'75 Fuel Inj. Bug surges after engine is warm

[DanD]

My son has a stock 1975 VW Bug with fuel injection that needs help. When the engine is cold, it seems to run fine for about 15 minutes (outside air temp = 50 degrees)then it will begin losing power for half a second then surge when power returns. It does this constantly once the engine is hot. Sometimes it dies at stop signs. The fuel pump, fuel filter, spark plugs, points and rotor have all been replaced in an attempt to fix this problem. Compression on all cylinders is 110-120.
We threw some money at a VW mechanic who came back and said that it's probably the fuel injection and recommends giving him $1000 to convert it to carberators.
There's got to be a better way. We're new to the VW world and could use some help.
Any suggestions would be apprecitated.
Thanks

[metropoj]

Most injection is the same for Bus or whatever running AFC ( Late bus, 914 1.8l 75 to 79 Beetle ( Air Flow Control ).

Instead of throwing money at a mechanic ( who I guarantee suggested carbs because he's a stumped as you are ), you and your son should sit down and read up on AFC and how it works. You probably spent WAY too much money replacing unlikely's on the engine which is why people start hating fuel injection. Since you're in a replacing mood, check the Cyl head temp sensor as well, cheaper than a pump, about 20 bucks ..... clean the electrical contacts with a good quality TV tuner spray, or if you really want to go nuts, buy some Stabilant 22, it works great !

I suspect vacuum leak or a bad warm up regulator. These things are supposed to allow more air in at start then close up as the car warms. Check all those vacuum hoses for leaks. Fuel pressure regulator may be faulty as well but I would bet on vacuum leaks .... There is lOT of info out there on AFC, that is your best bet ..... If you think you have problems now, wait till you have to balance out carbs, keep them running clean, rebuild them etc.

I've also had bad seals in the oil filler cap cause the engine to stall out as well .... the AFC required that NO false air gets into the system through leaks, etc.

Good Luck ....

[DanD]

Thanks for the info.
I'm thinking we should probably replace any remaining original vacuum hose.
Since it ran fine when the engine was cold and then all of a sudden kicked in with the problem, I was thinking it might be some sort of temperature related sensor/valve or something like that.

[Speedy Jim]

"Since it ran fine when the engine was cold
and then all of a sudden kicked in with the
problem, I was thinking it might be some sort
of temperature related sensor/valve or
something like that."

As the other poster suggested, check the
Cyl Head Temp Sensor. It's a single wire
down by #4 spark plug. Pull the wire off
and measure resistance with an Ohmmeter
from chassis to the sensor stud. Should be
roughly 2500 Ohms Cold and go down to 100 Ohms Hot.

This sensor has a huge effect on mixture.

You will want to buy the Bentley Official
Manual for this car; covers not only the FI
but all the mechanical details as well.

There is some electrical info on the FI
on my web site although it is not intended
as a primer on FI.

Speedy Jim
http://www.nls.net/mp/volks/

[DanD]

Success !!!!
The cyl head temp sensor tested out fine so we replaced all the vacuum hoses.
It seems to be back to normal. It's driven without problem for the last couple of days.

Thanks guys. My wallet is breathing a deep sigh of relief.

[regis101]

I'm glad to hear you discovered the problem. The maintanence of stock FI is not hard as long as we have a working knowledge of the system. If you wanted to go further, checking the resistance readings of the remaining components will give you the whole picture. You could also loosen up the black toothed wheel inside the AFM 2-3 clicks to make it a little richer. Also try to find out how old the CAT converter is. But at l;east it's running for ya.

[regis101]

Please try to find another mechanic. Just my opinion

[DanD]

No doubt on the new mechanic suggestion.
Thanks for the additional info.