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just started surging. a noise when accelaerting , goes away while coasting. Surges and downshifts rough while braking. Engine idle varies while foot on brake. Can’t find any obvious vacuum leaks. No check engine light either. Scary when braking and downshifting as car tries to surge forward.any ideas? 1989 with 5liter 302 engine

Hmm, A bit away from my area but check the engine temp sensor. What has your fuel economy ben like? just a thought.

just started today so cant speak to mileage. not sure it fits with surce during braking

I think that you probably need a new brake booster or perhaps just a booster check valve.

Sounds interesting but I don’t know how to connect a leaky booster to the surging in engine speed. Can you explain it for me? also what is a booster check valve and what does it do?

It lets more air into the engine, just like opening the throttle would. Since your 89 does not have a MAF, the PCM responds to the extra air by putting extra fuel in, so the engine surges.

OK one more piece of info. started the car cold , waited 10 seconds for idle to even out, stepped on brake pedal and no surge. does the engine temp. change the results?

Another possibility is that the idle air bypass valve may be sticking.

The engine runs richer when cold so there is more fuel going in to begin with. I can’t tell you whether or not that can cover up a vacuum leak. But, in essence, a leaking brake booster amounts to a vacuum leak - hence the surge.

In you original post, which was very unclear and not exactly brimming with info about the car’s state of maintenance, you wrote: “Engine idle varies while foot on brake” That sounds to me like a leaky brake booster. You also mentioned the connection of weird running with brake application.

OK makes sense Ill mentin it to the mechanic when I take it in. Cars in excellant shape no rust no dents good interior and always serviced, Thanks for the help I know it’s late there only 8 here. any idea of cost to replace