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  #1  
Old January 5th, 2015, 07:52 AM
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measuring mis-fire

I'm struggling to fix my mis-fire which is non-existent at cold startup but after driving under load for 10mins becomes severe enough that the truck won't run.

I've measured, tested and swapped just about everything methodically but am still at a a loss.

Apart from watching the strobe light what is the mets way to accurately measure the spark being regular?

There is a $200 snap-on oscilloscope on my local craigslist and I'm tempted to go this route ...

Any other ideas?
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  #2  
Old January 5th, 2015, 07:59 AM
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Jason, first of all, the obvious question: are you sure it's misfiring? What exactly are you experiencing? Any unusual physical signs (leaks more than usual, smoke, etc)?
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Old January 5th, 2015, 09:46 AM
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When it starts cold it's as smooth as it's ever run.

It will rev to beyond 5k rpm with no issues initially and that's better than it's ever run!

When I take it on a road test ( about a mile or two ) it will start to be mildly hesitant ... then under load ... laboring up an incline it will start to feel like it's missing and will not pull cleanly.

As I return from the test drive it will likely backfire mildly

When I get it back to the shop and rev it up it will burble on the overrun after revving up ...

If I hookup a timing light on either the coil lead or #1 and watch the flash i perceive ( although can't be sure obviously )

The truck has :

New plugs ... no change in behavior
New Genuine Cap ... no change in behavior
New Genuine Rotor ... no change in behavior
Newish leads ... no change in behavior
New Bosch Coil ... no change in behavior

Tried 2 ECU's
Tried 2 ignition amps
reset the air gap
no horizontal play in dizzy
dizzy resistance is about 3.5K ohms which is what it should be

Battery is fully charged and alternator is tested producing a good voltage.

Dizzy has correct voltage and all values measure as per the 14CUX test procedures.

ECU throws no codes. Tested with Rover Gauge and ECUMate.

Even tried a test run without O2 sensors connected but made no difference.

Thinking an oscilloscope is my only option ...
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  #4  
Old January 5th, 2015, 09:50 AM
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Also ... Checked dizzy advance operation. Tested vacuums is being pulled using vacuum gauge.
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Soapy water / KY jelly, etc. is is basically a must. Yes, good idea to remove trim panels - only takes 5 more minutes to do so.
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  #5  
Old January 5th, 2015, 10:42 AM
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If you stay on your road test, does it eventually die, continues to run but misfiring, or run but condition gets worse but doesn't die?
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  #6  
Old January 5th, 2015, 11:09 AM
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It died while accelerating uphill, I think it died when you took your foot off in that little parking lot when we pulled off the road as well, right?

It certainly sounds very similar to my symptoms a while back Jason. Remember I'd intermittently get spark, then no spark, then spark? We may be looking at the same thing with your vehicle even though resistance checks out per spec.
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Old January 5th, 2015, 11:38 AM
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I didn't see the condenser mentioned. Since its just a capacitor, its possible that heat is changing its insulating properties and shorting out the - post of the coil. They are cheap.
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Old January 5th, 2015, 01:14 PM
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Quote:
Originally Posted by robertf View Post
I didn't see the condenser mentioned. Since its just a capacitor, its possible that heat is changing its insulating properties and shorting out the - post of the coil. They are cheap.
I can just remove the condenser as it's job is to suppress electrical noise for the radio or does it have another purpose?
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Soapy water / KY jelly, etc. is is basically a must. Yes, good idea to remove trim panels - only takes 5 more minutes to do so.
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  #9  
Old January 5th, 2015, 03:29 PM
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Quote:
Originally Posted by leastonce View Post
I can just remove the condenser as it's job is to suppress electrical noise for the radio or does it have another purpose?
it is to suppress electrical noise for anything that is affected by voltage drop, like an ECM

