TheSamba.com :: View topic - Fluctuating idle speed 1978 fuel injected beetle

Fri Sep 14, 2012 10:31 am

I live in Ireland but have owned a 1978 US spec convertible Super Beetle for the last 20 years. For the past four months I have been trying to track down the reason why sometimes the car revs normally at idle, sometimes very rapidly and other times it revs so slowly that it cuts out. Recently the problem has got worse in that sometimes as I slow down with my foot off the accelerator the engine cuts out. The cold start valve seems to be functioning correctly as the revs drop when the engine heats up. I have collected quite a few parts over the years and have tried replacing the cylinder head sensor, the rubber elbow attached to the Intake air sensor, all the rubber pipes and tubes I could get at, the temperature sensor on the cylinder head, the auxilliary air regulator and the decel valve. I have checked anywhere I could think of for air leaks and the accelerator cable for free movement.
I built the engine myself about 10 years ago and it is mostly standard apart from removal and blocking up of EGR pipework. The ditributor is a single vauum unit from aircooled.net with petronix repalcement for points. I also have an old 009 with petronix and have tried it but the problem persists.
Any suggestions would be greatly appreciated.

Fri Sep 14, 2012 12:49 pm

From your description it does sound like a air leak. Process of elemination is the only way to go. So far we know
1.engine rund ok when cold
2.idle fluctuated when warm

This tells me one thing

Engine idles fine when warm, because the cold start injector is giving enough fuel to compensate for the additional air. I am assuming you checked decel valve, head temp sensor ect with the help of the AFC manual in the tech section here?

Did you check the fuel presure at cold and when warm to see if there's a drop in pressure?

Fri Sep 14, 2012 1:23 pm

Thanks for your comments. I replaced the head sensor with a new one. I found it difficult to test as it gave the reading in the Bentley manual when cold but changed hugely when the engine was warm. I checked my decel valve according to the manual and was suspicious that it was causing a problem so replaced it with a spare that seemed to check out.
I used the car since my first posting and it idled fine on this occasion when warm.
I have no idea about the fuel pressure as I don't have the equipment to test it.
When it decides to idle fast I would go along with the air leak idea and have spent a huge amount of time looking for one. While I'm no expert, I have owned the car for a long time and traced a few performance issues in the past to an air leak. An intermittant air leak makes less sense to me but I am happy to be corrected

Fri Sep 14, 2012 3:32 pm

Air leaks in the intake system are the usual suspect. I would consider swapping out the AAR valve and the decel valve on at a time. Either of those acting goofy could cause the symptoms you are experiencing. [1] You can run the engine and drive the car without either of those connected so long as you plug the holes left by pulling the hoses. The AAR valve is essentially the fast idle cam on a carb, so it won't idle well until warm with that disconnected.

I think you have the fuel pressure regulator connected to the wrong vacuum port. This could be causing unwanted fuel pressure variation at idle. The vacuum port on the front of my throttle body is supposed to be connected to the vacuum retard on the stock DVDA distributer. I have the vacuum port low on the left side of the intake manifold connected to the fuel pressure regulator and the distributer vacuum advance hose hooked up like you have it.

[1] I have the decel valve and vacuum retard disconnected. The decel valve went goofy and even slight backing off of the throttle opened it. This caused some erie sensations when trying to hold a steady speed on rolling roads. The vacuum retard was then disconnected because having it without the decel valve results in mild backfiring out the exhaust.

Sat Sep 15, 2012 3:26 am

Before reading your post I had been having another look at the decel valve and when I pinched the hose the revs dropped. I had relaced it already with a spare so your post got me thinking about wrong connections. I think you are correct about the vacuum connection, but I'm also embarrased to say that I think I have switched the other two pipes at some point also!!! However, I have been driving the car for years without a problem. If I can get it to run properly will post again.
I really appreciate your help

Sat Sep 15, 2012 4:05 am

Looking through an old VW parts book I now think that while I had the vacuum connection wrong the other pipes were connected correctly. Perhaps I have two faulty decel valves.

Sat Sep 15, 2012 5:57 am

sounds like you dont have the bosch books
on the L-jet tronic system? (L= Luft for air) eg . uses an air meter....

its free to view.

keep in mind it may have 2 or more problems. and each hiding the other.
kinda common after 40 years.

here is one example of 2 wrongs making a right.
the engine temp sensor reads low. (wrongly 1)
so ECU adds fuel
the air meter reads high (lean) (wrongly2)
they kinda cancel each other. see? too rich +too lean equals kinda ok.
On EFI?
we check is it connected up the right way. by the book. every inch.
we check fuel pressure 1st
the fuel regulator 2nd. (it must regulate !)
if those 2 things are wrong, all the other parts are near useless.
The ECU assumes those 2 things are working and calculates (sic) the fuel injection rates on RPM , Air flow , load and temperatures.
The ECU (brain) is an analog computer. (not digital ( arcane it is ))

not to mention air leaks, or idle speed controls bad, causing leaks.etc.

most relic L-jet owners swap to carb so they don't have to figure all this out. ,
to me it be fun but is , long hard work, end to end.

we have data on all sensors. for sure.

