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1988 Ford Mustang GT 5.0L V8 Misfire - Replaced many components.

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01 Jun 2019 10:46 #30285 by tom7603
Hello,

I'm a new member. I'm reaching out to community for help. I have an intermittent misfire that occurs often across full range of rpm's. I could use help with direction to go next with the technical diagnosis. I've replaced numerous parts (see list below).

Vehicle: 1988 Ford Mustang GT/5.0L V8/5-speed manual/Air-conditioning, 170,000 original (well maintained) miles, owned since 1991.

Problem: Intermittent misfire that occurs often across wide range of rpm's. It is most noticeable driving in first gear just as you begin to let off the throttle. Then there is a sudden jerk. It acts as if the ignition momentarily cuts out. Also, in general I've noticed uneven acceleration. Sometime better than other times.

First, basics that were Verified/Checked:
Fuel pressure
Battery voltage
Good clean fuel
Clean air filter & air intake

Parts replaces:

Ignition-
Coil
Spark plugs & all new spark-plug wires
Cap & rotor
All new distributor w/pickup
Ignition control module (component tested and verified/passed...AND to be absolutely sure I replaced it with a new one)

Fuel-
Fuel injectors (8ea)
Fuel pressure regulator
Fuel filter

Sensors-
MAP (located on firewall)
TPS
IAC (idle air control)
EGR
O2 (new Bosch both left & right)
New computer (EEC-IV), swapped with a new one and misfire still remains
Various temp sensors (gauge, coolant, etc)
...and by the way, there is no MAF on this vehicle.

I've been working on this problem for a couple years. As you can see, I've tried many things already. I'd like to get some feedback before I try anything new. Please feel free to comment. Thanks!

Tom
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01 Jun 2019 11:54 - 01 Jun 2019 14:46 #30286 by John Curtis
Are you sure it’s a misfire and not stumbling/bogging down?
Have you performed a compression test and leakdown test?
I’d be suspicious of weak valve springs.
Have you verified correct base timing and ignition timing?
Do you have a scope to test the crankshaft position sensor?

Thinking out loud always helps me in the process.
Last edit: 01 Jun 2019 14:46 by John Curtis.

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02 Jun 2019 09:17 #30314 by tom7603
John,

Thank you the interest in this and quick reply.

1) I have not performed a full compression or leakdown test yet. I'll post the results of that separately.

2) I don't think I would describe it as stumbling/bogging down. It may be an uneven acceleration at times but has enough power to accelerate normally to speed and does not diminish or fade away. Rather, it is a momentary drop-out/cut-out that happens when you are up to speed just as you lift off the accelerator after steady cruising awhile. It results in a sudden jerk but then power comes right back on.

3) Base timing is set correctly per vehicle spec. Also, you can hear this cut-out at idle if you know what you are listening for. You can see it very clearly on the harmonic balancer using the timing light. When it misses, in that instant you will not see the timing mark side of the harmonic balancer. But then returns to normal until the next misfire. At idle, the duration between misfires is irregular but a good average is 15 sec.

4) Regarding scope. I do not have one. But last summer I took it to a shop that did. I don't recall the full list of tests they performed but I know they did look at the pickup signal inside the distributor (I believe that's how this car reads crankshaft position) and it was "normal". In fact, tech was pretty confident that the mechanicals of the engine including valves were operating normal based on his data. His opinion was that this was a "lean misfire". His next suggestion was to evaluate O2 sensors (which are brand new). I removed the O2's, leaving them plugged in to computer, and drove it down the road. It had the same problem but perhaps occurred less frequent.

Again, I'll post the compression test findings soon. I would like to know that for myself anyway. Thanks again.

Tom

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04 Jun 2019 17:15 #30365 by John Curtis
Have you happened to check/clean your Throttle Body?
Do you have a scanner capable of displaying live data so we can determine if it is running lean?

Thinking out loud always helps me in the process.

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04 Jun 2019 21:14 #30373 by guafa
Hi guys,

I agree with John Curtis.

In case you don't have scanner, you can monitor O2 sensor activity and TPS voltage (at ecu pin).

It seems like PCM is receiving intermitent voltage from TPS or TPS voltage is out of range/calibration.

I would expect a lean reading on deceleration

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05 Jun 2019 10:10 #30402 by tom7603
Yes, TB cleaned & intake inspected. I only have an inexpensive multimeter.

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05 Jun 2019 10:34 #30405 by tom7603
Thanks. I haven't calibrated or checked TPS voltage. I'm sure I can tap into it. Since I only have an inexpensive multimeter can you tell me what voltage range to expect or how to identify this fault?

You also mentioned reading the O2 voltage. Will I be able to read lean condition with this meter? Seems like I'd be looking for an average voltage less than 0.5v??

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05 Jun 2019 16:44 #30419 by John Curtis
You can use a Multi-meter to test 02 sensor and your TPS.
TPS value at idle should be .9 to 1.1

Narrow-band 02's range and oscillate between .100 to .999
.1 = lean
.999 rich
.450 is ideal.

If you see it stuck lean/rich then force it the other way.
Force lean by disconnecting a vacuum line.
Force rich by snapping the throttle.

