News

246 BRAKE -VACUUM PROBLEM

Discussion in '206/246' started by DINOMR, May 7, 2019.

  1. DINOMR

    DINOMR Rookie

    Aug 6, 2017
    9
    CAN LOW COMPRESSION IN A 246 MOTOR WITH HOT CAMS CAUSE BRAKE PEDAL FADE,OR ARE THERE OTHER MORE OBVIOUS CAUSES ?
     
  2. synchro

    synchro F1 Veteran

    Feb 14, 2005
    8,922
    CHNDLR
    Full Name:
    Scott
    Does this feel like a brake fluid bubble?
     
  3. pshoejberg

    pshoejberg Formula 3
    Silver Subscribed

    Dec 22, 2007
    1,066
    Denmark
    Full Name:
    Peter H
    Leaking vacuum hose, Leaking booster diaphragm, leaking seal between booster and brake cylinder or hanging one way valve in vacuum line.

    Best Peter
     
  4. DINOMR

    DINOMR Rookie

    Aug 6, 2017
    9
    Thanks Peter and Scott
    The motor runs great with lots of top end power despite low compression in some cylinders
    It was only rebuilt 3,000 miles ago .
    From your answers -I presume the brake pedal fade is not from this low compression causing lack of vacuum ??
    The brake fade only commenced after the callipers were rebuilt and new pads were installed.
    I have been advised that the engine requires another rebuild to create enough vacuum for the brakes.
    Does this sound correct?
     
  5. DinoLasse

    DinoLasse Formula Junior
    Silver Subscribed

    May 26, 2009
    525
    Sweden
    Full Name:
    Lars
    No, that does not sound correct at all. And that is putting it politely.
    I don´t claim to be an expert in this matter, but in my experience, even an engine with a miserably low compression will still have enough pumping action to generate sufficient vacuum for the booster.

    But I think we have a problem with terminology here. Brake fade normally refers to braking distances getting longer and longer after repeated hard braking. Brake pedal fade as a description, is not so clear. I believe that what you are experiencing is a soft, spongy feel in the brake pedal, coupled with poor braking power. Is that correct? If so, it probably has nothing to do with the vacuum boost, it is a problem with the hydraulic system. Air in the system is the most likely cause, since you mentioned that you just had the calipers rebuilt and new pads installed.

    It sounds like you should go back to the people who did the brake work and tell them to fix the problem they have caused - instead of giving you you silly excuses about engine compression.
     
    Nuvolari likes this.
  6. Nuvolari

    Nuvolari F1 Veteran
    Sponsor Owner

    Sep 3, 2002
    5,074
    Toronto / SoCal
    Full Name:
    Rob C.
    If the problem started after the brakes were re-built then the issue probably lies in the braking system and not the engine. Start with the simple stuff first:

    1. Make sure the brake master cylinder is bench bled FIRST. These master cylinders are difficult to get bled through the calipers so you need to make sure they are properly bled.
    2. With the master bled then properly bleed the braking system and road test the car.

    If there is still no resolution you can have the brake booster checked and check all the lines to the brake booster although I would say this is unlikely.

    Lastly if the brake pads you are using are of a very different compound than before then this can greatly alter the braking feel.

    I would do all these things first and greatly doubt any issue with vacuum causing your braking issues.
     
    DinoLasse likes this.
  7. racerboy9

    racerboy9 Formula 3
    Silver Subscribed

    Nov 3, 2003
    1,825
    I would be more concerned that you have low compression on a newly rebuilt and run-in engine. I would have whoever rebuilt it do a leak down test and while they are fixing that have them look at your vacuum/brake problem.
     
    dm_n_stuff likes this.
  8. TonyL

    TonyL Formula 3

    Sep 27, 2007
    2,470
    UK
    Full Name:
    Tony
    Vacuum is created in the induction manifold and inlet valve recession can cause a drop in vacuum and cause it to pulsate rapidly.
    Taking compression readings can be hit and miss if using the hand held tester which relies on a strong hand to hold back 160psi.
    I would guess, as other have, that they haven't bled the brakes properly, they do need a few attempts at securing a good pedal, cheap pads can give you a fade in performance if braking from high speed.
     
  9. Bluebottle

    Bluebottle F1 Rookie
    Silver Subscribed

    Oct 15, 2012
    4,617
    Exeter and Cirencester
    Full Name:
    John
    That would seem to be the key to your problem.
     
