Pull the valve cover , make sure the valves are opening & closing the same amount as the other cylinders. Set the engine on fast idle & see if 8 is firing then . Make sure there's no obvious vacuum leaks ( hose, etc.)
Trying to chase down a misfire
This is for a 350 Chevy in a 77 Vette.
It seemed to be running a little rough so I did a cylinder drop test by pulling a plug one at a time from the distributor.
Pulling the wire from cylinder #8 didn't affect the idle. So here is where I am chasing the prob down with that cylinder:
Could it be that cylinder isn't getting fuel? Cylinder 6 is next to it and fine so that makes me think that corner is getting fuel.
Its stock except for the Edelbrock intake manifold and Carter AFB carb.
Ideas?
It seemed to be running a little rough so I did a cylinder drop test by pulling a plug one at a time from the distributor.
Pulling the wire from cylinder #8 didn't affect the idle. So here is where I am chasing the prob down with that cylinder:
- There was spark at the terminal for #8 but changed the cap anyway, it looked iffy.
- Traded plug wires and spark plugs between cylinders 6 and 8. No effect, 8 was still dead and 6 was fine. The plug I took out of 8 also looked normal compared to 6 so it was running normally until recently.
- Checked compression and cylinder 8 has 180 psi.
Could it be that cylinder isn't getting fuel? Cylinder 6 is next to it and fine so that makes me think that corner is getting fuel.
Its stock except for the Edelbrock intake manifold and Carter AFB carb.
Ideas?
1 - 20 of 28 Posts
Inspect the distributor cap for carbon arc paths from that terminal to another or to ground. or the presence of moisture. Rocker cover off, carefully check the rocker movement in search of a failing lobe and lifter. Replace the plugs and gap them at .030 inch, even good looking plugs can go bad.
Bogie
Bogie
Thanks for the replies, never got my notifications, will have to look into that.
Cap looked good but put a new AC Delco cap on anyway, no change. I traded the spark plug with one from another cylinder and it didn't make a difference, so wouldn't that eliminate the plug as a suspect? I just don't want to throw parts at it and would like to understand what is happening.
Cold compression test for that cylinder shows 180 PSI, is there some problem that could kick in when the motor is running that would then drop the compression?
Will pull the valve cover and do that and report back. Will try fast idle as well. After I created the post I went on the hunt for a vac leak and found one, fixing the leak didn't change the misfire unfortunately.Pull the valve cover , make sure the valves are opening & closing the same amount as the other cylinders. Set the engine on fast idle & see if 8 is firing then . Make sure there's no obvious vacuum leaks ( hose, etc.)
Will pull the rocker cover and check.Inspect the distributor cap for carbon arc paths from that terminal to another or to ground. or the presence of moisture. Rocker cover off, carefully check the rocker movement in search of a failing lobe and lifter. Replace the plugs and gap them at .030 inch, even good looking plugs can go bad.
Bogie
Cap looked good but put a new AC Delco cap on anyway, no change. I traded the spark plug with one from another cylinder and it didn't make a difference, so wouldn't that eliminate the plug as a suspect? I just don't want to throw parts at it and would like to understand what is happening.
Cold compression test for that cylinder shows 180 PSI, is there some problem that could kick in when the motor is running that would then drop the compression?
check for broken valve spring
Ok tried some things.
Ran cylinder drop test at fast idle and 8 was still dead.
Took off the valve covers and ran the motor, everything seems to be moving the same as the other cylinders. Didn't measure but ran my finger along the same edge and felt the rocker rub the same spot on my knuckle for all the cylinders. Not the most scientific I know.
I don't think there is a broken spring from what I can tell, I made a video showing it idling with the cover off.
YT vid here.
Maybe someone will spot something I am missing.
If not next will try a leak down test. This is frustrating.
Ran cylinder drop test at fast idle and 8 was still dead.
