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-   -   Testing methods for Light Misfires (http://productforum.autorepairdata.com/forum/showthread.php?t=11101)

snapon20r 11-11-2015 05:41 AM

Testing methods for Light Misfires
 
Hello everybody, I am a tech at a gm dealership. We are running into lots and lots of light misfires on engines ONLY AT IDLE that are resulting in valves not sealing properly / bad valve guides. We do have the snapon verus, have not been able to find a good test to pinpoint this problem. We just been ruling everything else out, then taking the heads off. We can put air in the cylinder, compression test etc, nothing pinpoints it. Anybody find a pinpoint test for these light misfire valve problems?? These are on gm engines usually 4 and 6 cylinder engines.

Just adding some more info, we know which cylinders are misfiring by watching the misfire data on the scanner, we are just trying to find a pinpoint test that tells us that it is a valve problem in that cylinder, after ruling everything else out. Performing an air leak down test and compression test are not showing us anything, so we assume its a valve problem, take the heads off and sure enough there is valve sealing issues and guide problems after our machinist takes the heads apart. We are just wondering if anybody has found a test that says this is a valve problem before taking the heads off. We were actually on the phone with gm assistance and they told us all you can do is rule everything else out, then take the heads off. So far this has worked, it has been a valve/guide problem everytime, I am just one of those techs that likes to pinpoint exactly what it is before tear down.

Witsend 11-11-2015 08:03 AM

2 Attachment(s)
Sounds like the back of the intake valves might be coking up with carbon from the crankcase ventilation system. The ultra low viscosity oil combined with longer change intervals , means more vapors getting past the baffles and separators and drawn into the intake plenums.
With the direct injection now a days for better fuel economy, you no longer have the benefit of the cleaning capabilities of what top tier gasoline used to do to keep the backs of the intake valves clean of intake valve deposits anymore .
Any Port Cargo 08 should be able to give you an idea of suspect cylinder with most GM's. Doubt an old Counselor could be much help on some COP ignition , but on older non waste spark systems higher KV bar graph and shorter burn time on the handy bar graph were useful features to have. I remember working on motorcycles that had a miss at idle . I would do the ole spit sizzle check on the exhaust head pipes to check which pipe was running cooler. Maybe a Port Cargo infrared thermometer pointed at the exhaust manifold outlets could detect significant temperature differences better, but bet the spit check be faster. :o

snapon20r 11-11-2015 12:47 PM

Just replying to witsend, so an infared thermometer would show this on the exhaust manifolds??

I haven't used an ignition analyzer much like the counselor, can an ignition analyzer show valve issues, even light misfires at idle??

Crusty 11-11-2015 03:10 PM

The much older real time ignition scopes (PRIOR to the digital ones) used to show the secondary ignition waveforms with the voltage required to start the plug firing, then the voltage and time to keep the plug firing (aprox 2-mil-sec) and the rise and fall along with the "hash" of that burn time, then the plug stopping firing and then the coil oscillations and finally the points close.
With experience we used to be able to see and hear the condition at exactly the same time which is now impossible due to the electronic "filtering" of the "digital ignition analysers' since the 1980's.
Cylinders would fire properly and then not fire properly and that older ignition trace used to show that if you watched closely. It's been one of my bugaboos for several decades and yes I am fully aware of what the new scopes can do, but I'm old enough to recognize what they don't do anymore as well.

Personally I believe the root cause of the worn valve guides is the overly ambitious oil drain intervals. Ya, ya, the new oils are so much better. Yes they are but the "new" oils claimed the same extended drain interval capability back shortly after the 1973 oil crisis AND ANYONE STILL IN THE TRADE SAW THE DAMAGE DONE back then.

Crusty 11-11-2015 03:13 PM

Quote:

Originally Posted by snapon20r (Post 51062)
Just replying to witsend, so an infared thermometer would show this on the exhaust manifolds??

I haven't used an ignition analyzer much like the counselor, can an ignition analyzer show valve issues, even light misfires at idle??

An infared thermometer is only going to see a dead hole that never fires and creates any heat. Or a cylinder that fires so infrequently that you probably don't need to check it that way in the first place.

Witsend 11-11-2015 03:49 PM

Quote:

An infared thermometer is only going to see a dead hole that never fires and creates any heat. Or a cylinder that fires so infrequently that you probably don't need to check it that way in the first place.
Reply With Quote
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.:)

diesel71 11-11-2015 05:10 PM

1 Attachment(s)
I would try a in cylinder running compression test, get a screen shot of a known good cylinder then compare the intake and exhaust, opening/closing humps in the wave forum. you should see a difference when/were and how long the valves open and close.
Attachment 20012

STURNER 11-11-2015 05:35 PM

Have you tried a relative compression test using the scope and the low amp probe?

diesel71 11-11-2015 05:51 PM

Quote:

Originally Posted by STURNER (Post 51076)
Have you tried a relative compression test using the scope and the low amp probe?

