Compression test to determine valve conditions.

Not sure what could come in contact with the plug...

Do you have access to a borescope?

The car is an hour away, and parked on the street and needs to go asap. I was going to bring a compression tester with me. No access to a borescope offhand. I was going to bring an insert kit and plugs, but I don't see how a new insert would even work if the old one popped out. He claims the new insert went in without any problems. However he (actually his friend, who seems to have no idea) didn't use loctite, just "pipe thread sealant". I still can't understand how the electrode would get bent immediately. Am I correct in assuming that the only way for this to happen is by a foreign object in the combustion chamber? There was no noise before the pop. New plug, tapping. Removed the plug, no tap. If there was a broken valve, wouldnt it still make noise without the plug?He isn't mechanically inclined and just wants to get rid of it. He thinks the plug seems "too long". I don't want to deal with a rebuild on a 420a, but I want to check it out on the chance that it could be something ridiculously stupid.

I would pass.

If the insert popped out, I would bet the spark plug hole is damaged, stripped thread and a new insert will not likely fix it.

With the tap and loss of compression, I would guess the insert and/or spark plug are being installed too deep and a valve is hitting it.

Now if you are getting the car on the low ball price, and have no issue putting in some wrench time once you push it off the trailer and dropping another $300-500 in the car once it is in your driveway. Then get it.

But I would say the head will need replaced, along with gaskets and timing...

I don't know about compression yet, but I assume the insert may have got pushed down too far (it wasn't the kind with the lip), and, assuming I could get it out, I was considering trying a helicoil since it's slightly thicker than the threaded insert. Here's the other problem. If the compressions good and I can get it for a couple hundred bucks, I have no way to tow it home, and I'm not going to pay a tow fee for 50 miles. Worse case scenario, what type of damage would happen driving on three cylinders, assuming I disconnected the injector?

A hellva miss and be pig slow...

Find a buddy with a full size truck and rent a tow dolly from Uhaul and drag it back.

With the insert popped out, I doubt a heli-coil will cut new threads and get a decent bite to stay in place.

If you get the car, go ahead and figure on a head swap..

My best guess is that the valve is bent and hit the spark plug bending the spark plug electrode causing the tap, and the tap went away when the plug was removed.

No IF I am correct and the intake valve is bent, driving 50 miles, even with that cylinder not firing, you stand a chance of snapping the valve head off and dropping it in the cylinder and damaging that cylinder even further.

420a with a bad head that needs to go asap. Sounds like a 200$ win to me!

I know, it's tempting. I'm curious as to why the plug popped out in the first place. If the valve is indeed bent, will this be apparent on the compression test? Theoretically, this is the only way the electrode could be instantly damaged, correct?

PlanZero said:

I know, it's tempting. I'm curious as to why the plug popped out in the first place. If the valve is indeed bent, will this be apparent on the compression test? Theoretically, this is the only way the electrode could be instantly damaged, correct?

Click to expand...

Compression test if good (155 psi+) will confirm good valves and seating provided you can get a good seal in the plug threads. Based on what you have said getting that seal is not likely.

If you want the car, the right thing to do is pull the head and see what is going on. My bet is there is a foreign object in there... part or a plug or insert is probably a safe guess.

Yeah I didn't even think about that. Well the car is in a suburb of Philly, so maybe I'll take a ride down there and check it out. If nothing else, I'll get some cheesesteaks. I guess if I can get it for a steal I'll make arrangements to get it home. I've never swapped a head before, so maybe this will be my learning project. I don't like having to "practice" on my Tsi. Thanks for all the help and input!

Good luck doing a compression test if the spark plug hole is jacked up... I'd pass. I owned a 420A GS for 12yrs and absolutely hated wrenching on it, seemed like such a PITA compared to the 4G63 and not too many parts are interchangeable. For a beater/DD, I'd get another GS-T/GSX/Tsi and keep it stock and maintain it if you're trying to stay in a DSM.

If you get a good enough deal on the car and can source an assembled head, I guess why not? You'd probably want to replace all the timing belt components anyway, I just wouldn't want to deal with that questionable spark plug hole and the variation of the N/T platform.