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Thread: Questions for you EFI experts

  1. #1

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    Questions for you EFI experts

    I have a customer with a donor build cobra that (randomly) will not start after it warms up. After much research I diagnosed it as a bad ecu. I swapped in a known good one and the problem went away. Sent the old one out to be repaired, re-instlled and everything was good. Started it hot at least 20 times. Customer drives the car to NH and when stops for gas it will not restart. Heres a clue. The only way to get it to restart hot is to trip the impact switch to shut off the fuel pump. Found this out by accident. I can duplicate this over and over. Why does shutting off the pump allow it to start?
    Is it possible something in the ECU is still bad?
    Thanks Mike

  2. #2
    Senior Member RickP's Avatar
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    Hey Mike. What kind of pump is it running? I’m thinking the PWM has something to do with it.

  3. #3
    mburger's Avatar
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    I’m not an expert at anything but I was going to suggest a bad PIP sensor and to pull the spout plug to see if it would start hot, but you lost me when killing the pump gets it to start.
    Mark
    Mk1, Frame #1929 Complete restoration/upgrade. BP 347 with Edelbrock PF4 439/420. 4 link with coilovers. 8.8 3.55, 15” Halibrand, New beefed up T5 w/short throw shifter, Power 4 wheel disc brakes, Custom original style steering wheel, shaft and boss, Heat/AC, Heated seats, PPW wipers w/washers, Forte’s throttle linkage, RT trunk hoop mod, Pusher cooling fans, full LED lighting, custom headrests, 5 point seat belts with sub pass through, Speedhut GPS gauges, battery drop box in trunk, LED courtesy lights, Breeze trunk cubby kit.

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    Senior Member seagull81's Avatar
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    I had a similar issue that turned out to be the TFI module.
    Steve
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    FFR3542K, 347 C.I., EFI, T-5, 3-Link, Miata Front Sway Bar, Red with White Stripes

  5. #5

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    I am familiar with the typical hot start issues related to the distributor. When this happens I am getting spark and power at the injectors.
    Mike

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    If the fuel pump cycles off shortly after the key is on. I would think it would be the same as the inertia switch being disengaged. My point, let the pump quit pumping before engaging the starter. Most people using fuel injection, go straight to start with the key switch. I really do not know what this is going to reveal, other than maybe eliminating the pump and leaving the inertia switch at question.
    My next guess would be a ground back feeding through a second circuit. Why it would do it in a hot start condition, I do not know, unless the starter temp requires more current draw.
    I use jumper clips to add a ground to the easy stuff. Like a juke box that plays better when one of the circuit panels is swung out for access.
    Might check the ground to the fuel pump.
    Not sure if the ecu activates a relay for the fuel pump, but make sure that circuit is fed by an independent circuit.
    20th Anniversary Mk IV, A50XS Coyote, TKO 600, Trunk Drop Box, Trunk Battery Box, Cubby Hole, Seat Heaters, Radiator hanger and shroud.

  7. #7
    EFI Rules and Carbs Drool Arrowhead's Avatar
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    Quote Originally Posted by michael everson View Post
    I am familiar with the typical hot start issues related to the distributor. When this happens I am getting spark and power at the injectors.
    Mike
    Sounds like an electrical issue to me. Are the injectors firing? Is it not starting because of lack of fuel then? - which is very odd if it is starting when shutting the pump off. Pump is definitely run off a relay correct?

  8. #8
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    In my car the ECU definitely controls the fuel pump through the FP relay. With ignition, the pump primes for a few seconds and then turns off until ECU sees engine revs and then turns it back on. If I trip the inertia switch when my engine is running it will stall in about two seconds. The ECU presumably sees engine revs from the distributor so I would check that signal.
    Phil: FFR 3121, Frame #2050, Mk 1.5, built 2002, 1991 donor with Blueprint 302 long block and 1993 Cobra intake, BBK 70mm T/B, no smog, power steering, T-5 and 3.55 rear.

