Diagnosing a possible misfire

[AlaskaTy]

Is it okay to have one different plug wire? Just curious b/c after the fire there was one plug wire that the insulation was melted a little, and was thinking about swapping it out for the time being until I can get a new set or replacement taylor.
It idles like a pig right now, it keeps dropping below 500 then surging up to 1000. The compression checks out, all cylinders either 130 or 140. It feels like it's got a misfire, so I'm gonna be running datamaster tomorrow, what would a misfire look like on the program? It's got new plugs, maybe 2k miles on the cap/rotor and wires. Thanks.

 

[Captain Morgan]

its ok to have a different wire as long as its good.

I had to put 2 stockers back on after my shitty 9mm Holleys crapped out on me. Went to Taylors after that, no problems.

 

[sy1907]

It doesn't take much to mess up a wire. I mean, you can go to a parts store and buy the cheap wires for 15.00 and you're problem could be diagnosed. I'd say switch out the wire if there's any doubt. You could just ohm them out too. That's what i do, works every time.

 

[AlaskaTy]

so is the best way to figure out a miss to just see how it idles? I mean that's pretty easy to do, then just go through the ignition parts (cap/rotor, wires, and plugs) right?

 

[sy1907]

If i think i have a miss, i just slap on new wires. If it's still there, go to the plugs. Still there, go to the cap/rotor etc. let us know what you find out.

 

[AlaskaTy]

It doesn't take much to mess up a wire. I mean, you can go to a parts store and buy the cheap wires for 15.00 and you're problem could be diagnosed. I'd say switch out the wire if there's any doubt. You could just ohm them out too. That's what i do, works every time.

I replaced the questionable wire and it didn't fix my prob (gonna check the cap and rotor contacts this afternoon). How do you ohm test a plug wire. I've got a multimeter with an ohm function, just wondering how to do it and what to look for?

 

[TYRODD]

I am actually picking up a new multimeter tonight, wierd.

The way I always test ohms is to set it on that function, and touch the two leads together, (I cant always remember exactly what it does)what you will see on the meter is what continuity your wires should have, if they show an all together different reading, prolly no good. Remember, your wires will have some resistance to them so the readout will be a little off, two identical wires will never have the same resistance.

 

[turbodog]

My experience has been that a bad wire will not affect idle nearly as much as WOT performance. I can hear a bad wire by tuning the FM radio to a weak station. I can hear high voltage arcing plain as day. I've gotten to where I can tell HOW MANY bad wires I have by how fast it snaps while idling.

For a rough idle (but OK WOT and no FM radio noise) I would go after a vacuum leak or the pickup coil in the distributor.

 

[AlaskaTy]

Does anyone know the resistance in Taylor wires and ATR wires? I'm going to be ohm testing my wires finally (truck's been down while I toyed with the IC). Or is there a range of ohms/feet that is within tolerance for our trucks? Thanks.

 

[AlaskaTy]

I ohmed some old wires (ATR with about 1500 miles) and some new wires (accel 8.8 race wires) and found the following:
ATR averaged 290 ohms/ft
Accel averaged 160 ohms/ft
I also found this testing some website that had a dodge stealth testing differnt plug wire ohms.
http://www.stealth316.com/2-msd-ignwires.htm

It looks like the Moroso super 40's have the least resistance of any plug wire, and the MSD 8.5 superconductors have about 50 ohms/ft. Is anyone running either of these plug wires? I'd like to hear comments on them. Add those to the autolite plugs that have 0 ohms, and you should be getting maximum spark.

 

[tyndago]

The resistance isn't going to make much difference in a miss....


I have found that around 5 psi of boost is when I find the miss... Idle will be OK... under load and 5 psi.. issues....

Change the wires.... I used to carry a couple extra wires with me at all times...

Would be changing #4 wire out a lot..... No matter what I did...

 

[AlaskaTy]

I posted what my plugs look like after 150 miles in "fuel smells like oil." I ohmed all my wires:
#1: 307 ohm/ft
#2: 316 ohm/ft
#3: 312 ohm/ft
#4: 304 ohm/ft
#5: 302 ohm/ft
#6: 279 ohm/ft
Coil: 288 ohm/ft

I haven't checked my cap/rotor yet, but the misfire wasn't coming from my wires, at least I don't think so.

Tyndango - the idle is soo erratic that it stalled yesterday while I was trying to pull into Burger King's drive through (PS the spicy chicken sandwich is not all that spicy :wink: )

 

[AlaskaTy]

I don't have a frame of reference here, but I just pulled my cap and rotor and they don't look horrible. The inside of the cap has some green corrosion build up on all the terminals, and the rotor has a bunch of metal shavings on the inside of the hole where it goes through the plastic to meet up with the distributer. The blade of the rotor had minimal corrosion.

Since my plug wires are all still low 300 ohms/foot should/could I put them back on with new plugs and cap/rotor? My new plugs are universals so they don't have the straight boot or 135* boot for #1 and #6 respectively.

