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      03-26-2023, 12:56 PM   #1
grannyknot
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Something strange going on with acceleration

So the car is back together and took the car out for the first long drive today.
I've found that while driving if I put my foot to the floor as soon as I get to around 4000 rpm the engine will buck, there is a complete loss of acceleration, then the power comes back on just as suddenly.
With my foot still to the floor the revs will continue to climb up to about 5000rpm then it will buck again.

If I let the accelerator up a bit I can sneak the revs right up to the red line but to the floor and it bucks violently.

This winter I rebuilt the vanos completely with all new Beisan seals, it is the 3rd time I have rebuilt a vanos for S54 engines so I'm as sure as I can be that I reassembled it and timed it properly. I haven't done any of the vanos testing procedures as I don't own that equipment. YET

The engine didn't do this bucking last year before I tore it all down.
A buddy suggested it sorta sounded like the vanos so that is my working theory but I'm open to any suggestions.
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      03-26-2023, 02:26 PM   #2
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Vanos rebuild was last work performed prior to taking it out? Almost sounds like a MAF issue, any codes?
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      03-26-2023, 03:20 PM   #3
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Quote:
Originally Posted by 3002 tii View Post
Vanos rebuild was last work performed prior to taking it out? Almost sounds like a MAF issue, any codes?
I rebuilt the entire car, the vanos was just one of the many systems that were overhauled.
No real codes, there is a transient air leak detected in the EGR system but it comes and goes occasionally with no pattern.
The maf sensor is new and I have had no codes relating to it so far.
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      03-26-2023, 03:44 PM   #4
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I would log the car using testo or Severn’s Flash App and see what sensors report or see the hiccup. But sounds to air and / or fuel related. But logging is your next best bet. Hopefully is something obvious that can be diagnosed with logging or throws a DTC to have cues to pinpoint the issue

I would also strongly suggest using INPA and ISTA as generic scanners won’t catch all errors.
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      03-26-2023, 04:10 PM   #5
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Quote:
Originally Posted by maupineda View Post
I would log the car using testo or Severn’s Flash App and see what sensors report or see the hiccup. But sounds to air and / or fuel related. But logging is your next best bet. Hopefully is something obvious that can be diagnosed with logging or throws a DTC to have cues to pinpoint the issue

I would also strongly suggest using INPA and ISTA as generic scanners won’t catch all errors.
Funny you should say that as I just did a data logging session today using Severn's version of Flashapp.
I haven't sent Josh the file yet because I thought I should get this accelerator foot to the floor bucking problem solved first.

I just tried to upload the file on to this post but I'm getting Invalid File notices. I guess it's too big.
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      03-26-2023, 05:47 PM   #6
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I dealt with a similar issue recently. Complete loss of power past 3k and the engine was idling extremely rough. There was a code stored for vanos timing being over retarded. Apparently the vanos solenoid pack is a common issue on s54. Beisan does sell refurbed units which are a lot cheaper than buying new. Repair procedure also involves thoroughly cleaning the solenoid body. After doing that, all the power came back.
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      03-26-2023, 06:38 PM   #7
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Quote:
Originally Posted by grannyknot View Post
Funny you should say that as I just did a data logging session today using Severn's version of Flashapp.
I haven't sent Josh the file yet because I thought I should get this accelerator foot to the floor bucking problem solved first.

I just tried to upload the file on to this post but I'm getting Invalid File notices. I guess it's too big.
You can do some basic analysis yourself. Open the log in excel using auto delimited feature, then by looking at the obvious parameters identify the time stamps at which bucking happened.

I would look at fueling, calculated load, mass flow, knock, there are more parameters you can log, i would add injector duty cycles.

If you want a more pragmatic approach then unplug the maf and see if the issue remains. Also ask Andrey to help with testing VANOS with ISTA.
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      03-27-2023, 05:48 AM   #8
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My car did that after I had the intake off. I used narrow screw clamps to attach it to the ITBs; one of them was interfering with the throttle actuating rods and not allowing full throttle. If I pushed the gas pedal down far enough it would buck violently; otherwise it ran smoothly. I rejiggered the clamp to allow the ITB actuator to have full range of motion and the problem was gone.
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      03-27-2023, 08:11 AM   #9
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Quote:
Originally Posted by wdb View Post
My car did that after I had the intake off. I used narrow screw clamps to attach it to the ITBs; one of them was interfering with the throttle actuating rods and not allowing full throttle. If I pushed the gas pedal down far enough it would buck violently; otherwise it ran smoothly. I rejiggered the clamp to allow the ITB actuator to have full range of motion and the problem was gone.
I remember when you warned me about that the first time and I had checked to see that there was no interference but I have had the intake plenum off many times since then.

I just went out to the garage and checked again and there was a small interference between the black plastic ball of the centre linkage and the hex head of the jubilee clamp right at the end of the linkage travel.
So in other words right where the linkage ball goes with "foot to the floor" acceleration.

Not much of an interference, I could easily push the linkage past it but the throttle actuator motor might be more sensitive to resistance than my finger.
And now that I think about it I remember reading somewhere that the throttle actuator won't fully open the butterfly plates until about 4000 rpm right where the bucking occurs.

This is looking like the smoking gun but I won't be able to test it until tomorrow, it is snowing at the moment.
Fingers crossed, a mechanical fix is so much nicer to deal with than a ghost in the machine.
Thanks
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      03-28-2023, 08:35 AM   #10
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Success! The problem was the throttle linkage bumping into the boot clamp.
Bucking is all gone.
Thanks wdb, I'm sure I would have spent weeks farting around trying to solve it before it occurred to me to check the linkage.
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      03-28-2023, 08:52 AM   #11
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Excellent news!
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      03-28-2023, 03:41 PM   #12
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Great news! You probably have some codes to clear...
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