KEY checks begin here.
http://ac-vw-remove.com/look/L-JET.JPG

The Perspective, Today , back to relic L-Jetronic era:
This system works just like modern EFI, that is, Air flow density systems.
The ECU, or FI controller. Engine control Unit.

works just like today's systems, but is very very simple.
There are no backups systems, no limp home modes.
if something fails, it goes, yes, dead. (depends on the failure)

it has no diagnostics or live data stream or data logging abilities.
All the brains are in the technicians, head. :shock:
one nice thing is , the injectors can be seen to squirt. unlike modern rail mounted injectors.
you remove it and watch it go,,
it has one nice feature, you only need to under stand the simple way it works and not all the odd things modern cars do.... so its more predictable
in your hands. (that is, its just simple inputs and outputs)

THE critical Base line:
This system assumes fuel pressure is right at all times. even on Pikes Peak.
http://www.arizonaedventures.com/uploaded_images/PikesPeakViewFromTop-761955.JPG?dur=269
(a great advantage over any carb)
This feature holds differential pressure across the injector at a constant.
(a key feature to understand in all EFI systems)

It then measures air, flow , air temp,and engine temps, , RPM.
note the missing Throttle Angle , and speed controls like on 88 and newer cars.
With accurate inputs the ECU then INJECTS (outputs) correctly !
Just like on new cars.

The FI controller , adds (injects) fuel based on air flow and temperature.
when cold the old injectors used then ,were not so good, they could not handle (it's more complex but that is a simple way to say it)

idle injection rates. cold. so they used a cold start injector to add extra fuel
cold, (a choke it is, or a fuel enrich device)
the cold injector must turn off , when the motor gets hot.
if the idle air is wrong, the ECU may not mix fuel right.
so no air leaks.
(just like new cars, they HATE air leaks after the MAF)

The Bosch books shows how to test each part
and the 2nd book is a flow chart of logic for diagnosis.
both are on line,

I repaired ECU for many years so find this one fun to examine.
It is like the first ever mass produced system. 1975.
The concepts in this box are still used today.

The LUFT based systems.

I hope this helps. i see few, theory pages on this system.
looking at it from carb's, must be scary.
im an EFI guy. (most cars are)

the carb is a computer. (sure is ,it's a hydraulic computer !)
Air is a fluid.
it takes air flow and computes fuel volume. (2 fluids)
but uses the Bernoulli' principals to pull that off.

Sat Sep 15, 2012 9:53 am

Marcas wrote: Looking through an old VW parts book I now think that while I had the vacuum connection wrong the other pipes were connected correctly. Perhaps I have two faulty decel valves.

If pinching the hose to the decel valve slows the idle speed then I'm pretty sure it's bad. I suggest disconnecting it and capping the resulting holes in the intake. Then readjust the idle and drive it for a week to see if the problems returns.

Sat Sep 15, 2012 3:06 pm

Thank you both for taking the time to reply. I think I will try to get hold of a fuel pressure guage. A lot of my fuel injection stuff is not 34 years old as I replaced a lot of stock parts, including the pump and pressure regulator, about 10 years ago. However, the decel valve certainly looks suspect and it is also about ten years old. I came across some instructions for adjusting the valve written by Ray Greenwood so I will probably try that before resorting to blocking up.

Tue Sep 18, 2012 10:08 am

Took the car for a drive to warm the engine before adjusting decel valve. when I came back home I kept the engine running and squeezed the hose from the bottom of the decel valve. The revs dropped to a low level and stayed there even when I released the hose. The level was so low that after around 10 seconds the engine cut out. It refused to start again until about an hour later when it had cooled down! I may try blocking up the hoses to verify that is the source of my problems. Any further thoughts would be welcome.

Tue Sep 18, 2012 10:46 am

dont run it lean
then drive it
or worse try to make it go by flonging it
it will over heat lean.. bad news.

lots of things go wrong.
and all parts are old. show me new parts? few there.
and what is wrong will be wrong. you nor anyone can change fate..
and more that 1 thing wrong, (i bet that)

the restart. that is trouble some.
loss of fuel
or flooded.?

Tue Sep 18, 2012 11:01 am

Thanks again. In the past the one job I have left to a mechanic is to set the idle speed and idle mixture with a CO2 meter. I could be talking nonsense but my guess is that the idle speed has been adjusted to take account of a leaking decel valve. The screw is screwed in almost as far as it can go. As far as I can remember the idle mixture screw has not been touched for years.
If I have understood the problem correctly a properly functioning decel valve would require the idle screw to be unscrewed.
Despite the problem the car still runs well and I drive it often. Recently it did a trip of around 500 miles over two days so I'm keen to get the problem sorted.