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Discussion Starter · #1 ·
Yes, me too. :(

Here's some background: This is a 3.0 Vulcan and was not running when I bought it. I could tell that the engine had overheated at some point -- all that lovely mottling on the upper intake kinda gave it away -- and sure enough I found a warped head on the rear deck. Fixed that, and the car's been running fine except for a rough idle, which I've been living with.

Threw a P0301 at me the other day, and since I've got some time off I thought I'd see what I could find out. Here's what I've done so far:

Replaced fuel filter. Yeah, kinda gunky. The car did seem to run better for a little while but is now back to the rough-idle bit.

Pulled and checked spark plugs. They looked good, nice and light tan on all six. I'm thinking I can rule out fuel injectors at this point.

Put a vacuum gauge on the upper intake. Here's what puzzles me. I'm pulling 15.5-16 inches of vacuum with a very fast needle vibrating between 15.5 and 16. If this were one of my hot rods from back in the day I'd call it valve overlap and move on, but somehow I don't see Ford putting that hot a cam in the Vulcan. Anybody ever seen anything like that before on one of their cars?

Tomorrow I'm going to pull and examine the coil (which is original as far as I can tell) and run a compression test. We'll see what that tells me.

I am curious about that vacuum reading, though...
 

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Good idea to use a vacuum gauge on your trouble shoot.

Which one of the following vacuum gauge scenarios best describes what your seeing on your vacuum gauge????
How to Use and Interpret a Vacuum Gauge
 

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Is the engine misfiring and running rough only at idle? Does it go away/smooth out after increasing RPM's?


Try swapping #1 plug and or wire with a good cylinder and see what happens.
Pull #1 plug wire from coil and check for spark.

When you do the compression test, I would probably go ahead and do all cylinders just to be safe. Ford has a strange formula for compression checks.
Straight from Taurus Service Manual:
The indicated compression pressures are considered within specification if the "lowest reading cylinder is within 75 percent of the highest reading on 3.0L (2V), 3.0L (4V) and 3.4L SHO engines. Refer to the separate Compression Pressure Limit Charts."
 

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Ford has a strange formula for compression checks.
Straight from Taurus Service Manual:
The indicated compression pressures are considered within specification if the "lowest reading cylinder is within 75 percent of the highest reading on 3.0L (2V), 3.0L (4V) and 3.4L SHO engines. Refer to the separate Compression Pressure Limit Charts."
That's pretty normal. Most manufacturers want each cylinder within 20% of the others. I guess they worded it differently.
 

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That's pretty normal. Most manufacturers want each cylinder within 20% of the others. I guess they worded it differently.
Yeah, they sure did word it differently. Of course that quote from the service manual is followed by several charts to compare different readings etc. to make it more confusing than it should be.

Tried to attatch the info from the manual but too large or I am just an idiot lol!
 

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Discussion Starter · #6 ·
Hey, thanks for the responses!

@PawPaw: That's a great site. I've always been a big believer in vacuum gauge trobleshooting. Very useful tool IMO. I would have to say that Scenario 3 is closest to what I'm seeing. It's a very rapid needle movement over a very small area.

@thecurrent: I would have to say sometimes. The rough idle is always there. Some trips it purrs like a kitten, but other trips there's a distinct miss at speed. Seems to occur most often at cruise and not so much during acceleration, especially if I use more gas pedal than is strictly necessary.
 

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Hey, thanks for the responses!

@thecurrent: I would have to say sometimes. The rough idle is always there. Some trips it purrs like a kitten, but other trips there's a distinct miss at speed. Seems to occur most often at cruise and not so much during acceleration, especially if I use more gas pedal than is strictly necessary.
Do you have any diagnostic equipment that can monitor the engine PID's? I have seen anything from a cracked sparked plug to a dirty fuel injector cause the same condition that you are experiencing. As a matter of fact, we just had a police cruiser with the exact same issue. Turned out to be the bank#2 catalytic converter partially clogged up. Under cruise load the exhaust gas was backing up and causing the buck/jerk/mis. When under heavy acceleration load, the exhaust gas had enough energy to push through the clogged converter and seem to run ok.

I would keep it simple to begin with and start with the basics.

Oh, are there any other codes current or strored?
 

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As has been said, a number of things can cause a P0301.

Have you done a cyl balance test, using a scantool????

Seeing as how you know the Vulcan hasn't been messed with, perhaps a mild case of scenario #5, sticking valve???? Maybe consider a Marvel Oil treatment in the gas tank at their prescribed 0.4oz/gal & see how it goes for a tank or two????

Would be helpful to know if you have any pending codes & what their number is if you do, as they can provide good trouble shooting clues.

Post the compression numbers you get.
 
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