C5 Engine Misfire
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8th Gear
Joined: Apr 2019
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C5 Engine Misfire
Just purchased my first Corvette. Love the car, fun to drive. 1999 convertible. Drove to town last week, cranked and ran fine. After getting spare key made, upon cranking, engine started running with a very rough idle. Check engine light came on and flashed off and on several times while driving. Left car at friends house overnight. Went back next day and cleared out DTC code P0300 C, Cranked and drove car home (about 16 miles) with only a mild engine miss. Check engine light did not come on this time. Checked diagnostics and again reset DTC code P0300 C. Car sit in garage for 2 days. Cranked and drove about 10 miles to auto zone to have them check with plugin diagnostic meter. Car cranked and ran fine with no engine miss, nor check engine light. No diagnostic codes showed up. Thoughts as to what issue is. Will try driving again tomorrow to see if I get another engine misfire code.
Moderator, Tech Contributor
Joined: Sep 2013
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From: Cape Coral, Florida
Congrats on your purchase !!...without a scan tool you’re really not going to be able to nail it down to a particular cylinder or cylinders. You can take some tap water (more minerals) and mix it in a spray bottle with some baking soda to make it more conductive. At night with the ending running at around 2000 RPM spray it around the plugs and wires...see if you see a lightning storm under the hood. It can also be fuel injector related but I would just stick to the ignition system for now. The coils are pretty sturdy on these cars but for piece of mind change them if you want !!...good luck !!...scan tools are pretty cheap these days...they really help in diagnosing these cars....if you can afford it pick one up !!
Tech Contributor
Joined: Dec 1999
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From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
Not knowing the maintenance history of the car, I would do the test that C5 DIAG recommended. If it is bad plug wires, get some AC DELCO GM Performance Red Wires (very good quality and not expensive) AND a new set of AC DELCO Iridium sparkplugs.
What wires are on the engine now. Are they stock? Take a picture if you can.. Until you change the wires and plugs, stay away from replacing coils and injectors.
Bill
What wires are on the engine now. Are they stock? Take a picture if you can.. Until you change the wires and plugs, stay away from replacing coils and injectors.
Bill
Last edited by Bill Curlee; Apr 29, 2019 at 03:08 PM.
Thread Starter
8th Gear
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Thanks foe feedback. Cranked car today and she purrs like a kitten. Spoke to a local mechanic and he is baffled because the engine is now running fine with no problems. The day I went to have keys made, it was parked on an incline. We wondered if there was water in fuel tank they got picked up and caused the rough idle. He suggested I add some "Sea Foam" to tank just in case. Will probably replace plugs and wires as insurance. Car has 82500 miles. Will post results when complete.
Thread Starter
8th Gear
Joined: Apr 2019
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Drove the car a couple more times after adding Sea Foam fuel additive. Have had no more DTC P0300 codes. I did go ahead and replace spark plugs (AC iridium) and spark plug wires. Engine continues to run fine with no problems. Will post late if there is any change in performance. Did I say I love driving my C5! My first corvette, but it want be my last one.
Thread Starter
8th Gear
Joined: Apr 2019
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After installing new spark plugs and wires, engine started engine miss again. This time there was no P0300 code after driving for about 20 minutes. Left at friends shop for them to check to see if they could find reason for engine miss. Thoughts on what might be issue?
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Moderator, Tech Contributor
Joined: Sep 2013
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From: Cape Coral, Florida
See if they can look at misfire "history"...they should KNOW this stuff !!...you shouldn't really have to tell them how to diagnose this issue !!...just sayin' !!!...also a good idea to put your location...you might live in the next town over !!
Last edited by C5 Diag; May 19, 2019 at 11:16 PM.
Thread Starter
8th Gear
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Update on engine misfire. The shop I took the car to found I had put the wrong size spark plug wires on. The wire plugs were not fully seating on spark plugs or coil packs. Replaced with correct set and it solved the problem. Drove to Florida and had no more issues. Thanks for all the suggestions.