Hi all,

When I was a mechanic I worked on 4cylinder engines only. There always was a little trick to check on engine:
run engine in idle. Pull the ignition wire of first cylinder. Check how much rpm (idle speed) goes down. Put cable back.
do same thing with 2nd cylinder and so on.
If you pull a cable and the rpms in idle dont drop then that;'s a sign for a bad cylinder. sparkplug or cable or compression or valve adjustment.

Sure as hell I tried the same with my 8 cylinder now.
When I pull the cable on some cylinders, the rpms drop. And on some it doesn't really.
I checked compression on one of the 'bad' cylinders, but that all seems fine.

Any opinions to this ?
Ps: I have new distri, plugs and wires but problem was there before already.

Thanks very much,

Kalli

 

when you pull 1/8 of the cylinders out, you won't really notice much of a difference.

 

that's what I thought.
every one of them should cause the idle to drop by lets say 50rpm.

what hapens though is that when i pull let's say cyl 3, then the idle stays the exact same and when I pull cyl4, then the idle drops by 100rpm and engine starts shaking

I can make a list of how the engine drops with each cyl if that's a help

 

You want to check the compression on all eight cylinders as the relative differences between the cylinders is the important factor. The difference between the highest and lowest cylinder should be less than 10%.

What are the symptons of the engine that you are trying to diagnose?

 

Hello Kalli!

Remove your sparkplugs and "read" them. They should be a medium brown, a deviation in color might give you an indication of what and where the trouble might be.

 

how does the color tell you whats wrong?

 

lots of info on the net about reading plugs, here's one
http://www.ngksparkplugs.com/techinf...q/faqread2.asp

 

Sometimes the way the intake runners are designed and where the vacumn takeoffs are positioned will affect the idle. What you are doing is called a cylinder balance test and confirms proper combustion at idle. You should be looking at a tach and not going by ear during this test. if compression is good and within 10% of the adjacent cyls, look for fuel problems. Looking at the plug is a good way to see what that cylinder is doing.

 

Hi all,

thanks for the responses so far
a) the plugs are new so I can't tell from them.
The old ones were proper nice deer-brown. they didn't really need replacing, but I bought new ones before checking the old ones (as I never replaced them)
b) compression test:
the compression gauge I have is ****e as you ncan't screw it into the cylinder, only hold it up against it (with rubber feet at bottom).
So from this the reading is probably inaccurate. But they all show about the same compression.
But I'll redo the testing on the weekend and I'll write up
- the compression values of all 8
- the amount of idle that the engine is going down with unplugged cable per cylinder

c) the symptoms ... ost people will probably shake their head reading this ...
- the engine starts very nicely warm and cold
- the engine drives smooth since I replaced distri, coil, plugs and cables.
that's the good news

the bad news:
the engine is a typical 289 with headers, flowmaster exhaust and a good distri. So it should do the factoty 200HP.
Somehow my bum-meter tells me that it don't feel like it. I had 120HP cars which were (felt) the same fast and somehow I believe the engine was stronger when I got her.
The clutch pedal used to pop back/up when accellerating. Not that I miss this effect but this was affected by some torque which now seems gone.

So the first thing that popped in my mind to quickly check the cylinders by unplugging them. And I was quite surprised that some of ten cyls didn't care when I unplugged them in idle.

Again, I'll measure and come back

Thanks for helping

Kalli