Testing methods for Light Misfires

[snapon20r]

Diesel71, what scope accessory do we need for our verus to do the running compression test you are showing in the picture?? What kind of scope are you using in your picture?

[diesel71]

Originally Posted by snapon20r

Diesel71, what scope accessory do we need for our verus to do the running compression test you are showing in the picture?? What kind of scope are you using in your picture?

that pic is not mine,just one i had handy to post and help show what i was trying to say. all of my work is on diesels but i have used this test on several gas engines with great success. i use the verus for all my scope needs, i just adapted my pressure transducer to a spark plug compression tester fitting. the only accessory's you need are a pressure transducer and the adapter that works with your verus. depending on what verus you have (wireless/pro/edge).
on the top of the this forum page,click on the link diagnostic platforms. there you can find your verus, click on it and you should find the accessory's for your machine. the pressure transducers and needed adapter. ill try and find my screen shots of the ones i have done, you don't need to run the engine very long only about 15-20 seconds. just long enough to capture a wave forum and diagnose.

[Crusty]

Originally Posted by Witsend

If the Head pipes on the exhaust manifold have enough length between the collector , and not shrouded completely by a heat shield , I can't see why the cylinder that is not consistently firing would show as high a temperature as the other cylinder's head pipes.

"cause a light misfire is still firing and metal tends to transfer heat. You MIGHT, just MIGHT catch it if you check it quickly from an overnight cold soak, but all that's telling you is one isn't right. If three or four cylinders all have turbulence issues, there will be several acting up. There aren't many engines with two foot header pipes ahead of the collector. You already know one (or a few) aren't right, that's why it's in the shop.

[Crusty]

Originally Posted by diesel71

I use the relative compression test almost every day. with this test your just going to know there is an issue with that cylinders ability to seal, with a light misfire at idle only I don't think this test would be much help. the in cylinder running compression test will show you when the valves are opining and closing. a bad valve guide is going to change not only the open/closing ramps on the scope but show compression lose and or gain in that cylinder compared to a known good one.

The only thing wrong with eliminating the spark plug is you lose the burn time turbulence which is the primary reason it's not firing properly.
With the pressures created within a diesel engine with much higher compression ratios I can see how the transducer may show something. With the "light misfires" the valves are sealing a lot of the time.

[snapon20r]

Originally Posted by diesel71

that pic is not mine,just one i had handy to post and help show what i was trying to say. all of my work is on diesels but i have used this test on several gas engines with great success. i use the verus for all my scope needs, i just adapted my pressure transducer to a spark plug compression tester fitting. the only accessory's you need are a pressure transducer and the adapter that works with your verus. depending on what verus you have (wireless/pro/edge).
on the top of the this forum page,click on the link diagnostic platforms. there you can find your verus, click on it and you should find the accessory's for your machine. the pressure transducers and needed adapter. ill try and find my screen shots of the ones i have done, you don't need to run the engine very long only about 15-20 seconds. just long enough to capture a wave forum and diagnose.

So just curious how you adapted the pressure transducer to a spark plug compression tester? Would you have a picture of it??

[jm43130]

In a reply to Witsend. ON the old scopes, didn't they use to tell that the flat plug firing line, after to spike to start spark plug, if it sloped upwards, didn't that tell that there was valve problems.

[Witsend]

In a reply to Witsend. ON the old scopes, didn't they use to tell that the flat plug firing line, after to spike to start spark plug, if it sloped upwards, didn't that tell that there was valve problems.

I'm pretty sure a gradual slope upwards is normal, but on a throttle snap ,a very short line that suddenly spikes upward " is a classic no fuel misfire"

https://www.youtube.com/watch?v=SUShO72Grq8

[diesel71]

Originally Posted by snapon20r

So just curious how you adapted the pressure transducer to a spark plug compression tester? Would you have a picture of it??

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[Witsend]

So on the end of the hose that normally would have threads you would screw in , instead you got an easy shove in non threaded end with an O-ring, you need to hold firmly in holes?

[diesel71]

Originally Posted by Witsend

So on the end of the hose that normally would have threads you would screw in , instead you got an easy shove in non threaded end with an O-ring, you need to hold firmly in holes?

you screw it in,there are threads on it. on that end you can remove part of it to use the bigger sized spark plug threads. i believe one size is 5/8 and the other is 13/16.