Old mustang won't stop misfiring

Jason Technician New York Posted   Latest   Edited  
Unsolved
Driveability
1988 Ford Mustang LX 2.3L (A) 5-spd (T5)
Misfire On Light Throttle Cruise

This car is killing me. I was 7 years old when it was built, and it's winning.

Customer uses this as an A to B, keeping his Marauder immaculate. For the past month he's been complaining of a stumble, and hestitaion while shifting. I noticed the misfire on test drive, and replaced the coil. It felt fine afterwards.

Customer takes car, and it dies 2 miles from the shop. Fuel tank took a crap. We towed it back, replaced the pump, but still get the hestitation during cruise if you give it a little bit of gas.

I've scoped the TPS with no dropouts. TPS was my initial suspect because I can make it happen at almost the same throttle plate angle everytime. ut the TPS is good.

I thought I saw a crack in 1 of the plugs, so I warrantied them (They were installed 2 months ago when the car was purchased). The cap, rotor, and wires were also changed at that time. It's also gotten a new CKP and timing belt.

I've unplugged the o2 sensor, with no change.

I've unplugged the EGR, electric, and vacuum, with no change.

I'm running out of options here.

If I cruise about 55mph, and slightly increase throttle it'll kind of fall on its face. Itll slowly lose speed. If I give it just a tad bit more throttle, it'll snap out of it, and run totally fine.

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Michael Owner/Technician
Utah
Michael Default
 

Have you done a sweep of the TPS using a scope or graphing multimeter?

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Jason Technician
New York
Jason Default
 

Yes, as I said in the OP, I did scope the TPS with zero dropouts. I used my Pico 4425.

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Michael Owner/Technician
Utah
Michael Default
   

Apologies, I missed that. MAP is the next likeliest culprit.

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Gib Owner/Technician
California
Gib Default
 

Check the vacuum port to the Map sensor. I've had issues in the past with the port being restricted, not plugged. It made the MAP slow to respond to throttle changes.

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Rafael Owner
Mexico
Rafael Default
   

Have you check the TPS minimum voltage at idle? Check timing belt again, compression, the crankshaft sprocket for worn keyway . Plugged injectors.

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Dale Inspector
Texas
Dale Default
 

Fuel filter changed with pump?

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Jason Technician
New York
Jason Default
 

Yes I replaced the filter. Inside of tank was surprisingly clean as well. This is a low mileage vehicle, 50k, original.

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Gary Owner/Technician
Colorado
Gary Default
   

Jason, It's hard to set codes on the old EEC-IV Fords, so you must rely on direct sensor readings than on codes. Just as a preliminary precaution, clean the PCM ground connection at the battery and battery negative at the frame and engine block. It's good if you can find an old 60-pin EEC-IV breakout box, but if not, connect a voltmeter to the oxygen sensor signal wire. If it switches from…

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Jason Technician
New York
Jason Default
 

I was sure it was a TPS issue myself. It's in my bay as I type this. I just did some hunting based on other comments. The MAP had previously been changed, but I went back to it anyway, low and behold the hose is now collapsed, and split at the end. I'm warming it up, and burping coolant. Then I'm off for a test drive. Thanks for taking the time to help!

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Mike Educator
North Carolina
Mike Default
 

If the MAP line doesn't get it fixed, it might be worth checking fuel pressure. If memory serves the regulator screens used to plug up which would keep FP high, and the fuel trim would reduce the pulse width to conpensate. Then on acceleration the vacuum biased regulator would not react and you could get a tip in hesitation.

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Jason Technician
New York
Jason Default
 

I like your theory. It fits. The customer took the car back for a few days but it'll be back soon. This will be my first test.

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Albert Owner/Technician
Pennsylvania
Albert Default
 

I would look at the MAF also if it has one

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Neil Owner/Technician
Indiana
Neil Default
 

If it has a MAF check if the barometric pressure readings are way off of the actual barometric pressure. A good weather app on a smart phone can tell you the actual baro pressure in your area. Lint on the MAF hot wire will act as a blanket keeping it from cooling with air passing by the hot wire. If this is the case the baro readings will be off.

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Craig Technical Support Specialist
Illinois
Craig Default
 

Hello Jason another item to check is the EGR position sensor, if my memory serves me well I believe that the sensor would wear out at certain points causing the EGR to over shoot and under shoot desired position mimicking a misfire, scope the output of the EVP sensor with a scope while applying vacuum and slowly opening and closing the EGR valve, if you see any bad spots change out the sensor…

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Stephen Owner/Technician
California
Stephen Default
 

Aftermarket MAF is probably wonky. I figured that collapsed vacuum hose was the problem, and I'm curious if you got any positive results when you changed it. Next, if you don't get data on that car, back probe the MAF signal wire with your DVOM set to frequency. KOEO should net you 159 Hz at sea level. (subtract 3 Hz for every 1K' of elevation) Readings below about 152 Hz will begin to exhibit…

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