Weak spark causing unburnt fuel?
 

Car will start up perfect but idle slightly low around 500-750. After about a minute or even after 10 minutes, it'll drop rpms and run rough until it coughs up unburnt fuel and stalls.

Changed the rotor, cap, wires, fuses, and the sensor under the seat after the issue developed and I saw sparks across my wires. Car was running perfect until I drove it in the rain... Any ideas?


81' Widebody SC

GT... sparks across what wires?

Across my spark plug wires, was a nice light show.

Unmetered air entering the intake. Did you experience a backfire?

Or you installed your plug wires in the wrong order, double check the firing order.

Arcing across the wires likely means you have bad wires, may be its time to replace them.

I am going to check the injectors next to see if I have uneven spray. Might as well put in new spark plugs too, wish we could buy them pre-gapped.

I fixed my vacuum leaks recently along with new fuel lines, fuel filter, and fuel accumulator.

I'll double check the order.

I had a bad vacuum leak before which caused a poor idle but never unburnt fuel to come out of my exhaust.. Interesting. No backfires here and I have a Popoff valve that is sealed good as well.

Just did that tonight, still same issue.

Check your spark plugs, you may have fouled plugs, and you could have water in your plug wells which may contribute to the arcing of your wires.

Gapping plugs is pretty straightforward, they usually come gapped correctly but it is always best to check them.

There should be a woven 1/2" ground strap in the upper drivers side of the engine compartment make sure those connections are clean and dry as many electrical and ignition issues can be caused by a bad ground.

Make certain the connectors are fully seated on the cap, the new wires will be tight and may create an air pocket allowing it not to seat all the way. Use a pick tool and some di-electric grease and let any trapped air out. Also you should feel and maybe hear a click when properly seated.

To answer your question, yes weak spark will burn less fuel.

If you want to insure you get the most optimal spark, pull spark plug wire and place a new plug for testing the current, ground the plug and have someone crank the car. You should see a blueish spark, if more yellow or worse orange then the wire or cap/rotor may be defective. Pull the fuel pump fuse to disable the fuel while cranking.

Jim

is it driveable?

stuck plunger in the fuel head
bad WUR
bad mixture
bad CSV-i dont really hear much about them being bad though

start it up, reach inside the air box and pull the sensor plate down. if it runs better it is too rich.
push it up, if it runs better it is too lean.

If you had a good idle before you fixed vacuum leaks, your idle screw might have been turned in.

The start idle speed should be 1200 or so, dropping to 950 after a bit.

You might need to open up the idle speed screw to allow the metered flow to increase and get the idle rpm back to where it needs to be.

Btw, new plug wires should fix the arcing. And eliminate the effect of dampness that you experienced.

Great hint. You might check for moisture inside your distributor. Seems like a very common problem after exposure to rain.


Cheers,

Joe

Thanks, I'm going to try everyone's suggestion one at a time and report back what I find.

Hopefully it's just an ignition issue and not a fuel issue.


81' Widebody SC

Tried the sensor trick while the car started dieing and it wasn't able to keep the car alive either way. I think my mixture setting is good and don't want to mess with it too much.


81' Widebody SC

Hard to imagine how improper mixture, vacuum leaks, WUR failure, injector spray patterns and plug gaps could suddenly appear as the result of rain. However, moisture infiltrating ignition components could easily create havoc causing symptoms as described.

Cheers,

Joe