Any harm from driving with cold start out?

[Smokey Eddy]

I know the cold start lever advances the timing of the pump. But is there any disadvantage to driving with it out all the time?

[jtanguay]

i saw what it does to the timing advance piston at Giles' shop.  ONLY use the timing advance for cold starts!!!  if it runs rough without it on, then adjust the static timing and or idle.

[Smokey Eddy]

Thanks for the heads up! what does it do?

[Rabbit TD]

Exactly what does it do to the piston and where that piston actualy located in the pump.  I've often wondered about that and I've driven with it out lots of times accidently and never noticed a difference in idle, pickup, smoke, noise or mileage after it warms up but I know it moves the timing about .20 MM looking at the dial indicator when it's out on my N/A pump.

[gldgti]

i dont see why it shoiuld be a problem.... the cold start timing advance only adjusts the timing by a static amount by moving a cam against the backside of the advance piston. according to one of the SAE papers (maybe the 1.5 one) the adjustment pushes it to the equivalent '2000 rpm' position. so, the timing is only advanced below 2000rpm. after that, the timing curve is normal. before 2000rpm, its a flat line from idle up to 2000rpm.

(IF my memory of the curve plotted in the SAE paper is correct)

[vanbcguy]

From what I've read on here the problem is apparently the "finger" on the cold start smashing in to the timing piston again and again every time it moves - IE every time the car goes over 2K RPM the piston moves away from the finger, then when the internal pressure in the pump drops as engine speed drops it smashes the piston in to the finger. 

If done repeatedly, well...

[burn_your_money]

I'll be back with pictures

[Rabbit TD]

I've only had a pump apart 3 times but from what I saw the advance lever rotates the peice that has the rollers on it that the cam plate runs against which it looks like to me just changes the position a little when the cam starts to lift but the lift is always the same amount isn't it?   From what I remember the only constantly moving parts in there other than the vane pump and the governor wheel is that cam plate and the plunger which are always against eachother under spring pressure which is the same all the time from what I see.  I could never understand though that after it's warm it doesn't seem to affect a thing.  But rotate the pump that same amount and it will and it will affect everything all the time.  I never understood that part

[gldgti]

But rotate the pump that same amount and it will and it will affect everything all the time.  I never understood that part

the cold start advance simply pushes the advance mechanism to a predetermined point - imagine the advance piston has a certain range which changes with rpm - at idle, or shutdown, the piston is at one end of its travel, and at max rpm it is at the other extreme of travel. the cold start mechanism simply moves the piston a little towards the advanced end of the scale - kind of like changeing the zero point a bit. now, the travel of the piston is STILL goverend by internal pump pressure, and works against the advance spring - SO, when the cold start advance lever is engaged, the piston only begins to move after '2000 rpm' (if all the settings are correct) because there is a preload against the piston from the advance spring.

NOW - if you rotate the pump so that the static advance stting is the same as the cold starting position, then you index the entire advance curve upwards by that amount. the cold start lever still works just the same, and will advance  the timing in the same way - you just may not notice it anymore, because the timing is advanced enough already.


further on driving with it engaged - when the pump is operating, we cant forget the internal pump pressure working on hte other side of hte pistin against the spring. i can't imagine that the advance piston would 'smash' back down to the advance lever cam very hard with fuel pressure resisting on the other side.

[lovinthedeez]

it's actually 2200 rpm when the timing advance stops.  courtesy of one of my former teacher's diesel books that he wrote.  has a super duper section on the bosch VE pumps.  I'll see if I can scan some of it. 
I drove my jetta conversion with it out all the time for a minute, because I didn't have the handle yet.  but since most of my driving is in town, the mileage suffers.  over time I guess it would be bad.

[jtanguay]

i've personally driven with mine out all the time because the car seemed to run like crap without it (it was scary pulling out in traffic  ) i almost want to dissect that pump and inspect the timing piston... i drove that car really hard too.  but i suppose every situation is different, and even if my pump showed no signs of damage, who knows about someone else's? after seeing what the damage looked like (looked like some type of metal eating creature was hungry  ) it got me thinking.  i know for sure that i won't be using the cold start on my mTDI too often.

[lovinthedeez]

i not trying to argue, but is it just louder at idle because the fuel detonates quicker, making for a longer time of combustion.  I'm gonna go take mine for a ride to the junkyard(ha, what an excuse ) and see. 

[burn_your_money]

Sorry it took so long

[burn_your_money]

That's not the only timing piston like that I've seen since working here. But no, the customer did not specifically say that they drive with the cold start on all the time. Although a lot of our customers just send us a pump with no contact info so we don't really expect info like that. 
My personal speculation as to what is happening is that the vane pump does not create a constant pressure. Instead, since it has 4 blades it makes pulses. I imagine that there is a very narrow RPM range (internal pump pressure, or advance range) where the timing piston has just moved off the cold start ramps. It's here that the piston pulsates back and forth (probably just a few tenths of a millimter) and eventually hammers the pin loose.

Also, when the control sleeve uncovers the spill port on the rotor, high pressure fuel is introduced into the pump housing. If this is in synch with the pulses from the vane pump the effects will be amplified even further.

[79rabbit4dr]

  this is going waaaaaay over my head.

So the answer is "inconclusive" thus far?

Because I've forgotten to push it back in a few times and wondered this myself. I've never noticed any difference in the affect it has when trying to start in cold weather (meaning it still works the way it should)... but maybe it just hasn't happened yet?

I'll keep with the theory that it should only be used for "cold starts" and should be pushed back in after that, but if I forget, I won't light my car on fire and claim it on insurance or anything