the torque is kicking in (problby inj advance) and the motor mounts are winding up and pulling on the throttle. Due to the mounts being relitively tight, the occilations are quick. I think I can see the throttle moving during the movie.

I would make sure to have enough sheith length in the throttle cable to move using the sheith only to pull the throttle, and not just between the van body and the engine, else this might occour even with more cable slack.

Good luck.
Regards
CB&Stokers

grahamaaron · Post by **grahamaaron** » Thu May 11, 2006 9:27 am

Thanks for your reply. The throttle was the first place I looked for a solution to this problem. I still haven't completely ruled it out yet. I have been waiting for a help to drive while I manually control the throttle level. This will rule out cable issues alltogether. I am curious about your last statement, "...I would make sure to have enough sheith ... and the engine, else this might occour even with more cable slack. "
Could you explain what you recommend again?
The cable is very free to move within its sheath, so there is no binding going on. The sheath is well mounted at both ends (i.e. pedal and engine). However the I could imagine that the overall length of the cable and its sheath could be a little short. When the fault occurs, I can feel the gas pedal under my feet pulsing, which has got to be a sign that something is wrong. Thanks again for your comments.

Stoker · Post by **Stoker** » Thu May 11, 2006 3:28 pm

Yes,
If the sheith is too short, and the cable inside it essencally makes a stright run from the throttle pedal, and streatches stright to the fuel pump, when the engine rocks back on the trans mount, it will open the trottle more.
If there is say an extra 12" of play and it loops around, (as an example) no matter how far the engine rocks on the mounts, it cannot change the throttle position on the pump.
Or try the gorrilla act standing on the exaust Or suitable lever, out the back and with the throttle held partly open, and see if the IP throttle dosn't move, dosn't take much to set up the occilation.
Hope this is of use.

Regards
CB&Stoker

grahamaaron · Post by **grahamaaron** » Thu May 11, 2006 6:05 pm

Thanks for your post. It helped me finally diagnose the problem (after nearly 2 months of searching). I disconnected the cable and found that when actuated by hand (getting my girlfriend to drive) the problem still occurred at the same speed. After playing a bit... I realized something that I had never noticedâ€¦ the problem occurs when the vehicle is stationary and in neutral. When I moved the level slowly up by hand increasing the REVs the comes a point (as small range in the lever travel), where the engine begins to pulse. Beyond and below that point and the oscillations fall away. And so I have a diesel fuel pump problem. My next step will be to take in for a service or tune, because I don't like my chances of repairing the pump on my own.

I have been longing to make progress on this issue. When I found out the specifics of the pump fault I will post it again here. Thanks again for your advice.

Stoker · Post by **Stoker** » Fri May 12, 2006 2:44 am

Well that is too bad.
When you find out what it really is, give me a holler, mine is doing something realy simmilar, but without the occilation. More of an advance problem. (I think).

Regards
CB&Stoker

Stoker · Post by **Stoker** » Tue May 16, 2006 9:34 am

I beleive I have the answer for you now.

After much head scratching I have a solution.
My truck started out just acting like the govener sticking, whilst pulling away from a stop. No problems up top. Progressivly got worse, untill it staarted looking exactly like yours. Knowing the pump didn't have any problem before rebuilding the motor, helped. The pivot bolt just below the four banjo fittings was gone. Just watched it vibrate violenly (the pump) without the bolt in place. Put in a new bolt, and as smoothe as silk. :>)

Hope this works for you.
Regards
CB&Stoker

grahamaaron · Post by **grahamaaron** » Sun May 28, 2006 2:03 pm

Hi All, replacing the pump with a reconditioned one sorted the problem. According to a couple of diesel specialists I went to, attempting to repair a pump doing what mine was doing could have been difficult with limited success. On a test bed, the pump could behave ok, but on the engine it would be still present. So a recondition pump was a more sure way of getting rid of my problem. For those who are interested, the recondâ€™ bosch pump cost me Â£ 235 and cost Â£100 to have installed with a new cam belt at the same time.
Thanks again to all that posted help.