86 lucas USA 5.3L

I have a miss when idling. Cranks perfectly, idles perfectly for a bit and then begins missing on one cylinder on bank B after 2-3 minutes. It generally keeps mission on that cylinder but will occasionally settle down for a while then recur.

I pulled the fuel rail today expecting to find an injector on B not spraying properly. Tested all of them on B bank and found pretty consistent delivery over 10 sprays each. Not highly scientific. I should have gotten my smallest syringe and used it as a graduated cylinder and measured the volume delivered. I eyeballed it in a small container.

I also sprayed each on a piece of cardboard from about 3cm away for a pattern test. Each produced a reasonably round pattern approximately 3cm wide with a few having a bulge to one side. 4B seemed to be less round and more a four-pointed star pattern.

I have a new injector and I'll replace 4B and see if it resolves this. I'd think that at idle it would run consistently with the injectors anywhere close to balanced and regardless of pattern as long as it's not a stream. Am I wrong in thinking this?

I also pulled 4Bs plug. It was a bit blackened on the ring surrounding the electrode. Not oily though. Dry. The electrode was like new ( they have a few hundred miles on them ) the porcelain surrounding the electrode was clean and not shiny. The ground strap was covered with a very thin layer of whitish powder.



Does anyone know what this indicates?

I'll pull all the B bank plugs to compare after a sandwich. Any suggestions while she's apart?

Oh I also finally moved the original injection harness out of the vee so that I can clean down there. I was afraid to with it in there. I was pleased to find it all flexible ( even the boots ) and insulated as far as I can tell. Bit hard by the sides of ( what used to be ) the AC unit. Hopefully I haven't introduced any new trouble when I reassemble. I will be making a new one but need to fab my new fuel rails first.

 

Have you checked the dizzy cap? Not that the mentioned cylinder has a bad contact inside the dizzy...

 

Check the gap while you have the plugs out should be 0.025"

To find which cylinders are not firing, pull ALL of the injector plugs off the A bank so the car runs on the B bank only, start the engine, then ONE AT A TIME pull the injector plugs off each injector (put it back when finished), you will hear the engine change when you pull a pug on a running cylinder, if you pull a plug and the engine does not change then that cylinder is not firing.

 

When you say " a miss when idling" do you a slight tremble or roughness, or something more severe?

If it's just a slight irregularity to the idle quality I'd say that one injector with an irregular pattern or slightly weak flow could indeed be the problem.




Plug looks good to me

Cheers
DD

 

A clear miss. Fingers on the tailpipes, A bank is fufufufufufu... B bank is fufufuPOOFfufu

I liked Warjons suggested analysis but it was too late. I'd already cut B4 and was refitting the new injector when I read it.

But she purrs! First time since I've owned her she really purrs. Maybe first time since 1995 when she had her last inspection before I got her.

Thanks for the suggestions guys. Filing them away for future use.