Took off the valve covers and ran the motor, everything seems to be moving the same as the other cylinders. Didn't measure but ran my finger along the same edge and felt the rocker rub the same spot on my knuckle for all the cylinders. Not the most scientific I know.
I don't think there is a broken spring from what I can tell, I made a video showing it idling with the cover off.
YT vid here.
Maybe someone will spot something I am missing.
If not next will try a leak down test. This is frustrating.
#8 intake not moving as much as the rest.
It does kinda look like number 8 intake isn't moving as much. It's moving enough that that doesn't explain the dead miss though. It may run a bit rough but probably not a dead miss.
I'd do a leak down on number 8 at the very least to rule out a mechanical problems but this smells like an ignition problem.
Also. a spray bottle squirt on the headers tubes is a good way to see what cylinders are problematic. If the cylinder is working well the water evaporates in 1 or 2 seconds or less with a noticeable sizzle and if it's dead it'll stay wet for a more seconds. This doesn't work very well on manifolds though. Another way of checking for spark is to use your timing light on the wire as close to the plug as you can. Flashy's indicate a spark. No flashy, no spark.
What does the plug on 8 look like?
I'd do a leak down on number 8 at the very least to rule out a mechanical problems but this smells like an ignition problem.
Also. a spray bottle squirt on the headers tubes is a good way to see what cylinders are problematic. If the cylinder is working well the water evaporates in 1 or 2 seconds or less with a noticeable sizzle and if it's dead it'll stay wet for a more seconds. This doesn't work very well on manifolds though. Another way of checking for spark is to use your timing light on the wire as close to the plug as you can. Flashy's indicate a spark. No flashy, no spark.
What does the plug on 8 look like?
Will still do a leak down, just couldn't get to it today.It does kinda look like number 8 intake isn't moving as much. It's moving enough that that doesn't explain the dead miss though. It may run a bit rough but probably not a dead miss.
I'd do a leak down on number 8 at the very least to rule out a mechanical problems but this smells like an ignition problem.
Also. a spray bottle squirt on the headers tubes is a good way to see what cylinders are problematic. If the cylinder is working well the water evaporates in 1 or 2 seconds or less with a noticeable sizzle and if it's dead it'll stay wet for a more seconds. This doesn't work very well on manifolds though. Another way of checking for spark is to use your timing light on the wire as close to the plug as you can. Flashy's indicate a spark. No flashy, no spark.
What does the plug on 8 look like?
I tried the spray bottle test and glad you mentioned it. Very different evaporations rate when I squirt the manifold where #8 is. Plus I then noticed the overall rust pattern is different on the end where #8 is which leads me to think this has gone on for awhile.
Went over all the ignition stuff I could think of to start and covered it in the first post, the plug looked fine but was traded with another cylinder's plug anyway. Also traded the wire itself and the misfire didn't move. Basically swapped everything with cylinder #6, but #8 still misfired.
I hooked up a vac gauge and see the needle dancing. I don't know how much is acceptable but I would guess the way it danced in gear is pretty bad. Here is a vid of it in action.
vac reading
Nice vid!
It reveals another potential problem.
Those mesh air cleaners are known to light on fire with the least little hiccup. Turf it.
Here’s a chart for your vacuum readings.
www.onallcylinders.com
It reveals another potential problem.
Those mesh air cleaners are known to light on fire with the least little hiccup. Turf it.
Here’s a chart for your vacuum readings.
Vac Visual: Quick Guide to Vacuum Gauge Readings - OnAllCylinders
Intake manifold vacuum is a key indicator of engine performance. By measuring vacuum under different engine loads and conditions, you can track down problems within your engine and make the …
www.onallcylinders.com
The air cleaner is just temporary, if I wasn't distracted by this issue I probably would have modified a stocker to fit the Carter AFB.
Thanks for the chart, to me it seems to point to valve issues. I'm going to see if I can get a dial gauge set up just to really measure valve lift and know if the cam is going out first before I consider a valve job.
