In the process of chasing down P0308 and P0302 Misfire DTC's, I decided to do a compression test on my truck. Replacing coil with #2 and #8 cylinders seemed to help but only 75 miles 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 new engine.

 

White looking indicates a lean condition.
How many miles?
Did you do a "wet" test to see if it is rings or valves or head gasket?
Did you have a good seal on the tester to the plug hole?
Yes it looks tired, but not enough to struggle as you said.

What else have you done to eliminate the 302/308?
How does the oil look/smell?

 

159K miles.

My compression tester is nice, new, no leaks. I am about to do over with wet test.

Oil looks clean. Changed less than 500 miles ago. When I bought the truck the top part of the engine ticked like crazy. I changed the oil and put in Motorcraft synthetic blend, the ticking almost completely went away but there is a slight rod knock type sound audible at idle. I figure the PO had heavier oil in there to hide the knock.

When I first bought the truck 3 months ago first thing I did was clean the MAF (which I notice is a refurbished unit) and then had it smogged - the numbers were INSANELY low, the engine ran very clean and passed no problem.

I didnt know where to start when I first got just the 308 misfire. Then I got a 302 misfire, realized they are on the same coil, and replaced just that coil. Miss disappeared for the 75 miles and then came back just the same, but only 308 misfire this time...

 

Well just shot a little bit of oil in the number 4 cylinder and ran the test again and compression jumped from 90 on 4 strokes to a 155 on 4 strokes. It climbed much quicker as well which indicates piston rings are worn, correct?

 

yup. Sorry.

Could also have a stuck ring if it ever was real hot. The ring can get stuck in the groove then when the piston cools it pulls the rings away from the bore and holds them there.

The two "white" plugs could also have made the temps get high and have the same effect.

Might try checking those two injectors and with the good oil you might get lucky for a few more miles outta her. That is if it is stuck rings, Ive seen them break free sometimes.

Good luck
Jim

 

What do you mean by check the injectors, like swap them around?

And stuck rings - does this explain the low compression, or the misfire, or both?

also, could the low compression itself explain a misfire? But, the one I keep getting is cyl 8 and it has good compression so....

Thanks

 

If the injectors are plugged that can cause a misfire (not enough fuel) and being lean can show up as white on the plug color. You want to see if they have a good spray pattern to them and aren't starving those cylinders.

The stuck rings would explain the low dry/high wet compression. The wet test tells you the compression is being lost around the piston and not the valves or elsewhere.

No I don't think the low compression would cause a misfire unless it was maybe in the teens or something. The fuel is still being burned but the energy of the burn is being sent past the rings instead of pushing the piston down.

Does that help?

 

Might want to check for heater hose leaks for that #8. If coolant is dripping in that area it could cause some issues.

Also pull the wire at the coil and check for moisture, that will also give you a misfire. While it is off you can run an ohms test on it to see if the wire is with in range. I've had wires go bad to marginal on me before.

 

Good Luck 98.