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2nd Gen (93-97) V6 2.5L Discussion of Maint Issues for V6

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Old April 3rd, 2017, 06:18 PM   #1
Typhoon McGoon
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No spark past 2500 RPM - SOLVED

Starts easily and idles nicely, but engine will not rev past 2500 RPM, spark just stops, and engine bogs and misfires. I used a simple spark test tool and watched the spark disappear as the engine misfired, also my timing light cuts out as the engine misfires.

Car is a 93 MX-6, with a KL-DE, MTX from a Probe, and the HEI mod.

Engine sounds funny at any throttle position.

Also, no matter what I do, I can't get the ignition timing advanced, even with the dist fully turned it's only just to about TDC. At throttle, the spark actually appears to retard, not advance, and it's all over the place.

No OBD codes are thrown (except 26, 28, and 29, EGR stuff must have fallen off when I was cornering hard).

Things I have tested and seem to check out:
  • Swapped out coils
  • Swapped out HEI ignitors.
  • Verified wiring to ignitor intact, including ground at metal back.
  • Swapped out ECU's (one is a KL01 and the other is a KL48)
  • Tested the crank/cam position sensors in the dist with a multimeter
  • Tested the resistance of the crank position sensor near the crank (520 ohms).
  • Tested the VAF with a multimeter
  • Verified the cam timing is correct (took off front covers).
  • Verified that the front cam is not broken and dist rotates as expected.
  • Verified grounds everywhere.
  • No metal powder or shavings in oil.
  • New fuel filter.

The dist has the larger rotor shaft cap. Rotor only fits one way. At TDC rotor points to about 15 degrees counterclockwise (retarded) from plug 1.

I have a hunch that the crank position sensor CKP2 is the culprit, but I can't be sure. I verified its wiring, tested blue and green wires for opens and shorts to ECU, verified resistance between them (1.4 ohms), tested ground, all looks good.

I thought the crank position sensor would use a +5 index voltage like the one in the dist, but I don't measure any volts with the key in run. Should there be one? Is there any way to test this sensor without a scope? Also, the blue wire seems to be grounded by the ECU, essentially another ground.

Thanks.

Last edited by Typhoon McGoon; April 20th, 2017 at 12:04 PM.
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Old April 4th, 2017, 12:51 AM   #2
Typhoon McGoon
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My gearhead buddy says that there may be no index voltage sent to the crank sensor on the crank, that the sensor may generate power, and that I should try to measure voltage using a multimeter in AC mode.

Digging through parts boxes for a spare crank sensor... already found used-known-good spare dist cap and rotor so I can swap test those too.

Part of my problem may be what a Frankenstein monster this car is, the dist was cobbled together out of a box of dist parts, and I really don't know what cars these ECU's originally came from. The coil is new though, 1993/OBD1 configuration.
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Old April 4th, 2017, 07:34 PM   #3
NickR
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Sounds a bit like this case, which turned out to be the coil: https://forums.probetalk.com/showthr...p?t=1700964298
But you have swapped out the coil. So I don't know.

The 93 and 94 are supposed to run OK even with the crank position sensor disconnected. Try disconnecting it?
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Old April 4th, 2017, 07:50 PM   #4
BLUEnoEQ
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Did you check if the cps was clean on the pickup side? When you put together the disty did you make sure to align all the bits under the top screw properly, the metal tags for the cam pos sensor. I don't think it will fire if it's not getting the correct cam pos signal. Just something to check
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Old April 5th, 2017, 11:06 AM   #5
Typhoon McGoon
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Quote:
Did you check if the cps was clean on the pickup side?
I did, and it was pretty grimy, even had picked up some metal shavings.
Was pretty disappointed when nothing improved after cleaning.

Quote:
When you put together the disty did you make sure to align all the bits under the top screw properly, the metal tags for the cam pos sensor.
Sure did, even swapped the two exciter wheels for a spare pair. I've got drawers full of spares (no crank sensor though wonder where those went).

