Missing cylinders during idle

[spikeygg]

So when my car is idling it sounds like it isn't running on all cylinders and the engine rocks a bit. I'm curious if you guys know of a good way to prove out whether it's the fuel or spark... here's a video I made this morning that shows the sound difference between the left and right exhaust pipes:

[jmettee]

Tell us a little about the engine. Is it stock or does it have cams (i.e. Stage 2 engine)? If it has cams, that sounds normal from what I can tell in the sound coming out of my PC.

Sounds like mine from what I can tell, here's mine after I installed cams:

[spikeygg]

Tell us a little about the engine. Is it stock or does it have cams (i.e. Stage 2 engine)? If it has cams, that sounds normal from what I can tell in the sound coming out of my PC.

Sounds like mine from what I can tell, here's mine after I installed cams:

My car has 9.5:1 pistons and Isky racing cams (102707HP) with SuperTrapp exhaust. Unfortunately, it seems to only be running on 4 of 6 cylinders. Listening to the exhaust pattern of the left exhaust pipe there is only one or two beats while the right side exhaust pipe has a clear three beats. Need to figure out why all cylinders are not firing.

Our cars sound similar... I can't really tell from the video, do you have stock exhaust? My exhaust pipes come directly from each set of three cylinders so I can get a clean sound signal from each bank. If you have a similar setup, I'd love to hear how each side sounds.

[davidc89]

If you have a temperature gun, you can point it at each one of the exhaust tubes on your manifold. If one is colder then all of the others, you will have singled out what cylinder is not firing.

[Citizen]

If you have a temperature gun, you can point it at each one of the exhaust tubes on your manifold. If one is colder then all of the others, you will have singled out what cylinder is not firing.

Or, if you are poor like me, you can check the plugs, like when you're in there anyway to clean them, like during a tune-up. In my case, I found 5 plugs that were very clean, and one fouled. Yep, while I identified the cylinder not firing, I still haven't determined why that particular one got so fouled, and not the others.

...

[content22207_2]

Vacuum test is marginally useful on an odd fire PRV due to the dead space in our firing sequence (needle quivers even on a healthy engine). But engine wide (central vacuum) the needle should only quiver an inch or so. I suppose a vacuum test on each side of the engine independently would be useful to ensure both banks of cylinders are identical. In that respect a PRV will work for you -- there's no crossover in the firing sequence, so you'll be looking at each side of the engine alone.

But even before going that far I would clip a timing light on each wire, next to the plug boot. Every miss I've ever found has been a bad spark plug, usually oil or carbon fouled, but I did manage to melt an entire bank of spark plugs on my DeLorean once when the HVAC vacuum line cracked (driver's side of the engine pulls vacuum from the passenger intake rail):

Plugs2.jpg

In fact it might not be a bad idea to pull all the plugs for a visual inspection . Short of pulling the heads off, spark plugs are the only visual evidence of what's going on inside your cylinders. But a timing light is by far the fastest and easiest way to ensure the plugs are firing. Note that a bad spark plug wire can also prevent a plug from firing (carbon fiber plug wires are bad about fatigue breaking). A crack in the insulation jacket can allow the wire to short circuit to the block. And of course you could have a problem with a terminal inside the distributor cap itself (or simply a plug wire that has come unseated -- our vintage style terminals don't hold on as securely as later HEI terminals).

If all your plugs are firing, pull the injectors to ensure they are all spraying equally and adequately. A weakness of multiport injection is one cylinder's injector can be spraying differently than all the others, causing behavior issues in that cylinder alone.

Bill Robertson
#5939

[spikeygg]

Thanks guys this is helpful information. If I want to pull the plugs I'm guessing that I will need to pull the fuel distributor thingie, is there a how to on that somewhere so I know what to expect? How should I go about clipping a timing light onto all cylinders spark plug wires, those front-most plugs seem to be buried...

[spikeygg]

If you have a temperature gun, you can point it at each one of the exhaust tubes on your manifold. If one is colder then all of the others, you will have singled out what cylinder is not firing.

This is a fantastic idea. I'll definitely try it when I get a chance.

[content22207_2]

Use a 12" extension to remove the spark plugs with the intake manifold in place:

KJetSparkPlugRemoval.jpg

Concerning Cylinder #4: the only reason the idle speed motor is attached to the intake manifold is to facilitate assembly line manufacture (so the engine can easily be dropped into a moving car). When I was K-Jet I relocated my Aux Air Valve to the driver side pontoon and connected it with long hoses (my Aux Air Valve was an electric solenoid, not thermal). Makes engine access much easier. especially Cylinder #4. Also makes fuel/air mixture unit removal/installation much easier.

Bill Robertson
#5939

[Bitsyncmaster]

One thing I found when updating my ignition ECU was I needed to add a capacitor at the 12 volt power input to the ECU to stop a miss fire.

Use a 1000 uf 16 volt or higher voltage as close to the ECU as possible. The negative side of the cap should got to the ECU ground wire.