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Poor manifold vacuum 355

Discussion in '348/355' started by Russ Gould, Mar 6, 2018.

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  1. Russ Gould

    Russ Gould Formula 3

    Nov 8, 2004
    1,073
    #1 Russ Gould, Mar 6, 2018
    OK so I have been told that my car won't idle smoothly because there is not enough vacuum to work with the (aftermarket Motec) ECU. First the theory was that some prior mechanic had advanced the intake cams 5 degrees. When I pointed out that I paid the current shop to inspect the motor and more specifically the valves and belts, and to do a leakdown test before installing the motor, the story changed. Now the theory is that the intake manifold is leaking "around the shafts, a common problem with these motors". "We did nothing to the intake manifold yada yada". I have never heard that before. Is the 355 prone to manifold leaks? Or is there likely some other explanation?
     
  2. SoCal1

    SoCal1 F1 Veteran
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    Jun 14, 2011
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    Tim Dee
    #2 SoCal1, Mar 6, 2018
    Yes they leak quite a bit, last 3 I checked did exactly like this. TB shafts, plenums. These things are 200 years old it's more likely then not it's sucking

    First thing I do before tuning a ecu is be absolute that it has no intake or exhaust leaks as they will cause too much overlap and bad signals to the ecu to properly work

     
  3. INTMD8

    INTMD8 F1 Veteran
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    Jun 10, 2007
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    Lake Villa IL
    #3 INTMD8, Mar 6, 2018
    A vacuum leak wouldn't effect map sensor reading unless it was such a big leak that it could no longer achieve idle speed without severely retarding the timing.

    (and of course it would only be from the throttle blades to the intake valves. Plenums leaking wouldn't effect the map sensor reading/vacuum.

    What is it reading for manifold pressure at idle and part throttle? If this is just an idle problem, certainly wouldn't matter to the Motec if it has little vacuum if tuned properly.
     
  4. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    9,724
    #4 Mitch Alsup, Mar 6, 2018
    Where are you taking your vacuum from?
    Hint:: you don't take it from the plenum, you have to take it from the lower portion of the throttle body.
    That is, there is only vacuum AFTER the throttle plate and the throttle plate in an F355 is under the plenum (not before).
     
  5. yelcab

    yelcab F1 World Champ
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    Nov 29, 2001
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    Mitchell Le
    #5 yelcab, Mar 6, 2018
    Let me explain what Russ is facing.

    He has a 308 body into which a 355 engine (2.7 Bosch version) was installed. The computer is a non-stock Motec unit with infinite variability. The donor engine is now in the recipient car and it has two problems:

    1) It does not hold a steady idle.
    2) The vacuum assist brake does not work.

    In general, we can say that the 355 engine does not generate very high vacuum due to too much valve overlap.
    ​
    On item 2, we know that the vacuum on the 355 motor cannot drive the brake booster. That is why on the later Bosch brake system, there is a secondary vacuum pump attached to the cam shaft on bank 2 to drive the brake booster.

    On point 1, the mechanic who did the install did not fully inspect the condition of the motor before the installation. He is trying to explain why the motor does not idle well by giving excuses that the excessive vacuum leak on the motor is causing the Motec ECU to not idle well.

    ​
    Russ does not have possession of the car and likely cannot answer very specific questions such as where is the point at which the Motec ECU is sampling the vacuum.
     
  6. INTMD8

    INTMD8 F1 Veteran
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    #6 INTMD8, Mar 7, 2018
    I guess we will have to wait for more info but if it had such a large vacuum leak it would be idling very high.

    Russ, if possible get a screen shot of the Motec at idle.
     
  7. Russ Gould

    Russ Gould Formula 3

    Nov 8, 2004
    1,073
    #7 Russ Gould, Mar 7, 2018
    I was told the motor pulls 67 at idle on the motec scale 0-100 where 100 is ambient. I am guessing the motec has a vacuum sensor and you can see the reading via the interface. This is above my level of technical knowledge. I have spent a ton of money on this project (enough to buy a two 355s), including a hefty chunk on replacing the stock ECUs with a motec (not my idea, I was railroaded into that solution), developing the map, and the brakes, and neither are working right. Suffice it to say the "mechanic" and I are not exactly the best of buddies at this point.

    So as far as solutions, I have been told (at various times) that I need to pull the motor to check/reset the cam timing; and/or to install an aux electric vacuum pump; and/or to pull the inlet manifold to seal the leaks. Which of these would you guys try first?
     
  8. INTMD8

    INTMD8 F1 Veteran
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    #8 INTMD8, Mar 7, 2018
    Last edited: Mar 7, 2018
    Can any of you obd2 guys scan your cars and check idle vacuum? 67kpa seems lower vacuum than I would expect but could be right all things considered.

    "It won't idle smoothly"

    Does that mean it won't hold a specific idle rpm, as in, it's fluctuating up and down or sounds rough/misfire at idle?

    I would take a video of how the engine sounds idling and of the Motec software at idle. With that I could give you a recommendation.

    Not sure why the suggestion of an aux electric vacuum pump was brought up? That would never be connected to the intake tract of an engine, would only be used to power vacuum assisted accessories if the engine did not create enough vacuum, not to fix a running condition.
     
  9. Russ Gould

    Russ Gould Formula 3

    Nov 8, 2004
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    #9 Russ Gould, Mar 7, 2018
    It's rough unless you get the revs up around 1500 then it smooths out. Supposedly has good top end but I can't verify that, the one time I drove the car when I put my foot on the brakes at the first T junction doing 15 mph, nothing happened, nearly wrecked the car. So I turned around and took the car back to the shop which was on a quiet road. On the way I encountered a speed bump. I pumped the brakes, one of the rears locked up and the rear end tried to swing around. I was not braking that hard. Was told that some valve in the brake lines was sticking. This car has stock 355 brakes on the rears, and stock 308 brakes on the fronts. Stock 308 brake booster.
     
  10. Russ Gould

    Russ Gould Formula 3

    Nov 8, 2004
    1,073
    #10 Russ Gould, Mar 11, 2018
    Mechanic also rigged up an exhaust flap for the third pipe (after initially skipping this valve and discovering a flat spot in the power delivery), using engine vacuum as the means for controlling the flap. I guess at high vacuum flap stays closed. When you open up throttles it opens to increase exhaust flow and alter the harmonics. Anyway now I am wondering whether this wasn't another mistake that compounds the low vacuum problem. Theoretically, I suppose provided the circuit does not leak it should not alter the static vacuum level. But I wonder how you get it to work with insufficient vacuum. (This is not the stock flap, as we did not use the stock muffler ... does the stock flap use vacuum?)
     
  11. yelcab

    yelcab F1 World Champ
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    #11 yelcab, Mar 11, 2018
    If you are asking about the bypass valve, then it uses the engine vacuum. Except that the engine charges a vacuum canister, which then feeds the vacuum to the bypass valve by a vacuum switch which gets the signal from the computer whenever the CPU wants the valve open.

    If he does not have the "when to open" logic in his Motec then it may be a problem.
     
  12. Russ Gould

    Russ Gould Formula 3

    Nov 8, 2004
    1,073
    #12 Russ Gould, Mar 11, 2018
    That makes sense so the flap opening or closing does not alter the manifold vacuum and cause the controller to try to compensate. I am not sure whether he wired this into the Motec logic, or just did a dumb install. Will have to look into that. Since this motor does not have the aux vacuum pump, I wonder whether there is enough vacuum to charge the canister if there is one. I have a feeling we are going to have to redo a lot of work on this car.
     

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