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.

Witsend 11-11-2015 06:38 PM

I would bring the suspect cylinder slowly to TDC and do a cylinder leakage test and verify if it's an exhaust or intake. Ford had a run of 2.0 SOHC heads where the valve seats could work loose and cause misfires

snapon20r 11-11-2015 08:09 PM

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 11-11-2015 09:21 PM

Quote:

Originally Posted by snapon20r (Post 51079)
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 11-12-2015 12:58 AM

Quote:

Originally Posted by Witsend (Post 51073)
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 11-12-2015 01:08 AM

Quote:

Originally Posted by diesel71 (Post 51077)
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 11-12-2015 03:58 AM

Quote:

Originally Posted by diesel71 (Post 51080)
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 11-12-2015 05:09 AM

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 11-12-2015 10:35 AM

Quote:

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 11-12-2015 11:12 AM

3 Attachment(s)
Quote:

Originally Posted by snapon20r (Post 51084)
So just curious how you adapted the pressure transducer to a spark plug compression tester? Would you have a picture of it??

Attachment 20033
Attachment 20034
Attachment 20035

Witsend 11-12-2015 11:37 AM

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 11-12-2015 11:47 AM

Quote:

Originally Posted by Witsend (Post 51103)
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.

crackerclicker 11-13-2015 01:14 AM

Quote:

Originally Posted by Crusty (Post 51069)
Cylinders would fire properly and then not fire properly and that older ignition trace used to show that if you watched closely. It's been one of my bugaboos for several decades and yes I am fully aware of what the new scopes can do, but I'm old enough to recognize what they don't do anymore as well

I'm fairly certain we've argued on this or a similar topic before, but I'm unconvinced you've truly explored all the options available. Personally I find only a little value in examining secondary the way you describe, however I believe what you desire is available in Picoscope's "rapid trigger" feature (other quality DSO manufacturers likely have a similar feature).

Crusty 11-13-2015 02:45 AM

Quote:

Originally Posted by crackerclicker (Post 51107)
I'm fairly certain we've argued on this or a similar topic before, but I'm unconvinced you've truly explored all the options available. Personally I find only a little value in examining secondary the way you describe, however I believe what you desire is available in Picoscope's "rapid trigger" feature (other quality DSO manufacturers likely have a similar feature).

I'm unconvinced you've stood in front of a vehicle watching the scope screen and both heard the misfire and seen the misfire at exactly the same time, and I do mean exactly, positively identifying the cylinder or multiple cylinders, which used to be a normal capability of older mainstream scopes prior to the digitizing that began in the early 1980's, not just one specialty scope using one feature.

Whatever else is going on, the desired result is in fact that secondary firing of the plug, in the hole, under compression, with fuel delivered, completing the burn and the expanding gasses pushing the piston down.
Whatever the primary ignition is doing does not always translate to the secondary doing what it is supposed to do.

snapon20r 11-13-2015 03:49 AM

Quote:

Originally Posted by diesel71 (Post 51099)

Thankyou diesel71 for your pictures and info, that will be a big help.

snapon20r 11-13-2015 03:50 AM

And thankyou to everybody else for your replies.

crackerclicker 11-13-2015 03:11 PM

Quote:

Originally Posted by Crusty (Post 51108)
I'm unconvinced you've stood in front of a vehicle watching the scope screen and both heard the misfire and seen the misfire at exactly the same time, and I do mean exactly, positively identifying the cylinder or multiple cylinders, which used to be a normal capability of older mainstream scopes prior to the digitizing that began in the early 1980's, not just one specialty scope using one feature.

http://www.mikejonesauto.com/

Give him a call, he's a big fan of Rapid Trigger.


Quote:

Originally Posted by Crusty (Post 51108)
Whatever the primary ignition is doing does not always translate to the secondary doing what it is supposed to do.


Agreed, not sure what your point is.

Joe Rappa 11-13-2015 04:47 PM

There are some folks that are VERY accomplished when it comes to analyzing vacuum waveforms from the intake and pressure waveforms from the exhaust. If it's a mechanical problem with the engine(and you're looking for sticking valves), you can certainly see waveform changes if you do those measurements. I've got a friend that swears by the Sen X First Look.

