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Old 11-11-2015, 05:41 AM
snapon20r snapon20r is offline
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Default 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.

Last edited by snapon20r; 11-11-2015 at 12:44 PM..
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Old 11-11-2015, 08:03 AM
Witsend Witsend is offline
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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.
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Last edited by Witsend; 11-11-2015 at 08:22 AM..
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Old 11-11-2015, 12:47 PM
snapon20r snapon20r is offline
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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??
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Old 11-11-2015, 03:10 PM
Crusty Crusty is offline
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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.
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Old 11-11-2015, 03:13 PM
Crusty Crusty is offline
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Quote:
Originally Posted by snapon20r View Post
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.
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Old 11-11-2015, 03:49 PM
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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.
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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.
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Old 11-11-2015, 05:10 PM
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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.
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Old 11-11-2015, 05:35 PM
STURNER STURNER is offline
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Have you tried a relative compression test using the scope and the low amp probe?
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Old 11-11-2015, 05:51 PM
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Quote:
Originally Posted by STURNER View Post
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.
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Old 11-11-2015, 06:38 PM
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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

Last edited by Witsend; 11-11-2015 at 06:57 PM..
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