Hesitation on WOT - Volvo 850 T5

jbcarpentier · Post by **jbcarpentier** » 20 Jan 2026, 14:03

Hello everyone,

I’m new to the forum, although I’ve been reading it for quite some time. I’ll briefly introduce myself (I’m not sure if there is a dedicated introduction section, so please let me know if there is!).
Last December, I bought a Volvo 850 T5 Estate. It’s an Italian-spec, automatic transmission model equipped with the B5204T engine. The car is olive green with a black leather interior and currently has 297,000 km. I purchased the car not running, without knowing the exact cause of the failure.

At this point, the car runs well overall and makes no abnormal noises. When I bought it, it would not hold idle, and I found several vacuum hoses with holes. I replaced the entire PCV system, which restored a stable idle.
However, I now have another issue. When I accelerate at wide open throttle, the engine will not rev above 3,000 rpm and feels like it is hesitating or misfiring. This problem does not occur when accelerating at half throttle or even three-quarters throttle.
The CEL is on, but no codes are readable with a scan tool except P0120 that I erase and it never comes back. This CEL seems to be related to a fault in the automatic transmission. Additionally, the TRACS OFF and ABS lights come on intermittently, but I don’t believe they are related to my current issue.

I have replaced the spark plug wires, distributor cap, distributor rotor, and spark plugs. This did not solve the problem, although the old components were clearly worn. I’ve read on this forum that many people recommend using OEM Volvo spark plugs, so I’ve ordered a new set of genuine plugs.

I have carefully checked all vacuum lines: no tears, and none of them collapse. I tried unplugging the MAF, with no change. I also tried unplugging the idle air control valve as well as the TPS, without any improvement. I’ve also read that the CBV could be a possible cause of this type of issue.

When accelerating, the wastegate actuator moves correctly. However, I can’t check whether the opening pressure is correct since I don’t have a boost gauge. It would also be worth checking the fuel pressure, to make sure the fuel pump and fuel pressure regulator are operating correctly.

I’m new to turbo engines. I’m not new to mechanics, but I’m far from being an expert.

I would like to avoid buying parts at random (I’m a student, so budget matters) in hopes of finding the issue, which is why I’m asking for your help.

Thank you very much for any assistance you can provide.
Best regards,
Jean-Baptiste

Post by **matthew1** » 20 Jan 2026, 16:20

Hello Jean-Baptiste, welcome to MVS.

I'd say it's a collapsing hose or this viewtopic.php?p=120369#p120369 - the big intake hose slipping off.

jbcarpentier · Post by **jbcarpentier** » 21 Jan 2026, 11:03

Could a collapsing hose or an air leak affect the car’s behavior even at 3/4 throttle? I should note that the car has full power at 3/4 throttle and only shows the issue at full throttle.
Also, wouldn’t a large air leak only reduce the car’s power rather than cause misfiring? These are just my assumptions, so please feel free to correct me if I’m wrong!

Post by **ZionXIX** » 21 Jan 2026, 22:44

jbcarpentier wrote: ↑21 Jan 2026, 11:03 Could a collapsing hose or an air leak affect the car’s behavior even at 3/4 throttle? I should note that the car has full power at 3/4 throttle and only shows the issue at full throttle.
Also, wouldn’t a large air leak only reduce the car’s power rather than cause misfiring? These are just my assumptions, so please feel free to correct me if I’m wrong!

You could operate the throttle from the engine bay while watching for any collapsing hoses.

Post by **hausmeister** » 22 Jan 2026, 11:50

While I had some similar issues, can't say. Could still be a boost or air leak or a hose...
If its misfiring then maybe something else.
You could start with putting in a (mechanical) boost gauge to see what is really happening. The stock one isn't much helpful as its also slow to respond while not showing any scale. Maybe pressure test the intake, kits for that aren't that expensive.

Does it rev cleanly through the band when not at WOT? how's the boost looking when you do that? Can you slowly feed it the beans to WOT at higher revs or does it always fall on its face?

Post by **matthew1** » 22 Jan 2026, 16:22

Feel around the big manifold air intake hose (blue, in my case because it's a Snaab aftermarket). Mine was barely off, just a sliver of lip off the intake, but the vacuum at part throttle made the opening to ambient much bigger.

: IMG_1769.jpeg (276.51 KiB) Viewed 265 times

Post by **jreed** » 24 Jan 2026, 06:14

Lack of power at wide open throttle could be due to insufficient fuel flow. Many years ago up north in Connecticut, I had water in the fuel and in the fuel filter and it froze during very cold weather. The car would idle OK but not accelerate. Once it warmed up, the car ran OK again and I put isopropyl alcohol in the fuel system to absorb the water and replaced the fuel filter just to be on the safe side.

Cees Klumper · Post by **Cees Klumper** » 31 Jan 2026, 04:29

Could the TCV be failing?

Post by **hausmeister** » 31 Jan 2026, 14:48

Is the video new or did I just miss that back then... sounds strange.
Maybe the turbo is chugging/fluttering or something... you sure all the turbo piping (vacuum hoses etc) are on correct?