The fact that the ECU throws no codes (other than EGR) continues to baffle me. It will throw appropriate codes if I disconnect plugs.

I'm going to do a compression test and if it looks OK, this weekend I'm going to drive/limp on the frontage road the 15 miles into the shop where my mechanic buddy works, we can put a pro-grade scope on this thing and finally figure out what the heck has happened.

I've never been so stumped over a car in my life!

Last edited by Typhoon McGoon; April 5th, 2017 at 11:08 AM.
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Old April 5th, 2017, 11:23 AM   #6
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Quote:
Try disconnecting it?
Yep and it does run, even worse, very rough at idle, throws codes.
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Old April 5th, 2017, 02:47 PM   #7
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Is it possible it jumped timing and works with the cam sensor and not the crank sensor because the 2 disagree?
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Old April 5th, 2017, 03:40 PM   #8
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Quote:
Originally Posted by Typhoon McGoon View Post
Yep and it does run, even worse, very rough at idle, throws codes.
So I'm pretty sure you can rule out the crank position sensor being the problem then. But I'm stumped too.
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Old April 11th, 2017, 03:58 PM   #9
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Well, i had a problem like this once in my 89 Firebird Formula 350. Problem was the coil. I swapped my brand new MSD Coil for a stock coil and it cured the problem instantly. you may be using the same type of coil depending on your HEI setup. hopefully this helps some. good luck.
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Old April 20th, 2017, 12:02 PM   #10
Typhoon McGoon
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SOLVED:

After two days of checking practically every wire in the harness against wiring diagrams, I finally found the answer - the shielding on the front crank sensor wire in the harness was ungrounded at both ends.

In fact, there was an entire group of ground wires that serve as "drains" for shielded cables that were not connected to any ground, only each other, the wire that led to ground was broken off from the metal crimp. From what I've read, an ungrounded shield actually works like an antenna, adding interference.

I reconnected the "drain" wires, reinstalled the harness, and now spark is acting normal, not dropping, able to set 10 degrees btdc and it's not jumping all over the place as before.

What a headache!
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Old April 20th, 2017, 08:37 PM   #11
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Quote:
Originally Posted by Typhoon McGoon View Post
SOLVED:

After two days of checking practically every wire in the harness against wiring diagrams, I finally found the answer - the shielding on the front crank sensor wire in the harness was ungrounded at both ends.

In fact, there was an entire group of ground wires that serve as "drains" for shielded cables that were not connected to any ground, only each other, the wire that led to ground was broken off from the metal crimp. From what I've read, an ungrounded shield actually works like an antenna, adding interference.

I reconnected the "drain" wires, reinstalled the harness, and now spark is acting normal, not dropping, able to set 10 degrees btdc and it's not jumping all over the place as before.

What a headache!
you should take a pic of that ground your talking about and post it might help others out .. plus i would like to see lol
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Old April 21st, 2017, 10:16 AM   #12
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Quote:
Originally Posted by Typhoon McGoon View Post
...there was an entire group of ground wires that serve as "drains" for shielded cables that were not connected to any ground, only each other, the wire that led to ground was broken off from the metal crimp. From what I've read, an ungrounded shield actually works like an antenna, adding interference...
This is entirely correct! Grounds on signal wires are ultimately important. And they are drains -exactly as you have said. They "drain" any magnetically induced voltages that may occur from any close proximity wires -or from things like alternator, coil or starter magnetic fields. And to do this properly -only one end should be "shunted" to ground.

Great find -and awesome on you for sharing the info for future Probe owners that might experience a very similar problem!
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Last edited by KLZE Porsche; April 21st, 2017 at 10:20 AM.
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Old November 13th, 2017, 09:49 PM   #13
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Does anyone know where these shielded wires are grounded? I'm troubleshooting s similar issue with ignition and would like to take a look at this.

In fact, pictures of the normal ECM grounds on the transmission mount would be good too, as the ones on this car seem to have been relocated or are missing (there is that flat connector with several grounds but it doesn't look like it's designed to be connected to anything?)
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