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    Senior Member brewha's Avatar
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    Fitech units use 15 amps of power to power the fuel pumps. Most of the Walbro types pumps I’ve seen use up to 18 amps.
    If this is the case then heat could be created by that circuit and shut off the pump until it cools.
    Depending on the EFI unit I would find out how many amps are sent to the fuel pump, find out put what type of fuel pump you have and how many amps it can draw and decide if a better power source for the fuel pump is necessary. Adding a relay with a separate power source and have the EFI unit controlling the relay takes very little time to install.
    Mark4 - 331 Stroker - Fitech 600 -TKO600 - Moser 3.55

  10. #10

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    So a couple of updates from the customer. Car still wont start when hot. He says it stinks of fuel when this condition occurs. While the car was here, it had spark and fuel injector pulse when this condition was present. I have no idea why shutting off the pump is making it start. He called me from a gas station when it wouldn't start. I had him trip the switch and crank it over. It ran for about 2 seconds and died. Then had a him crank it some more while someone depressed the button on the cutoff switch, and it fired right up. He made it home with no issues. Would not start once he got home. Starts every time when its cold.

  11. #11
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    not an expert on this but it sure sounds like leaking injector(s) or a bad fuel pressure reg. I assume he is not touching the accelerator pedal on start up.
    David W
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  12. #12
    Senior Member brewha's Avatar
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    Checking the inertia switch should be easy. You can check for 12v before inertia switch and it should be 12 volt after the switch. The differences should be within .2 to.7 volts if not the same. Anything else would be a faulty switch.
    Mark4 - 331 Stroker - Fitech 600 -TKO600 - Moser 3.55

  13. #13
    Senior Member brewha's Avatar
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    One other thought. The inertia switch provides the power to the fuel pump but it’s the EFI systems job to control the ground. Make sure that the ground is good through the EFI and not a loose connection. Good luck...
    Mark4 - 331 Stroker - Fitech 600 -TKO600 - Moser 3.55

  14. #14
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    try bypassing the inertia switch and see what happens....

  15. #15
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    This advice is based on my understanding of the EFI system as it pertains to factory installs on Fox Mustangs. I apologize if some of this doesn't apply due to changes made from the original fatory setup.

    The EFI setup is notoriously hard to restart when warm when the engine is pig rich or floods out. One way to get around this is to kick the throttle to the floor and crank it over with the engine at WOT. The TPS will see the WOT condition, and signal the EFI to not fire the injectors during crank. Usually this helps clear the flood and the engine will start.

    If this method DOES allow the engine to stop, likely culprits are a bad ECT or ACT sensor. I've yet to wire up a FFR car myself (yet) but if the stock factory ECU test plug is present, it's possible to pull the engine codes and see what sensors (if any) are failed.

    As for the fuel pump issue. My question is, does the fuel pump cycle continuously with key on? It shouldn't. It should cycle on for 2-3 seconds when you turn key on, and then only begin pumping when the engine starts. When you pull the inertia switch, is it actually killing the fuel pump?

    My thinking is that you are flooding out the engine somehow, but trying to understand how. Normally in this case, on an EFI Mustang, I would dump the codes, check fuel pressure and do a leakdown on the injectors.

    What was bad on the ECU? Trace for pin 46? Which ECU is this? A9L?

  16. #16
    EFI Rules and Carbs Drool Arrowhead's Avatar
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    Quote Originally Posted by michael everson View Post
    So a couple of updates from the customer. Car still wont start when hot. He says it stinks of fuel when this condition occurs. While the car was here, it had spark and fuel injector pulse when this condition was present. I have no idea why shutting off the pump is making it start. He called me from a gas station when it wouldn't start. I had him trip the switch and crank it over. It ran for about 2 seconds and died. Then had a him crank it some more while someone depressed the button on the cutoff switch, and it fired right up. He made it home with no issues. Would not start once he got home. Starts every time when its cold.
    Starting to sound more like a fuel delivery problem now rather than electrical as mentioned in the post above. I'd look to the pressure regulator and vacuum lines / map sensor. If the map sensor is bad or not getting a vacuum signal it will dump fuel, vacuum leak will have a similar effect. (I'm thinking even with the MAF it still has a MAP sensor but I could be mistaken on that.) I think it's flooding initially so when you cut the pump the pressure goes down and runs till the pressure is drained from the lines.

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