New guy with a question on compression testing a 4.6

Greetings, all..

My son and I are working on a 99 P71 with 160,000 miles. I have been chasing codes (mostly PO30X series). We have inspected resistance in the coils, replaced plugs, swapped around coils and injectors mianly to see if the misfire codes moved with a specific component.

No real rhyme or reason to them. Car will start right up, idle ok ( a little lumpy) and go down the road, but usually around 30 mph and again in the 50-55 range it will hesitate and buck.

While replacing plugs, all looked pretty decent, really, except #8. It was wet and appeared gas fouled. I pulled that coil and stuck a plug in and grounded it. It seemed to be sparking just fine. I have suspicions of the compression in that hole.

I have a nice Sun compression tester but not quite sure how to get it threaded in tight enough twising on the hose to properly seal it. I sure don't want to just catch a couple threads and then blow it out while cranking and bugger up the threads in the hole.

Is there an extended fitting used by Ford mechanics when checking compression on these motors with the buried plug access?

Also, if anything about these symptoms jumps out at you, we're all ears!

Thanks for a great forum! :biggrin:

Mark

I would try a noid light and check for power to the coils. Bad wiring can cause these types of problems.
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Greetings,

I had the same problem about 8-9 months ago. Mine is a 93', I found it was the plug wires. Yours has the coil packs on each cylinder, but I can tell you what I did.

1) Remove the plug wires labeling the cylinder for each.
2) Make as good a measurement of length as possible,
3) Measure and record the resistance of each wire,
4) Divide the resistance by the length.

Each wire should have relatively the same resistance/unit length. Mine had a lot of variation (I don't have my garage notebook with me or I could tell you, i think one differed by over 100 ohms/in). I went to O'reilly, bought BWD and repeated the procedure. I think the resistance was around 30 ohms/in (+/-5 ohm).

(Some people will say only use Motorcraft, but I have used these and haven't had a problem yet). The network won't allow me to respond from work, but let me know how it goes.

Thanks, guys. I picked up a comp tester with the extension tube to reach the plugs and I'll eliminate the basics first (since I now have something I trust to check compression. The thing that scares me is that I keep getting a PO308 even as I swap different coils and injector to the hole.

Hopefully I'll find good compression and i can chase other less invasive fixes! :biggrin:

Reults of compression test

Results are dry/wet...

1) 130
2) 110/130
3)130
4)110/125
5)110/125
6)130
7)110/125
8) 0/0

Obvioulsy a valve issue or head gasket. I am leaning toward a valve in my best guess mode, but I would appreciate some pointers as to what to check next. I have never messed with an overhead cam motor before.

I am really hoping for a fix that doesn't involve head removal...

Thanks!

Remove the valve cover gasket look at the valve springs. If you have a broken spring you can put some rope in the plug hole and roll over by hand to hold the valve closed. Then replace the valve you need a spring compressor.
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