My 1998 GT, automatic, is having a random misfire. But no trouble codes are issued and my certified mechanic is stumped trying to find the cause. It has well over 200K on the odometer so we did the obvious regular maintenance items (i.e. fuel filter, belt, hoses, plugs, wires, coil packs, etc.) but the problem still persists. If it's in gear, foot on the brake, at idle you'll feel it in seat of your pants every few seconds. In motion at around 45-50 MPH it's noticeable and increases with speed. We've removed each injector and tested them and all 8 pass. Fuel pressure is good as well as tested fuel pump; all check out okay. Put an infrared thermometer to each cylinder with no noticeable difference in any of them. The car is factory stock. I've had it since new and this is a new issue. I'm stumped. Any help or insight would be greatly appreciated.
4.6L2V random misfire with no codes
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wmburns
SN Certified Technician
Also has your "certified mechanic" look at the misfire data stored on the ODB2 mode6 data page? In there are stored the counts by cylinder of misfires occurring.
Here's some information on a very affordable ODB2 Windows based USB dongle ($30). This will give you an easy way to check all of the available ODB2 data yourself now and forever in the future.
ForScan ODB2 scanner w ELM327 USB
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What have you done to confirm that the alternator isn't creating excessive AC ripple (bad diode)?
Howto perform charging system voltage drop test
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Have you checked the CKP sensor to be sure that it's not loose?
Have you checked the crank damper for damage or a loose bolt?
Here's some information on a very affordable ODB2 Windows based USB dongle ($30). This will give you an easy way to check all of the available ODB2 data yourself now and forever in the future.
ForScan ODB2 scanner w ELM327 USB
ForScan ODB2 scanner w ELM327 USB
While working on your car have you ever wanted: to find an ODB2 operational PID value (say fuel pressure or MAF)? How about graph a PID value over time? Or compare multiple PID's over time? Access a bi-directional PCM control such as test a...
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What have you done to confirm that the alternator isn't creating excessive AC ripple (bad diode)?
Howto perform charging system voltage drop test
Howto perform charging system voltage drop test
To anyone else reading any of my posts I usually include the line: Today's cars simply will not run right without a strong battery and charging system! Cut corners here at your own risk I'm a big believer in starting with the basics. As such I...
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Have you checked the CKP sensor to be sure that it's not loose?
Have you checked the crank damper for damage or a loose bolt?
Hi and thanks for the reply. Yes, he plugged in his Matco scanner and said there was nothing consistent stored on the OBD. In fact the only thing out of the ordinary was a code 12 for the air-bag/seat belt sensor resistance. The car has always been impeccably maintained but after 20+ years I know things are bound to creep up. He did a complete compression check on all 8 cylinders with good results, particularly for 200k+. He tested the fuel flow and pressure from the pump to the filter and from the filter to the injector rail with no issues. Next was inspecting each injector and O-ring and all passed. Ran a BG top end cleaner thinking maybe a gummed up sensor. Nothing has helped. Put a multi-meter on the TPS, Mass air, and idle speed sensors; all passed. We are absolutely stumped. The damper was replaced with the timing belt service 15k miles ago. To my knowledge the torque setting has not been checked since nor has the crank position sensor. I'll drop by after the holidays and have him take a look and see. Thanks for the info.
wmburns
SN Certified Technician
You didn't answer the question about the ODB2 mode6 misfire data.
I have worked on more than one Ford SOHC 4.6 that "seemed" like it had a slight misfire at idle. But runs perfect otherwise. ODB2 mode6 data showed ZERO misfires.
It doesn't make sense for a motor to ONLY misfire at idle and otherwise run perfect at all other power. The conclusion? In my case what might be thought of as misfires are not.
I have worked on more than one Ford SOHC 4.6 that "seemed" like it had a slight misfire at idle. But runs perfect otherwise. ODB2 mode6 data showed ZERO misfires.
It doesn't make sense for a motor to ONLY misfire at idle and otherwise run perfect at all other power. The conclusion? In my case what might be thought of as misfires are not.
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I can't be sure about the Mode 6 data as I did not do the scan myself. I wasn't standing over his shoulder during the diagnosis. But I'll be sure to specifically ask when I go back for the crank sensor and balancer check up. I really trust this guy and have used him for over 15 years on all of my family vehicles. He's always been very thorough and honest. So I'm inclined to believe the he did his best. But as I've learned over the years; You're never too old to learn something new. I appreciate the feedback and I'll keep ya posted after the new year.
For anyone keeping track, the mysterious misfire has been exercised from my GT. Thanks to the tip on Mode 6 Data my mechanic was able to determine that the Cam Sensor was reading out of spec but not enough to code. Upon inspection of the sensor it was found to be 100% functional, so he kept looking. Finally he noticed that the power to the sensor was "dirty". It had an unusual wave pattern where it should have been continuous smooth power delivery. He traced the "dirty" signal back to the alternator. The alternator was fairly new having been replaced with the timing chain service. So he tested it separately and sure enough the thing was overcharging and causing intermittent electrical problems. Changed the alternator and she purrs like a kitten once again. Thanks for the information. It was driving me nuts trying figure it out.
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