Rabbit Diesel '81 does 60.6 MPG
 

Hello EcoModders,
I've got a 1981 VW Diesel Rabbit I've been trying to get some decent MPG out of for years. I always remembered when they where new hearing people say "Yeah drove to so and so and back and got 55 MPG" but I never could do that well. I live in Tempe AZ, which means the A/C is normally on high and the petal is to the metal, which explains a lot. A couple of months ago the crap they call Diesel these days got to the seals in the injector pump and I had to pull the pump to R&R some seals on the advace mechanism. Put her back in the car and sure enough the distributer head seal leaked like a sieve. Well Virginia, I can tell you for sure you can R&R that seal in the car. Leave two screws in place at a time to keep the parts from flying apart. Along the way I discovered there is conflicting information regarding the timing. From the results I seen I'd say pay no attention to VW factory literature or Haynes which I have always used before, Chilton's call for more advance and the improvement is significant! So I drove up to Globe AZ and back and got 54 MPG, but that is a mountainous trip and coasting is illegal in AZ. Inspired by that result, I switched to Full Synthetic Rotella T 5W40 engine oil. Then I drove to Tucson AZ and back which is pretty flat along with about 10-15% city driving and 3 cold starts - 245 mi. 4.04 Gal for 60.6 MPG. Mostly trying to hold 52-55mph, but drafting a few trucks that were going 60-65. Whoa Who! Pretty annoying in a way, it has me obsessing about what to do next to improve it, when I really need to finish a few little cosmetic repairs and sell it. Its got 80 K on it and would make a great SVO biodiesel WVO project, it already has an auxilliary tank. But I keep think about taking off the vacuum pump, putting on smooth wheel covers, adding a fuel solenoid interrupt switch to get engine braking at zero fuel flow, adding a solenoid to increase idle speed with a/c on so the base idle could be reduced, switching the tansmission oil, etc, etc.

that rabbit sounds like a treasure.
i have an '86 accord. welcome.

Nice.
Welcome To EcoModder.
I hope yours is as loved as my slightly newer "rabbit" Diesel,

I used to own an 81 diesel rabbit. The only way you can drive it is pedal to the metal, especially on hiils. I got about 45 mpg.

Make sure the injector opening pressures are up to spec. In the old Mercedes of the same era the injector pressures would gradually drop from 1800 PSI new to under 1500 PSI, which cause it to smoke and run less efficiently, due to poor atomization of the fuel.

regards
Mech

Yeah I hear you. Thanks for the suggestion. Most designers of springs never consider fatigue in their design much less creep. Most material suppliers don't even have the data required. Mil Handbook 5 is a great source of this data. I suspected the injectors and replaced them with rebuilds from Parts Place. Unfortunately, their supplier used 1 of four bodies from a different crack point injector and I was about to send them back but I got a Mil surplus injector tester off e-Bay and was able restore it to operating condition and confirm they all had the correct crack point. No improvement so far as I could tell. Its all been in the timing. Too bad it is a challenge to instrument the MPG on this vehicle. I've thought of setting up a % of throttle indicator, so I could tell if drafting was of benefit. That, combined with an EGT gauge, might make optimization within the 2.5 degree range of the cold start advance possible real time.

My dad use to own a Diesel Rabbit/Golf in the '80. They run forever. Post some photos so we can enjoy as well.

Those were great cars. Welcome aboard.

Injector pump timing
 

Hello,
I currently have a 1981 vw rabbit p/u and was wondering if you remember what the chilton manual recommeded for the timing?

Thanks,
Chris

Hello Chris,
Yes, they recommend 0.036 to 0.038" for the NA, and interestingly, 0.038 to 0.040" for turbocharged engines. I set mine to 0.038".

BTW, I've been able to get 65MPG since, and almost every tank since has been over 60 MPG! I am working on some schemes hoping to get 75MPG.

Injector pump timing
 

Thanks, I'll give it a try.

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

... a "NewBie" joins in here... !
 

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

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

... Injection Pump "Case Pressure"
 

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

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.

... Injection Pump "Case-Pressure" is used to control TIMING !
 

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

case pressure myths
 

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.

... Case Pressure<-Myths...???
 

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