Help me with spark plug analysis [NPC]

rick-l · 03-08-2003, 09:00 PM

No Porsche Content - Porsches do use spark plugs however.

These are spark plugs out of a 98 GM product with 90,000 miles on them out of their scheduled 100,000 mile life. I got the Service Engine Soon light with misfire cyl # 5 Guess which plug was in #5.

What happened? Notice the ceramic is missing on the #5 plug. The engine never was knocking that I could hear. All the other plugs looked normal like the one on the left.

The engine has also started to use oil. About a quart per 1000 miles. Am I in financial trouble as in rebuild or buy a new one time?

dickster · 03-09-2003, 12:38 AM

dont know the cause but you've got damage on #5.

can you see smoke from the exhaust when driving?

change the plug, it'll be ok until its oil soaked again.

Britwrench · 03-09-2003, 05:28 AM

There you go, believing that 100K plugs WILL actually last 100K.
New plugs, drive the car and change them at a more realistic interval and all will be well with the world.

jmohn · jmohn's Garage

My first impression is (as Britwrench says) plugs simply do not last 100,000 miles. Number 5 just happened to fail first. Put in some new, good plugs (I like NGK, although I use Bosch in the Porsche) and monitor the oil useage. You could have a problem (bad oil control ring, etc.), but if the plug failed just from normal attrition it would look much the same. BTW, 1 quart of oil per 1000 miles is annoying but probably within spec. and not unheard of in a 90K mile engine; I would hope you're not following an "extended" oil change interval similar to the extended plug replacement interval.

Good luck,

Jerry M
'78 SC

tryan · 06:34 AM

3k on the engine oil and grease fittings (or yearly). 10k on air and fuel filters. 25k on transmission, transfer case, differentials, spark plugs, cap, button and wires. two years on brake fluid. three years on coolant. this works for me.

if the pieces of the plug beat the cylinder up, i would trade in the car.

Doug Zielke · 03-09-2003, 06:40 AM

Quote:

Originally posted by tryan
....if the pieces of the plug beat the cylinder up, i would trade in the car.

Yeah.....then it would be someone elses problem, right?

Find a mechanic who has a Boroscope and take a look inside the suspect cylinder for damage.

dickster · 03-09-2003, 06:47 AM

sorry i misread the post. i didn't realise you'd actually gone 90k on one set!

10k is long enough for plugs.

chances are the bits have been blown out a while back, else the damage is done. personally i'd stick in new plugs and keep an eye on the no5.

i don't believe the plug would look like that unless its oil soaked. if it was unburnt fuel, the plug would merely be wet, but still clean - not black like that.

rick-l · 03-09-2003, 07:31 AM

Just a short note in my defense. This is a GM U platform minivan. To get to the back plugs you have to rotate the engine forward and get the plugs out strictly by touch, abrading your arm jamming it behind the engine and stretching as far as you can. Not a fun job and it took several hours. A Porsche engine drop would probably be easier. 100,000 is the first scheduled major maintenance for this vehicle. 5,000 for oil changes.

I haven't had much luck with these american cars and 90,000 is the most I get out of them. The laast minivan momma drove (95 Windstar) lost a head gasket and transmission at this mileage.

tryan · 03-09-2003, 07:33 AM

if your dealer likes you, you might talk them into a new motor for free. see if you have an emissions warranty in your state.

doug, i didn't mean it like that. when i trade a car in at a dealer, i leave my phone # in the car. if someone is looking at the car they can call and discuss issues. out of the last 19 cars (not including vw's) in the last 14 years , 10 have called and i tell them everything. how many times the car has been in the body shop, motor issues, quirks ect.

slater · 03-10-2003, 09:24 PM

About spark plug condition, check out this site. This is the same chart that is in the back of a lot of repair manuals. http://www.gnttype.org/techarea/engine/plugs.html

stormcrow · stormcrow's Garage

Looks like the gap is too wide on the one plug, and oil fouled on the other. You probably have a bad valve stem seal on that one.

Steve

"A Porsche does more then just go fast in a straight line"

john70t · 04:19 AM

That's right, good for 100k miles on the tranny oil and plugs. After the warentee expires, the whole car is engineered to break simultaneously.

Seriously though, no fluid or component performs that well with so many miles. It looks like the engine did a Chrysler minivan on you and shredded its rings, or the #5 ring is sticking from not having it's 75k mile oil change.

New plugs will last a few hundred miles before doing the same thing. You could do an oil change with Rislone, put in a hotter plug in that cylinder, and hope for the best, but keep smelling for burning plastic from the carpet because your CAT is going to glow like a light bulb.

A compression test will tell you if the rings/head gasket is seating, and the other source left would be the valve guides.