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Hesitation on hard acceleration (Problem Solved!)

Hubnut

Hubnut

1/2 ton status
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Dec 21, 2008
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Location
Gainesville, FL
Please help me diagnose this problem. My 91 TBI 350 Jimmy hesitates on hard acceleration. It idles fine, drives fine. If you accelerate gently, not too noticable, but when you accelerate hard, it hesitates pretty badly.
Changed the distrubutor cap, rotor button, fuel filter and in-tank (holy crap that's hard) fuel pump. Still hesitates on hard acceleration. TBI bad? I'll check the timing tomorrow, but it has been fine. Don't know why it would change all the sudden. Any ideas??
Frustrated!

Thanks to all for your suggestions! Disconnecting the battery overnight cured the problem. I suppose the computer resetting was all it needed. I was doubtful--but it's running fine again...no hesitation at all. I wished I'd checked here before spending money ($100) for a pump, rotor cap and button AND doing all that unecessary work. Live and learn! I don't ever want to replace that fuel pump again!
 
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I woud say timing and plugs (not listed above) then reset the computer due to the air/fuel mixture maybe being messed up. Then check the fuel pressure, maybe its the pump not keeping up.
 
Disconnect the neg bat cable is what resets the computer. I would check the fuel pressure, the fuel pattern on the injectors, and the timing. Another think is possible is if you did a thermostat change find out if you put in anything other than a 195* thermostat. That will affect your engine performance.
 
I just went out and disconnected the neg battery cable. I'll leave it off overnight and try her out. Now that you guys mention the computer, the problem actually started IMMEDIATELY after I installed a winch. Could briefly disconnecting the batt terminals have caused a change in the computer or something??? Thanks for the feedback and help.
 
yo mr pickle....i've got a similar problem with my 78 350 carbed but rough idle like a vac leak the carb and gasket are new and all vac lines are attached also when i mash on it.....the tranny kicks down..the secondaries open but it just makes more noise it doesn't pick up speed:confused:
 
Heck of a lot easier to reset the ECM by pulling the fuse. Need it out about 2 seconds, or the time it takes to pull and install the fuse. Nothing in there stores data after power is lost.
 
roger acord said:
yo mr pickle....i've got a similar problem with my 78 350 carbed but rough idle like a vac leak the carb and gasket are new and all vac lines are attached also when i mash on it.....the tranny kicks down..the secondaries open but it just makes more noise it doesn't pick up speed:confused:
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When you mash on it and it makes a bunch of noise but doesn't pick up speed, is the motor running smooth or is it rough? If it is smooth then I would say something might be wrong with the torque converter or clutch, whichever you have. That is based on tires and axle ratios being close to stock (not running 44s on stock axles). If the engine is running smooth and you mash it, it should pick up speed, so something is being lost when putting that power to the ground and the torques conveter or clutch would be the #1 suspects.

For the rough idle it could be a number of things, did you check the basics? (cap, rotor, plugs, plug wires, coil connections, etc) You say the carb and gasket are new, have you tried re-torquing the bolts? Maybe there is a slight gap. Since it is a 78 I am guessing it has a bunch of the prehistoric smog stuff on it and trying to find problems in there is like xmas present for the in-laws, you don't want to do it and it usually ends with frustration and lots of drinking. Check every random tube for crakcs and such. Maybe you will get lucky and find one. Sorry for not being too much help.
 
scooter122983 said:
Disconnect the neg bat cable is what resets the computer. I would check the fuel pressure, the fuel pattern on the injectors, and the timing. Another think is possible is if you did a thermostat change find out if you put in anything other than a 195* thermostat. That will affect your engine performance.
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X2 on the battery terminal.

Ok I have to ask, how will a stat affect performance? Once the vehicle heats up to op temp, usually around 200, it will stay at whatever temp the cooling system decides, not the stat. I understand maybe it running bad/weird if the stat is at like 220 but if the stat is at around 180, I don't see it making a difference. It will heat up to 180, open and then keep going to whatever temp the engine wants to run at (based on outside temp, usage and radiator size) Not trying to be an a$$, just asking.
 
Problem solved! Thanks to all for your suggestions! Disconnecting the battery overnight cured the problem. I suppose the computer resetting was all it needed. I was doubtful--but it's running fine again...no hesitation at all. I wished I'd checked here before spending money ($100) for a pump, rotor cap and button AND doing all that unecessary work. Live and learn! I don't ever want to replace that fuel pump again!
 
hold on i'm still cracken up over the x-mas present for the in-laws;;;;boy do i know what you're saying,,,,anyway the torque conv. sounds like a good possibility,,,,i replaced the front seal on the tranny because of a red oil leak;;;;the old seal looked new and undamaged;;;i replaced it any way;;put it all back together;;;still tranny fluid on the ground in front...maybe we gotta crack? mean while i'll check out all the smog crap and intake.........thanks for the come back.....later
 
Mr. Pickle said:
X2 on the battery terminal.

Ok I have to ask, how will a stat affect performance? Once the vehicle heats up to op temp, usually around 200, it will stay at whatever temp the cooling system decides, not the stat. I understand maybe it running bad/weird if the stat is at like 220 but if the stat is at around 180, I don't see it making a difference. It will heat up to 180, open and then keep going to whatever temp the engine wants to run at (based on outside temp, usage and radiator size) Not trying to be an a$$, just asking.
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Your t'stat should control the temp. If your engine keeps getting warmer after you t'stat opens your cooling system is to small. I would understand if you were towing in HOT weather the engine will get warmer but under normal conditions the engine shouldn't get much warmer than what the t'stat is rated for.
 
Hubnut said:
Could someone tell me where the computer fuse is located?
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Sorry for the late response, I believe the trucks have the ECM fuse on the panel as ECM A and ECM B(?). Not sure which the ECM is on, TPI has the fuse in the harness elsewhere.

Pulling the fuse has the added benefit of making sure you don't lose your presets and time.:)
 
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