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-   -   Help reading spark plugs (https://forums.turbobricks.com/showthread.php?t=336164)

940 16 valve 09-18-2017 03:16 PM

Help reading spark plugs
Hey Crew

The 16v turbo has been idling rough for some time. Started doing preliminary investigations yesterday before pulling the head (hopefully tonight). Compression check came up 145/150/150/150. Had a look at the plugs and thought the difference between them is pretty interesting, but I don't have a lot of experience reading plugs. I did some reading online, but I still don't feel like I have a confident idea what would explain this.
Setup is stock B234 with 16T running Fred's 16v chips @ 12lbs boost on Saab APC. green giants, stock FPR. Have been running 3" AMM which does have it a bit lean at onset of boost.

Cyl 1 is at top of image, 4 at bottom.

JohnMc 09-18-2017 03:24 PM

Photobucket ate your pic.

linuxman51 09-18-2017 03:38 PM

did you drop one? why is the gap closed on the one on the left? (at least on the left of the pic I could see)

gross polluter 09-18-2017 03:42 PM

Plug reading doesn't work very well when the plugs have 50k miles on them.

940 16 valve 09-18-2017 03:47 PM

Great catch, I did drop one, and recall thinking good thing I'm not reusing them.
Also a good point, the car sat for quite a while so the plugs are quite old, and probably have around 5000 miles on them.
Interested in thoughts on what can cause 2 cylinders to look so different than the other 2.

linuxman51 09-18-2017 06:12 PM

the intake manifold if it's the b234f one

doucheNozzle 09-19-2017 08:34 AM

Fred's chips are old school. Have you checked for vacuum leaks? Sounds like you've had the setup for awhile.

wujek 11-18-2018 06:08 AM

Is everything lookin' fine?


freevolvos 11-18-2018 04:44 PM

The only time I have read plugs is a full pass then stop engine and read the plugs with no idle time. But I would say your plugs look fine

142 guy 11-18-2018 06:05 PM

I think the point has been made that if you are trying to get a mixture read on the plugs they need to be fresh plugs and the plugs need to be pulled immediately after the full speed or part speed pass that you are trying to do the mixture assessment on.

That said, the insulator tips on #1 and #3 look reasonable. Depending on when they were pulled out of the engine (after a full throttle run or after 10 minutes of idling?) #1 looks like it might be a tad on the rich side of 14.7 and #3 a tad on the lean side of 14.7. Its hard to tell; but, #2 looks like it has some serious deposits on it compared to the others. Its hard to tell the color. If it is shiny and blackish then you may have excessive oil consumption on #2. If its not shiny and its dark you may have some fuel fouling / running rich which is lowering the combustion temperature. The reduced combustion temperature can result in deposits not burning off the insulator tip in normal operation. #4 is obscured by the Photobucket advertising.

Have you had your injectors cleaned and flow tested? #2 might be flowing more than you expect. Get the injectors cleaned up if they have never been cleaned and flow tested and then put a new set of plugs in and then do a full throttle run (if possible) and pull the plugs. That will give you an idea as to whether your full throttle fuel mixture is balanced (I am assuming that you have a wideband for monitoring the aggregate AFR of the cylinders). Put the plugs back in and drive for a few hundred miles and see if the deposits re appear on #2.

wujek 11-19-2018 03:13 AM

They have +/- 300 miles 80% running on full throotle (20T @ 1,7 bar boost).
Injectors have cca 1000 miles (bosch 440cm3). But because of their bargain price I will check them anyway.

What Im thinking about is this little yellow deposit, Ive never had this before.

Will make better shot of #4.

Dirty Rick 11-19-2018 11:03 AM

Cant read plugs saturated with oil.
Todays pump gas does not color the plugs like the pictures in the old books.
Any coloring on todays gas is too rich or oiling, a greyish ash like deposit is all you get when things are right.
Look for changes caused by temperature, blueing or burning of the ground electrode, how far the heat comes up the threads.
Those plugs are not worn, note the sharp corner still present on the center electrode and no erosion on the inside of the ground electrode.
The yellowish brown deposits are a cylinder that was not firing right, did not get hot enough to burn off deposits.

Compression and leakdown test before teardown? Or some further diagnostics before teardown, like swap injectors, plugs, & wires/coils

142 guy 11-19-2018 12:36 PM


Originally Posted by wujek (Post 5884697)
They have +/- 300 miles 80% running on full throotle (20T @ 1,7 bar boost).
Injectors have cca 1000 miles (bosch 440cm3). But because of their bargain price I will check them anyway.

What Im thinking about is this little yellow deposit, Ive never had this before.

Will make better shot of #4.

I was referring to the photo in the original post. Your plugs look pretty much like the plugs that came out of my old 1987 745 T (or pretty much any emission controlled car running at 14.7 AFR), so OK for normal operation. The 2nd plug from the right looks a little darker than the other plugs so you might want to check your injector flow rates.

80% x 300 miles = 240 mile at wide open throttle? Most engines would be worn out after 240 continuous miles at wide open throttle. If your plugs came out of the engine right at the end of a high speed run I would expect them to be a darker tan. Even emission controlled engines tend to operate at lower AFRs at wide open throttle at the horse power peak.

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