I have an '87 GT with 120,000 miles on it. I have had to do virtually nothing to it since buying it in 1993. I drive it to work everyday. Lately I have noticed a VERY slight surge when cruising at about 45 mph. It is barely noticeable. Most people would not even notice it. If I accelerate at all it is gone. Idle is fine as usual.

Sound like a fuel thing or ?

Its probably just the EGR valve kicking in but among other things there are random possibilities. If you were running the A/C the cycle switch may have toggled, that will cause it to twitch a little. If you felt like replacing your fuel filter that’s always nice insurance too.

quote Originally posted by Emc209i: Its probably just the EGR valve kicking in but among other things there are random possibilities. If you were running the A/C the cycle switch may have toggled, that will cause it to twitch a little. If you felt like replacing your fuel filter that’s always nice insurance too.

The EGR and fuel filter are a possibility. I did not have the AC or defroster on. In fact, I do not even have my AC lines charged. I would have thought the fuel filter would be worse under an acceleration condition due to fuel starvation.

I will check them both though.

If the you turn the AC on and the lines aren't charged the compressor will engage and disengage on off on off on off because of the cycle switch reading low charge. But you said it was off so we'll set that issue aside. I'd think it was the EGR if it was a slight surge. Other things that come to mind are plug wires being old, dirty injectors(maybe), spark plug missing a fire, etc etc.

Other possibilities, Map sensor, Fuel pump or pressure (check with gauge), TPS, or Ign. module.

Are you using cruise control? if you are a vacuum leak is the most likely problem.

------------------

Signature courtesy of MinnGreen.

quote Originally posted by Emc209i: If the you turn the AC on and the lines aren't charged the compressor will engage and disengage on off on off on off because of the cycle switch reading low charge.

If there is no charge in the lines, the A/C compressor will do nothing when the A/C switch is turned on.
If there is a charge, but not enough, then you will get the cycling described.

Kevin

Based on the symptoms you describe, a worn Throttle Position Sensor could be the culprit. Excessive wear is most likely to show up where the TPS spends most of its time, right at moderate throttle. The result is that "noise" from a worn TPS may be telling the ECM that the throttle is changing position when it's not.

As others have suggested, a malfunctioning EGR system is the other most likely suspect.

[This message has been edited by Marvin McInnis (edited 03-23-2007).]

Cheak your motor mounts as well as your TransAxle mounts.

I thought I would check the tips I read on my ride to work this morning. I could not make it surge at all. I tried with defroster on and off, cruise, accelerate, idle. No surge. I will try isolating next time I feel the surging.

By any chance is this an automatic? I think the torque converter is supposed to unlock somewhere around 45 MPH. This would cause the engine RPM to jump a touch.

The TPS is also a good suggestion, and about the only other thing that comes to mind.

quote Originally posted by Jax184: By any chance is this an automatic? I think the torque converter is supposed to unlock somewhere around 45 MPH. This would cause the engine RPM to jump a touch. The TPS is also a good suggestion, and about the only other thing that comes to mind.

No sir, this one is a 5-speed.

I replaced about everything and then I decided to run the computer codes(the Check Engine light was not lit up). It gave me a speed sensor code. I replaced the speedometer and the surge (like you described) went away.