1993 GMC C2500 intermittant hesitations after 2400 RPM

JOHNRHAYNES

MEMBER
1993 GMC C2500
V8
4WD
AUTOMATIC
160,000 MILES

My suburban runs good up to 2400 rpm in D (overdrive) but then after 2400 the tac intermittently jumps up 50 rpm and you feel a slight lag in speed. (So you kind of go pulsing down the road if over 66mph in Drive.) Doesn't happen at a lower gear and the same rpm.
454 engine with factory replaced trans at 60K.

6 Replies

2CARPRO JACK

MECHANIC
11,533 POSTS

Is that a throttle body or port fuel injection? Has it been tuned up recently and if so what brand parts were used? Sounds like it is ignition related if it jumps

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Friday, January 25th, 2008 AT 3:19 PM

JOHNRHAYNES

MEMBER
4 POSTS

I have throttle body. Seems strange that it runs so smoothly until it hits exactly 2400rpm and above. Then the 50rpm jump is at the same time you feel a slight reduction in speed. Lasts for a second or so and then drops back for a second or so etc.
Last tune up was maybe 6k ago and generic parts were used.
If you are cruising at 70 on level grade and the torque converter lock goes in and out, how much rpm change would there be in and out?

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Friday, January 25th, 2008 AT 7:25 PM

2CARPRO JACK

MECHANIC
11,533 POSTS

That would be about the right amount for the lock-up. If you are giving it any throttle it shouldnt lock the converter, might go in and out of OD depending on the amount of throttle being given. What gets me is the reduction in speed, almost like an ignition misfire causing the tach to jump and the engine to lose a slight amount of power. By generic parts, I hope you dont mean Bosch platinum plugs or their wires. Try to stick with the OEM stuff, costs a little more, but the ignition system is designed around the resistance in certain ignition parts.

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Saturday, January 26th, 2008 AT 7:54 AM

JOHNRHAYNES