Hi All. I've posted before, with no solutions, but now I have more information. I have a 98 Cobra. I have two open issues on the car. Anyone ever have a situation where you turn the key and the car kinda makes a pop sound and just goes completely dead electronically? All I have to do is disconnect and reconnect the battery to get it back. It might start normal or do it again. Eventually, it'll start just fine and then run like it never happened. That aside the bigger mystery is this issue I'm having with fuel pressure. The car will run normal and great. no issues driving, no issues WOT. Then randomly, at idle and only at idle, the car will just cut out. I was able to verify that it has spark still when cranking, but the fuel pressure is gone. down to about 3 psi. On attempts to restart, the usual fuel pump prime sound has, in the past, not happened at all until it comes back when it feels up to it. This last time, i heard something under the hood, a different sound, like a relay or something is trying to energize the pump, but still no fuel pressure. Given a rest period, it comes back to life. 30 psi on prime (with a more robust sound under the hood), starts like nothing ever went wrong. Forscan is showing no codes at all. This has only ever happened with the car was idling. it's not repeatable, there are no common circumstances when it happens and it has, so far, always been fine after a 20+ minute rest. this is a garage kept car. never driven in snow, rarely ever driven in rain. This happened about 8 years ago and I swapped in new OEM fuel pump and it ran fine for another 8 years, but something in me just doesn't feel like it's the pump as it never hiccups at all when driving and always comes back after a rest. I haven't been able to catch it in the act where I can then go connector by connector in a diagnostic, but if this sounds familiar to anyone, suggestions would be welcome. THANKS!
Fuel Pressure cuts out intermittingly at idle - comes back after 20 minutes
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wmburns
SN Certified Technician
IMO you should start by doing a proper voltage drop test to the fuel pump on the positive and ground side to CONFIRM the electrical circuit's ability to carry a REAL current.
While you are there looking at your wiring, look for evidence of water or rodent activity.
Howto perform charging system voltage drop test
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Do yourself a favor. Don't cut any corners with regards to the basic health of the charging system and the cleanliness of the electrical connectors.
Once you are certain that the charging system is in tip top shape, things are likely to get more difficult. If this were my car, likely I would start by using an "add a circuit" to the fuel pump fuse. Use the pig tail to extend the wiring inside the cabin where you can hook up a VOM meter. The idea is to have an easy way to monitor voltage while the car is operating.
While you are there looking at your wiring, look for evidence of water or rodent activity.
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...
www.stangnet.com
Do yourself a favor. Don't cut any corners with regards to the basic health of the charging system and the cleanliness of the electrical connectors.
Once you are certain that the charging system is in tip top shape, things are likely to get more difficult. If this were my car, likely I would start by using an "add a circuit" to the fuel pump fuse. Use the pig tail to extend the wiring inside the cabin where you can hook up a VOM meter. The idea is to have an easy way to monitor voltage while the car is operating.
OneSick99GT
Active Member
Make sure your battery connections are solid and wires are in good shape going to the battery. I had my battery for the positive start to crack which led to wierd hesitation, no start conditions. Once I tightened down the connector to the positive terminal it broke. Replaced the terminal connectors and everything worked perfect. Battery and alternator test is a must before you do anything else.
Thanks for your reply. A few years ago, I put in a quick disconnect on the positive battery cable since this car sits all winter and I feel something related to this has something to do with the pop car dead issue. I think the best thing to do is replace that cable, which feeds power to two different parts, as I don't need the disconnect anymore since I put in a storage charger. However, I can't think of how this affects the fuel pump given the car can crank over and has spark, which are high demand draws when it's not running. what is shutting the fuel pump down, only ever at idle and requiring a 20 minutes rest before it'll come back on line is just baffling. and over the course of about 500 miles this summer, it only happened 3 times and completely at random. just so odd
wmburns
SN Certified Technician
If you really want to know what's going on I highly recommend going the ODB2 route. Data log some key PID's such as RPM and MAF. What the goal is to find out what PIDs are changing when the problem occurs.
ForScan ODB2 scanner w ELM327 USB
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What if the problem has nothing to do with the fuel pump at all. For example. What if there's a problem with the CKP sensor signal at the PCM? The PCM uses the CKP sensor to sense if the motor is running. If the motor is not running, the PCM will not run the fuel pump.
However I still stand by my previous advice. I have seen charging system "issues" cause some of the strangest unrelated problems.
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...
www.stangnet.com
What if the problem has nothing to do with the fuel pump at all. For example. What if there's a problem with the CKP sensor signal at the PCM? The PCM uses the CKP sensor to sense if the motor is running. If the motor is not running, the PCM will not run the fuel pump.
However I still stand by my previous advice. I have seen charging system "issues" cause some of the strangest unrelated problems.
Thanks. I have a forscan scanner and all systems register perfectly normal when it's running. if I knew I could repeat the issue, I could be watching it, but it's running normal one second, the next, it's gone and then comes back on like nothing happened and could operate fine for weeks. it throws no codes. However, I'm putting in a new battery cable. I just ordered it. cheap part to at least eliminate one possibility. The car is garage kept, so all the wiring looks great, but I also ordered shop manuals and i'm going to do a comprehensive check of all connectors related to the fuel system.
OneSick99GT
Active Member
I'm with burns on this! I've had wierd issues that led me to replace parts only to find on the battery connections weren't good enough. Make sure to take the connections completely off and do a thorough inspection of all the parts before putting them back on.
