My first 412... Pulling out my hair!

[raygreenwood]

The clymer manuals are generally 90% wrong. In fact...like Chiltons, many of the illustrations are not even type 4...they are type 1 . The pressure switch for exhaust air injection? Sure yep! But chances are you do not have air injection...right?

No...the return line Only goes to the tank. Look at the illustration again very carefully. From the tank (1)...to the fuel filter (3) to the fuel pump (2) to the little un-numbered "bisquit" shaped thing...which is the fuel pressure accumulator...to the fuel injectors (9). All of this goes up the RIGHT side of the car ONLY!
Next (still following the diagram) to the cold start injector (10)...to the fuel pressure regulator (4)....(you are now on the LEFT side of the engine compartment)....to the FUEL TANK.
Now....notice the extra line coming from the pump...that horizontal dashed line that "T"'s into the return line in the diagram between the regulator (4) and he tank? That is the return line from the D-jet pump. Since you don't have a D-jet pump....you don't needto worry about that "T" or the return line coming from the pump. BUT....you MUST have a return line coming from #4 (the fuel pressure regulator) to the tank!!!!

Don't worry about doing anything else...until you fix the fuel return line. The car is not drivable at all without a return line. You will light it on fire....NO KIDDING! Also, until you get it running, there is no way to tell what else to do yet. Ray

[Kombi]

I get it, return line from pressure regulator to tank.
That was done and checked last week.
The lines run as indicated in my diagram.

Green lines checked and running through proper parts.

Fuel fed from tank, through filter, through pump, to fuel accumulator (biscuit thingy), up the passenger side to the right hand injectors, to the cold start valve , to the left hand injectors, to the regulator, through the firewall, down the left of the car to the nipple on the fuel tank.

Fuel pressurised on passenger side and returns on Drivers.

As per the diagram with green lines does the return line marked 'Bolt' pose a problem?


The engine runs without sputtering coughs etc. It sounds great! There are only minor oil drips and NO fuel leaks. From start it drives strong through first and into second. It is at that point ( around 30 mph) the car begins to bog down. It is not an "electrical power loss" but more like I took my foot off the gas. If I apply more gas the car wants to stall.
When started and driven dead cold, on maiden trip after rechecking lines I was able to accelerate to 40 mph on a gentle downgrade. But when it came time to come back up the hill the problem returns.

Has anyone had a similar problem?
Internet diagnosis is an enviable art (especially when talking to a 412 newby) I really do apreciate all help.

It's probably amazingly simple I just haven't found it yet...
Head temperature sensor cutting off fuel?
I don't know that's why I'm asking...

[raygreenwood]

Hmmmm. It actually sounds alot like the sock filter problem in the tank....or possibly water sloshing in the relay for the FI (strange but can happen).....or...a bad plug on the fuel pump.....

Have you drained the tak yet? It can also be large amounts of water in the tank. Its actually quite common. If you drain the tank, pull off the fuel lines....unscrew the big nut, pull out the fuel tube assembly....cut-off that damn sock filter and re-install everything.


Wait.....you have an L-jet style pump...right? Enters from one end....and out the other? First...make sure the flow direction is correct. Next..post the part # off of it. If by chance it is a digifant pump or a CIS pump ...they look almos identical...neither will function without a secondary feeder pump installed in the tank. They will cause the exact problem you are speaking of. Ray

[vwbill]

Ray can a throttle switch do that or leaking vacuum advance? Bill

[Kombi]

Have you drained the tak yet? It can also be large amounts of water in the tank...cut-off that damn sock filter and re-install everything.

The tank was drained (as it had 2 year old gas in it). By the previous owner. (he bought it and never drove it)
But I will check for the 'sock'.

Wait.....you have an L-jet style pump...right? Enters from one end....and out the other? First...make sure the flow direction is correct. Next..post the part # off of it. If by chance it is a digifant pump or a CIS pump ...they look almos identical...neither will function without a secondary feeder pump installed in the tank. They will cause the exact problem you are speaking of. Ray

The pump is almost certainly a generic pump, (also previous owner)
I did take a second pump down that I know works with the D-jet. I'll swap it out.
Now this is something I can work on.
Thanks!

