North American Turbocoupe Organization



Fuel pump cutting out and tach spiking
BradM Offline
Member
#1
I have an odd electrical issue and I'm curious if anyone has some thoughts on what direction I should go. My 87 TC sometimes goes lean and backfires under certain hard pulls (usually about every 10th 2nd gear aggressive pull around a corner). I thought it might be fuel pick up and since I have a PiMPx and can data log, I installed a fuel pressure sender to monitor the issue. During the backfire, the fuel pump appears to be turning off for a fraction of second (this is not the same as fuel cutoff for over boost). Fuel pressure goes low and then spikes when turned back on, all within a few hundred milliseconds. I notice too that the rpm spikes to ~24,000 but my actual tach doesn't spike (but it does seem to visually jump a bit). Whatever electrical issue I am having seems to be related to both of these events. The fuel pump is new and I actuated the fuel pump cutoff switch a number of times thinking of bad contacts. Battery voltage is rock solid. Again, this only happens during certain pulls and I've logged 7k miles of normal driving without seeing this issue. It doesn't act like a simple loose wire. It's a head scratcher.
1965 Mercury Comet Caliente; 1968 Mercury Monterey; 1969 F100 Ranger; 1987 Thunderbird Turbo Coupe
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spittinfire Offline
Member
#2
That's an interesting one and unfortunately one that will drive you nuts. Finding intermittent problems is always a challenge. My first thought is a bad connection or a broken wire but what you're describing in the data log makes me think something within the EFI.
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Jeff K Offline
Administrator
#3
Had the same issue 20 years ago. Problem turned out to be the inertia switch being intermittant. Discovered the issue by hooking a test light with LONG wires to the inertia switch contacts and running the test light into the cabin where I could see it. Light would occasionally come on indicating open inertia switch. My thought was the switch contacts were damaged from the higher current drawn by the Walbro 255 lph fuel pump.

Replaced the inertia switch with a new one (they were still available new years ago) and put a 30 A relay in the circuit so the inertia switch only sees the 150 mA relay coil current. Problem solved.

Similar issue a few months ago with my 95 taurus beater. Brief miss with a brief CEL indicating fuel pump primary circuit intermittant. I wacked the inertia switch to pop it open and then reset it a few times thinking dirt / corrosion on the metal ball that completes the circuit. No issues since wacking the switch.
Jeff Korn

88 Turbo Coupe: Intake and exhaust mods, T3 turbo at 24 psi, forced air IC, water injection, BPV, Ranger cam, subframes, etc., etc.
86 Tbird 5.0 (original owner): intake, exhaust, valvetrain mods, 100 HP N2O, ignition, gears, suspension, etc., etc.
05 Taurus SEL Duratec daily driver
04 Taurus Duratec (wifes car)
02 Pontiac Grand Prix GT
95 Taurus GL Vulcan winter beater
67 Honda 450 Super Sport - completely customized
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BradM Offline
Member
#4
Thanks. This is good to hear this. I was planning on bypassing the inertia switch just to see what happens. I also was going to hardwire the fuel pump relay (switched of course) if all else fails. By the way, the last big backfire cost me my T3/T4 turbo. It rammed the turbine into the bearing retainer which caused the compressor wheel to grind into the housing. By the time I figured it out, there was a lot of aluminum going through my engine (all checked out fine though).
1965 Mercury Comet Caliente; 1968 Mercury Monterey; 1969 F100 Ranger; 1987 Thunderbird Turbo Coupe
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firebirdparts Offline
Member
#5
That is a very believable reason. Hopefully that's it.
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