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Guest Message by DevFuse

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Low Voltage at Coil (poor mpg cause?)

ignition coil fuel mileage

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2 replies to this topic

#1 chaseman


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Posted 24 June 2013 - 09:30 AM

Hello , so I've been only getting like 20 mpg if I drive really nice (less if I'm driving normal) in my 1986 4wd GL Weberized wagon.

I've done the usual tune up stuff (plugs , cap , rotor , wires, fuel filter , cleaned air filter) The weber is running the recommended tuning I got from loyale27turbo's guide. 

I figured we were both at similar elevations so it seemed close enough. 

I've also recently replaced the (roaring) bad front wheel bearings it had , with some sweet sealed ones.

I run normal sized tires at right below their max psi. Everything has fresh oil. All the front cv joints have been regreased and booted in the last 2 months. I've also replaced the timing belts (they're spot on) and changed the oil pump gasket. (not that it helped the ticking at idle , I think those cam tower o rings are bad) Ignition timing is set at like 16 degrees btdc. 

Battery voltage reads in the 14's with the car running 



Anyhow here's the actual issue , so someone gave me the idea to check the voltage at my coil. It's only 12.7v with the engine running around 1200 rpm and goes progressively down with more rpm. It also heads down with whatever accessories I turn on (heater fan , headlights , turn signal , etc) to a low of 12 volts at 2000 rpm. I bet it even heads lower with an actual load on the engine and not just revving in place.


Also I dunno if this is related but my voltmeter jumps with the turn signal. 


Any ideas? I mean I thought about doing a relay set up to get full voltage to the coil (using the same logic as the headlight relay set up. Bypassing all the various connectors and ignition switch that add resistance and rob voltage)

Edited by chaseman, 24 June 2013 - 09:43 AM.

#2 CarpeNoctem


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Posted 26 June 2013 - 06:50 AM

reading the start of this a relay was my initial thought as well. just make sure that voltage doesn't go low enough that it cuts out completely. Most likely what you're seeing is voltage drop due to increased load put on the coil for higher rpms. putting the power through a relay can only be beneficial to this though. If battery voltage remains constant while coil voltage drops it's just load doing it. these aren't the newest wiring harnesses. test the relay system using a light to make sure it won't cut out if you have accessories on with it driving. if not temporarily wire the coil through it and recheck mpg results. If things improve finish of the wiring job to make it permanent.


other option is to do an spfi swap. but that is a lot more work

#3 Cougar


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Posted 28 June 2013 - 04:11 AM

Try cleaning the ground connections under the hood area and see if that helps. Some voltage loss is due to normal wire losses.

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