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

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01 Jun 2019 09:46 #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 10:54 - 01 Jun 2019 13:46 #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?
Last edit: 01 Jun 2019 13:46 by John Curtis.

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02 Jun 2019 08:17 #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 16:15 #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?

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04 Jun 2019 20:14 #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 09:10 #30402 by tom7603
Yes, TB cleaned & intake inspected. I only have an inexpensive multimeter.

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05 Jun 2019 09:34 #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 15:44 #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.

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09 Jun 2019 00:51 - 09 Jun 2019 00:54 #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 00:54 by jreardon.

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