 

[TYRODD]

I don't have a frame of reference here, but I just pulled my cap and rotor and they don't look horrible. The inside of the cap has some green corrosion build up on all the terminals, and the rotor has a bunch of metal shavings on the inside of the hole where it goes through the plastic to meet up with the distributer

Metal shavings???? Oxidation(I take it that its a conrad-brass turns green)
for twenty delores you could rule out the cap/rotor, I dont think there should be be matal shavings in there.

 

[turbodog]

Might as well put a cap n rotor on it. Then put it all back together (be careful with those wires, be a drag to damage one puttin' them back on... would really confuse things)
then watch timing (w timing gun) and see what happens when it stumbles. Disconnect tan wire and check again. If timing is steady at idle, I wouldn't rip into the distributor.

There's at least 3 stumble/bad idle threads going right now, I assume you're looking at them and checking things like park/neutral switch, etc. If only stumbling after warm-up (closed loop), might suspect the o2 sensor.
Also, if your high RPM/high load performance is OK, I'd look somewhere besides spark.

 

[AlaskaTy]

Might as well put a cap n rotor on it. Then put it all back together (be careful with those wires, be a drag to damage one puttin' them back on... would really confuse things)
then watch timing (w timing gun) and see what happens when it stumbles. Disconnect tan wire and check again. If timing is steady at idle, I wouldn't rip into the distributor.

There's at least 3 stumble/bad idle threads going right now, I assume you're looking at them and checking things like park/neutral switch, etc. If only stumbling after warm-up (closed loop), might suspect the o2 sensor.
Also, if your high RPM/high load performance is OK, I'd look somewhere besides spark.

I already have a new cap and rotor, plugs, plug wires. Just wondering if I should (or could get away with) using my old wires (not quite 2k miles on them). I have a new neutral safety switch, I don't have a timing gun, but timing was checked at the same time as the wires were put on (verified 0*). I have datamaster and it gives spark advance, could I use that?
When I start it, it jumps to about 1300 RPMs and then slowly comes down, after 10 seconds or so when it's down to idle (about 600) it starts to misfire: will get to about 400 and try and die and then rev up to about 900-1000 before it drops down again and starts all over.

 

[turbodog]

I was suggesting the timing gun to look for noise/instability from a distributor problem. DM will not show it unless it is bad enough to corrupt the timing pulse.

If it starts to misfire within 10 sec of a cold start, then o2 sensor is not at fault (ECU is open loop when cold).

What are IAC counts? if low, might indicate a vacuum leak/lean mixture. If high, might indicate rich mixture.

If system voltage is low at idle, it can cause IAC to not respond.

 

[AlaskaTy]

I was suggesting the timing gun to look for noise/instability from a distributor problem. DM will not show it unless it is bad enough to corrupt the timing pulse.

If it starts to misfire within 10 sec of a cold start, then o2 sensor is not at fault (ECU is open loop when cold).

What are IAC counts? if low, might indicate a vacuum leak/lean mixture. If high, might indicate rich mixture.

If system voltage is low at idle, it can cause IAC to not respond.

When I first fire up my truck the IAC is high 40s to low 50s. After some driving and everything's warm they oscillate from 13 to 20 whith the RPMs oscillating (when the RPMs go up, the IAC goes down and vice versa). My voltage at idle (when warm) was mid to high 13s.

 

[turbodog]

These numbers are suspiciously low. Left my CD at work, but IIRC, range is 0-140. 70 (natch) is normal. 20 cold is way too low, and even 40 warm is low.
[pause while I go out, hook up and start the Ty, take readings, walk back]

Yep. Mine went straight to 75 cold and drifted up to 83 warm. I don't have the troubleshooting guide with me, but I'm sure it will say to check for vacuum leaks. I believe low IAC could also mean not enough fuel. Your fuel pressure OK? What coolant temp does your scan tool say when starting cold? Same as outside air temp? If CTS reads high, cold start mixture will be too lean.

HTH!

 

[AlaskaTy]

These numbers are suspiciously low. Left my CD at work, but IIRC, range is 0-140. 70 (natch) is normal. 20 cold is way too low, and even 40 warm is low.
[pause while I go out, hook up and start the Ty, take readings, walk back]

Yep. Mine went straight to 75 cold and drifted up to 83 warm. I don't have the troubleshooting guide with me, but I'm sure it will say to check for vacuum leaks. I believe low IAC could also mean not enough fuel. Your fuel pressure OK? What coolant temp does your scan tool say when starting cold? Same as outside air temp? If CTS reads high, cold start mixture will be too lean.

HTH!

My fuel pressure is set to 46psi (stock chip/map sensor/injectors). When I first started my truck coolent temp was 86* (it was about 80 out, so now gross discrepency there). What vac hoses would cause the IAC to be low if they were leaking? Also, I think it was in the stumble post, but someone said you can test the tps sensor with the key on (truck off) and rotate the throttle open and dm will record it (does it sync up with the ECU when the trucks not on?) My max tps is about 97%, but once it hits that it drops to 92%. But that's a different problem.