1988 Ford Mustang GT 5.0L V8 Misfire - Replaced many components.
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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- John Curtis
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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?
Making Pressure Differential Sensors (PDA Sensors) for pressure pulse diagnostics.
Currently servicing Central Texas.
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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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- John Curtis
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Do you have a scanner capable of displaying live data so we can determine if it is running lean?
Making Pressure Differential Sensors (PDA Sensors) for pressure pulse diagnostics.
Currently servicing Central Texas.
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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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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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- John Curtis
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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.
Making Pressure Differential Sensors (PDA Sensors) for pressure pulse diagnostics.
Currently servicing Central Texas.
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@10:30
@12:56
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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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- Andy.MacFadyen
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" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)
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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
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.
"Ground cannot be checked with a 10mm socket"
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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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This era of Ford engines (EEC-IV) is very difficult to work on without the diagnostic tools that the folks on this website are familiar with. I've learned to test parts before purchasing new ones. I've learned that new parts are not always good parts. Also, some electronic parts may require special equipment in order to test.
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I can finally report that the misfire is gone after a full summer of driving. There were two things I changed after thoroughly going back over each of the key components discussed above.
1) Replaced catalytic converters
2) Replaced fuel pump solenoid
I can't say definitively it was one or the other since the misfires seemed to diminish over several weeks of driving. But the issue is gone and the car runs much better overall.
Thanks for the help!
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