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97 12v running down the highway at 65 the truck beginns to surge when staying at the constant speed, but it doesn't surge durring acc. surges harder around 70.
 

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I had the same problem and the mechanic said it was tranny temp(which on the 95 is calculated based on the engine temp and mine was low so the replaced thermostat and lowered collant level). That held up for a day, then they replaced tstat again and it held up for a few months but truck temp still fluctuates. After a few months it was still doing it, but too busy to take it in. Then when i went to get the tranny serviced the next guy said tq converter was tore up and coming out of lockup so replaced tranny and put in with a low stall tq converter. No problems yet. But i went back to the mechanic that changes tstat and collant cuz operating temp still fluctuates and the part was warranteed but they charged $78 labor and temp stayed same so i asked why they replaced the tstat if it gives same reading and they gave money back. Still no surge like it used to...

But the other day I was passing at about 75 on the freeway and it did it... (not to mentioned I became very dissapointed). It only did it while I was at that speed and went in and out and again until i slowed down.

I have read many articles on bad throttle position sensor causing this issue,
but recently, a thread that caught my interest said that and over-adjustment in injection pump timing advance can cause this at high rpms as it will "skip a beat" (or two or three...). I got my truck used so I don't know if timing has been adjusted, but I suspect it has or, like many guys say, was bad from the factory.

But other than the surge I get bad mpg (but afc has been tampered with) so I don't know what my problem is, but I hope I gave you some Ideas to think about.
 

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oh... i didn't realize how old this thread is, but if you haven't solved the problem yet, maybe this will give you a significant 'BUMP'
 
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