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Intermittent Hesitation At Low Rpms on 1998 Volvo S90

S90 vacuum line - intermittent hesitation

Finding the cause of the intermittent hesitation

bloodrootfc says:

“After getting fed up with “try this” from 4 mechanics, I started doing my own diagnostics with a Torque app and scan tool, and calling the shots. My mechanic works faster than I do, and he does a good job so I tell him how to spend his time, rather than do the repair myself.

What I have discovered from monitoring the intake manifold vacuum with my scan tool is that I have a high vacuum at idle (24-25 mm Hg as opposed to a healthy 22 mm). WOT test confirms this is not a compression issue, quite the opposite actually. The vacuum drifts up and down 1 mm with the rpms at idle, which is indicative of the ecu trying to reconcile an improper fuel/air ratio. The only other explanation is:


“I should clarify, the problem I have is intermittent hesitation at low rpms. It only happens after coming to a complete stop, and only 5% of the time. I also get “System Too Rich” code 100% of the time whether there is a noticeable hesitation or not. The knock sensors I had installed made my engine hum nicely at idle, but didn’t fix the problem. They checked the function of the ignition coils, fuel pressure, and transmission shifting. All good.

This morning I finally diagnosed a problem: I used my Torque app and scan tool to graph the front oxygen sensor through time. I have determined that the oxygen sensor is being affected by the application of the brakes. If I continuously pump the brakes in and out, I can make the front O2 stay lean (<0.1). After releasing the brakes, the system compensates by flipping too rich for the same period as the lean condition was detected, before returning to normal oscillation. Furthermore, I was able to log the exact same behavior when the hesitation occurred during a normal drive. I am having the brake booster system examined for leaks.”

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1998 S90 hesitates at low RPM