Saga of misfire on #4, or is it?

[strupgolf]

As some of you know, I've been working on a "miss" or "popping" from my 3.2 86 Targa for awhile. Its there at startup, but as it warms, the popping goes away. But, its replaced by a miss at steady speed, no torque on the engine, just a miss that is noticeable around town. Now I took it to a small shop in town and they mentioned a bad fuel injector. This because it smells of gas when ideling, and I know gas is going out the tailpipe unburned. Thus, the "popping "I've been thinking of this as the cause. Replaced that, same thing, no change. All wires, cap, rotor, checked out ok. Great compression on # 4, where I think it is happening. Changed all the plugs again. NOW, when I pull off the plug on #4, the popping stops, and the engine sounds like it always does. I notice no change???? The engine has a missfire at low RPM, but at full throttle, its fine, cant run better. What could cause this guys? I'm at my wits end to figure it out. Thanks for any reply's.

[asphaltgambler]

Couple of things here. What are the actual compression numbers? VERY important

When checking the ignition wires did you ohm them?

How did you check the cap and rotor? Visual inspection may not reveal hairline cracks or carbon tracking from #4 terminal to ground.

Also if the plug is not firing it will be wet with fuel on the tip even with good compression, was that one (#4) wet?

Even with a stone dead miss the engine will seem to sound ok at high rpm's because crank speed / inertia.

[Early_S_Man]

First of a all ... I wouldn't trust any shop whose tech or service rep. 'mentions' bad injectors! Gee, who would have guessed that? That is just a 'dumbo' response looking for a sucker like P.T. Barnum used to talk about ...

My suggestions are multi-pronged, and I would suggest asking to see the evidence firsthand if you consult another shop, but it would be better to do the troubleshooting yourself ...

1. Either put the ignition system on an old Sun Analyzer [or equivalent Delco system like GM dealers used to use] ... so you can compare the ignition waveform on all six cylinders simultaneously ...

2. Put an oscillosope on the six injector control lines [ I know they are 'ganged' in two sets of three] to see if there are any differences at the injectors at idle.

3. Put a mechanic's stethoscope at all of the injectors, comparing the sound at #4 with all of the others ...

4. Pull the injectors and send them off for cleaning and flow testing ...

[strupgolf]

The shop teck tested the injector for current. All had about 28 ohms, but # 4 had 4 . All plug wires tested ok. The compression on the cylinder # 4 was 135 running. When I've changed the plugs, #4 does not seem to be any different from the others, not wet. And I guess when the engine is at higher speeds and revs, its still missing but I dont hear it. Just wishville thinking. Do you think it could be needing a valve job? I believe the last one was done about March "04 by the PO not having more than 4000 miles since. Another P-guy said it might be the camshaft lobe not opening the valves ???. Thanks for any thoughts.

[Early_S_Man]

If the #4 injector had 4 Ohms resistance while all five others had 28 Ohms ... that points to a defective injector, conclusively! The absolute value measured is not important, but the difference is statistically very significant!

[strupgolf]

He installed a new Pelican injector in #4 and it still runs the same. Also switched both ends of the plug wires on 4 and 5, and it still misses. What sounds odd is that if you rev the motor higher and unplug #4, the missing stops. What in the world could cause that. Of course, #4 isnt firing, but the engine does'nt sound any different. ???

[john walker's workshop]

i mentioned a sucked in intake gasket in the other thread. happens all the time and you won't find it by spraying anything around it because it's quite low below the fan shroud. good compression, plenty of fuel, ignition, plenty of air, and dead at idle is the usual symptom. the right side comes off easy also.

[strupgolf]

Thanks John. He said he spayed something around the intake, but didnt notice anything. Now, how hard is it to change the intake gasket, is it something I could do or does it take a shop with all kinds of special tools.

[ruf-porsche]

Originally posted by strupgolf
Also switched both ends of the plug wires on 4 and 5, and it still misses.

I don't think that you could just switch wire with an efi system and expect it to work correctly. With CIS because all the injectors are opening at the same time it doesn't matter if you have the injector from 4 switch with 5.

[strupgolf]

No, no. He just removed the plug connector on 4, put it on 5, and changed the 4 and 5 at the distributor. Just to see if it could be the wire shorting out. It didnt change.

[john walker's workshop]

Originally posted by strupgolf
Thanks John. He said he spayed something around the intake, but didnt notice anything. Now, how hard is it to change the intake gasket, is it something I could do or does it take a shop with all kinds of special tools.

remove airbox and meter, unplug injectors, undo fuel line fittings front and rear, remove 6 allen nuts and washers, loosen clamps on center rubber sleeve, pull off the right side. usually it's the gasket below the phenolic spacer. takes about 20 minutes, then you will either be fixed or you can eliminate that from your list. order 6 gaskets.

[strupgolf]

John, until I do tear it down and put some new gaskets on, whould it be good or helpful to put some sealant around the #4 intake shaft so air does not get in? Sounds like a good idea, but I've had other good ideas in the past and ?????? So, let me know. Thanks

[john walker's workshop]

you can't get anywhere near the leak if indeed it's the lower gasket. that's why spraying stuff around it won't detect the leak.