Ignition vs. Gas mileage

[The Dark Side of Will]

When I bought my Formula last summer, it had been sitting in a barn for 6 years. Amazingly, everything worked... clutch, brakes... even the parking brake. The dizzy had been cannibalized for something and a dizzy base installed in its place. I built a dizzy for it, new module, cap, rotor, wires, plugs... everything except the coil. After replacing the gummed fuel pump, the car started and ran so I declared the operation a success. That was last fall.

It initially gave me up to 27 mpg on the highway, but after a month or two that dropped to 25 and then to 24, with around town mileage dropping as well. In the last couple of months, the tach started to quiver. I figured that the cheap module I'd used to get the car on the road was about to give up. Last Friday the tach was quivering worse than it had been. I went to Walmart that evening and when I came back out, the car fired and ran for a couple of seconds and then died. The tach indicated while cranking, but it wouldn't even try to hit. I checked spark at a plug and found none. I checked spark at the coil and found none. Tach indicates + no spark = bad coil. I replaced the coil and the car started instantly... more easily than it had been starting. It popped and gurgled less on overrun.

Now that I've run a tank of gas through it, my gas mileage went up from 21.2 on the previous tank to 24.8 on this tank with similar driving. I have to run a couple more tanks through it to average, but I'm quite pleased with the improvement. I'm also quite surprised that the coil could degrade that far before failing and have such a large effect on gas mileage.

[p8ntman442]

the effect of mileage is no surprise at all. think about it. your nut burning efficiently so you requrire more fuel to do the same work.

Coils can loose power through eddy current losses, as the lamination on the outer coil breaks down. After so much, the resistance increases and your making heat now rather than power. Then the windings finally short out.

[The Dark Side of Will]

I'm not surprised that weak ignition affects gas mileage. It is exactly as you've stated... less efficient burn means more fuel is consumed to make the same power. I'm surprised it can degrade *that much* and still operate.

[p8ntman442]

well My fiance's car started acting up, under hard acceleration into traffic, it would almost die, and stutter. So i got a set of plugs, wires, and a coil pack, and prayed It wasnt gonna be the fuel pump or filter. Turns out #4 cyl was a severed wire, that was intermittent if fuctional at all. I had noticed with 3/4 of the cyls firing properly gas mileage went from 32mpg to 28.

[The Dark Side of Will]

This hasn't been the end of this situation. Shortly after the new coil, the car started bucking and stuttering, sometimes with a miss when cold and humid. It would mostly straighten itself out after 30-45 minutes of driving. Putting a timing light on the coil wire showed that the coil was dropping out. Since that coil was a $20 autozone special, I upgraded to the $60 coil. That didn't help. I upgraded from the cheap ign module I'd installed last fall to the $60 ign module. That didn't help. I replaced the pickup coil (OE) and cleaned the coil mounting bracket to assure a good ground connection to the block. That didn't help. Cap, rotor, plugs & wires are new as of last fall.

Last night it died, but I won't have time for serious troubleshooting until next weekend. Gas mileage had dropped to mid 25's on the highway. Grrr...

[p8ntman442]

replace the plugs. you could have a lemon in the year old set. This could lead to other ignition component failures.

[crzyone]

Or ditch the 2.8 all together. I thought you were building a short stroke tdc?

[Shaun41178(2)]

He should and use my 2.8 dis crank I have laying here.

[The Dark Side of Will]

I tried cranking after recharging the battery. The computer does not register RPM when cranking, according to my TECH 1. Since the ignition module is practically new, to me this indicates either a wiring fault between the module and the ECM or a dead input pin on the ECM. I'll troubleshoot more as I get time.

[Shaun41178(2)]

I bet its your module, or the wiring harness going from your module to your coil.

I just replaced my little harness. I thought my module went bad too as I had no tach movement. It turned out to be that harness. I got a new one from the fiero store as I wasn't able to repair the current one. THe original factory peice was rusted from the inside out when I cut the sheathing off. It was just powder.

[The Dark Side of Will]

Not just no tach movement... no RPM according to the computer... two different problems.

The module reads the pickup coil directly and outputs reference pulses to the computer. The computer sends back the EST (electronic spark timing) signal that tells the module when to fire the coil. The tach signal comes from the primary side of the coil.

IOW, if it were what you think it is, the computer would understand RPM, but the tach would not indicate and there would be no spark. In my car, the computer does not understand RPM. This means that the reference signal is not getting to the computer. I just spent $60 on a brand new module, so it had better not be the module. I just replaced the pickup coil with no effect on the problem (ran then, but poorly), so neither the old nor new pickup coils are bad.

What's left: wiring between module & computer or the computer itself. I can isolate the computer by scoping the signal at the ECM connector. If there's signal at the ECM pin, but the ECM doesn't get it, then that input pin is bad. If there's no signal at the ECM pin, then the wiring is bad; binary search.

[p8ntman442]

Note to self: Build a small inductive timing light with a choke and LED to take the place of this "scope" will is talking about. Those cost money.

[The Dark Side of Will]

That would work too. Got a quick sketch handy?

Scopes only cost money if you don't already have them.

[whipped]

I bought a 20mhz scope back in 98? for $150. Nowdays they're on ebay for $20. I love ebay.

[p8ntman442]

[The Dark Side of Will]

Anyway, going through the GM troubleshooting tree...

Cranks but does not start checks for spark. No spark calls the ignition system testing tree. Ignition system testing tree leads to either bad module (both of the ones I've had in the car test OK), or insufficiently magnetized dizzy reluctor. We can get spark by putting 6V on the pickup coil terminal on the module, so I'm wondering if the magnet has gone kaput. It still attracts screwdriver tips and I can feel a difference in force between the side of the magnet and the reluctor pins. The troubleshooting tree asks if the reluctor is still magnetized, but doesn't say how much is necessary. My dad said he was in an HEI class once in which the instructor put one hand on the + terminal of a battery and touched the pickup on the module with a finger on his other hand and made spark. The module ought to be sensitive enough to pick up just about any magnetization on the reluctor.

The ECM does not enter into this trouble shooting tree because IF everything in the ignition were good, it would have spark while cranking, but it does not.

[whipped]

Sounds like a pickup coil problem. You sure it's good?

[The Dark Side of Will]

700 ohms, no ground. I had just replaced it about 1/2 hour of driving time before the car died and left me with this mystery.

[whipped]

Get a magnet and see if you can shock yourself.

[The Dark Side of Will]

Works.