[raygreenwood]

If its a Bosch pump...(the one you have in the car) find out the part #. If it is a CIS pump, or digifant pump....then not having a feeder pump is exactly whats happening. I had to think back to the symptoms of what happens when the feeder pump either fails or is not there. It does this exact thing. The standard CIS pump does not have suction ability. It must be force fed. It will start to draw...give you pressure...then cavitate as pressure builds.

VWBill....no the throttle valve switch cannot cause this. The advance can cause stumbling problems, but since you also have centrifugal advance internal to the dizzy, it will not keep you from driving relatively normally. Ray

[Longbeach412]

I know this is too simplistic, but I'll toss it in anyways; Have you checked that your fuel filter is good & clean?

[Kombi]

I know this is too simplistic, but I'll toss it in anyways; Have you checked that your fuel filter is good & clean?

Yep that was changed with a new one.

Update: Running problems persist.
However contrary to my initial belief It does not run forever while iddling. Further testing found that it would die just sitting there.

It had a fuel pump FP4 by L????, I replaced it with one from my fastback.
Bosch 0 580 463 010 VW 043 906 091
Either way The engine is getting fuel. Which makes this all the more frustrating. 15 Years of driving VWs and a car has never given me this much grief

The head temperature sensor read about 350 ohms resistance when cold and 150ish when the engine cuts out. I'm replacing that sensor even if it's not faulty just for piece of mind(and I can use it on another vws as well).

It was sugested that the fuel pump kicks out when warm, but you can here the pump working just after the engine dies, so the engine is cutting out before the pump. (the pump is not warm to the touch)

Ok here's a new one... after a couple hours of fiddling and checking, the engine still died after running 6-7min but the throttle body was actually cold (damn near icy). I assume this is from fuel evaporation but does this help with diagnosis?

Next time I go down I'll have to arrange a tow if I can't get it running

[Longbeach412]

Are you sure the injectors are spraying as they should? you know the pump is pumping, but is it pumping gas? can you see gas in the filter? is your gas tank clogged? is your gas old and diluted? Can you verify in the first few moments that the injectors are indeed spraying? somewhere along the fuel delivery lines there is blockage that builds up with time & pressure. Just some food for thought from my D Jetronic mindset.

[Kombi]

Are you sure the injectors are spraying as they should?

All four injectors are spraying

you know the pump is pumping, but is it pumping gas?

Yes Gas is beeing delivered to the engine.

can you see gas in the filter?

No, the filter is grey plastic.

is your gas tank clogged?

Worked through that.

is your gas old and diluted?

No old gas was drained, new gas in tank.

Can you verify in the first few moments that the injectors are indeed spraying?

Yes

somewhere along the fuel delivery lines there is blockage that builds up with time & pressure.

Agreed, but fuel is getting to the engine.

My dad borrowed a pressure guage today and may have found the problem.

RAY it may be your original diagnosis, the return line.
I made sure the return line was there, but did not check if it was cloged.

With the pressure testor conected the fuel pressure reads around 30 psi.
As the engine warms the psi spikes to over 100 psi then dies.
(This is where I believe Ray would say 'you will be lucky if you do not blow a line and light the car on fire').

So tomorrow I will be getting a call saying one of two things...
The car is working.. or
your car caught on fire and burned down my garage

I sure hope it's working, we've got our End Of Summer show this Sunday, and I know there won't be any type 4s...

[Kombi]

I will be swaping fuel pressure regulator from my type 3 If lines are clear.

[vwbill]

Hey, if you need one let me know the number. I think I have a few from a tin deal. Hope Ray can get you in the right direction! These cars are great if you know them and what makes them run! If the parts are right this car is great just like any car that is 30yrs old,lol! Good luck! Bill

[Kombi]

Car is now running strong.
The original fuel return line is crimped or blocked somewhere.
When a new line was run (along the A/C line) from the regulator to the tank the car drives like it should.

Thanks to raygreenwood, raygreenwood, Longbeach412, vwbill, oh and did I mention raygreenwood for all your help!

[Longbeach412]

Congrat-you-lations!

[DeathBus]

GOOD DEAL congratulations!