Ignition Coil test

[rcrawley]

2000 Explorer, 4.0 engine

Checked the resistance oft the coil. The secondary readings were 12.5, Haynes manual gives an upper limit of 11.0 Is the coil bad? Checked a new one at NAPA and it was also over 11.0

 

[Runnin'OnEmpty]

You mean 12.5K ohms, right?
If the testing was done when the coil was
warm, the coil's OK. Temperature will effect
the readings you get, and different multimeters
will also give different readings, especially the
cheaper ones. The meter battery voltage will
also affect the readings.

Check the primary resistance at the plug end.
If memory serves, it should be around 1.5-2.5 ohms.

 

[rcrawley]

The Haynes manual gives a range of 0.3 to 1.0 ohms for the primary resistance. The coil was 0.9 ohms on all post.

I have not ran the codes again but before the codes indicated mis-fire on cylinders 3 and 4.

 

[Runnin'OnEmpty]

Cylinders 3 and 4 are paired on the coil, so that
does make the coil suspect. Before replacing it
though, check the resistance on both #3 & 4 wires.
This should be no greater than about 10-14K ohms
per foot of wire.

You can also check the plug's resistance. It should be
about 6-8K ohms measured from the tower connector
to the tip. If it shows an open or infinite resistance, then
the plug is bad.

The misfires could also be caused by faulty injectors.
Did the misfires start together, or did one occur and
then the other start later...?

 

[71cutty455]

Also you can try swapping the coils around, if the misfires moves to another pair of cylinders then you know the coil is bad.

 

[rcrawley]

Checked the codes today, P0171: Bank One System Lean, P0174: Bank Two System Lean, P0303: #3 Misfire, p0304: #4 Misfire.

New wires and plugs.

Another person suggested switching the wires around on the coil. Would thing this will throw the firing order off? As he suggested would this give me an answer about the coil being good?

 

[koda2000]

Checked the codes today, P0171: Bank One System Lean, P0174: Bank Two System Lean, P0303: #3 Misfire, p0304: #4 Misfire.

New wires and plugs.

Another person suggested switching the wires around on the coil. Would thing this will throw the firing order off? As he suggested would this give me an answer about the coil being good?

yes it would throw the firing order off to just switch the wires around at the coil. perhaps he's talking about switching the plug wires around (coil and spark plug) to see if it's a wire issue, but as you say they are new... ??? the lean codes are very often related to intake manifold gaskets/o-rings, or other vacuum leaks. they typically cause a rough idle.

 

[BubbaFL]

Set the misfire codes aside for now - troubleshoot the lean codes first. Lean mixture can cause misfires.

Could be a vacuum leak - possibly intake manifold gaskets or a vacuum hose. I'd check that first, then move on to looking at the fuel system.

 

[rcrawley]

Tried three times to find vacuum leaks with starter fluid. Twice by me and once by mechanic with no success. Could misfires cause the lean fuel codes?

 

[1992fordgreen]

Usually running lean wont cause misfiring but its possible.

To test the coil, just grab it and start the car. lol

 

[BubbaFL]

I've never had luck finding vacuum leaks with the carb cleaner or propane methods.

I'm not sure about the Ex, but on other Fords (i.e. the 3.8) TSB's specifically say that the carb cleaner method won't identify intake manifold gasket leaks.

A smoke test is your best bet to rule out vacuum-related problems.

I'd also suggest a compression test, to rule out any major internal engine problems. I had a bad experience last year chasing an intermittent misfire that ended up being a due to low compression from a bent connecting rod.

 

[rcrawley]

Replaced the coil and cleared the codes. Two days later misfire #4 code shows up but nothing else. Replaced #4 plug with no improvement. Rough idle when cold, runs very rough upon slow acceleration (at any speed). Rapid acceleration or driving at highway speeds runs great. Getting around 16 mpg, not the best but not extremely low. Any suggestions?

 

[jremington59]

In order to test with starter fluid the engine must be cold with a rough idle. If it's a small leak once it heats up the gasket can seal.

 

[BubbaFL]

Any suggestions?

At this point, I'd definitely do a compression test.

If compression is good, try swapping the #4 injector with another position and see if the misfire moves.