It has always in the 6 yrs I've owned it puffed out some smoke when started a couple of hours after a drive, so figured the rubber valve seals were going bad, but maybe the guides etc are too.
Thanks for the chart, to me it seems to point to valve issues. I'm going to see if I can get a dial gauge set up just to really measure valve lift and know if the cam is going out first before I consider a valve job.
It has always in the 6 yrs I've owned it puffed out some smoke when started a couple of hours after a drive, so figured the rubber valve seals were going bad, but maybe the guides etc are too.
Can't figure out how the shiny spot on the pushrods for 4 and 6 seems to be above the slot in the head. I guess the pushrods might have run with guideplates at some time.
Yeah I have to admit I stared at that for a couple of mins really wondering WTF.Can't figure out how the shiny spot on the pushrods for 4 and 6 seems to be above the slot in the head. I guess the pushrods might have run with guideplates at some time.
Vacum video, my guess is the Valve lash is not set right or Your cam is Bad or going bad, Or worst case it needs a Valve job.
The Older Hydraulic Flat tappet cams have a tendency to go bad reasons why I wont go into.
The Older Hydraulic Flat tappet cams have a tendency to go bad reasons why I wont go into.
I'm as sure as I can be on the valve lash being correct.
Unless the leak down test shows something I will get set up to measure actual rocker arm movement at the head to compare to a couple of good cylinders.
I am worried about the cam with the whole modern oil thing, if its bad and I can convert to roller I may go that route.
Unless the leak down test shows something I will get set up to measure actual rocker arm movement at the head to compare to a couple of good cylinders.
I am worried about the cam with the whole modern oil thing, if its bad and I can convert to roller I may go that route.
Could be #8 intake gasket sucked in and pulling crankcase air instead of mix.
i had a similar problem with my 460 in my motor home, #4 made absolutely no change when i pulled the plug wire, i could hear the miss from the exhaust, i changed valve springs, did a complete tune up on it and it was still there, it has a holley carb on it so i decided to adjust the carb a little, as soon as i turned the a/f mixture screw out a little the miss went away and the engine smoothed out, turned out to be a lean miss,
Could be sucking air in from the crankcase due to a sucked in gasket. Easy to check. Pull the PCV valve, and stuff a vacuum gauge in the hole.Plug the filtered air intake on the opposite side of the motor, on the valve cover.
If it builds vacuum its leaking from crankcase to intake runner.
If it slowly builds a little pressure, then it does not have an intake leak . Easy peasy to check.
If it builds vacuum its leaking from crankcase to intake runner.
If it slowly builds a little pressure, then it does not have an intake leak . Easy peasy to check.
Someone mentioned that to me yesterday so I backed out the idle mix screws, I could smell it running richer but it didn't fix the miss.i had a similar problem with my 460 in my motor home, #4 made absolutely no change when i pulled the plug wire, i could hear the miss from the exhaust, i changed valve springs, did a complete tune up on it and it was still there, it has a holley carb on it so i decided to adjust the carb a little, as soon as i turned the a/f mixture screw out a little the miss went away and the engine smoothed out, turned out to be a lean miss,
Could be #8 intake gasket sucked in and pulling crankcase air instead of mix.
I will try that test, since this is a smog car there is enough stuff to make pulling the intake a PITA but beats a bad cam.Could be sucking air in from the crankcase due to a sucked in gasket. Easy to check. Pull the PCV valve, and stuff a vacuum gauge in the hole.Plug the filtered air intake on the opposite side of the motor, on the valve cover.
If it builds vacuum its leaking from crankcase to intake runner.
If it slowly builds a little pressure, then it does not have an intake leak . Easy peasy to check.
I have things apart for a leak down test and so far it looks good percentage-wise, but I want to test again. I have to do this in the driveway and my neighbors started to make a racket when I wanted to listen for escaping air so took a break until they are gone.
1 - 20 of 28 Posts
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