Bucking??

Comatose · 03-28-2002, 12:36 AM

86 GT H.O. 5.0EFI, 5spd.

It doesn't matter if I'm cruising through town or on the highway, sometimes when I let off the gas at lower rpm (less than 2200) the car bucks. It doesn't do it all the time, but enough to bother me. Sometimes while cruising at 25 to 35mph it will also 'buck'.

Any Ideas. I cleaned the IAB recently, complete tune-up about 10k miles ago. Mods are Hypertech chip and cat-back exhaust.

Thanks,
Jeremiah

sn95gt19 · 03-29-2002, 12:14 AM

i would say a small vacuum leak, when you let off the gas the motor is trying to rev up for a sec. then bucking, either that or you just need new u joints

PKRWUD · 03-29-2002, 12:40 AM

You need to inspect your TPS. Disconnect it at the harness, and attach a DMM to the TPS green and black pins. Set the DMM to ohms, and watch the bargraph while you SLOWLY rotate the throttle. If the bargraph jumps backwards at all, replace it. It should gradually increase, very smoothly.

Take care,
-Chris

sn95gt19 · 03-29-2002, 11:14 AM

if your prob ends up being the tps you should set it at idle and it should be about .95 -.98 just make sure that it isnt above 1 volt, hope that helps you.

chris91LX · 03-29-2002, 12:17 PM

I had one guy tell me it was a fuel thing, because my car runs a little lean below 3300 rpm, a/f is at like 16 below 3300 and just a hair over 14 from 3400 to 5600rpm. I bumped up the fuel pressure and it still bucks at really low rpm like yours, usually on decel but also if I'm trying to coast in gear. Another friend told me it's the cam. I'm running the TFS Stage 1 and he said i might need a chip to smooth it out. I'd already checked my tps and it was set at like .985 so I think that was fine.

I don't know either way so I've decided to do my level best to keep the rpms up so i don't have that problem.

Grn92LX · 03-29-2002, 04:19 PM

Driving at a low rpm in a car with a cam, the car will buck sometimes. Thats normal with a cam. Your car is also running lean from the a/f readings you posted. In the upper rpms, low to mid 13's is considered the norm a/f for n/a cars. I have the TFS 1 cam and it'll buck sometimes in a low rpm in a high gear, but mine doesnt seem to do it like your does. Whats the rest of your combo? Sounds like it needs tuning.

JRowlette · 03-29-2002, 08:59 PM

GRN92LX:
I just installed the exact same combo you have. Are you still using the stock 19# injectors? What fuel pressure are you at? My car is also bucking like a mule at low rpms.

Grn92LX · 03-30-2002, 03:14 AM

I'm running 24# bosch injectors. They are flow matched. Playing with fp at the track, I have found 37psi to be best for my car. I havent played with timing at the track yet. My car doesnt buck much, but like I said lugging the engine with a cam will make it buck like your describing. Cant really help with fp for 19# inj, but at the track, try 43-45 psi and see how mph is affected when you change it.

Thad · 03-30-2002, 09:26 AM

I also have bucking at low rpm's. i've done all the tunings that would relate like timing, tps, fp. it helped some but it still bucks a little bit. i'm thinking it's partially my combo and my u-joints.

03-28-2002, 12:36 AM	#1
Comatose Registered Member Join Date: Oct 2001 Location: Goldendale, WA, USA Posts: 84	Bucking?? 86 GT H.O. 5.0EFI, 5spd. It doesn't matter if I'm cruising through town or on the highway, sometimes when I let off the gas at lower rpm (less than 2200) the car bucks. It doesn't do it all the time, but enough to bother me. Sometimes while cruising at 25 to 35mph it will also 'buck'. Any Ideas. I cleaned the IAB recently, complete tune-up about 10k miles ago. Mods are Hypertech chip and cat-back exhaust. Thanks, Jeremiah __________________ Jeremiah 1986 Mustang GT K&N Filter, Removed Air Silencer, Black Magic Fan, 65mm Explorer TB, 110lph BBK Fuel Pump, F/M Force II Cat -Back, O/R h-pipe, U/D's Timing@14* 1986 Mustang LX SSP--new project car AOD, 3.27's, Dumps, 238k miles

