Pressing brake drops Idle speed a lot

GoToGuy · Nov 6, 2022

454cid said:
Early trucks could be vacuum, but his is a 2000.

So if your gonna say " early ", ya gotta be more definite.
Early as in my '86 K3500 has hydro boost, and does have it. The 1948 two ton has a huge vaccum boost can for brake boosting.

454cid · Nov 6, 2022

GoToGuy said:
So if your gonna say " early ", ya gotta be more definite.
Early as in my '86 K3500 has hydro boost, and does have it. The 1948 two ton has a huge vaccum boost can for brake boosting.

GMT-400

1998_K1500_Sub · Nov 7, 2022

454cid said:
Early trucks could be vacuum, but his is a 2000.

Yup. Our 1988 5.7 SRW K3500 has vacuum assist.

Airmanwoody · Nov 7, 2022

Be aware, that hydroboost taxes the power steering to add stopping power. So, when you stop the pedal hard at idle, it puts a higher load on the engine.
As long as it is only 50 rpm, I wouldn't be worried. You should get a similar drop if you hold the steering wheel against the stops.

I know my 05 Avalanche (had hydroboost) would drop a few RPM's if you pushed the pedal to the floor at idle. I don't think my diesel GMT-400's did, but don't have one to confirm with.

missouritig · Nov 7, 2022

Airmanwoody said:
Be aware, that hydroboost taxes the power steering to add stopping power. So, when you stop the pedal hard at idle, it puts a higher load on the engine.
As long as it is only 50 rpm, I wouldn't be worried. You should get a similar drop if you hold the steering wheel against the stops.

I know my 05 Avalanche (had hydroboost) would drop a few RPM's if you pushed the pedal to the floor at idle. I don't think my diesel GMT-400's did, but don't have one to confirm with.

I think im all good now. The leak out of my manifold from the evap line was basically soda straw sized. I think under load that caused the performance issues since I dont have a visible leak in the p/s system.

It drops from park just a few rpm. Before I plugged the leak it was dropping several hundred

Airmanwoody · Nov 7, 2022

I must've missed how big the drop was originally. Glad you found the leak and fixed it.

GoToGuy · Nov 7, 2022

It as an engine driven hydraulic pump, therefore when a load is applied depending on what's required at low rpm you may notice that load by rpm drop. Same if you activate every electric device installed, wipers, heater on high, high beams, seat heat, heated mirror's.

Pressing brake drops Idle speed a lot

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