1998 S70 T5M 240k the Road to Stage 0

[abscate]

I was left hanging for 21,days; now I am 15 cm taller

[WhatAmIDoing]

I'm back again after disappearing down the rabbit hole for 2 month (had to double check I wasn't still down the rabbit hole once I reemerged). Took the T5 out for a 4 hour marathon down windy back roads. All went well until I was heading back home. Pulling out of the drive-through, the idle got really rough and started bouncing all over between 400 and 1500 rpm. Engine would hesitate when I tried to pull away from traffic light. All seemed good once I was above 2k rpm, so I tried to hold it there whenever I stopped, or else it would nearly stall. When I got home, I put the car in neutral and checked for vacuum leaks, but I didn't hear or see anything obvious like I was suspecting. By now the idle had settled down, but it was still fluctuating by about 100-200rpm. When I would step on the brake pedal, the idle would fluctuate slightly, but whenever I released the brake, the rpms would shoot above 1200 then come back down to fluctuating in the 'normal' 800 range.

Other details that may be pertinent to the incident: When it started I was in city stop and go traffic. It had just started raining heavily. I had the A/C on, but turned it and the blower off when it started and the idle did not improve. Scan tool found 3 codes but they were for the disconnected rear O2 sensor and disconnected purge valve. Engine would hesitate initially on acceleration, and would nearly stall out when coming to a stop. When I was checking for smoke at the tailpipe, it sounded like the fuel pump cut out before the idle stumbled and came back on just before the idle came back up.

My thoughts: large vacuum leak, possibly in the brake booster circuit? Low fuel pressure, possibly due to failing fuel pump or yet another failed fuel pump relay (similar symptoms to my last run in with one)?

Thoughts?

[abscate]

Large induction leak. You popped something off , probably using those cheap,clamps like I did.

[bmdubya1198]

Agreed, this sounds like a vacuum leak. Might be worth checking fuel pressure anyway, it’s easy enough.

[WhatAmIDoing]

So, thought I was chasing an induction leak, but this is weird. Getting intermittent rough idle. Worse with A/C on, but I think that's more to do with the extra load on the engine. Seems to pop up after long drives at moderate to low speeds, after idling for more than a few minutes, and after moderate periods of stop and go traffic. Also only seems to happen when in closed loop (but it only happens when hot so makes sense). I think I need to rig up a fuel pressure gauge, because I have a gut feeling I've buggered another fuel pump relay. Or maybe I just need to clean my IAC. Also need to check and regap my spark plugs so might try a hot compression test. I have a spare relay from the parts car, but symptoms never show up when it's a convenient time to swap it.