Misfire Issue Options

[PaukST]

So, about 3 weeks ago my 94 ST started getting a misfire. Initially, it was only under mid-heavy load, like going up a hill at relatively low revs. I replaced the spark plugs and it got better for a few days, then went back to the occasional misfire. However, a few days ago it suddenly got way worse. I pulled up to a stoplight, but when I went to take off, it would barely go. It's nearly undrivable. It's pretty much a constant misfire now. It seems to be cylinder #2. It tries to die if i pull a spark plug wire/unplug an injector from 1, 3, or 4, but nothing changes when i unplug either from cylinder #2.

I had changed out my spark plugs and wires probably less than a year ago. I rebuilt this engine June of last year, and have had a VERY slight miss with this engine all along. I assumed it was due to the ever-worsening crack in my exhaust manifold (located closest to the cylinder #2 exhaust port).
So:
---Rebuilt 18 months ago
---Replaced spark plugs about 2 weeks ago, the old ones weren't broken or particularly noteworthy
---Replaced spark plug wires within past year
---Switched over the injectors from my other 7A-FE thinking one may have been clogged, no change

Anyone have any ideas? Advance tells me they can't check the CEL code on anything pre-96.

Second edit to add that I replaced my distributor cap/rotor around the same time as my spark plug wires. I took the dist cap off today and cleaned up the contacts, they looked fairly bad again. Unfortunately that changed nothing.

This post has been edited by PaukST: Dec 12, 2013 - 6:15 PM

[Special_Edy]

Use a multimeter to check primary and secondary resistance on the coil.

Remove the valve cover and check the valve shim clearances with a set of feeler gauges. They should of been checked when reinstalled, they tend to get smaller gaps till the valve is being held perpetually open

[Special_Edy]

Use a multimeter to check primary and secondary resistance on the coil.

Remove the valve cover and check the valve shim clearances with a set of feeler gauges. They should of been checked when reinstalled, they tend to get smaller gaps till the valve is being held perpetually open

[PaukST]

Use a multimeter to check primary and secondary resistance on the coil.

Remove the valve cover and check the valve shim clearances with a set of feeler gauges. They should of been checked when reinstalled, they tend to get smaller gaps till the valve is being held perpetually open

When i rebuilt the engine i had the valves done by a local machine shop, but it can't hurt to give it a check. I'm a little concerned that the misfire is on one cylinder specifically, wouldn't a problem with the coil affect the others as well?

[Smaay]

could be the distributor cap, could be a valve out of spec.

[PaukST]

changed out the spark plug wires, back to normal. >.< those ones weren't even that old. Oh well.

[Island_Racer]

Use a multimeter to check primary and secondary resistance on the coil.

Remove the valve cover and check the valve shim clearances with a set of feeler gauges. They should of been checked when reinstalled, they tend to get smaller gaps till the valve is being held perpetually open

May i ask what the correct range is, for the resistance that shud be observed in the plug wires (3rd gen 3sgte)

This post has been edited by Island_Racer: Dec 15, 2013 - 4:13 PM

[Special_Edy]

I was referring to the resistance in the ignition coil rather than the plug wires. Usually in the ohms for the primary side and in the 1k's to 10k's of ohms for the secondary resistance.

I imagine the plug wires are in the thousands to hundreds of thousands of ohms but I cant say for sure.

[Smaay]

spark plug wires should have very low resistance. ohms is the measurement of resistance. you want that to be very low so that voltage will travel at its best.

[czwalga]

spark plug wires should have very low resistance. ohms is the measurement of resistance. you want that to be very low so that voltage will travel at its best.

Not necessarily true. Low resistance provides a ton of EMI interference at those voltage ranges. Most OEM spark plug wires have very high resistances per foot.


EMI can have negative effects on your ECU/Gauges etc. Also most 'low resistance' spark plug wires are made with spiral wire instead of solid core wiring. In transmission lines, the majority of the power flows through the outer most portion of the wire; so if you have stranded wire its actually hindering the transmission.

This post has been edited by czwalga: Dec 16, 2013 - 2:14 PM

[DeviantCorndog]

Use a multimeter to check for excessive resistance in a wire. Excessive resistance in a wire limits the wire’s ability to carry voltage to the spark plug.
In most cases, a spark plug wire should have approximately 4,000 ohms of resistance per foot of wire. When testing spark plug wires, you may sometimes get a resistance reading that’s so high the meter can’t measure it. This type of reading is called an infinite reading.
. An infinite reading usually indicates that the spark plug wire is broken somewhere along its length, so electricity can’t flow through the wire. Any wire that produces an infinite resistance reading must be replaced

Hope that helps.

[DeviantCorndog]

You also said your Distributor cap was dirty again which also isn't a good sign what kinda of dirty was it. This may help.

The major defect found in distributor caps and rotors is carbon tracking.Moisture that accumulates inside the distributor causes spark flash over inside the cap, which produces a path of carbonized material between two of the cap terminals. Carbon tracking in the cap frequently causes a constant engine misfire. A carbon track looks like a dark line in the cap. The rotor may also develop a carbon track under the rotor input terminal due to high-resistance or old plug

[VavAlephVav]

Toyota factory book says the resistance for wires and cap together should be 25k ohms max.
the coil primary is 0.4-0.5 ohms, and the secondary is 10.2K -13.8K ohms