mkriete

... or that is what an error code might say if I had a computer.

I've been fighting a misfire on my '80 US. Last fall, I replaced the plug wires with new Beru, after I found a bad wire. I also replaced the green wire. The cap and rotor looked fine and tested fine, so I cleaned them up and reinstalled them. All was good. Yesterday, the misfire reared it ugly head. It was particularly noticeable during the idle. I have dual exhaust, so I put a piece of paper at each pipe. The left bank had an irregular "puff." I figured this was from ignition with the exhaust valve opened or after burn. I pulled the plugs on the left side and #7 was a little wet. All the plugs looked fine otherwise. I hooked up a timing light on left side. I had a consistent light on #5, #6, & #8, but #7 was was sporadic, weak, and often flashed rapidly. I assume this an arcing issue in the rotor cap and the cap needs to be replaced, but I'm still a noob at this. Is there anything else I could be missing?

soontobered84

You might start the car at night in the dark and look for fireflies under the hood around the #7 wire. It could be something intermittantly shorting.
It doesn't cost anything to look, but I'd agree that the distributor cap seem most likely.

M. Requin

Also, be sure the wire to cap and wire to plug connections are good. I would use a DMM to check the resistance and make sure it is within WSM values.

Wisconsin Joe

How old are the cap & rotor?

Despite "looking and testing fine", they may still be bad.

Dr. Gonzo

Check the small SAE nuts on top of the spark plugs. If they are loose - no spark.
Cheers.

Lizard928

Replace the cap and try again.

mkriete

I ran the car in the dark and sprayed water around the plug wire. I saw no "fireflies". Roger sent me a new distributor cap, rotor, and fuel injector relay (my pins were pitted). I installed the cap and rotor and let the car idle in the driveway for 15 minutes, and the engine was smooth. This smooth behavior continued for about 5 miles. Then the vibration and exhaust pooping came back. I installed the new relay, but no change.

Couple things to add. Cold starts have requird a rather long time cranking. My plugs are Cooper NGK with around 2,000 miles. I checed the spacing when I pull the left side plugs.

I'm left just scratching my head. I can't understand how just one plug or wire could be misfiring. It seems I have eliminated all the potential areas of failure. I'll do some more testing this weekend. I plan to check the plug wire resistance, and to reconfirm it is still #7 that is misfiring. Are there other thing I should be considering?

OTR18WHEELER

burnt valve?, a compression check is needed.
today's cheap fuels are not 928 friendly, low octane gas burns hotter, ignites too soon, pre '87 cars don't have knock sensors, so >=86 years need to run premium gas.
btw, just replaced 31 year old Oxygen sensor, original plug wires, and changed out the 10k mile plugs, New intake plenum boots.
....car still runs great, no noticeable change, but I always pump 92 or 93 octane gas, ...
Did I ever mention that my '85 auto with 220 diff chirps the 9" rears when it shifts to 2nd at WOT?
oh, and I use castrol GTX 20w50 year 'round

Mrmerlin

add a bottle of techron to the fuel,
add in 6 oz of MMO,
put in the Bosch plugs 7900 silver tips
Always use premium fuel from a name brand

mkriete

Stan, thanks again! I purchased a bottle of MMO and Techron. I'm not familiar with MMO, but I assume I want to use it as a gas additive. Just wanted to confirm.

I have a nearly full tank with a bottle of seafoam. I guessing it would not be wise to add any other additives at this point. I'm thinking I should run compression tests to make sure I don't have a burnt valve, before I run this tank out. Thanks OTR18wheeler

I checked the signal at #7 and it flash is regular now with a timing light.

For a little more background. I'm using Castrol 20w50 and have running Shell 89 octane. Sounds like I should switch to 91.

I found a good description of my problem on another forum (Honda-tech). I thought this description was much better than my attempt to describe the "puffing":

"During idle there is generally even exhaust pulses but randomly about every 2-3 seconds there will be a larger exhaust pulse. It's louder (nothing like a backfire but you can hear it if standing near the outlet) and there is a single burst of higher exhaust volume. The engine and exhaust system will shake when this happens as if the combustion in that one pulse was larger than normal."

No solution to the problem, though.

I just completed a timing belt job. Is there anything I could have done while I was changing the belt that could have caused this?

Mrmerlin

check the belt timing ,
replace plug #7 ignition wire.
with a siphon, drain out the gas and put it in your truck,
refill with 93 octane and 6 oz of MMO and a bottle of techron

OTR18WHEELER

Marvel Mystery Oil - Wikipedia
linky no worky, but now you know what mmo means.

OTR18WHEELER

had to fify, my neighbors have diesel trucks, ....

mkriete

MMO, Techron, and 93 octane in, 89 octane and seafoam out. Slight tightening of the timing belt. Cam gears are still aligned with the marks. 5 miles in, No change; not that I expected instant. .

I'll switch the plug wire out when I get home.

From others experience, how long does the MMO and Techron take to have an impact on the valves and injectors?

dr bob

Marvel Mystery Oil, Seafoam and Tecron will do nothing to help solve an ignition miss. Since your timing-light testing shows an ignition problem, stay focused on ignition. Those fuel additives may help with a partially-plugged injector, but that's really unrelated right now.

With the new cap and rotor, go back with the inductive timing light and see what it shows now, particularly on #7.

The inductive light shows when current flows through the wire to a plug that's firing or shorted, but doesn't show current to a plug that's open. The plugs have resistance built in, mostly to even out the firing voltages and therefore keep the timing consistent between the short and long wires. Once in a while, a plug will get through manufacturing with a crack or other defect that raises the resistance to the point that the plug doesn't fire reliably. Or it was dropped sometime in its life, even in the box. Or the plug gap is too big (or too small). Bottom line: New plugs are really really cheap and easy in the cost-to-test decision tree. Replace at least that one, preferably all of them, and know for sure that it's not the #7 plug. Poor/cheap/immediate diagnostic option is to swap the #7 plug with another on the other side and see if the miss "moves" to the other tailpipe. Considering the cost of plugs vs labor and headscratching time, a new plug is needed. Bosch copper was the original plug in WR7DC flavor. IIRC the NGK BPR6ES is the listed equivalent. I like to use what the car manufacturer used, so I keep a set of the Bosch WR7DC plugs in the service parts bin JIC.

Also: On the S4 plus cars, there are resistors in the plug terminals on the wires. 16V cars don't use the same ends as the S4+ 32V cars, but may have resistors in the wire ends anyway. A decent multimeter will tell you the total resistance end-to-end for each wire. If the #7 wire has higher resistance than the others it deserves some extra attention.