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Old 06-06-2022, 04:44 PM   #1
intothelabyrinth
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Default 91 240se +t subtle power oscillations

I recently completed a +t on my 92 240se wagon. The thing seems to run great, and rips hard. Starts every time, and never even thinks about stalling. Great idle... I noticed yesterday that the power seems to slightly oscillate at moderate throttle inputs. Like, if I step on it it rips hard and I have no problem hitting almost 14psi. But when I'm cruising I can feel a constant, subtle, pulse that I see on the vacuum gauge as well. I am thinking it must be misfiring? I am going to test fuel pressure, and see if that is in spec, as well as really check over everything for vacuum leaks. What does that sound like to you guys?
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Old 06-06-2022, 05:32 PM   #2
ZVOLV
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Air leaks are likely. I'd do a smoke test.
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Old 06-10-2022, 09:03 PM   #3
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Smoke test yielded no leaks, not even brake booster or evap. Fuel pressure next. It seems to be getting worse. Just not an even power delivery.
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Old 06-11-2022, 01:04 PM   #4
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Have you upgraded the intank pump? The stock one can be a little weak to supply a turbo engine. Volvo spec'd a more powerful pump for the 740 turbos and you can install that in the 240. I think that's the ipd upgrade kit they sell.
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Old 06-12-2022, 09:47 AM   #5
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Nah, I haven't even touched the fuel system yet. Probably should do the filter and look in the tank. 740 turbo in tank pump is probably a good idea. I'm thinking it has to be fuel related.
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Old 06-26-2022, 12:44 PM   #6
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Ok so I finally got around to investigating this. Hooked up my homemade fuel pressure gauge and it's working great! Got a reading of 45 psi running pumps with engine off. With engine running I get 45 psi with vacuum to fpr disconnected and when I plug it up its rapidly bouncing from 35-38. This was a bit of an aha moment for me. My boost gauge has always fluttered and I thought it was just the gauge! But now I know that it is indeed engine vacuum fluttering, causing the fuel pressure to flutter as well. Under boost this goes away and it rips smooth because with no vacuum the fuel pressure is steady full blast!

This leads me to investigate the cause of my fluttering vacuum. I'm thinking, check timing and do a compression test? Maybe valves need adjustment, or my cam timing is off?
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Old 06-26-2022, 01:27 PM   #7
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It sounds like a misfire could be the issue.

How'd you perform the smoke test? A small airleak could cause you to go lean at cruise and misfire.

Maybe start with a good cap, rotor, plugs and wires if they haven't been done in a while.

Maybe try another MAF too.
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Old 06-26-2022, 01:28 PM   #8
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I forgot I had the boost gauge dampened. I took that off and it oscillates even under boost.
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Old 06-26-2022, 01:31 PM   #9
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I made this awesome smoke tester that puts pressurized smoke into a vacuum port.
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Old 06-26-2022, 01:34 PM   #10
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Im almost thinking that it could be a burned valve. The vacuum at idle is bouncing super fast.
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Old 06-26-2022, 01:35 PM   #11
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I'd be interested to see pics of smoke machine. If you are just going into a vacuum port, that sounds like you aren't really smoke testing the entire induction system. You may wanna do a good strong smoke test starting close to the turbo.

IC hoses leak commonly, idle air hoses leaks commonly at the ends of the hoses, soggy MAF intake hose can leak at the bottom, etc.

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Originally Posted by intothelabyrinth View Post
Im almost thinking that it could be a burned valve. The vacuum at idle is bouncing super fast.
Perform a leak down test.
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Old 06-26-2022, 01:35 PM   #12
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I'll check my plugs and see if they tell me anything. I can also do a compression test.
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Old 06-26-2022, 01:38 PM   #13
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I was putting 5psi of smoke into the evap port on the throttle body. It filled up the whole intake and started coming out the air filter. Now that I think of it, that wouldn't really put smoke out the intake manifold gasket would it! Do that again and open the throttle?
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Old 06-26-2022, 01:40 PM   #14
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Im gonna smoke test again. Any tips on where to plumb the smoke in?
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Old 06-26-2022, 02:00 PM   #15
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You could cap off the intake hose at the MAF, crack the throttle and try again. Maybe something will show up that didn't last time.

Another trick is spraying around with brake cleaner at idle and seeing if you can get the engine to stumble. On modern cars, I spray around with brake cleaner and monitor fuel trims with a scan tool to find air leaks. It works well.
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Old 06-26-2022, 04:13 PM   #16
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Pulled the plugs and they all looked fine and the same. Tested compression dry and got
Cyl 1 143
Cyl 2 150
Cyl 3 150
Cyl 4 146

Changed plugs to bpr7es and put on new wires.
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Leakdown test next. I'm really starting to suspect a burned exhaust valve. This had to have happened before I got the car because the bouncing vacuum deal was there from the moment I put the boost gauge on, which was before the +t was used at all. Maybe the po rode around with a vacuum leak forever?
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