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1985 Ford Crown Victoria Tuning - Ignition System
Question: Ignition Module failed on this 302CID (5.0L) V8 engine. The failed module was found to be an aftermarket replacement marked "Standard", with no other identifying marks. I've replaced the module with two similar Motorcraft units from similar vehicles with unsatisfactory results -- also replaced plugs and checked rotor, cap & wires. I followed, or tried to follow the correct timing sequence and yet the problem acts like a timing or weak spark issue. Engine fires instantly and appears to run perfectly under no load conditions. However, as soon as I put the transmission into Drive, that's when the trouble begins. Initially, one can hear a roaring sound from a unit mounted on the intake manifold between the throttle body and the firewall. When driving, the engine misfires badly under load, most noticeably on the wide gear step to overdrive -- this misfire diminishes gradually as speed increases and the need for lots of torgue is lessened -- you can almost feel the engine regaining proper firing one cylinder at a time as it smoothes out but the roaring sound never goes away completely. You can smell exhaust type fumes from the noisy intake manifold unit. Fuel economy sucks. Any ideas?
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Technican: How did you check the plug wires? This problem could be caused by bad plug wires or a weak ignition coil.
Could you describe in more detail the part that makes the roaring noise? Does it have an electrical connector, metal pipe, vacuum hose, etc?
Owner: Firstly, the intake manifold unit:-
It's electrically controlled with connections at back of a tennis ball sized cannister almost at the firewall. This rounded cannister is attached to an oblong unit between the cannister and the manifold that appears to be measuring coolant temperature -- there's a small coolant line tapped off a heater hose that runs into this oblong unit and back into the water jacket part of the intake manifold. The entire assembly connects into the manifold immediately below the throttle body on the back side. There is no vacuum hose involvement as far as I can see.
Unfortunately I have not been able to obtain or access a shop manual for this beast but I suspect that this apparatus is part of the emissions control system. I have no idea what it is called. I suspect that, when incomplete combustion is detected, probably by the oxygen sensor, a valve opens up in the cannister to permit a massive injection of extra air below the throttle body and that this open valve is the source of the roaring sound -- sounds like a blown exhaust gasket.
You're obviously on my wavelength by suspecting a weak spark breaking down under load. The strange thing, however, is that everything was functioning perfectly prior to the sudden and complete failure of the Standard ignition module -- total loss of spark. I've tried going through the ignition one step at a time to try and eliminate/isolate the present weakness. I wish I knew what voltage I should be looking for on the primary circuit to the coil but, like I said, I have no access to any manual.
Initially I replaced the plugs due to the potential for a wider gap due to wear. In reality, the old plugs were good for further service if I had just re-gapped them but I figured I might just as well replace them with new, gapped to OEM specs, for a fresh start.
As far as checking wires, etc., (remember everything was working perfectly prior to the module failure) I have taken several approaches:-
I've substituted rotor, cap and wires (borrowed) from two other perfectly working 302 engines,
I've substituted the distributor and ignition module from one of those engines,
I've replaced each ignition wire, one cylinder at a time with a new one to see if I can isolate any particular cylinder(s) or wires(s) causing the problem (ie- looking for individual wire problems as opposed to random breakdown across all cylinders),
I've inspected the new plugs for signs of richness or uneven combustion between cylinders.
Thus far, nothing that I've done has produced even the slightest improvement. It's my suspicion that the replacement Motorcaft ignition modules put out a lower primary circuit voltage to the coil than the aftermarket Standard unit that failed or, putting it another way, the Standard unit is/was better than the Ford OEM unit by putting out a higher primary voltage -- prior to the sudden failure, this old wagon ran fabulously, giving me as much as 24 MPG on highway trips at high speed. But that does not explain, of course, why the rest of the ignition system is failing somewhere.
My next thoughts, unless you can come up with anything else, are as follows:-
1) To try replacing the ignition coil (borrowed)with one from either of the two engines known to be working properly and/or
2) To try closing the plug gaps below specs to see whether I can get a stronger spark from what appears to be a diminished spark input/coil output.
What do you think?
Mike.
Owner: Further update:
Just tried a substitute (borrowed) coil and main HT wire from coil to distributor. No change -- still breaking down under load and roaring through the manifold valve.
Mike.
Technican: Lets consider the other half- fuel. Have you checked fuel pressure? Observed injector spray?
If you have a donor car try swapping the map and throttle position sensors to see if there is any change. I assume this car is fuel injected?
Hmm, on the mystery unit. I wonder if it may be for the heater controls. Regardless try unplugging the electrical connector and see if the car runs differently. Also note if you lose functions to the heater.
The following link contains a generic repair manual for your car. Generic because it covers a 20 year span.
autozone.com/servlet/UiBroker?ForwardPage=/az/cds/en_us/0900823d/80/10/29/3c/0900823d8010293c.jsp
If you want a better repair guide there is alldatadiy.com This site is not free. Here's How To Get Guaranteed Solutions In Minutes
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