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engine missing

Posted by bmans05 (My Page) on
Sat, Jun 14, 08 at 13:49

I have an 81 chevy K-20 with a 87 t0 94 305 motor with a bad miss. I bought the truck as a project a couple of years ago,It seems to have no fire on #6 cylander ( the way i checked was hooking my timing light on each plug wire). I checked the plug wires for resistance and they all read the same, I replaced the cap an rotor and plugs, still missing. Talked to the auto parts store and they sugested replacing the pickup in the distribitor, samething.If anybody could help me figure this out it would be greatley appreciated.
Thanks Brian


Follow-Up Postings:

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RE: engine missing

Check for spark by installing a tool in the wire and hold it near ground. You want to see it. If it just misses at idle, check for a vacuum leak at the intake for that cylinder by spraying carb cleaner or some such thing in that area. Check the compression. If the valve cover is easy to remove from that side, take it off and observe how the rockers are moving. Get the little clips at NAPA to keep oil from squirting all over the place. Some GM engines around that time seemed to wear cam lobes for some reason. If you have some rockers that aren't moving much that's the problem. Engine will still be quite, but if a valve isn't opening, You'll have a dead hole. That's all I can think of for starters. Good luck.


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RE: engine missing

Lets reiterate what you have reported:

o Have replaced cap, rotor, and plugs.

o You are checking for spark by using a timing light, moving it from plug wire to plug wire.

o You have measured the cold resistance of the plug leads and all measure the same? These should differ according to their length. The long ones will have more resistance. However, maybe all your leads are about the same length.

You still see no spark on no.6.

As unsensible as it may seem, you may need new plug leads.
Do you have any old spare leads that were removed from other tune ups, but were good when removed? Try replacing no. 6 with a spare that measures good for resistance. If that clears up the no-spark problem, this suggests that the offending lead was shorting to ground.

Once you have cleared away the spark issue, you may still have a cylinder that is low on output as suggested by the poster above - worn cam lobes.

Run a compression test (if you have enough battery to spin the engine fast enough). To do this, disable the ingition and remove all spark plugs. It will be much easier for the starter motor to spin the engine. A cylinder with badly worn cam lobes will be down on compression compared to the other cylinders - however, this is not the only cause for low compression.


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RE: engine missing

Using a timing light to attempt to locate a cylinder that is missfiring is actually a very poor method, you probably realized that by now. Asking to a parts store counter person for advice makes using a timing light to locate a missfire look like a good idea. You probably realized that by now as well.

Missfires often occur a different times during engine operation. The time the missfire is occuring is quite often a clue to what is causing it. For a cylinder to fire you must have, spark, fuel, compression, and one thing many overlook, oxygen. They also have to be present at the right time and in the correct amounts.

So first question, when is the missfire occuring? Idle? Driving? Accelerating? More than one, or all?

Have you attempted to pull plug wires off of the cap to see which one does not make a difference in how the engine is running? Using insulated pliers pull #1 wire and note if it makes a difference in how the engine is running. If the missfire worsens, then it WAS firing and you need to put it back on and move to the next wire. If your getting shocked, turn the engine off, re-install the wire and restart the engine and move to the next wire to repeat the test. This test works specifically if the missfire is constantly there at idle. Missfires under acceleration and cruise require different testing.

Once the missfire is located , pull that spark plug and one more from a cylinder that was firing and measure compression on those two and compare the readings. Report back to us what you found.

One thing Jem didn't get exactly right is there is a very specific failure where a camshaft lobe wears down and it will not have an effect on compression. I'll stop here and he can work on figuring out what that would be.


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RE: engine missing

About pulling off spark plug wires:

You should not "open circuit" the secondary circuit of the ingition system with the engine running (pulling off a spark plug wire). This impresses undue voltage in the secondary circuit and can begin breakdown of the insulation and components. In older electronic ignition circuits, this can destroy electronic compnents. It is best to short the plug in question rather than open circuit it. This can be done by inserting a bare wire alongside the plug wire at the distributor cap. Using insulated pliers or other tool, cause the free end of the wire to touch metal on the engine block. If the leads fit too tightly in the cap to allow this method, then install the shorting wire(s) at the plug(s). Use care to prevent sparking some other circuit or wiring harness. Do not use a sharp point to pierce the lead.


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RE: engine missing

Hi Jem. Good thread!!

