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Engine surge when hot



My problem is this... On a 90+ degree day, with the AC on, and in stop and go traffic, the engine will start to surge. The temperature will get up to 225 or more, and the engine will surge, then rev a little, then go back to normal. After a few minutes, it will surge again. Last week I let it keep doing it in hopes it would throw a check engine code, but it never did (the light does work). In fact it surged to the put that it actually stalled once. Once I got back on the road, and the temperature came down, it ran perfect again. I flushed the coolant and changed the thermostat, the fans all kick on when they are supposed to, so I don't think that is the issue. I have also noticed that when the car is first started, the volts are at 14.5 with the AC on, as the car gets hotter, the volts will sometimes drop to 12.9. To the point that at idle, my turn signals will blink slowly. This is when it will also start the surging. I had the alternator checked, and they said it was fine, although I think the amps are too low at idle. It runs perfect until the temp gets on the high end so something seems flaky. I'm considering the lower temp fan switch, or maybe the idle is too low when hot, any thoughts?


Another thought... I was just reading up on the throttle position sensor. When it gets hot, it will also sometimes idle a little rough, is it possible the TP sensor goes wacky when it gets overheated, causing rough idle and the surges?


I have having a problem that is very similar. I get the surges at idle (usually over 1K RPMs but sometimes over 2K RPMs) when the temp gets up around 225. I have had it in a shop many times - replaced the O2 sensors, IAC controller, plugs. Checked the injectors, every electrical connection, and much more. Diagnostic scanner shows nothing other than IAC count jumps way up when problem happens - which is why the surge occurs. I have not noticed the volt issue you describe. My mechanic, who has worked on vettes for over two decades, is stumped. He has sent of the diag snapshots to contacts he has at GM but has not received any suggestions back yet. This problem has been occurring all summer and it is very frustrating.

Have you made any progress on your problem?


On my 91 Vert, I also have been down this road, only when the Outside Air Temp exceeded 90 degrees and I was in heavy traffic and I found that when I cycled the A/C from "OFF" to "AUTO" AND BACK to "OFF" again , that the problem followed. If I kept it in AUTO the problem went away, so I R&Red the TPS and the problem. Wala problem solved....NAPA sells the Delco Remy OEM switch for around $64. Ecklers wants $79 for the same switch

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