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Discussion Starter #1
Hey guys I know this isn't a forum for the trucks but its still the same ol' NPI 4.6L with a different intake.

In the process of chasing down P0308 and P0302 Misfire DTC's, I decided to do a compression test on my truck. Replacing the coil back with #2 and #8 cylinders seemed to help but only 75 miles later the misfire returned. Truck also generally has low power and really struggles accelerating at all up hills even with overdrive off. Plugs installed by previous owner were autolites but looked ok with the exception of cylinders 2 and 8 spark plugs are very white looking.

Truck is a 98 XLT with 4.6L V8.

Driver Side Back to Front

Cyl 8 - 155 (155)
Cyl 7 - 170 (170)
Cyl 6 - 150 (170)
Cyl 5 - 165 (165)

Passenger side back to front

Cyl 4 - 90 (100)
Cyl 3 - 120 (135)
Cyl 2 - 105 (140)
Cyl 1 - 155 (180)

Numbers outside of parentheses are numbers after 4 strokes, numbers inside ( ) are after 6+ strokes. The three rear cylinders on the passenger side all sounded quite different from the 5 cylinders with good compression, and they also built compression quite slower. I suspect some sort of valve problem with cylinders 2 and 3 at least, because the compression test was quite erratic, they would lose pressure in between compression strokes.

I am pretty sure that a low of 90 and a high of 170 is quite far out of the suggested tolerance. Is this motor just tired? I am not sure if I should just buy new motorcraft plugs and wires and see if that helps the misfire or just start thinking about a PI engine swap. I would sure like the extra power and less weight with the aluminum block like with my Tbird swap, it really made a huge difference.

Thanks for any advice or help you guys can give me.
 

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how many miles on the motor?

Put some oil in the low cylinders and see if it comes up. You could have rings sticking.

If there is any change put seafoam in the oil for 200 miles, then change it, and maybe run it thru the intake as well. Lots of info on the web on how to do this.
 

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White plugs indicate a lean condition, swap the injectors from #2 & #8 to different cylinders and see what happens, our new fuel has a tendency to clog injectors. The other thing I've seen is the condensor mounted on the coil bracket causing similar problems, just unplug it and see what happens. As long as it's not burning oil, it's probably not "worn out" I've seen 4.6's in taxi cabs with over 350,000 miles on them and still run great. To find out the real condition of cylinder health requires a leak down test.
 

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Discussion Starter #4
The motor has 159K on it.

I guess I am going to order new spark plugs and wires and when I change those I will swap the two injectors around.
 

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Discussion Starter #5
I've been busy working on my Bird so I haven't been able to mess with the truck. But I was looking at my compression numbers and I realized something.

The 3 cylinders that have low compression indicating worn rings are the 3 cylinders at the rear of the engine on the passenger side. Could this be proof that these motors do indeed "heat soak" at the rear part of that head because of low coolant circulation? Engine oil was changed frequently and is very clean, and the temperature gauge reads below half in the normal range. Of course, I am sure the sender is on the front of the intake and that the coolant temps could be drasticially different elsewhere in the system.

I did the freeze plug cooling modification at the rear of my PI engine that is in my Tbird, re-routing that coolant back to the radiator because I had the engine out and I thought it was a great idea when I read about it. I sure am glad I did that when I had the engine out after seeing theses compression numbers on my truck motor.
 

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Could this be proof that these motors do indeed "heat soak" at the rear part of that head because of low coolant circulation?
negatory.....the water pump pulls hot coolant out of the rear of the pass. side cylinder head (via the heater core, but still)
 

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High-Mileage 4.6L Thrasher
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negatory.....the water pump pulls hot coolant out of the rear of the pass. side cylinder head (via the heater core, but still)
Not on the 4.6L trucks. It's pulled from the passenger side front of the coolant crossover, through the heater core, and then into the tube under the intake back to the water pump.
I don't think that's his problem though-if it were,then the driver's side would be just as bad.
JL
 

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High-Mileage 4.6L Thrasher
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96 LX, I'd bet that you've got some valve sealing issues on those cylinders. The '98 4.6L F150's are notorious for pinging-my old '98 was terrible about it,and the knock sensor was bad too,so that made it even worse. It's very possible that the previous owner just held the throttle down while it was hammering away pinging. That could cause enough valve and piston damage over time to cause the symptoms that you're seeing.
JL
 

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Not on the 4.6L trucks. It's pulled from the passenger side front of the coolant crossover, through the heater core, and then into the tube under the intake back to the water pump.
I don't think that's his problem though-if it were,then the driver's side would be just as bad.
JL
gotcha....coulda sworn I had seen a truck like the 4.6 cars, guess I was confusing it with memories of mine...thanks for the correction
 

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High-Mileage 4.6L Thrasher
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gotcha....coulda sworn I had seen a truck like the 4.6 cars, guess I was confusing it with memories of mine...thanks for the correction
Well, now that is true-the 2007 F150's with the 2V 4.6L got a plastic Mustang style intake,but they still feed the heater core from a nipple at the front crossover-almost exactly like the Explorers did.
JL
 
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