You could remove it to test drive it, but I'd at least have a new one on order.
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  #10  
Old January 5th, 2015, 04:35 PM
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Quote:
Originally Posted by robertf View Post
it is to suppress electrical noise for anything that is affected by voltage drop, like an ECM You could remove it to test drive it, but I'd at least have a new one on order.
Thanks!
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Soapy water / KY jelly, etc. is is basically a must. Yes, good idea to remove trim panels - only takes 5 more minutes to do so.
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  #11  
Old January 5th, 2015, 04:39 PM
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Jason, I'm not sure what else to recommend from an electrical standpoint. You've changed/swapped things out that I would've done to troubleshoot the problem. I'm thinking fuel system now; am i premature to move to this line of thinking at this point in time?

I'd swap in a fuel temp sensor.

Then, I'd start checking power to the fuel pump. I clipped on a trailer light bulb to the fuel pump contacts of my wife's MGA. It was having the same problems as you Jason, she would drive it around and then when it warmed up sufficiently, it would sputter like it was misfiring and then eventually die. At first it seemed like it would only do it uphill, so I was checking compression, etc. etc. But when the trailer light died right before a breakdown, then I knew it was the fuel pump. I cut off the old electrical connector, spliced in a new set of terminals, and presto. Never had the problem again.

For us defender guys, the pump is not as accessible as a 1960 MGA. Maybe checking it at the fuel rail would work, but the problem is you won't have a "live feed" for when it happens again; the trailer light did that for me.
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  #12  
Old January 5th, 2015, 04:41 PM
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Did you try running with MAF unplugged?
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  #13  
Old January 5th, 2015, 04:53 PM
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what trims are you seeing on each bank in rover gauge?

That'll help narrow it down to spark or fuel cutting out

I had a similar problem on my 95 D1 and it was corrosion on a few of the injector plugs. They would intermittently cut out and the thing ended up maxing out the fuel trims.
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Old January 19th, 2015, 05:26 PM
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so ... in order to fix this I made a small investment ... I bought a old but working Snap-on MT1665 Oscilloscope from craigslist

A few minutes reading the instructions and a few scans of the screen looking for something 'odd' together with a five minute conversation with a LR tech and ... hey presto ... faulty lead on #4.
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Soapy water / KY jelly, etc. is is basically a must. Yes, good idea to remove trim panels - only takes 5 more minutes to do so.
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  #15  
Old January 19th, 2015, 06:22 PM
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Dig it.
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  #16  
Old January 19th, 2015, 06:34 PM
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Kewl, what plug wires were you using?
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Old January 19th, 2015, 06:40 PM
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Quote:
Originally Posted by rover4x4 View Post
Kewl, what plug wires were you using?
crappy ones ... probably broke it removing it at some point. I had to make up a temp lead using some spares and will buy a decent set tomorrow.
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Soapy water / KY jelly, etc. is is basically a must. Yes, good idea to remove trim panels - only takes 5 more minutes to do so.
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  #18  
Old January 19th, 2015, 06:42 PM
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Glad you got it fixed, now you can diagnose my cold start misfire, even though I'm pretty confident its an injector.

------ Follow up post added January 19th, 2015 06:43 PM ------

Quote:
Originally Posted by leastonce View Post
crappy ones ... probably broke it removing it at some point. I had to make up a temp lead using some spares and will buy a decent set tomorrow.
Genuine ones are super cheap last I checked.
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  #19  
Old January 19th, 2015, 08:26 PM
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Quote:
Originally Posted by leastonce View Post
crappy ones ... probably broke it removing it at some point. I had to make up a temp lead using some spares and will buy a decent set tomorrow.
I thought I asked you like five times if you were using the stock plug wires and you'd said yeah.
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  #20  
Old January 19th, 2015, 08:35 PM
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Quote:
Originally Posted by rijosho View Post
I thought I asked you like five times if you were using the stock plug wires and you'd said yeah.
I did too, and he said they were newish and Genuine IIRC. Plug wires of any brand can fail at any point in my experience.
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