Hello all; kind of at a loss. I recently purchased a '97 GT with low miles (was a sheriff DARE car). The car has been sitting, and has a hesitation under speed. So far I have replaced the alternator, fuel pressure regulator, fuel pump, fuel filter, camshaft crank sensor, plugs/wires,and battery. All coils show good. The last 2 shops that have had it on their computers say nothing shows up, and are at a loss. Could it be the PCM? It's a very clean car, and has not been abused. All stock internally, and an auto. I've owned Foxes for 30 years, but this is my first SN95. Any thoughts??
'97 4.6 hesitation
- Thread starter lbburris
- Start date
wmburns
SN Certified Technician
IMO given the fact that the cars drives for the most part the chances of this problem being caused by a bad PCM are very low. So low as to be approaching zero.
But why guess? Test! Stop replacing parts on a guess trying to fix such a problem that might be caused by a TON of "possible" causes.
If there's something "causing" the problem the odds are that it can be tested for and either ruled in or ruled out. What is the best way to do this? With an ODB2 scanner. For a fraction of the $$ already spent with no fix, it's possible to have a full function and powerful ODB2 Windows based unit.
ForScan ODB2 scanner w ELM327 USB
https://www.stangnet.com/mustang-forums/resources/forscan-odb2-scanner-w-elm327-usb.57/
With an ODB2 scanner you can quickly answer questions such as:
- what is the fuel pressure
- How many misfires
- what are the values for long term fuel trims?
- Are the O2 sensors working?
- Is the MAF value reasonable?
- What is the EGR flow?
Has the MAF sensor been cleaned? Use only a product designed for the job. No brake clean.
Not a bad idea to perform a compression test and cylinder leak down test. Then you will know if the base motor health is good.
Have you actually measured the fuel pressure? Is the fuel pressure intake vacuum reference line connected and leak free?
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It’s had every scanner/computer attached to it known to man. It shows zero misfires, throws no codes whatsoever. Fuel pressure regulator is new, fuel pump is new, as are all filters. This was done due to the car sitting. It only has 25k miles on it. The local Ford dealership couldn’t even tell me what was wrong with it.
wmburns
SN Certified Technician
Just because there's been an ODB2 reader attached means nothing (other than no DTC codes). Since you didn't supply the any of the requested information is it fair to assume that none of that operational data has been recorded or analyzed.
IF you are expecting the ODB2 scanner to TELL you what to do, then all hope for a fix is left to happen stance parts chaning or luck to find a competent professional.
Think of an ODB2 scanner in the same way as an X-ray machine. Will taking an X-ray tell the doctor what's wrong with the patient? In the same way that taking someone's blood pressure, temperature or any other vital sign is really going to create a treatment plan. It necessary for the Doctor to interpret the test results by understanding what is normal and what's not. More importantly, what does a variance mean?
Perhaps it's best to try an independent professional shop. Many will charge a one hour diagnostic fee that will be deducted from any repairs done. Maybe work an arrangement where you don't pay unless their recommended repairs actually fix the complaint.
If looking for an educated guess, for a car that sat for an extended period of time, think fuel injectors clogged with varnish gas. I have had great luck with InjectorRX.com. Once cleaned and flow tested they are as good as new. Cheaper than new.
Or maybe go back to your original thought and replace the PCM.
OBTW, here's another recent thread from someone with driveability issues and NO CHECK engine light.
https://www.stangnet.com/mustang-fo...out-on-hard-acceleration.905641/#post-9134192
IF you are expecting the ODB2 scanner to TELL you what to do, then all hope for a fix is left to happen stance parts chaning or luck to find a competent professional.
Think of an ODB2 scanner in the same way as an X-ray machine. Will taking an X-ray tell the doctor what's wrong with the patient? In the same way that taking someone's blood pressure, temperature or any other vital sign is really going to create a treatment plan. It necessary for the Doctor to interpret the test results by understanding what is normal and what's not. More importantly, what does a variance mean?
Perhaps it's best to try an independent professional shop. Many will charge a one hour diagnostic fee that will be deducted from any repairs done. Maybe work an arrangement where you don't pay unless their recommended repairs actually fix the complaint.
If looking for an educated guess, for a car that sat for an extended period of time, think fuel injectors clogged with varnish gas. I have had great luck with InjectorRX.com. Once cleaned and flow tested they are as good as new. Cheaper than new.
Or maybe go back to your original thought and replace the PCM.
OBTW, here's another recent thread from someone with driveability issues and NO CHECK engine light.
https://www.stangnet.com/mustang-fo...out-on-hard-acceleration.905641/#post-9134192
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Had the "check engine" light turned off, and it came back on this weekend. I cleaned the MAF meter, and that seemed to help the hesitation, but only briefly. If I'm "in the gas," the car runs great. If I'm cruising along, or if it's idling, that's when the hesitation comes in to play. I'm in a rural community, so there just aren't that many "competent" shops that I trust. The gas tank has also been dropped and cleaned/flushed by a Ford dealer (done prior to me taking delivery of the car). Thanks for the information thus far.
wmburns
SN Certified Technician
It might be handy to know what the DTC code is.
In the opening reply you were given several "low tech" methods to possibly rule out excessive EGR flow. What one symptom of excessive EGR flow? How about low power at part throttle operation.
Also you stated that the fuel pressure regulator was new as if that's a guarantee of correct operation. Besides what if the installer forgot to hook up the intake vacuum reference line? Or it leaks? Or what if the fuel return line is kinked? If true this would make the fuel pressure chronically high. Guess what one of the expected symptoms would be? Hesitation. We would also expect to see drastic changes in the LTFT values as the throttle is tipped in or out. The delay would cause the AF mixture to be out of wack as the PCM dialed it back in. Can you say hesitation?
My 2000 GT "project car" sat for two years before I got it. One of the first things done was to drain the stale gas. Before reusing the fuel injectors they were sent out for cleaning and flow testing. The before report showed big differences in flow between the injectors. The after report showed even flow with all injectors flowing more. Remember that each bank of the V8 has a single O2 sensor to monitor. The PCM "assumes" that all of the injectors have an even flow rate. Can you see how uneven flow will cause the PCM to attempt to average out the LTFT on that bank? In practice it more often than not, the STFT/LTFT will "hunt" rich and lean. I personally feel strong enough about this repair as to say it's a requirement when a car has been setting for a prolonged period.
Given the above I would say that having an ODB2 scanner is not a luxury but a requirement. At only $30 for the USB adapter it will more than pay for itself with the first avoided "parts change". At the very least, think about how much time it will save not having to drive to your local auto parts store to have the DTC codes read.
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