Hey everyone, I have an 04 C5 with about 90k miles, a few intake, and exhaust mods, and an appropriate tune. I'm having an issue where it cranks fine and runs well at times, but it has a bad engine skipping issue, especially under decent acceleration. The issue is intermittent and gets "better" as the engine warms up. Intermittent meaning it happens pretty much every time I drive it but isn't constant.

A similar issue happened about 6 years ago just after I bought it. After reading great articles on this forum, the solution was cleaning the ground connections. I cleaned the main one under the battery, under the brake master cylinder area, and the two over the front wheel wells, and that fixed it.

When this started this time I cleaned them again and they still looked pretty good. and the issue continued. I've changed plugs and wires, gone through several cycles of Tecron fuel treatment, cleaned air filter (Hurricane Cold Air system), and I recently installed a new Optima battery but no change.

I haven't checked the remaining ground points, as they are more challenging, especially the one on the backside of the driver's side head...how the heck are you supposed to access that??? After a bunch of reading I understand certain ground points are for certain systems, so before tackling those, are there certain ones that may cause the issue I'm having?

Two questions:
Any suggestions to check other ground points that may be related to this issue, or anything else?
Any suggested shop in the North Atlanta (Alpharetta) area in case I can't fix it?

Thanks in advance for your time and suggestions!

 

Do you have the check engine light illuminated ??…if it is a misfire it could be a few things….plugs, wires, injector, or a wiring issue or a mechanical issue and without some specialized equipment it comes down to changing parts…this may also be an issue with your TPS sensor…find a diagnostic shop in your area…the only diagnostic shop I know is Royalty Auto Service in St. Mary’s which I believe is over 300 miles from you…these guys are sharp !!…they have their own YouTube channel now.

 

How old are your wires one may be arcing out intermittently. Some times you can test a bad wire by spraying them down with soapy water. But without having the car to diagnose the most I can do is guess at possible problems.

 

Thanks for the feedback so far! I should have mentioned there are no active codes.
Hey @v8srfun what wires are you talking about...plug wires? I have changed them with no difference.

Getting back to the ground points...other than the 4 I've cleaned, are any others grounding a device that could cause this? I know some are for lights and accessories, etc., and those "shouldn't" cause an issue. I'm concerned about the one on the backside of the head on the driver's side...what connects to this and could it be the issue? No idea how to access that one without pulling parts out to get access...trying to avoid.

This issue is driving me nuts!

 

The 2 wires behind the left cylinder head are the grounds for each bank coil packs…if there was a broken wire on one of those 2 ground wires 4 coil packs on that particular bank would not work…I doubt that is your issue but if you look at the white connector the black/white wire (arrow) goes to the back of the cylinder head…if you think that is your issue connect a 12 volt test light to battery positive and back probe that black/white wire or disconnect it and front probe it…if the test light illuminates the ground is fine.

 

The ground at the back of the drivers side head is for the coil packs. There is also a harness that runs near the bracket at the back of the fuel rail that has wires for the fuel injectors that can be damaged and cause intermittent drivability issues. You can wiggle it while the car is running to see if affects anything.

 

Okay, it's been a while on this one but finally got it fixed! It was a bad cable on the crankshaft position sensor. Replaced the sensor and the cable and it runs great again. My mechanic fixed it and said the cable was a PITA to get to and replace. It's on the back of the engine on the passenger side and had to remove the header and the starter to get to it.

It never threw a code to help troubleshoot, just trial and error moving cables and when he touched this cable the engine died. After a long time chasing an intermittent issue, the mystery is now solved.