North American Turbocoupe Organization



missfiring above 5500 rpm
88cat Offline
Member
#1
My 88 t-bird missfires and starts shaking real bad when it hits 5500 rpm. I have replaced the spark plugs, tps, coolant sensor, o2 sensor, distributor and nothing. I have tried another coil, pcm, bap sensor, vam, and have yet to figure out the problem. Can anyone help me out? thanks
88 Cougar Bostonian
88 Cougar (351c under construction)
88 TurboCoupe
88 TurboCoupe
87 Turbocoupe: Holset H1C 25 psi, fmic,n2o,stinger header, 38mm ext wg, 3.5 exh, wms sds, innovate wb, c4 3000 stall, 3.55's, p&p head, 70mm tb custom upper intake, ported lower, Alky kit, line lock, all autometer gauges, kirkey seats
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Pete D Offline
Administrator
#2
How many miles on the head components? Might have weak valve springs and the valves are floating??
Pete Dunham


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88cat Offline
Member
#3
Well this problem started ever since I changed the headgasket. The other head that was on it was craked replaced it with a good turbo head. The car already had a roller cam but I do not remember if I changed the valve springs or If i installed the same ones. When I rev it to 5500 the engine pops and then starts shaking and missfiring.
88 Cougar Bostonian
88 Cougar (351c under construction)
88 TurboCoupe
88 TurboCoupe
87 Turbocoupe: Holset H1C 25 psi, fmic,n2o,stinger header, 38mm ext wg, 3.5 exh, wms sds, innovate wb, c4 3000 stall, 3.55's, p&p head, 70mm tb custom upper intake, ported lower, Alky kit, line lock, all autometer gauges, kirkey seats
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Pete D Offline
Administrator
#4
Let's see if anyone else has other ideas. "Engine popping" sure sounds like valve float, especially since it's not occurring until fairly high rpms and you have replaced a lot of the usual suspects already.
Pete Dunham


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Jeff K Offline
Administrator
#5
Should have come here first before throwing parts at it..... O2 sensor output is ignored over 3000 RPM if load exceeds 10%, so the O2 isnt the problem. PCM for sure is nt the problem. I agree with Pete... valve float, especially if it happens at the same RPM every time. Also, have you chwcked your fuel pressure under loadto be sure the pump is keeping up?
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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88cat Offline
Member
#6
I have not yet checked the fuel pressure yet but It does smell gasoline after i rev it and starts missfiring.
88 Cougar Bostonian
88 Cougar (351c under construction)
88 TurboCoupe
88 TurboCoupe
87 Turbocoupe: Holset H1C 25 psi, fmic,n2o,stinger header, 38mm ext wg, 3.5 exh, wms sds, innovate wb, c4 3000 stall, 3.55's, p&p head, 70mm tb custom upper intake, ported lower, Alky kit, line lock, all autometer gauges, kirkey seats
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