Daf 44 fuel economy

[deskjockey]

Hi all,

I've run through a few tankfuls now and I must say fuel economy is not fantastic at under 30 mpg.

I've read a few posts about the Variomatic vacuum system and have proceeded to replace a few hoses (the fat one between the electric valve and the manifold had cracks at the engine end).

I've blown my eyeballs out checking the Variomatic lines but I'm not sure what is OK and what is not. I can feel some resistance up to a certain pressure and a faint hiss as air comes back up to the electric valve, depending on the position of the rotary valve linked to the throttle.

What is the symptom of a vacuum leak somewhere in the system?

The engine seems to be revving a bit high until I get to 50 mph and ease up on the throttle but due to my previous experience with automatic cars I may be biased into thinking that an engine should not be revving this high.

When I coast from 50 mph to a stop there is a clear downshift at around 25-30 mph (sound of engine revs increasing).

Also, pushing the electric downshift switch does not seem to make any difference at all. The electric valve works, I've tested it separately.

DJ

[pauldaf44]

Aug 11, 2012 10:25:41 GMT deskjockey said:

Hi all,

I've run through a few tankfuls now and I must say fuel economy is not fantastic at under 30 mpg.

I've read a few posts about the Variomatic vacuum system and have proceeded to replace a few hoses (the fat one between the electric valve and the manifold had cracks at the engine end).

I've blown my eyeballs out checking the Variomatic lines but I'm not sure what is OK and what is not. I can feel some resistance up to a certain pressure and a faint hiss as air comes back up to the electric valve, depending on the position of the rotary valve linked to the throttle.

What is the symptom of a vacuum leak somewhere in the system?

The engine seems to be revving a bit high until I get to 50 mph and ease up on the throttle but due to my previous experience with automatic cars I may be biased into thinking that an engine should not be revving this high.

When I coast from 50 mph to a stop there is a clear downshift at around 25-30 mph (sound of engine revs increasing).

Also, pushing the electric downshift switch does not seem to make any difference at all. The electric valve works, I've tested it separately.

DJ

That MPG is on the low side. Gladys used to return 40mpg on average and I managed 46mpg on a run once.

Changing down sounds right and they do rev abit. But backing off the throttle should cause all to go nice and quiete. DONT press the electric switch above 30mph overwise if all is working correctly it would cause the engine to rev beyond the red line! Sounds like there is a slight vacuum leak to me. Also what colour are you spark plugs?

Oh and the best way to achieve good mpg out of a DAF is to drive it like you stole it ;D

[Richard DAF Webmeister]

Without wishing to appear a spoilsport, I would personally be very cautious about this "drive it like you stole it" idea.

Whilst there is no doubt that if you've got a DAF that is set up correctly and all is working well, then they are an absolute delight and hoot, and are well capable of showing other cars what they can do, much to the annoyance of others. And yes, in my younger days with the 55, I did do traffic light demos etc.

But, I assume that the statement implies fast pedal to the metal type of behaviour?

We should remember that even the best maintained and good condition cars are now very old, so should, in my opinion, be treated with, shall we say, understanding... Also, you might be introducing stresses and strains on parts of the car such as the transmission itself but also the driveshafts.

As I say, I'm not being a killjoy here, but caution, I think...

[dafdaffer]

i agree Mr B, i have been driving biddy with a little more respect now days, she don't go any faster building up the speed with a light foot then planting it but this may be because the earlier 44 have bigger bob weights and change up quite high quite fast.

this is the main reason i have never driven one as an every day motor, modern and diesel is a must for me

[deskjockey]

I agree that old ladies must be treated with due respect. Beside, I don't think I can be backsided to drop the engine every month for repairs If I can help it...

I'm quite easy on the Cloggie Kart and I tend to feather the throttle in order to get the maximum acceleration/cruise from as little pedal pressure as feasible.

That's why I find mileage to be borderline lousy at best.


Hence my initial question, what are the consequence of a vacuum leak on engine behaviour?

DJ

[spunkymonkey]

When the revs do drop as you ease up around 50 do they sound about "right" for the road speed at that point? If they do, but she doesn't want to change up if you lift the throttle earlier then it's quite likely to be a badly adjusted vacuum valve linkage.

The rod that controls the mechanical valve behind the carb is adjustable for length and will affect the minumum speed (well, throttle opening really) at which the vacuum will assist in changing up. If it's too short then you need the throttle quite a way open before any vacuum is passed to the transmission, which translates to a later change. If it's too long then it'll try to change up too early and become sluggish even for a Daf.

The book's way of adjusting it involves a vacuum gauge and rev counter but try altering it one turn at a time (the minimum adjustment) and see what difference it makes. To adjust, you pop one end of the rod off the ball it's clipped to, loosen the lock-nut on the rod, and turn the joint. Don't worry about doing the locknut up again until you're happy with it.

If it improves things then keep going till you're happy with how she feels. You should get a definite up-shift when you ease off at anything over about 20-30mph depending on how you like her feeling. Keep a note of how many turns you've made so you can put it back if it doesn't make a difference!

[dillyman]

Thank you SM for such a simple and lucid explanation of the effects of varying the length of that rod. I have always shied away from attempting any alterations to that component. I shall experiment forthwith on Becky who does tend to be too revvy at 30-40 m.p.h. Report to follow!

[deskjockey]

Hi all,

Thanks for your advice.

I finally got around to doing a little tweaking on the rotary valve rod today. I lengthened the rod by two turns and went for a drive.
It seems that I'm now getting steadier speed-up through 20-30 mph on less throttle, and the engine is a bit quieter.

I'll try to see how much more turns it can take before performance starts deteriorating.

I'm still puzzled about the downshift switch, which does not seem to do much...

I found and purchased the DAF 44 shop manual at an antique car parts sale the other day. Apart from problemes of pages falling out, the description of the Variomatic drive provided in the book is for model year 1968, which includes a valve connected to the brake lines and a vacuum reservoir, which my model (1975) does not have.

The electric valve works (clicks) when switched on, but maybe something else does not so I'm not getting low gear lockup...

DJ