Garibaldi500

Hi All!
I'm 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 I found the PCV tubing completely rotten and replaced it with new spareparts but 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 around this time I stopped getting the Check Engine and P0300 disappeared.

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.

I would be grateful for any ideas or advice that can help me find out what's wrong.

Cheers,
G.

C5Z06CE

I had a similar problem and there was a spark plug wire especially on bank 7 that is close to my headers. Using a fiberglass wire protector sleeve and obviously replacing the wire helped . However, I also had the problem after that but about half as much. It turns out the remainder of the problem was from a loose ground wire. Check all the ground connections for the car and battery and make sure you don't have one vibrating causing misfires. Tighten them a little even if it seems like they are tight. Make sure they don't touch anything else. My misfires were mostly random but USUALLY bank 7 more than the others. I hope that helps. Good luck.

Pounder

do a compression test ???

punz

Pull valve covers, check springs.

feeder82

look carefully

C5 Diag

As others had mentioned a compression test may be in order...you can try a “cranking compression test”...push the accelerator pedal to the floor...engine will not start...engine will only crank so listen for a even sounding cranking over of the engine...if a dead hole you’ll hear a different sound !!

killian96ss

What did you gap the plugs to and did you also install new wires? Even though you've got new plugs have you tried swapping #7 with another plug? P0300 is a random multiple misfire code, meaning more than 1 cylinder was misfiring randomly, not just #7, so maybe you fixed one, but now your problem is #7. If you have confirmed your getting fuel and spark the only other possibility is mechanical like broken valve spring, rocker arm, lifter, etc. If it gets to that point start with the valve cover since it's fairly easy.

Steve

Vetteman Jack

Posts like this really belong in the C5 Tech section. Moving this thread over there.

Garibaldi500

I just went out to record the sound of the cranking compression test. mp3 attached. Can something strange be heard, you think?

C5 Diag

Without being right there it’s hard to tell just by listening to a recording...watch the video below and you can hear what a bad one sounds like...I’d check the plug itself and maybe swap that out too...if the plug is good it not difficult pulling the valve cover off and taking a peek !!

Garibaldi500

Thanks for answering. I already watched you video with much interest. Your measurement of the starter current is a brilliant idea and inspired me to plot the audio recording waveform.

C5 Diag

This is certainly not my idea but this is the procedure to do a "relative compression" test. I really don't think an audio waveform of the cranking is as accurate as a oscilloscope which takes MILLIONS of voltage samples per second...you would also need a refence cylinder to see exactly which cylinder or cylinders is bad !!

Garibaldi500

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

Garibaldi500

Put it all back together again and gapped the spark plug 0.040. They were less than that before.
No the engine run worse than ever and CEL is back again.

I guess the next thing will be to take a look under the valve covers

C5 Diag

Did you push on the plug wire until it clicked ???...getting #7 plug wire on securely is a challenge sometimes !!...if you have a scan tool and your long term fuel trims are really high (positive) you most likely have an injector misfire !!

Garibaldi500

I will check the cabling again tomorrow. I think I did it right but you never know.

Do you have any comments on the compression values?

Pounder

our 02 Z went into limp mode out of the blue, all kinds of codes, map, misfire,tps and trying to find the issue I realized there was a tapping noise in the engine. decided to find that first. broken valve spring. car had 107k on it and for the most part was a dog !!! replaced all the springs and got the misfire code again, pushed all the injector plugs in tighter and it was fixed !!!! after driving it a while realized it had a ton more power !!!!! (I think the valves were floating at 4000 and above,bad springs) for what all this is worth ?????

C5 Diag

Looks OK !!...with 185 being the highest anything less than 130 would be an issue !!...forgot to mention about the plugs...both looks like they may be carbon fouled...probably ignition misfire.

Garibaldi500

Thanks all of you for your advices. It helps me a lot.
I will keep on investigating.
I'm determined to find the problem!!!
Would a scan tool help me further, you think?

C5 Diag

A scan tool that can see misfires data and fuel trims would be better than nothing in diagnosing a misfire...an oscilloscope to look at secondary ignition waveforms and fuel injector current and voltage can nail it down pretty quickly !!...in the pics with the scope you can see my



injector opening (current-draws a little over 1 Amp),closing (voltage) and what the ignition looks like !!