throttle/compression check

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What is the relationship between having the throttle wide open and doing a compression check? The cylinder isn't firing the fuel.

I suspect my FI of being innacurate due at least to a cracked-wire CHT sensor. Will this have an effect on the compression readings?

Also, how do I know if I can attribute any sluggishness to the ignition system. When looking at info. about the FI system, checking the ignition system is the first recommendation. Given a van starts well and fires when it is supposed to, how does the ignition system contribute to poor performance?

Thanks,

Hans

[ratwell]

1) For a compression test the throttle position has to be at WOT because you want to allow the engine to suck the maximum volume of air in and try to compress it. With the throttle plate in the closed position you have a restriction to the air flow. Compression ratings are posted at WOT so you need to reproduce the conditions at least in order to compare results. The gasoline engine design of the last 100 years is essentially an air-pump.

2) Compression is independent of the fuel delivery system. It depends on the seal between the piston rings and the cylinder walls, the valves sealing against their seats in the head and the spark plug being snug. If the rings or cylinder walls are worn then the compressed air will blow by the rings into the crankcase and the compression will be low. If there is a problems with the valves seating, compression will be low. If the valve's are not adjusted correctly and do not close completely compression will be low.

A CHT sender it easy to test and inexpensive to replace. You'll never get a rock steady idle without a working one.

Do the follow steps to test compression for FI:

- break torque on each plug and snug them up again (do not retorque)
- put transmission in neutral
- warm up the engine for a 4-5 minutes then turn off
- disconnect battery negative
- disconnect double relay (both plugs)
- remove 12v from the coil
- attach remote starter to terminal 30 and 50 of solenoid or have a friend sit in the front seat.
- remove all the plugs (careful can be hot)
- reconnect the battery and test each cylinder which entails:
- threading in the compression tester hose (I hate that part)
- set the throttle position to WOT
- crank engine for 10 pulses of the compression gauge
- take reading, reset the gauge then repeat on same cylinder
- repeat for the other cylinders
- when you're finished, regap the plugs if needed and put everything back together.

Removing power from the coil disables the ignition system. Disconnecting the double relay disables the fuel pump and the cold start valve so you'll get no fuel in the cylinders. The injectors will be disabled at the coil also.

3) If the ignition is faulty the FI system can't compensate. All it can measure is rpm, temperature and air volume (and in some models, oxygen content in the exhaust) in order to meter the correct amount of fuel.

You need a dwell meter to check the ignition system. As the rotor in the distributor turns the points open and closed 4 times. Divide 360 degrees for one rotation by 4 and you get 90 degrees per cylinder. Dwell is a measurement of the time that the points are in the "closed" position during that 90 degrees. VW says the dwell at idle should be 48-52 degrees. What this really means is that during this time the coil is building up a "charge" inside it and when they open the ignition circuit on one side is broken forcing the other high tension side to send high voltage to jump the spark plug gap and ignite the fuel. The dwell is directly proportional to the gap between the points. As the gap increases the dwell decreases and vice versa.

Ignition timing contributes to poor performance in several ways:

- if the dwell is too low then not enough spark energy will build up and the weak spark will only be able to ignite a portion of the atmoized fuel in the combustion chamber.

- if the dwell is too high then too much time was spent charging the coil and although it will produce a nice spark it is going to happen too late in the combustion cycle.

Timing changes when you rotate the distributor relative to the piston position. Because igniting the fuel starts a chemical reaction that doesn't occur instantaneously the spark needs to occur before the piston compresses the air fuel mixture to maximum at the top dead center position. To get the most from the engine, the timing is advanced, meaning that the spark occurs before the piston is at TDC and the amount required is largely governed by the engine speed. If the timing is late then the piston could already be on it's way down by the time the spark dissipated and less power would be produced to push down on the piston. If the timing is too early there there is a risk of the detonation of the fuel occuring and this can damage the piston crown (hole!).

To answer your question, you need to verify compression to make sure the engine has the ability to pump. Then you need to test the ignition settings to see if you are getting the most out of the spark. The quality of spark is also determined by the age of the components: the cap, rotor, points and plug wires all wear out over time so you need to read the owners manual and find out the replacement interval. Points are the worst offender and wear out long before they are do: at some point they are almost impossible to adjust and you'll have points will a dwell that will make the bus run sluggishly. I must say that an in tune bus set to 50 degrees dwell almost feel racy. At least I mean that the timing range for performance is small and sensitive.

After you have done both of those you can start to diagnose the FI system to find out which component is not performing correctly. You didn't really provide any details as to when during driving you are having this performance problem.

BTW, one of the easiest modifications you can make it so drop an electronic module into the distributor to replace the points. It's will modernize the ignition somewhat and you'll never have to adjust the points again. Very popular upgrade and vw put it in every bus starting in 1979 with the CA model.

[NeverHadaBeetle]

Damn dude that is some nice writing. Kinda' simple like Muir, but technically inspired like Bentley. You kind of had me thinking old school rabid stock purist then right there in the last paragraph you dropped the bomb on installing an electronic ignition! Style, grammar, twisted story line, and hell you can even spell! We oughta keep bumping this post up to the top and tell all the newbies to print it out and keep a copy in their glove compartment. Nice job ratwell!