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Discussion Starter #1
Hi All!
I new to this site and it's my first post.
I have a 1998 C5 convertible which has started to act up.
It started to misfire and the check engine light came on and P0300 showed up.
Judging from my feelings it some times runs smooth but not most of the time.
I started to troubleshoot and changed all of the sparkplugs with no change in performance.
Installed a brand new battery. no change

then found the CPV tubing completely rotten and replace it with new spareparts and no change in performance.
Disconnected the spark plugs cables one at the time while idling and noticed degradation in performance on all cylinders except when I disconnected the cable to cylinder 7.
Swapped the 5 and 7 spark plug cables. problem still remain at 7.
At some point arounf this time I stopped getting the Check Engine and P0300 dissapeared.

Swapped the 5 and 7 coils and problem still remains with 7
Checked all the supply voltages at fuel injectors, all 12 V.
Fuel injector cleaning fluid in the tank and no change.

Removed the fuel rail and the nr 7 injector. Tested it with a battery and it clicks. Tried blowing through it and when activating it with the battery it passed the remaining fuel and then air. I guess it's ok.
Swapped nr 5 and 7 injectors and the problem still remains with nr 7.


Any ideas are welcome.


Best
Garibaldi
 

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Discussion Starter #3
Thanks a lot for answering!


I will have someone check the compression on all cylinders.
One question, though, since the motor run on all cylinders for short periods of time, can it still be a problem with the compression?
 

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short answer, yes....
but a good deal depends on how your loosing compression if thats the issue,
so theres no sense in getting into potential details,
until your compression test verify's or excludes,
the loss of compression issue in a single cylinder.
at this point your simply eliminating or verifying potential reasons your pulling a trouble code,
once its verified we work on curing the issue/making repairs:thumbsup:
Id also verify the cam lobe lift on both lifters on that #7 cylinder
as a worn cam lobe or defective lifter, or defective valve spring or rocker, might contribute to the issue and not be making much noise.
 
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Discussion Starter #6
Ok here are the compression values

1 2 3 4 5 6 7 8

Dry 165 185 170 180 180 175 170 185

Wet 170 170 170 175 175 175 170 175

I also attach a photo of the driver's side spark plugs
Notice the darker color on 5 and 7, even the white isolation is dark.
They were all new two weeks ago and I have only started the motor for test drives
 

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If a misfire was detected on #7 then the code should be P0307 not P0300. P0300 is a random misfire and P0307 is cylinder #7 specific. Is the code still gone now? It appears that #7 plug is darker than #5 and #7 could be running rich. Because of the overlap of the camshaft some of that rich condition could be contaminating #5 plug through the intake .

If the code is gone now how does the car drive? Can you feel a miss? A miss would definitely have a bad feel to it. Have you checked the fuel pressure? I would be tempted to put #1 and #3 plugs and injectors into cylinders #5 and #7. Go for a 20-30 mile test drive and check the plugs again.

As Grumpy stated, this would be a good time to check the lift and make sure that you haven't wiped a lobe. That would be a rare problem on your engine but it's a possibility. A damaged lobe would limit the amount of air into the cylinder and the injector may still be allowing what it "thought" was the right amount of fuel. This would cause a rich condition like you appear to have.

There's not much left to check. Good luck and let us know what you may find.
 

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Discussion Starter #8
Put it all back together again and gapped the spark plug s 0.040. They were less than that before.
Now the engine run worse than ever and CEL is back again.
Feels like more than one cylinder is misfiring

I only have P0300
 

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https://www.summitracing.com/parts/a...yABEgI8WPD_BwE

https://www.carparts.com/details/Chevrolet/Corvette/Replacement/Crankshaft_Position_Sensor/1998/REPC311802.html?TID=gglpla&origin=pla&gclid=EAIaIQobChMIvrHSmcC-4wIVg5-zCh2XRwEVEAkYASABEgKIoPD_BwE&gclsrc=aw.ds


I might be wrong, but Ive had similar issues that were resolved with a new cam position sensor replacement,
and since its not that expensive or hard to swap out....both sensors......
Id have tried replacing them.
frankly I talked to several of my more knowledgeable friends and
many suggested you swap out BOTH THE CAM AND CRANK POSITION SENSORS


and yes I agree with 1963SS
If a misfire was detected on #7 then the code should be P0307 not P0300.
 

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please let us know if you decide to replace those sensors and if the change results in eliminating the problem,
I know it might seem like a waste of your time and cash,
but Ive found that at times, you tend to find long shots pay off,
time, spent aggravation avoided and less effort, is involved,
if the parts don,t cost that much, or are hard to install,
to simply swap them out eliminating the potential. :thumbsup:
 
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