Thinking out loud always helps me in the process.

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09 Jun 2019 01:51 - 09 Jun 2019 01:54 #30540 by jreardon
Could parallel induction crossfire be a possibility?

@10:30
https://youtu.be/9OhO2TH-c14?t=628

@12:56
https://youtu.be/Bnx8aBYN4Tc?t=774
Last edit: 09 Jun 2019 01:54 by jreardon.

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27 May 2020 16:01 #40229 by tom7603
1) Thank you for the suggestion on induction misfire. Unfortunately, that did not resolve the issue. I believe it is the 5 & 6 wire for this motor (13726548 firing order). I can hear the cut-out/misfire at idle so I was able to space out other wires too to make sure it wasn't occurring somewhere else. Nothing related to wire locations seemed to make a difference in causing this issue to occur.

2) I did replace the TPS after testing it. Thanks for the instructions to do that. It helped with the throttle response but did not fix the issue either.

3) I discovered something that seemed to help. I was able to make an improvement when I replaced the battery. I had the old battery checked and it tested "marginal" and due to its age I went ahead and replaced it. I noticed a nice improvement in the driveability with the new one. However, issue does still occur. I have noticed it occurs more often and more severe when the engine is idling when I have all of the electrical accessories turned on, such as air conditioning fan, lights, and flashers. Also, I see the dash volt meter drop well below the normal 12v. So, I'm now convinced that there is a correlation to voltage drop.

I have replaced the alternator not that long ago. When I put in the new battery this month, I had NAPA run an electrical system test on the car to evaluate battery, starter, and alternator. Their test device simply connected to the battery terminals and reported battery, starter, and alternator strength and all were 'acceptable'. But now I'm thinking of taking the alternator off and having it tested separately. Possibly focus on voltage regulator?

Before I go down the path of testing electrical system components. I was wondering if anyone has experienced this before or even think it is possible that low voltage might cause such an issue? I'm not an electrical person what so ever but it seems like the ecu could encounter difficulties and possibly introduce error with ignition. Does that make any sense? Thanks!

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27 May 2020 21:01 #40234 by Cheryl
Wonder if the alternator is putting out to much ac voltage and screwing with the system.

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28 May 2020 02:28 #40238 by Andy.MacFadyen
If the flash from the timing light disapears when it skips at idle it is an ignition electrical issue, that it skips when the throttle is lifted points to a intermittent connection that gets moved probably with engine torque . To me it looks like a wiring harness short or open issue.

" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)



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02 Jun 2020 20:53 - 02 Jun 2020 20:57 #40421 by Noah

Andy.MacFadyen wrote: If the flash from the timing light disapears when it skips at idle it is an ignition electrical issue, that it skips when the throttle is lifted points to a intermittent connection that gets moved probably with engine torque . To me it looks like a wiring harness short or open issue.


The ignition coil connector on the Fox Body cars tend to have big sloppy female pins.
You still have the plastic cover over yours. Nice B)
Try tugging and wiggling that connector to see if you can induce the misfire.

You may know that the "salt & pepper" connectors behind the plenum can be stressed when engine torques, but I don't believe anything in those circuits is spark related.

It really is a shame that there's no way to pull live data from those ECU's.

Too much plus more equals just right.
Last edit: 02 Jun 2020 20:57 by Noah.

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21 Jun 2020 17:00 #40793 by tom7603
Update on latest things I've tried:

1) Alternator was rechecked and is good. I checked voltage at battery and AC voltage as well. All seem to be within spec. I also moved around the coil connector and other ignition wires to see if that would bring about more/less problems which didn't seem to make any difference.

2) Found an unrelated issue with the ignition switch connector which was not holding the wiring bundle/terminals tight to the ignition switch. It did not correct the misfire/cut-out issue.

3) I found "Bang Bronco" video titled "Ford EEC-IV Ignition Misfire - description, operation, testing part 1" posted on YouTube by ScannerDanner on June 7, 2016 and this describes my issue very well. I do not have a scope to confirm my signals but by removing the SPOUT connector like they did in the video, it did make a noticable difference in the elimination of misfire/cutout. So it does not seem to be present in "base timing" mode. However, the conclusion that was drawn in the video was that it was necessary to replace the distributor or the crankshaft pick-up sensor. My distributor is brand new. To make absolutely sure, last week I replaced the pick-up sensor inside of it with a brand new Motorcraft replacement. Issue still remains.

Has anyone experienced this same issue of faulty clipping between the PIP and SPOUT signal? Are there other likely causes, ICM, MAP sensor, or something else??

I've had ignition control module tested at Autozone but is that possibly still the issue? My ICM average voltages with engine running and relative to battery ground for pins #1-6 were as follows: #1 (PIP)=6v, #2 (SPOUT)=7v, #3 (IGN/IDM)=0v, #4 (IGN/RUN)=14v, #5 (IDM/COILNEG)=14v, and #6 (IGN GRD)=20mV. My coil voltages were 14.5 and 15v and reference voltage at the TPS was 5v.

Just to reiterate, I did also try a new computer about the same time (2018) as the new distributor.

Again, thanks for your input and please feel free to share experiences/advise.

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