Erratic ETM apparently connected to loss of brake assist

[helmbelly]

Quick backstory: 2001 XC70 190K miles, I cleaned the ETM three years ago and the idle has been steady ever since. I started noticing a surging a few months ago when in Drive but at a stop with foot on brake, like she wanted to go. Eventually it would start but run very rough, coughing and misfire- and here is the unique part- I would lose all brake boost. Hard pedal.

So I'd turn engine on and off and she'd start up correctly and run fine, brakes working normally. Got worse to the point I'd have to cycle the engine on and off 20 times to get a "good" start. Then it drives fine until its shut off and next start becomes a lottery even if its just off a couple mins.

it throws error P0121 TPS, so I'm looking at either 1. a contactless kit on ebay or 2. an OEM Bosch TPS replacement which seems far simpler. I only expect 70K of a replaced sensor but I can live with that if it works.

But what I dont understand is those brakes - losing power assist - I mean I know that the ETM controls the air mixture but losing vacuum seems strange. They are linked bc when it starts well the brakes are always working - brakes work= engine runs. Brakes don't work= engine cough and misfires.

Appreciate any thoughts

Caleb
New Orleans
2001 XC70 190K

[abscate]

You caught it exactly. The ETM controls air flow. When the engine runs roughly due to loss of synch between the air fuel mixture, excess air means low manifold vacuum. Low manifold vacuum means no power brakes.

On next years model, in 2002, Volvo installed a brake vacuum booster to overcome this