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· Premium Member
22 Posts
Discussion Starter · #1 ·
2008 GXP. 5 speed. 60k miles. All stock.

Ever since I bought this car I've noticed some stumbling at idle and strange surging while in high (3/4 to full throttle) boost. Well I finally decided to look into it and found that I am getting significant misfires from cylinder 1. On a really hot day, I've seen over 500 misfires on that cylinder and on a few occasions the cylinder has completely dropped out at idle. Even that doesn't set off the CEL. I do see part throttle KR ( up to 4 degrees, no KR at WOT) and my fuel trims seem a bit unstable. So this led me to believe the problem was a fueling issue.

So here's what I have done to solve the problem:

1. Moved and then changed spark plugs
2. Swapped coil packs
3. Replaced injector 1 with a new injector and injector seals
4. Cleaned the sludge off the intake valves while the intake manifold was off during the injector swap.
5. Leak down and compression test all look very good.
6. Probed injector harness at injector 1 with a data logging oscilloscope. Signal looks good, no dropped signal to injector during misfire.
7. Ran a borescope down cylinder 1. No chipped pistons or burnt valves.

I think I have covered my bases here. The problem isn't significant enough for me to take it into the dealership, but I am planning on adding power and I need to get this figured out. I haven't pulled the valve cover off yet to inspect the cam and lifters. This might be the next step.

I'm kind of baffled on this one. Maybe an LNF expert can chime in and head me in the right direction.


· Registered
5,172 Posts
Seems to me I remember someone else having this issue. I think it was Red_Devil if memory serves me correctly.

EDIT: How was the pressure coming off of your HPFP? If memory serves me correctly, that was his issue.

· Premium Member
10,583 Posts
What plugs did you use and what gap?
Also check the cables going to the coil pack
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