1. Replacing a VW golf cambelt

Replacing a VW Golf 1 cambelt

If you've reached this page from a search engine (say www.google.com), then you are here for one of two reasons:

No offence to the reader, but my money is on the second option above :-).

It's a fact of human nature that we tend to try things first, then look up how to do it properly. Either way, I'm going to explain how I did the change on my VW 1300 engine after losing all the markings once the old belt was off. This is the worst-case scenario that one can possibly recover from.

The worst case scenario that one cannot recover (cheaply) from is when you do it wrong and damage the valves, the pistons and your self-confidence; but I won't cover that for the very obvious reason being which, if you have encountered this scenario, no amount of advice over the internet can possibly turn your heavy metal paperweight back into a running engine.

Note that this guide is *not* specific to the VW Golf 1 engine; this guide will apply to any 4 stroke internal combustion engine that uses a cambelt to control the valve timing (even, theoretically, diesel engines, should they have a cambelt).

Contents

  1. Engine Timing Theory
  2. Preparation
  3. Setting the pistons and valves with no marks (An unedited copy of a posting I made to usenet early in 2006, when I replaced my cambelt)
  4. Common problems

Engine Timing Theory

The 4-stroke internal combustion works on a very simple principle: When the compressed fuel/air mixture is ignited, a controlled explosion takes place forcing the piston down and away from the explosion.

So, once the piston is at the top of its travel, the spark plug generates a spark which ignites the mixture, forcing the piston down again. The 4-stroke term comes from the fact that a single piston has to complete 4 distinct strokes for a single cycle of the engine state to take place.

After the 4th stroke, the 5th stroke sees the piston back at the same spot it was in when at stroke 1, but more importantly, then engine as a whole is back at the same spot; the valves on the 5th stroke is at the same spot as the 1st stroke, the other pistons relative to the one in question are also at the same position as on the 1st stroke, the rotor is at the same position, etc.

Obviously, for all this to take place, the engine needs to be engineered like clockwork to make sure that everything is where it should be at any given moment in time.

Thats where belts and chains (and sometimes rods) comes in; A geared, toothed wheel can exactly control the rotation of another gear and/or toothed wheel with knife-edge precision. The cambelt is one such belt.

The cams in the cylinder head are a number of off-centered roughly egg-shaped wheels mounted on a shaft which control when the valves open and close. You can usually easily rip off the tappet cover on your car and you'll probably see a shaft with all these wheels on it. Note that they push down on the valves when rotated, thereby opening and closing the valves as the shaft is rotated.

The only thing that needs to be controlled is the rotation of the shaft, and this is where the cambelt comes in; it connects the main crankshaft to the camshaft with a certain number of teeth calculated to produce the precise rotational ratio between the crankshaft and the camshaft that causes the valves to open in a certain order and each at a certain time in the engines cycle.


Preparation

Well, the first thing you should do is wash the engine really well, especially around the area of the spark plugs, as once you take the plugs out you don't want grime and dust falling into the cylinder.

Once the engine is dry again, disconnect the battery.

(you should, as a rule, always do this before undertaking any large or medium scale work on the car)

Locate the fasteners for the cambelt cover. These are usually three or four screws or bolts that hold the cambelt cover on. On the VW engine, you may need to loosen the tappet cover itself to get the cambelt cover off.

On rear-wheel drive cars, there is probably a fan in front of the cover, and this will have to come off as well; front wheel drive cars, with some exceptions[1], generally have a transversally-mounted engine which does not allow the radiator fan to be driven off the the engine itself[2].

Setting the pistons and valves with no marks

The following is a post I made to usenet early in 2006 when I changed (and messed up) my cambelt on the vw 1300 engine.

Hello all

I'm writing this down in the hope that it will help other
home-mechanics
who search google for an answer.

The problem I've found is that the cambelt also turns the distributor.
When I replaced the cambelt, the gear which turns the distributor (off
the cambelt) had turned, so the ignition timing was off.

Since I had already messed up the valve *timing* while trying to figure
out what was wrong, I now had both the valve and ignition *timing* out,
and the cambelt was off the engine.

1. I "found" TDC by using a long screwdriver down the #1 cylinder. The
procedure is to get the piston near TDC, and rock back and forth over
TDC while the screwdriver was in the spark plug hole. I marked the
point where the screwdriver stopped upward motion (turning the engine
with a spanner), marked off the position where the screwdriver started
going down and took the middle of the two marks as TDC.

2. I set the distributor by merely spinning the toothed gear near the
crank gear until the rotor was under one of the wires on the
distributor
cap (any wire will do, you then just call that #1).

3. I turned the camshaft (spanner again) and "rocked" #4 (this means:
turn the camshaft until the inlet valve and exhaust valve for #4
are just about to  open/close. Ideally the cams would be holding the
valves in the same position with no clearances). This ensured that
I had #1 valves at the maximum clearance. I marked of this position
on the outside of the head and the camshaft gear wheel.

4. I then worked out the firing order from the valve positions for
all the cylinders (on my car, this worked out to 1-3-4-2).

5. I installed the new belt, making sure after the belt was tensioned
that none of the gear-wheels had shifted position (which is why I
marked
them all off before starting) and that the tensioner was tight.

6. I set the firing order on the distributor cap.

7. I turned the engine with a spanner two full revolutions. This I did
to check whether any of the valves were going to touch the pistons. If
you turn the engine like this first, then at least you won't damage the
valves because you'll feel the resistance if a valve touches a piston.

8. I started the car (it started on the first turn off the key -
YAY!!!)

9. I then switched the car off, and set the ignition *timing* statically;
This is done by turning the engine until the *timing* mark is visible and
near the point of reference on the cambelt cover and when #1 is getting
ready to fire (i.e. the rotor must be about to fire #1). I took a spark
plug, put it on the #1 wire, held it against ground and twisted the
distributor back and forth until I got a spark on the plug (obviously
the ignition must be on while you do this). I turned the distributor
and set it at the point that a spark was produced. This is good enough
to ensure the car starts.

Later this week, once I fix the leads on my *timing* light, I'll set the
*timing* proper for unleaded petrol.

Whew... Hope that this post is helpfull to anyone else with a
4-cylinder
watercooled VW - AIUI most of the VW engines from 80's & 90's have the
same design, with the cambelt controlling the rotor (first car I've
worked on that had this arrangement was this car! All the others had
a different crank-gear or *cam*-gear arrangement for turning the rotor);
hopefully anyone else who runs into the same problem will read this
(google is your friend:-) and realise that the cambelt is on proper,
it's just the rotor thats off.






Common problems

Well, there are a few, but I haven't the time right now to detail them, so drop me a note (see the contacts page) and let me know what problems you've had.

Alternatively, you feel that I got something wrong? Is the process inaccurate? Well, then help me set it straight and send me a correction.


[1]The older Audi 100 and 500 1.9L and 2.2L engines were front-wheel drive cars with longitudinally-mounted engines. This meant that they had an extra long wheelbase forward of the front axel as the gearbox had no propshaft behind it, but had two CV joints coming out each side of the gearbox, which itself was mounted directly under the gear-lever. It also meant that there was very little (or no) torque-steer problems.

[2]Which is why your FWD card has a fan that has to be electrically operated. The transverse-location did not allow for the more reliable and efficient mechanical/hydraulic fan of longitudinally-mounted engines.