About pulling wires to "open circuit" secondary, when someone chooses to use an ohm meter to test a wire, exactly what fault are they really looking to see if it has occured?

The insualtion of the secondary ignition system definately should be able to prevent spark leakage due to an open plug wire, or by opening the secondary. In many cases today there is little else that can be done in the way of finding the source of a missfire except to open the secondary as one of the steps. At the same time we use an open secondary to allow us to measure peak ignition coil performance. We do that by normally looking for a spike in excess of 40k, and then count the occilations as the energy dissipates back into the vehicle charging system. Generally speaking if a car comes into thne shop with a driveability problem, it HAS to pass every test, and tbat includes stressing the ignition system. If we fail to test it, by stressing it and don't detect a looming failure th customers hold us responsible. So open circuit secondary is not only OK, but a required step.

BTW attempting to ground a plug/wire with a probe weakens the insulation between the two. I worked with a guy that did that regularly back in the 80's. It was normal to replace all of the plugs and the wires within a few months of his testing method. Your method of using a "tag" lead will work without causing too many problems. Except accessability may play a role, as well as because at the spark plug end the boots dry out from the heat. Many wires end up failing from the effort required to remove them. Plus the dried out boots scratch the plug porcelin, and now you start the carbon track game.

Jem, did you figure out the camshaft issue yet this morning?


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RE: engine missing

John_G

About detecting a bad cam lobes using a compression test. Yes, you are correct in that this is not a difinitive test as its sensitivity is too low for most cases - the difference between good and bad is too low to be of reliable use.

I did encounter one engine where it did work. My daughter had GM 4 cylinder, 2 L engine, and one set of cam lobes were worn so far down that if you spun the engine fast enough, the difference was detectible. The key here, the engine must be spun up to 150 rpm or more to see it. I saw it when all the plugs were removed except for the cylinder under test. Most of the time, the engine under test spins too slowly for a low lobe to make any difference in a compression test.

Admitdely, the better test is to remove the valve cover and inspect the valve action of the running engine.

When I tried to remove a lifter on the bad cam lobe, the lifter had peened over so badly, that burrs sticking out would not let the lifter come up through the hole. It was a mess.


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RE: engine missing

ANS. An exhaust camshaft lobe worn off will simply not allow gases in the cylinder to exit. Meanwhile the intake valve opens and allows a full charge to occur, thereby causing at the least no compression loss as compared to the other cylinders, but in fact causes higher compression on an engine running compression test. You are correct that engine speed plays a role in a compression test, thats why we not only test engine cranking, but engine running. An intake lobe worn off will possibly still allow enough air to be pulled in when cranking. But start the engine up and we look for a 50% drop from cranking compression and you will likely see 30% or less than the cranking spec for that cylinder. Of course this also works for valves that are sticking in the heads or valve guides.


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RE: engine missing

First of all thanks for all the help.The miss was all the time. I checked compression there was only a 5 pound differance in one clyander. I pulled the valve covers off put the clips on the rockers and ran the engine they all seemed to be moving the same amount. I took my test sparkplug (i dod'nt know why i did'nt use it in the first place instead of the timing light)and no spark to the #6 clyander.Bought a new set of wires and fixed the problem.I never suspected the wires because i bought them about a year ago and the truck has only been run down the street a couple of times since then. This has been a project i have been working on for about 2 years in my spare time. Thanks again for all your sugestions.


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RE: engine missing

Glad you got it working. Bet you won't be checking for spark with a timing light again! Feels good when you finally figure it out though, don't it?


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RE: engine missing

I have used a timing light to find a spark plug with no spark. It only works when the current pulse in the plug lead is too low to trigger the timing light. It will not find a fouled plug.

But here's whats been missing in the treads above. Not all timing lights will work for this precedure - only the ones that have a pickup coil/core that clamps around the sparkplug lead in question. These use the current pulse in the spark plug lead to trigger the lamp.


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RE: engine missing

I wouldn't use a timing light for that purpose no matter what. So much easier to pull off a wire from a plug and see if it jumps that gap at the point were it needs to. I have a little tool that clamps between the spark plug and wire that allows you to see how good a spark is there. A pulse that lights a timing light but doesn't make it to the plug for whatever reason just means you took an extra step that can lead you down the wrong road in your head as it did with the original poster. jmo


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