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Discussion Starter · #1 ·
1998 Taurus SE with 3.0L Vulcan engine, VIN U designation, 265 thousand + miles.

Bought as a "cash car" for town use. Ran a little rough but had plans to freshen up vital systems in the usual way. Had a bit of a rough idle and if you punched it it would drop RPM and stall, so it was best to ease in to acceleration in the meantime. Then one day before we were to start on the plugs and etc. it just quit. Stopped running like a switch was flipped and had to be towed home. Spin over fine but no start. Inline spark tester showed spark in all 6 cylinders but kind of odd and twinkly. Decided to take a look at the plugs and they were well destroyed. Changed out the 3 easiest to get to, gapped @ .044 per specs and lo and behold it fired right up. The engine didn't even get a full revolution and ka-voom, on it came. Revved up beautiful and idled like a dream. Yes, this is it, I believed. Changed out the other three spark plugs - and nothing again. Would spin over and act like it wanted to start, but like the timing was somehow off. When trying the "flood override" trick by holding the pedal to the floor while cranking, it would pop, bang and backfire. A lot of raw fuel funk coming out of the tailpipe and raw, unburnt fuel on the plugs. Even pulled plugs to air everything out, pulled relay to pump with carb cleaner being squirted in as the only fuel source. Nothing.

Examined all critical components, tested them, did all the diagnostics that could be done under the circumstances and came up with nothing that made any sense. Did discover that the entire camshaft synchronizer assembly was fairly new, locked firmly in place, was certainly pristine on the inside, along with the sensor. Cleaned up ground cable contacts on engine block & body. Fuel system is falling into spec, doing it's thing. Coil pack was factory - 23 years old - and had a crack from the bottom running up the side on the 1&5 coil position. That was changed out. Tester showed a better more decisive spark impulse resulted from that but nothing else.

I don't have an oscilloscope or such so I referred to an old mechanic trick that has been used for time immemorial - is over a 100 years old. Took a new sparkplug, grounded it good, hooked a new plug wire up from the coil, made sure all was connected good, cranked the engine over to see what kind of spark. Was getting a terrible little brown spark, so bad I had to squint - and this was after dark. Not bluish white, not yellow - brown. Moved the plug around to get ground at different spots, different coil holes, tried the old spark plug wires also - same result. Hooked old original coil pack up, still brown and acted worse than the new pack. Coil pack is getting 12 volts with key in on position, just a few tenths down from a fully charged ready to go battery. Drops to 10 1/2 volts wavering around during starter engagement, which is to be expected considering the amperage draw.

When the car was first looked into after getting it, it came with a P1744 code in storage, but not pending. No weirdness by the transmission was ever felt or noted. Later on a P0300 code joined it. After doing the procedure to reset the PCM, both codes were gone and have not reappeared, even during the brief time it was running again. No dashlight was ever on warning of them, though it does work.


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Detected weirdness with the injectors early on. The software I was using isn't very friendly with such an old PCM, though a handheld diagnostic tool was able to scavenge a little more live data.

At idle it was showing closed loop with the short term fuel trim bouncing around a bit, which is normal


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But going off idle and opening up the throttle would induce a steady 19.5% short trim in open loop - add in the long term trim value, it was just below the threshold for triggering a DTC code.


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Now days with the key in the on position the short term trim fuel bank stats now show the fuel injectors are just sitting at 46.9% which makes no sense and during cranking the engine jumps to 52.3% and then 99.2%. Wasn't doing that when it was startable and running.

Am out of ideas - it should be running. It ran once perfect during this ordeal, which I cannot explain. I tried to backtrack thinking I may have disturbed wires or connectors somehow during the plug change but came up empty on anything that would be obvious. Only speculations I have is bad new coil - is some no-name brand from an auto parts chain store, squirrely injector circuit issues jamming the PCM with bad info, or a wacked PCM, though I think unlikely. Am leaning towards something that is feeding it bad or incomplete info to work with.
 

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Spark timing is controlled by the PCM from reading the crankshaft position sensor as it reads the teeth on the reluctor wheel on the crankshaft pulley/balancer. This tells the computer when to fire which cylinders. Coil pack is pretty simple, it's a waste spark system so when the computer fires one cylinder, it fires a pair at the same time (1 and 5, 2 and 5, 3 and 6). From the info you're giving me, I suspect your no name parts store coil is no good (these engines are very picky, Motorcraft or BWD are the only known good ones), or it's not able to ground properly because the negative battery cable or its connection to the engine block is severely corroded. Have you done a quick voltage drop test? Measuring the voltage from the negative battery post to the engine block gives you some good info. If the difference is really high from normal battery voltage, it's safe to assume there's a grounding issue.

