MIssing at idle, weak spark

Tiny
GREASYFINGERS
  • MEMBER
  • HONDA CIVIC
I have a 1987 honda civic, carbureted 1.5 engine.
I am battling with an idle misfire that I can't seem to resolve.
Heres how it looks, the idle fluctuates rather than holding steady by maybe 100 rpm, Intake vacuum is not steady but bounces between 17.5 and 16.5 "hg, it does become steady at higher rpms. The exhaust pressure at the tailpipe is steady for a few seconds and then makes a "puh" when the pressure falls off.

I checked my O2 sensor output voltage and it seems to indicate a lean condiition at.1 volts, but closing off the choke makes the sensor respond quickly up to.9 volts. I put a spark plug test light inline with each spark plug 1 at a time and the lamp glows pretty steady but will occasionally glow brightly. When I rev the engine up to say 2k rpm the lamps is always bright and there is no sign of a misfire.

new plugs, cap, rotor, wires.
In desperation I replaced the ignitor.
plugs are denso, cap and rotor are quality parts, the wires are the top shelf wires from aurtozone.

I suspect my carb is the cause of the lean condition, but the weak spark has me concerned that my distributor may be shot.

I checked the ignition coil resistance and it is really close to dead on with the factory service spec, I pulled the coil wire from the cap and the spark jumped about an inch and was reddish blue. I then pulled the wires (at the cap) to each plug and they acted the same, they would jump about an inch and were the same color as the coil spark. The wire that had the test light in it seemed to give a brighter glow when I held the connector a little out of the cap.

What should I do?
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Tuesday, December 5th, 2006 AT 4:49 PM

7 Replies

Tiny
BRUCE HUNT
  • EXPERT
Are you getting a PGM-F1 light? Have you checked the PCV valve? Can you narrow it down to a particular cylinder that is causing the problem? I am suspecting a vac problem. I am not too familiar with a carbed Honda car engine but there are several things that I think about. You mentioned the lean and that could be a timing issue. Try looking at the timing and vac lines, etc. See if the timing makes any difference to the idle.
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Tuesday, December 5th, 2006 AT 5:47 PM
Tiny
GREASYFINGERS
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There is no light for this one since it isn't fuel injected. More history on the car, I got it with a blown head gasket and shredded waterpump. I replaced the gasket (checked the head, ok) and the waterpump/timing belt. I also replaced the PCV while I had the Intake Man off. Afterward I replaced all the typical maint type items fuel/ air/ oil filters, cap/rotor/plugs/wires.

I have adjusted the valves to spec and the valve seals were replaced when I did the HG. I do have 2 cyl with lower compression, but the deviation is within spec (20 psi from the highest cyl, max is 28)

I did have a vac leak at the carb insulator which I replaced and no longer get any feedback when spraying carb cleaner aroung the intake manifold and vac hoses. I still think my vacuum is low, but I cant easily find out why. I have checked my EGR which is not funtional due to leaking diaphragm but the valve is clean and stays closed

I get the same results with my spark tester on all the cylinders, dim glow and occasional bright glow, so I haven't been able to catch one cylinder missing occasionally that way. The miss seems to be random.

I have the timing set at the 15 deg mark on the crank pulley but it bounces due to the idle problem.
I had it really advanced and that was bad, should I try retarding it?
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Tuesday, December 5th, 2006 AT 6:06 PM
Tiny
BRUCE HUNT
  • EXPERT
You mention the EGR valve and that it stays closed. I found this and am wondering if the valve is possibly causing some of the problems.

"If it sticks in the open position or fails to close all the way, it acts like a vacuum leak resulting in a rough idle, hesitation and possible stalling."
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Tuesday, December 5th, 2006 AT 6:53 PM
Tiny
GREASYFINGERS
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I had the EGR valve off and cleaned it up today, it moves freely and the spring seems to hold it closed. Still acts the same.
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Tuesday, December 5th, 2006 AT 10:05 PM
Tiny
BRUCE HUNT
  • EXPERT
One other thought; you said you had the intake manifold off. Did you put on a new intake manifold gasket? I have experienced a rough idle and surging on the FI engine, due to a leak at the intake manifold. A fragment of old gasket can cause a leak of air that mixes with the fuel.
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Tuesday, December 5th, 2006 AT 10:42 PM
Tiny
GREASYFINGERS
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I replaced the gasket since it came with the headset, I did a pretty good job of surface prep as well. I sprayed down the top of the IM with carb cleaner looking for leaks, but I will try getting at the bottom as well
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Tuesday, December 5th, 2006 AT 10:51 PM
Tiny
GREASYFINGERS
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After several more hours of poking around, I have determined that the spark is the same all the time and not the reason for the misfire. I don't know why I get an occasionally brighter flash.

I retarded my timing a bit so that when I artificailly richen the mix by shoving my finger into the idle secondary venturi, the timing is dead on and rock solid. Of course as soon as I take out my finger it advances and jumps around.

Last intake vacuum reading was nearly 20" but the idle may have been a bit high.

Any vacuum leaks are either tiny or internal and will be devilish to locate as the last 2 were.

I may just rebuild the carb in desperation, but first I am running a tank of gas treated with seafoam.
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Wednesday, December 6th, 2006 AT 7:37 PM

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