pjensen

Hi everyone,

Driving 15 miles home in my 2004 C5 Corvette, suddenly the engine starts shaking and the engine light comes on. I drive the last 3 miles at 35mph and make it home. Since then, it miss fires all the time - immediately on start up.

Pull the codes (using option and fuel button) and only the P0300 code shows. I get a code reader and get the same code.

I've replaced the spark plugs and wires - no change. Old spark plugs looked (gap ok) except one side of the the insulator is black (all plugs). Pulled a new plug after a 1/4 mile drive - same pattern.

Cleaned the air filter (it is 6 months old). Checked the fuel pressure at the fuel rail (54psi on turning on the key, 62 psi when engine is running). Battery is charged up.

Following the service manual, I removed the drive belt and checked all the accessories and pulleys turn freely. I replaced the belt anyway.

Corvette is stock and has run perfectly every day for 7 years.

Any ideas?

pjensen

Spark plug pictures:



and the other side of the same plug:

dougbfresh

Do you get any P030[1-8] codes from the code reader?

dgrant3830

If you don't get a specific code for a specific cylinder as mentioned already, one item to check is the ground on the rear of the driver's side head. The bolt there, grounding the wires, seems to come lose from time to time on some cars. If there's a specific cylinder, then check the plug wire and insure its plugged onto the plug all the way.

pjensen

No, it just showed P0300.

Used two different code readers (one rental and a pc based one) - same p0300 code.

All the old plugs have an identical pattern as seen in the picture.

pjensen

Went out and ohm'd between that wire lug and battery ground - measured 0.02 ohms (lowest meter scale). Also checked between the alternator case and that wire lug - same 0.02 ohms.

Thanks for the suggestions.

Tech

This thread may be of some help...
http://forums.corvetteforum.com/c5-t...ont-start.html

BLOWNBLUEZ06

Broken valve spring is a common cause of misfires.

Also not too difficult to fix.

dougbfresh

Seems to be more of an issue with the spings on the Z06's but worth investigating anyway.

pjensen

Checked the ground A on the two spark plug harness connectors. Both are 00.2 ohms to battery ground.

Running the engine for a few minutes, I noticed more exhaust on the driver's side tail pipe. Using a digital temperature laser gun, these exhaust pipes were 106 dF and 94 dF. The passenger side exhaust pipes were both around 74 dF. Both are pushing air, but it seems the driver side exhaust is stronger.

dougbfresh

Does not mean a lot with an H pipe up stream. Use the laser on the exhaust ports, see if one or more cylinders are off.

pjensen

After a few minutes of idling, all the exhaust port temperature (near the spark plug) are hot. They vary from 120 to 190 dF - and it varies where the laser dot is placed on the exhaust port (the top or the side).

I'm ready to remove the valve rocker arm covers to look for a broken valve spring.

Is there anyway it could be a bad sensor causing the misfiring?

pjensen

Doing more testing - found that there is no manifold vacuum at all (while idling). I hooked vacuum gauge to a tee between the manifold valve rocker cover and the throttle body. Pulled the gauge off and could not feel any suction. Double checked the vacuum gauge with a vacuum pump and it checks out.

What to look for now?

BLOWNBLUEZ06

If you don't have any vacuum and the car isn't over revving, I might think you have clogged cats although I would expect the car to throw other codes. I've seen it happen and no other codes were thrown. Have you tested the gauge? If you had a valve hung open or broken, it would cause misfires and kill your vacuum too.

lucky131969

If you have a stock setup, and connected the vacuum gauge between the forward rocker cover nipple(passenger side) and the throttle body, I would expect the exact same result, as that is the fresh air intake for the crankcase. If you want to test manifold vacuum, that should be tested at the back of the intake, or at the reservoir.

Jim in Mich

pjensen

Looks like you may be correct, sir.

Here is a picture of what looks like a broken spring - just took off the passenger side manifold valve cover.



And another view of the same spring (center).



Thanks for the help! Now what to do?

pjensen

Yes you are right - I have the stock setup. And yes I did the test on that fresh air intake. Duh. Still learning a lot here.... Thanks for the help.

heavymetals

Do the trunion upgrade also since you have to remove them as well. IMO

Also, go buy yourself a lottery ticket.

You are so lucky you didn't drop the valve into the cylinder.

You could/would be looking at major $.

lucky131969

A 2004 should already have the corrected rockers.