Joe

Crusty 11-14-2015 03:04 AM

Quote:

Originally Posted by Joe Rappa (Post 51137)
There are some folks that are VERY accomplished when it comes to analyzing vacuum waveforms from the intake and pressure waveforms from the exhaust. If it's a mechanical problem with the engine(and you're looking for sticking valves), you can certainly see waveform changes if you do those measurements. I've got a friend that swears by the Sen X First Look.

Joe

Good point Joe. I've been so used to hooking up vacuum gauges to see how the engine is breathing that it's second nature and I had forgotten to even mention it. Fundamentally the valves dictate how the engine breathes.
It is getting harder and harder to access a vacuum source on todays' engines but worth the effort.
Diesel engines don't need the spark plug to complete the combustion so I can certainly see where the pressure transducer in place of the glow plug would work just fine. It eliminates a very necessary part of the entire cycle in a gasoline application though.

Question; does the First Look system have a quick and easy way to measure both the air entering the engine (via air in the intake duct, OR, the vacuum) and the exhaust out at the same time, and then graph the two well on a scope screen-??

Crusty 11-14-2015 03:08 AM

Quote:

Originally Posted by crackerclicker (Post 51134)
http://www.mikejonesauto.com/

Give him a call, he's a big fan of Rapid Trigger.


Ummm it's still one system, not the mainstream norm the way secondary ignition scopes used to be


Agreed, not sure what your point is.

Ummm misfires...

crackerclicker 11-15-2015 01:40 AM

"You can lead a horse to water . . . "

Crusty 11-15-2015 03:58 AM

Quote:

Originally Posted by crackerclicker (Post 51160)
"You can lead a horse to water . . . "

Young bull, old bull...

Witsend 11-15-2015 12:49 PM

F@ck the lack of OE manifold vacuum ports . You could Drill or melt some holes in dem plastic fantastic intake runners as close to the intake ports as possible and thread in some Yamazuki screws with orings or adapters with rubber caps and hook up a Motorcycle Manometer -Carb synchronize r setup.:D

Joe Rappa 11-15-2015 04:38 PM

Quote:

Originally Posted by Witsend (Post 51167)
F@ck the lack of OE manifold vacuum ports . You could Drill or melt some holes in dem plastic fantastic intake runners as close to the intake ports as possible and thread in some Yamazuki screws with orings or adapters with rubber caps and hook up a Motorcycle Manometer -Carb synchronize r setup.:D

I'm not sure whether it is time for you to stop taking your meds, or get back on them!
Either way, you're due for a change. :D

Joe

Joe Rappa 11-15-2015 04:43 PM

Quote:

Originally Posted by Crusty (Post 51141)
Question; does the First Look system have a quick and easy way to measure both the air entering the engine (via air in the intake duct, OR, the vacuum) and the exhaust out at the same time, and then graph the two well on a scope screen-??

Crusty,
The SenX tool is just a pressure/vacuum probe that's sensitive to small pressure changes. It makes a nice, clean signal while doing it. I imagine you could add MAF or MAP to make the signal more useful.

Joe

Dr Dave 02-13-2019 07:01 PM

Vacuum tranducer
 
You could experiment with a vacuum transducer in the intake or maybe while your going a leak down you use a vacuum transducer in the intake or breather to monitor the leak. The best part is you have access to known and an good engines.knock yourself or and keep us in the loop

Dr Dave 02-14-2019 11:28 AM

WHATS GOING ON with the fist sceen shot
 
whats going on with the first sceenshot very sharp and abroupt lines

BRIAN617 02-14-2019 12:44 PM

Quote:

Originally Posted by Dr Dave (Post 63466)
whats going on with the first sceenshot very sharp and abroupt lines

Dr. Dave, do you realize most of these posts your'e replying to are 3-4 years old?

sbreland73 02-14-2019 03:00 PM

Quote:

Originally Posted by BRIAN617 (Post 63469)
Dr. Dave, do you realize most of these posts your'e replying to are 3-4 years old?

Playing catch-up.

Dr Dave 02-15-2019 07:02 AM

ya old post i know
 
ya I was just trying to see if anyone still interested . I read through and I see no solution from the originator. So just kicking around ideals if anyones looking ,but really mostly I,m looking for knowledge . thanks is all good stuff. I actually did start a new post on vacuum testing equiptment .

Steve6911 02-15-2019 07:27 AM

Here is a link to a recent Motorage video, basic but good stuff

https://www.youtube.com/watch?v=vlhABxdLI9I


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