  10. DINOMR

    DINOMR Rookie

    Aug 6, 2017
    9
    Thanks everyone for your expert advice- very much appreciated
    Lars- the brakes work reasonably on initial application then on repeated immediate pushing of the pedal the power of brakes fades significantly
    The theory proposed to me was that the hot cams are causing loss of vacuum and also combined with the low engine compression and that the remedy is to replace the hot cams with standard cams and then likely also rebuild the engine.
    The brake pads that came with the car were racing pads and worked well but after the calliper rebuild - poor braking ensued and these were then changed to standard pads with no improvement in braking .
    Has anyone ever seen a lower compression motor (that runs very well with good power) cause lack of vacuum for the brakes ??
     
  11. Rory J

    Rory J Formula Junior
    Rossa Subscribed

    May 30, 2006
    826
    WA
    Full Name:
    Rory
    Agree with others that if the problem started after a caliper rebuild, your issue is likely localized to the brake system itself.

    If you aren't able to go through the brake system yourself to find the trouble, it sounds like you may need a new mechanic with fresh eyes to look things over. I had an experience once where I kept a car at a mechanic that didn't know what he was doing for far too long, throwing good money after bad on a relatively simple cooling system issue. Where are you located? Folks near you may be able to suggest someone.
     
  12. Ken Ivey

    Ken Ivey Karting
    Silver Subscribed Rossa Subscribed

    Jan 6, 2013
    104
    Portland, OR, USA
    Full Name:
    Ken Ivey
    I know it wasn't the original question, but given your multiple references to low compression on a recently rebuilt engine, I have to agree with racerboy9 - there is no reason for that engine should not to have excellent compression. By 3,000 miles everything should have seated and be running properly. Get a second opinion on the compression also.
     
  13. pshoejberg

    pshoejberg Formula 3
    Silver Subscribed

    Dec 22, 2007
    1,066
    Denmark
    Full Name:
    Peter H
    As mentioned by Rob insufficient bleeding is most likely the culprit. The master cylinder and the rear brake force regulator valve is notorious for being difficult to bleed and air in these components will give you a spongy feeling. Both components should be bench filled before assembly in the car and first hereafter the system should be bled. Good luck.

    Peter

    Brake regulator valve
    Image Unavailable, Please Login Master cylinder: Image Unavailable, Please Login
     
  14. DINOMR

    DINOMR Rookie

    Aug 6, 2017
    9
    Thanks Peter ,Ken and Rory
    The motor had 2 independent compression tests with very different results
    The mechanic has also disconnected the crankcase ventilation pipe to stop oil
    re entering the inlet manifold and fouling the plugs
    Could this also have an impact on the vacuum created for the brakes??
     
  15. racerboy9

    racerboy9 Formula 3
    Silver Subscribed

    Nov 3, 2003
    1,825
    You need a leak down test as well. Compression test tells you which cylinder is low. A leak down test tells you why it is low. You shouldn't have to disconnect that crankcase vent pipe. You could always use a vacuum gauge to see how much vacuum you have.
     
  16. DINOMR

    DINOMR Rookie

    Aug 6, 2017
    9
    Thanks Peter ,Ken and Rory
    The motor had 2 independent compression tests with very different results-
    1-Range 150 -120 ,one cylinder 105
    2-range 120
    The mechanic has also disconnected the crankcase ventilation pipe to stop oil
    re entering the inlet manifold and fouling the plugs
    Could this also have an impact on the vacuum created for the brakes??
     
  17. DinoLasse

    DinoLasse Formula Junior
    Silver Subscribed

    May 26, 2009
    525
    Sweden
    Full Name:
    Lars
    No, it will not affect the brake assist vacuum. There is a thin vacuum line connected with the crankcase breathing system, but It is taken off of a different intake runner than the brake assist vacuum.

    But now we are talking about several different problems in several different areas of the car. Things are getting very confusing. If I were you, I would try to concentrate on the brake system first, and then continue step by step until the source of the problem is found.

    Here is how I would go about it:

    First I would try to determine whether the problem is in the hydraulic system, or the vacuum assist.
    You should be able to tell by the feel of the brake pedal. With the car stationary and the engine off, press as hard as you can on the brake. Does the pedal go down further than it did before the brake repair? Does it feel softer, more spongy than before? Does it go close to the floor under max pressure? When you keep the pressure on, does the pedal slowly sink? If yes to any of those questions, you have a problem in the hydraulics, as suggested by Rob and Peter above. It is after all the most likely scenario, since the system has been worked on. In that case, have the mechanic address those problems.

