Spark plug fouling. Bad coil or cylinder problem?

Post by **FireFox31** » 09 Jun 2017, 10:28

Hello, please help me determine the cause of a spark plug fouling on my 2000 V70 manual.

A month ago, my car experienced a sudden loss of power on the highway, thankfully near an AutoZone. I immediately replaced all the spark plugs and found the one in the driver's side cylinder was destroyed. The plug was loose, caked in black, its ceramic was missing, and the cylinder hole was caked in brown. Surprisingly, the car ran fine until this sudden loss of power.

I installed NGK Iridium IX plugs. On my first drive, I felt a few significant misfires. Every day after that, I'd feel a gentle misfire at idle once every two seconds, and a few slight misfires while driving. I assumed the NGK plugs were to blame, so I planned to swap them with OEM Volvo plugs.

Today, one month and 1500 miles later, I checked the driver's side cylinder and found it fouled. The car still drives fine except for the slight misfires described above.

What could be causing this plug to foul so quickly?
And for reference, what are the numbers of each cylinder?

To help me diagnose this, I swapped the driver's side coil with its neighbor, and the driver's side fouled plug with the center cylinder. This should help me identify if the fault is with the driver's side cylinder or wires or the coil which was in the driver's side cylinder.

Post by **abscate** » 09 Jun 2017, 11:06

Plug number 1 is at -the car RIGHT, passenger side
Plug 5 at car LEFT, driver side

Left hand drive of course

That plug isn't that bad and will still fire.

Any misfire codes?

Compression test might be next

Post by **rguzz** » 09 Jun 2017, 12:39

First pic looks like oil fouling to me, second less clear. I'd go with a compression test to try to distinguish, as suggested.

Post by **FireFox31** » 09 Jun 2017, 12:58

Thanks for the input. No misfire codes. Heck, there were no codes even when the plug was loose in the cylinder, when stepping hard on the gas caused violent misfires enough to make the CEL light flash momentarily. However, I replaced all the plugs within 10 minutes of detecting the failure.

Also note, that blown out spark plug was at least 100k miles old. My fault for not replacing them sooner.

What data will I receive from a compression test? What does that data mean? My local Volvo dealership is not that great, so I may have to hand-hold them through this process.

Post by **erikv11** » 09 Jun 2017, 13:09

You can do the compression test yourself, it's pretty easy and stores like AutoZone will let you use the gauge for free. It will tell you how well the composition chamber is sealing - health of piston rings, intake valves, exhaust valves, head gasket can play a part in compression.

Post by **FireFox31** » 11 Jun 2017, 20:24

Interesting, I will research how I can do it myself.

Can the compression test tell which component is at fault - piston rings, intake valves, exhaust valves, head gasket? If compression shows there's a leak, how do I further identify the problem?

Post by **rguzz** » 12 Jun 2017, 07:03

Post the #'s here after reviewing the proper technique and someone will help guide you from there.

Post by **FireFox31** » 12 Jun 2017, 12:43

I'm confused by the various compression test instructions that I have.

The Haynes UK book tells me to, "Disable the ignition system by disconnecting the RPM sensor wiring just below the distributor. Also disconnect the wiring to each fuel injector." I see the RPM sensor but can't trace its wire to a source. How do I disconnect it?

And are these the fuel injector wires? Must I pull the retaining wire away from the plug to free them? That will be tricky when the engine is hot.

Alternatively, the compression tester tool's box says to disconnect the ignition coil., singular. Does this simply mean to disconnect each of the five coils which connect to the spark plugs? Do this even though the coils are off the plugs and the plugs are out of the engine?

Lastly, a thread here says to pull the fuel pump fuse or relay.

Which of these steps must I do to prepare for the compression test? Also, please confirm the reasons for them so that I may learn why they're being done. Thanks.

jeffsws · Post by **jeffsws** » 12 Jun 2017, 12:54

This should help:

Post by **erikv11** » 12 Jun 2017, 13:25

You want to
(1) avoid unnecessary spark in the engine bay (without stressing out the coil(s))
(2) stop fuel from being added to the non-firing cylinders, as this may wash the oil off the cylinder walls and lead to reporting of a falsely low compression value. Hence pulling the fuel pump fuse or relay.

I actually don't know the best way to disable spark on an engine with coil packs, I've only compression tested the 98 and earlier cars.