03-29-2002, 12:40 AM	#3
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	Get a DMM w/ a bargraph You need to inspect your TPS. Disconnect it at the harness, and attach a DMM to the TPS green and black pins. Set the DMM to ohms, and watch the bargraph while you SLOWLY rotate the throttle. If the bargraph jumps backwards at all, replace it. It should gradually increase, very smoothly. Take care, -Chris __________________ Webmaster: Rice Haters Club Jim Porter Racing Peckerwoods Pit Stop Support Your Local RED & WHITE!

03-29-2002, 04:19 PM	#6
Grn92LX Registered Member Join Date: Mar 2002 Location: Long Island, NY Posts: 55	Driving at a low rpm in a car with a cam, the car will buck sometimes. Thats normal with a cam. Your car is also running lean from the a/f readings you posted. In the upper rpms, low to mid 13's is considered the norm a/f for n/a cars. I have the TFS 1 cam and it'll buck sometimes in a low rpm in a high gear, but mine doesnt seem to do it like your does. Whats the rest of your combo? Sounds like it needs tuning. __________________ TFS heads/FTI custom cam/TFS street heat, Longtubes, 373's, MM lca's, Tokico springs/shocks

03-30-2002, 09:26 AM	#9
Thad Registered Member Join Date: Oct 1999 Location: Stuart, FL US Posts: 192	I also have bucking at low rpm's. i've done all the tunings that would relate like timing, tps, fp. it helped some but it still bucks a little bit. i'm thinking it's partially my combo and my u-joints. __________________ 90 LX 5.0 ,bored 302, MSD 6AL, Twisted Wedge TrickFlo HDs, World class T5, BBK fender cold air induct., 70mm TB,Edelbrock performer Intake, aluminum Drve shaft, 3.55 Gears, Long Tube Headers to Flowmasters.

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03-29-2002, 12:14 AM	#2
sn95gt19 motors done......woohooo Join Date: Mar 2002 Posts: 799	i would say a small vacuum leak, when you let off the gas the motor is trying to rev up for a sec. then bucking, either that or you just need new u joints

03-29-2002, 11:14 AM	#4
sn95gt19 motors done......woohooo Join Date: Mar 2002 Posts: 799	if your prob ends up being the tps you should set it at idle and it should be about .95 -.98 just make sure that it isnt above 1 volt, hope that helps you.

03-29-2002, 12:17 PM	#5
chris91LX Registered Member Join Date: Oct 1998 Location: Suburban Chicago, Hanover Park Posts: 695	I had one guy tell me it was a fuel thing, because my car runs a little lean below 3300 rpm, a/f is at like 16 below 3300 and just a hair over 14 from 3400 to 5600rpm. I bumped up the fuel pressure and it still bucks at really low rpm like yours, usually on decel but also if I'm trying to coast in gear. Another friend told me it's the cam. I'm running the TFS Stage 1 and he said i might need a chip to smooth it out. I'd already checked my tps and it was set at like .985 so I think that was fine. I don't know either way so I've decided to do my level best to keep the rpms up so i don't have that problem.

03-29-2002, 08:59 PM	#7
JRowlette Registered Member Join Date: Apr 2001 Location: Bowling Green, KY Posts: 95	GRN92LX: I just installed the exact same combo you have. Are you still using the stock 19# injectors? What fuel pressure are you at? My car is also bucking like a mule at low rpms.

03-30-2002, 03:14 AM	#8
Grn92LX Registered Member Join Date: Mar 2002 Location: Long Island, NY Posts: 55	I'm running 24# bosch injectors. They are flow matched. Playing with fp at the track, I have found 37psi to be best for my car. I havent played with timing at the track yet. My car doesnt buck much, but like I said lugging the engine with a cam will make it buck like your describing. Cant really help with fp for 19# inj, but at the track, try 43-45 psi and see how mph is affected when you change it.