    But if the answer is no to all, and the pedal is rock solid and consistently stops at the same level, and all four wheels pull evenly under braking, then the hydraulics can be pronounced healthy. Preferably, that should also be confirmed by a second mechanic to be on the safe side.

    After you are 100% certain that the hydraulics are perfect, then you can move on to the vacuum assist.

    If the vacuum assist seems leaky and weak, I would first consider all the points mentioned by Peter in post #3 above. Those are much more common than the far-fetched theories about engine vacuum. Vacuum is easy to measure and leakages should be easy to trace for your mechanic. He can also connect an external vacuum pump to verify that the booster is OK.

    Then, in the unlikely event that the vacuum measured directly at the engine is close to zero, I would first check for a leak at the manifold (O-rings, etc.) Finally, if that cylinder is still not producing any vacuum to speak of, the engine internals have to be diagnosed. But only as a last step.
     
    pshoejberg likes this.
  18. DinoLasse

    DinoLasse Formula Junior
    Silver Subscribed

    May 26, 2009
    525
    Sweden
    Full Name:
    Lars
    I just wanted to add that if the mechanic has removed the thin vacuum line associated with the breather system, one has to assume that he has plugged the open connector. But even if he has not, it will not have any effect on the brake vacuum. That was my point.
    Also, it makes no sense whatsoever to disconnect the crankcase breathing system. It does not cause plug fouling. If there is a plug fouling problem, that should be dealt with as a separate issue. Also in this case, a second opinion from another mechanic would be highly recommended. Just my opinion. I hope that helps and I wish you luck with the process.
     
  19. DINOMR

    DINOMR Rookie

    Aug 6, 2017
    9
    Lars thank you again very much for your help and advice
    Will ask the mechanic to follow up according to your list of investigations .
     
  20. swift53

    swift53 F1 Rookie
    Rossa Subscribed

    Nov 17, 2007
    4,483
    E.S.
    Full Name:
    Alberto
    Potentially, and I am not saying it is a truism, might it be possible that the mechanic that is looking
    after your car, is not too familiar with Dino?

    Regards, Alberto
     
  21. DINOMR

    DINOMR Rookie

    Aug 6, 2017
    9
    Thanks Alberto - the mechanic is very experienced with Dino’s !
     
  22. TonyL

    TonyL Formula 3

    Sep 27, 2007
    2,470
    UK
    Full Name:
    Tony
    All pretty poor, 160psi is a good range to be in, you say your mechanic has disconnected the CVP, if you mean the large pipe attached to the airbox then you have serious problem with excessive crankcase pressure if oil is entering via this pipe, I take it you haven't bypassed or removed the condenser!

    If you are referring to the small 5mm tubes from the induction manifold to the small limiting valve tee piece on airbox then remove them anyway and blank off the 3 no induction tubes at the manifold (I use a small piece of hose with a blanking plug in one end, don't silicone them whatever you do!) leaving them open you will get rough running and idle + loss of vac!

    Plug fouling can be lots of things and you could have multiple issues such as bad carb setup, it may accelerate OK but that really proves nothing etc. More info needed?

    Vac is created in the induction circuit so if your car runs "pretty good" then you will have enough vacuum for the brake booster that's unless you have a serious vac leak. Disconnect and check the vac line to see if you have brake fluid in the pipe that connects onto the inlet manifold, that could indicate a failed master cylinder and leaking (internally) into the brake servo.

    Why hot cams?

    Tony
     
  23. racerboy9

    racerboy9 Formula 3
    Silver Subscribed

    Nov 3, 2003
    1,825
    Any chance the vacuum check valve is in backwards? There is an arrow on it.
     
  24. synchro

    synchro F1 Veteran

    Feb 14, 2005
    8,922
    CHNDLR
    Full Name:
    Scott
    Be careful of compression tests, they are not all the same and should be taken with a grain of salt.
    For example, If you do not open the carb' butterfly the reading can be skewed.

    I saw The Corbani car fail a compression test in one cylinder, spun it and got a solid reading - thought possibly it was a carbon particle obscuring sealing.

    I get the impression your mechanic wants to start replacing the most expensive parts first ....

     
  25. DINOMR

    DINOMR Rookie

    Aug 6, 2017
    9
    Thanks Tony /Scott and Racerboy9,
    I believe it is the crankcase ventilation pipe that was disconnected - I don’t know whether the condenser has been bypassed.
    The hot cams were put in by a previous mechanic when the engine was rebuilt -not my choice.
    Will have the points you suggested checked and let you all know the outcome.
    Thanks again
     

Share This Page