The coil pack gets a hot 12V in run from the W/LB wire, then there's three trigger wires coming from the PCM for each pair. I'd personally visually inspect the crankshaft position sensor and it's wiring to the PCM just for peace of mind. Then I'd look at the ground point for the PCM itself, it's under the passenger side windshield cowl. That 8mm bolt is known to corrode and the PCM has a hard time grounding properly. I wouldn't worry about the fuel trims until you have it running normally, since vacuum leaks on these engines with their age is super common.
 

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Grounds if it is a northern car.

You probably bumped a critical wire from block to chassis when you changed those first 3 plugs, temporarily giving you a clear path for voltage. Then things returned to normal.

Back to basics. Find your main grounds and clean them up.
 

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Pic of PCM ground and the ground strap from the engine block/Alt to the body. Issue was bad idle speed, measured bad TPS signal and erratic volts on the sensors "return"/ground.
Fixed. Fix does not cause loss of memory as there is a parallel ground to the fender by the battery.
-chart-
 

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Discussion Starter · #8 ·
I suspect your no name parts store coil is no good (these engines are very picky, Motorcraft or BWD are the only known good ones),

When it cranked up and ran after the 3 plug change-out, the original (cracked) Motorcraft coil was still on it. The no-name coil was taken off and swapped back for the original momentarily as noted in paragraph 3, still no joy. The no-name is called MasterPro, made in mainland China. Tppppth.


or it's not able to ground properly because the negative battery cable or its connection to the engine block is severely corroded.
That was attended to as noted in paragraph 2. Properly, so there should be no hassles with that.

Have you done a quick voltage drop test? Measuring the voltage from the negative battery post to the engine block gives you some good info..
Slipped my mind to do that. Will have a look.

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I'd personally visually inspect the crankshaft position sensor and it's wiring to the PCM just for peace of mind.
That was attended to before even posting here. The face of it facing the tone ring was covered with a heavy coating of gunk, that was attended to and it ohmed out at 689, which is in spec. It's in the loop and gives a pulse, is all I can tell you.


Then I'd look at the ground point for the PCM itself, it's under the passenger side windshield cowl.
Already inspected that, but I'll do it again.
 

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Discussion Starter · #9 ·
Grounds if it is a northern car.

You probably bumped a critical wire from block to chassis when you changed those first 3 plugs, temporarily giving you a clear path for voltage. Then things returned to normal.

Back to basics. Find your main grounds and clean them up.
Main grounds were attended to as noted in paragraph 2 of my post. They weren't bad to begin with but I worked them over anyway to be sure. One of the things I do, mainly with hi-po cars is add additional ground straps to be sure. Not sure if that is warranted in this case, but will find out.
 

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Discussion Starter · #10 ·
Pic of PCM ground and the ground strap from the engine block/Alt to the body. Issue was bad idle speed, measured bad TPS signal and erratic volts on the sensors "return"/ground.
Fixed. Fix does not cause loss of memory as there is a parallel ground to the fender by the battery.
-chart-
Already worked the ground cables over as reported in the 2nd paragraph, but the question remains if the PCM is getting it like it should. Will have to investigate.
 

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Discussion Starter · #12 ·
Check your engine temperature sensor with code reader to see if it is a real temperature so mixtures are ok.
I can monitor live data with the key in the on position. Sensor data output varies with the ambient air temperate, off by 5 degrees which is within variance. It's functional.
 

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Do you have a proper spark now on all 6 wires?
Perhaps doublecheck that wires 5 & 6 are correctly in the coilpack. Their positions are counterintuitive.
 

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Discussion Starter · #14 ·
Have you done a quick voltage drop test?
Can't do a textbook version of that since the engine is not running, but running probe leads off the battery positive post and on the block shows full circuit all over, even in the spots where the sparkplug was clamped for the rough spark test. So there is grounding all over the engine & body.
 

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Discussion Starter · #15 ·
Do you have a proper spark now on all 6 wires?
Perhaps doublecheck that wires 5 & 6 are correctly in the coilpack. Their positions are counterintuitive.
Wires have never been mixed up. As for accurately judging spark energy, wish I had a 'scope but I don't.
 
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