1998 mustang GT hesitation that I can't solve.

BooStang89

New Member
Jul 14, 2019
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St. Augustine
So I have a 98' GT that drives great up until 15-20 mins down the road it starts to hesitate and buck, total loss of power. However I can clutch in and shut the car off and restart it then it drives great for another 15-20 mins. I can tell you that I went completely through the entire car as far as crucial components are concerned when I first got the car. Fuel pump, fuel filter, ccrm, plugs, wires, coils, O2 sensors, cam sensor, crank sensor etc. I wanted everything new when I got the car on the road. No trouble codes in the PCM only one code in the PATS module but I don't have any trouble starting the car. I've seen two other threads here that had the same issues however nobody ever posted a solution. Any help is greatly appreciated.
 
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wmburns

SN Certified Technician
Aug 14, 2009
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What have you done to rule out excessive EGR flow?

Have you verified that the charging voltage is stable? No ripple?

What is the fuel pressure when this problem occurs? Have you checked the voltage at the trunk mounted IFS switch when the problem occurs?

What is the MAF flow when this problem occurs.

Do the long term and short term fuel trims (LTFT STFT) change when this problem occurs?

What is the timing advance when this problem occurs?

Do you have an ODB2 scanner that is capable of monitoring Ford operational PID's? If you did this might make short work of this problem.

ForScan ODB2 scanner w ELM327 USB

Likely you can't find any specific solutions to this problem online because there are multiple "possible" causes. For the average DIY'er it can be a difficult nut to crack. Most people will "assume" that because a part has been replaced that this rules out it being a possible cause. They are further handicapped because they won't look at the role that dirty/loose/corroded sensor connectors might play. Same for dirty/loose/corroded grounds. Add to the mix the role that parts getting hot could play and this can quickly lead to a large number of "possible" causes.
 
Last edited:

BooStang89

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Jul 14, 2019
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Fuel pressure normal at time of malfunction, charging system checked and rechecked also has a new alternator. I'll take the car out and hook up the scanner and check the timing advance when it does it. But I'm telling you it's not a thermal failure of a sensor. Because I can shut it off and instantly turn it back on and runs perfect for another 15-20 mins. Not the average joe either. Been building mustangs for 15 years and building mod motors for 13 years. Most likely if you can think of a test to do I've done it. I've checked all 5v reference and signal voltage on all sensors. I'll report back on the timing adv. I'm leaning on a bad pats module but I've never heard of one disabling an engine after its been started.
 

wmburns

SN Certified Technician
Aug 14, 2009
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Ford PATS does not disable the motor once started. However some aftermarket alarm systems do insert themselves within the ignition system.
 

BooStang89

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Jul 14, 2019
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Timing advance normal when engine begins to hesitate. Only thing I found while data logging was when the engine begins to hesitate the long term fuel correction goes up to maximum % and EGR duty cycle goes to roughly 85% under throttle. Fuel pressure still normal at time of malfunction. Thinking now it may be and EGR issue however I figured she would've thrown a code for excessive flow. Let me know what you think
 

Racin8de

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May 8, 2016
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Fuel pressure normal at time of malfunction, charging system checked and rechecked also has a new alternator. I'll take the car out and hook up the scanner and check the timing advance when it does it. But I'm telling you it's not a thermal failure of a sensor. Because I can shut it off and instantly turn it back on and runs perfect for another 15-20 mins. Not the average joe either. Been building mustangs for 15 years and building mod motors for 13 years. Most likely if you can think of a test to do I've done it. I've checked all 5v reference and signal voltage on all sensors. I'll report back on the timing adv. I'm leaning on a bad pats module but I've never heard of one disabling an engine after its been started.
GEM MODULE I BET
 

BooStang89

New Member
Jul 14, 2019
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Why do you suggest the GEM if you don't mind my asking. Only reason being that all the on board electronics work well as they should at all times even when the engine begins to hesitate/bog
 

Sluggie24

Active Member
Apr 8, 2017
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Try disabling the egr valve and driving the car to see if the problem continues. If the problem stops then you know the egr system is causing it. If the problem continues then that obviously isn't the issue.
 

wmburns

SN Certified Technician
Aug 14, 2009
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Houston Texas
The quick/easy way to disable EGR is by disconnecting and plugging the vacuum line to the EGR valve.

This method does have the downside that it won't catch a leaking EGR value. But one can rule this out by measuring the temperature of the EGR line down stream of the EGR valve. If there's an EGR flow then it has to be hotter downstream because of the EGR flow.
 

CovidStang98

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Jun 2, 2020
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hey all first time poster here. I am having the same issue with my 98 GT. First project car ever and looking to help! I want to keep this thread ongoing i have scoured the internet for a solution. I have been going through the whole checklist.

Ny specifics is power loss. I bought the car used with an automatic transmission issue. I replaced the trans. And car drives and has power under light load. As soon as i go uphill or stomp to get on highway it bucks and loses power. If I shut the vehicle off and turn back on I will have normal power back until under load again.
I changed spark plugs, wires, coil pack, no change.

Cleaned the throttle body, and maf sensor no change.

I checked fuel pressure and turn key on without start, pressure will go to 36 psi. If I start the igniton pressure drops to 28 psi at idle. If I disconnect the vacuum line it shows 38 psi. Haynes manual shows I should be between 30 and 45 psi at idle, and between 40 and 50 psi with vac hose off. Also if given any throttle it drops slightly to about 36 and will only go as high as 33 psi with some revs.

I suspect it is fuel system since when I turn the car off and back on, the pump will repressure the lines and give me power until it struggles to keep up again. Bought a fuel filter and fuel pump and attacking those next. Will post results.

Other specifics about the car from previous owner is magnaflow cat back exhaust, and what appears to be an aftermarket upsized throttle body. Anyone know these upgrades would cause a lack in fuel? Maybe I should upgrade to high flow fuel pump?

Hopefully I can find what solves my issue and can help other forum users down the road!
 
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