Rabbit Diesel '81 does 60.6 MPG

[cgnap]

Thanks, I'll give it a try.

[climbinghalfdome]

I too just started getting into the 60's on my MPG! Keep after it.!
Kevin

[MrSafety]

1/ I just discovered this FORUM...
2/ I just registered...
3/ This is the first THREAD I have read !
4/ This is my first POST !

Although I am a relatively "newbie" to diesels... NOW:

[1] My Wife's "daily driver" is an 1980 MB 240D that I started with ~ 5 years ago.
I have every system but cruise control working!
[2] I put my youngest Son in a 5-Cyl 1980 MB 300D and spent 4 years making it a daily driver.
I had every system working but cruise control...
and was getting ready to have a complete fresh paint job when a bit Mountaineer rearended him and totaled the 300D!
This sturdy car protected both my Son and his Wife from any real injury...
so it did what I expected it to!
I then put this same Son into an '02 TDI Jetta and learned a lot about the TDI injection pump/system making this "salvaged" TDI roadworthy!
[3] My personal project vehicle now is an '81 VW Rabbit P/U that I've done a lot of work on and
now getting ready to swap out the 4-spd manual for a 5-spd manual that I purchased for $100 but
then spent ~$600 having it completely rebuilt and even newer upgraded parts added!

Now onto the subject of this THREAD:
My studies into both the early BOSCH "distributor" style IPs in these A1 Rabbits and my studies into the TDI IPs used by VW...
these studies make me agree with Crazyrabbit's belief that making a simple repair to your A1's IP's timing system can make a great deal of difference in MPGs!

I also think that understanding and similarly changing out another simple and inexpensive (~$30) part on either of these IPs also has the potential for helping your MPGs.
I'm talking about 1 or both of the IP's "case pressure" regulating devices on these IPs.

Let me know IF I need to expand on my "case pressure" subject!

Anyone know about replacing my high-failure diaphragm vacuum pump with the newer style rotary vacuum pump?? I have power brakes that need a more reliable Vac. Pump!
Regards,
Sam Ross

[JethroBodine]

MrSafety- How about starting your own thread on the subject of case pressure. There are a few of us with these pumps.

[MrSafety]

Originally Posted by JethroBodine

MrSafety- How about starting your own thread on the subject of case pressure.
There are a few of us with these pumps.

OK... as soon as I catch up on several projects!
Sam Ross

[Crazyrabbit]

Sam,
VW Parts, VW Diesel Parts, VW TDI Parts @ Parts Place Inc. offers the later vane type vacuum pump as a drop in. Not too sure what you mean by case pressure. Is that the lift pump discharge pressure? The injection pressure is controlled by the injectors.

[MrSafety]

Crazy Rabbit,
Yes, the "Case Pres." comes from the
1st-Stage Rotary-Vane pump...
and it is regulated by two components:

(a) one next to the IP's fuel inlet fitting...
Pressure-control valve ~$30
The pressure-control valve is connected through
a[n internal] passage to the upper (outlet)
kidney-shaped recess, and is mounted in the
immediate vicinity of the fuel-supply pump.
It is a springloaded spool-type valve with which the
pump’s internal pressure can be varied
as a function of the quantity of fuel being
delivered.
If fuel pressure increases beyond a given value,
the valve spool opens the return passage so that
the fuel can flow back to the supply pump’s
suction side. If the fuel pressure is too
low, the return passage is closed by the
spring.AND

(b) the other at the fuel return fitting.
Overflow restriction - Less than $30...
and rarely fails... just clogs up!
The overflow restriction (Figure 6) is
screwed into the injection pump’s
governor cover and connected to the
pump’s interior. It permits a variable
amount of fuel to return to the fuel tank
through a narrow passage. For this
fuel, the restriction represents a flow
resistance that assists in maintaining
the pressure inside the injection pump.
Being as inside the pump a precisely
defined pressure is required as a function
of pump speed, the overflow restriction
and the flow-control valve are precisely
matched to each other.

IF not already available via this FORUM,
I can forward to you an excellent Bosch 101
instructional booklet (PDF file) that does a great job
of explaining these and also has very good graphics!
It's title is:
"Diesel-Engine management;
Diesel distributor fuel-injection pumps"
and is ~60+ pages... 4th Ed. ... last released in 1999
bright yellow cover !
Let me know if you want a copy
of this PDF file... less than 2mb
Regards,
Sam Ross

[Crazyrabbit]

Sam,
Now I understand. If the pressure control valve and bleed orifice let the case pressure get too high, the lift pump will take too much power to drive, making the IP inefficient. If they let the case pressure go too low, the pressure pump may not be able to fill completely, which could limit maximum power, and could cause cavitation damage within the pressure pump. There is no connection to injection timing. That is set by the cam position (I like mine at 0.038"), and the cold start device, which can advance the injection timing by 2.5 degrees.

So, measuring and adjusting the case pressure to the low limit could improve MPGs. Does your document address adjusting case pressure, and give limits?

This reminds me that I was thinking of installing an exhaust gas temperature gauge and while driving at a constant speed on level ground, see if an optimum advance over the 0.038" setting could be seen within the range of the cold start device.

[MrSafety]

Originally Posted by Crazyrabbit

Sam,
Now I understand.
If the pressure control valve and bleed orifice let the case pressure get too high,
the lift pump will take too much power to drive, making the IP inefficient.
If they let the case pressure go too low, the pressure pump may not be able to fill completely,
which could limit maximum power, and
could cause cavitation damage within the pressure pump.
(1) There is no connection to injection timing.
That is set by the cam position (I like mine at 0.038"), and the cold start device, which can advance the injection timing by 2.5 degrees.

So, measuring and adjusting the case pressure to the low limit could improve MPGs.

(2) Does your document address adjusting case pressure, and give limits?
This reminds me that I was thinking of installing an exhaust gas temperature gauge and while driving at a constant speed on level ground, see if an optimum advance over the 0.038" setting could be seen within the range of the cold start device.

CrazyRabbit,
Response to (1) above - I'm pretty certain that case pressure increases with RPMs and thus this is the physical parameter that this originally mechanical only IP uses to vary timing well beyond the 2.5 deg. advance used in the mechanical Cold-Start device.
Response to (2) above - No I do not remember reading anywhere about specific pressure ranges that are normal... but I think I might be able to get this from the IP shop in Portland, OR that I have used twice now!

I too am inclined to think in such R&D terms... but right now I'm still busy trying to get my '81 Rabbit p/u to a reliable Daily-Driver condition and will very soon be swapping the rebuilT 5-speed tranny! I just received my custom fabricated shifter linkages and boshings/bearing for the shifter system! Next I want to do my first timing allignment fine tuing using the Dial-Indicator tool I have purchased for just over $100!

Let me send you this 1.65mb (60+ pages) PDF instruction booklet from Bosch!
Q - Can you tell me if this PDF file is something this FORUM would want to make available to it's Members?Regards,
Sam Ross