jtjerry

Since I have my vette (4 years) there is same problem: sometimes car run normal (and it is for about 98% times I drove). But there is THAT GREAT 2% when car runs MUCH better like rocket.
When that happen I can feel that engine running much smoother, feel much stronger and much quiet. Whet it run better I can see that my instant fuel economy is better about 20%.
That 2% great running happen when it want, there is no correlation with weather, with temperature, no matter car run long or just started, simple that better happen suddenly and last sometimes 1 minute, sometimes one hour and gone as come: suddenly.
For 4 years I was trying pinup that problem (I want that 2% all the time!!!)
I changed plugs (3 times, ignition cables, rotors, caps, ECM and another)
Still nothing.
And last I saw that spark plug at 8 cylinder had a lot black oil and another sparks was in perfect condition.
So I replace 8 cyl spark plug with 1 cylinder spark plug. Drove some and checked. And again spark at 8 cyl was oil black wet and another 7 was good. So no spark plug!
Second I done the same with ignition cables and I found that no cables are reason.
In last week I checked that 8 spark plug in that 2% great running condition, and that plug was clean as 7 anthers!
When car next again turn in 98% mode I checked 8th cyl plug and it was black again.
SO PROBLEM IS WITH 8 TH. CYLINDER
You think it is bad injector or something mechanical (valve perhaps?)???
How check?
Any suggestions?
Car is 88 with 80 k miles

Sorry for my English
Hope you understanding what I try say.
Jerry

Togo

jerry, oil can be 2 things....i have the same problem.
in cylinders 4&6 i get oil coating all the time. its either valves or rings. i don't think there is any other way to get oil in there! as far as testing, i have no ideas.....maybe pull the heads off and have a valve job done (although thats not testing) and while they are off, check the cylinder walls for scoring?!
sorry i couldn't be more help, but my solution is i am just gonna replace the whole motor!!!!
good luck

jtjerry

thanks for answer,
but there is pssibility checking that without pulling heads off????
Jerry

merrick

If you are sure what you see on the #8 plug is oil and not fuel, then you should do a compression check. If the compression check is good, then you are getting oil from around the valve stem (need a new valve stem seal and/or valve guide).

85 Dave

Also do a leak down check on all the cylinders.

jtjerry

Today I made compression check. It show no more than 2 - 3% difference between cylinders. Of course when I drove to that test vette run quite good (I know when it run good, I can feel it from the start.)
So test show nothing and that 8 cylinder plug was in good condition (I was sure that, cause today my vette runs quite nice)
I am thinking about change injectors (form 8 cylinder to 1 and from 1 to 8) and when check what will happen .
That crazy. It run bad most of the time and today when I drove for that test she was good running. Ironic...
Sorry for my English
Jerry
leak down test, what it is?

merrick

Leak down test is putting air pressure through the spark plug hole on the cylinder at TDC. The tool used has dual pressure gages and separated by a calibrated orfice. The leakdown test compares the pressure readings on the gauge. You can also listen for air leakage from the rings or valves.

jtjerry

thanks for info
Jerry

wegozoom

Does your car have a cold start injector that may be sticking open?

Joe C

jerry -
this would be a good test to check out injectors!
another test, try swapping out the #8 plug for a hotter plug (one or two ranges), to see if you get that 2% drivability. second, measure the resistance across each injector. they should be (i think) about 5 ohms. if you have an oil problem in #8 cylinder, the hotter plug will help burn off deposits. hope this helps -


[Modified by joe ciechanowski, 1:32 PM 4/16/2002]


[Modified by joe ciechanowski, 1:32 PM 4/16/2002]

jtjerry

I checked resistance and driver side all 4 are 14.7 and passenger side all 4 are 16.2. Strange? hhhhmmmmmm....
I have 9 th cold start injector cause it is '88.
Ok, I can disconnect it.
But cold start injector leaking can cause spark plug at 8 th cylinder have that ugly black wet deposit?
Jerry