It's gotta be a vacuum leak ... right?

Discussion in 'Workshop Forum' started by Huey, Jun 24, 2019.

  1. Huey

    Huey True Classic

    Location:
    Quakertown, PA
    My '87 is just not running right. Low RPMs it has no power and it backfires. Runs OK at high RPMs but it has a slight misfire (or what sounds like a misfire.) I'm thinking it has to be vacuum leak but for the life of me, I cannot trace it. I've checked all the vacuum lines and they are in tact. In fact, if I unplug them, I can definitely hear a difference in how it runs so I know they work.

    My next thought is unmetered air. I've tried the starting fluid trick but can't seem to locate it.

    Ideas?
     
  2. carl

    carl True Classic

    Location:
    Virginia
    Did you check the ignition timing?
     
  3. Huey

    Huey True Classic

    Location:
    Quakertown, PA
    I did the ignition timing. It is possible the adjustable cam pulley is not set right (I did the best I could) but backfiring with lack of power at low RPMs usually indicates vacuum leak or lean condition.
     
  4. JimD

    JimD Waiting for Godot... Moderator

    Location:
    Missouri, USA
    Hi Huey,

    Maybe check out this thread which seems similar. Earlier when I saw your post, I queried to Karl to see if he ever figured it out.
     
  5. kmead

    kmead Old enough to know better

    Location:
    Michigan
    I believe I have fixed it but will know better later this week when the heat pours in here. I cleared the vacuum port at the base of the throttle plate which was completely clogged. In general driving over the last month I have not had part throttle hesitation occurring.

    Likely not your issue.

    Removing the access panel behind the spare makes tightening the clamp bolt a breeze on the distributor and the better access makes it easier to get the distributor in the correct position.

    I would chase around the various hoses, particularly the intake hoses which can get a nice crack in them, particularly if your upper engine torque strut is worn. At some rpms it opens the crack and then others it pushes it closed.

    I once drove my Passat two miles in reverse when the intake hose cracked and the compliance in the engine mounts when driving forward opened the crack leaving it running at idle but when I drove in reverse closed the crack and it ran fine. I found I could drive pretty fast in reverse at 4k...yes I am that stupid.
     
  6. Huey

    Huey True Classic

    Location:
    Quakertown, PA
    The intake hose is suspect. The one that came on it was definitely cracked so I bought a used one from MWB that was better but likely not perfect. Although, I sprayed starting fluid around various parts of it and did not notice a change in RPMs.

    It could be that I still have the valve timing off. I got a machined head and cam tower from MWB, which forced me to go to an adjustable cam gear. I tried to get it into spec as best as I could but it could be off. The ignition timing is dead on according to my timing light. But it fits all the criteria of a vacuum leak so that's why I was going there.
     
  7. Kevin B (Asheville NC)

    Kevin B (Asheville NC) Administrator Moderator

    Location:
    Asheville, NC
    since intake hose air leaks only manifest themselves when the motor torques against the mounts, you may have to put you hand on it and raise / lower the hose slightly while spraying starter fluid to find any leak there.
     
  8. Huey

    Huey True Classic

    Location:
    Quakertown, PA
    Hadn't thought of that. Thanks.
     
    kmead likes this.
  9. lookforjoe

    lookforjoe True Classic

    You removed the aux air slide & hoses? Those can crack & allow massive unmetered air leaks that will definitely create the symptom. Not likely to be a small vacuum line. That or the ribbed intake hose seem the most likely culprits.
     
    kmead likes this.
  10. Dan Sarandrea (Phila)

    Dan Sarandrea (Phila) Waitin' On Parts...

    Location:
    Philadelphia, PA
    A leak in the intake snorkel would technically be "false air" as opposed to a vacuum leak, since it is upstream of the throttle plate. (But the effect would be much the same as a vacuum leak, running lean.)

    Same if you have a leaky oil dipstick grommet, or cracks in the oil breather hoses, and/or aux air valve hoses.

    If the aux air hoses are airtight but the AAV itself is leaky (they can be finicky) you technically don't have false air or a vac leak since all air squeezing past the AAV's blockoff plate has been metered by the AFM---you'll just have a higher idle that will be hard to adjust out.

    On later FI cars, the way the evap hoses are routed, there is a planned vacuum "leak" that pulls vacuum thru the charcoal canister into the FI plenum.

    Can't remember if your car has A/C fitted, but if so, there is the vacuum system that runs much of the function of the A/C controls--if any of this is perished you will have a true vacuum leak.

    One often overlooked possibility for vac leaks: throttle shaft seals.
     
    nichol01 likes this.
  11. carl

    carl True Classic

    Location:
    Virginia
    ...and everyone thinks us carb guys are a bunch of cavemen.
     
    Brayden_connolly likes this.
  12. Mxgrds

    Mxgrds True Classic

    Location:
    Netherlands
    Close up the airmeter, set the engine so that all valves are closed and pressurise the system from the fuelpressure hose. Hear where air is escaping.
     
  13. Huey

    Huey True Classic

    Location:
    Quakertown, PA
    I removed the air intake hose and found a crack. I fixed it but no difference at all.
     
  14. lookforjoe

    lookforjoe True Classic

    Did this start after the cam gear install? You may need to revisit the basic cam timing. Maybe it's off a tooth.
     
  15. Huey

    Huey True Classic

    Location:
    Quakertown, PA
    Yeah that’s my fear. What a pain it was to do that.
     
  16. JDExSquid

    JDExSquid Not quite a Daily Driver!

    Location:
    Stockbridge, GA
    I once had a similar problem. On mine it turned out to be the bakelite seats that the injectors plug into: two were cracked, allowing unmetered air in.
     
    kmead likes this.
  17. Rod Midkiff

    Rod Midkiff True Classic

    Location:
    Eugene, OR
    would that not show up with the starter fluid test?
    (the bakelite seats being cracked and leaking)
     
  18. JDExSquid

    JDExSquid Not quite a Daily Driver!

    Location:
    Stockbridge, GA
    I'm not sure. I don't remember doing the starter fluid test back then. I believe I was just replacing the short injector hoses and found them. The car ran MUCH better after replacement! Ran like crap before, but I had recently purchased the car and was in the process of going through it.
     
  19. Huey

    Huey True Classic

    Location:
    Quakertown, PA
    Ok I played around with this a little. I tweaked the ignition timing and can get it to run out better. It has its power back and it cruises nicely. But it idles too fast and I have the idle adjuster screwed all the way in. Timing it to where it idles nice and it bogs down starting out in 1st gear.

    Either way it sounds like it has a misfire — sort of a pop pop pop at idle. Maybe I don’t have the adjustable cam gear adjusted correctly? I did the best I could but it’s damn near impossible to get a dial indicator on the valve tappet and be out of the way of the cam lobe.

    Maybe I’ll try making small micro tweaks to the cam gear. This is getting frustrating.
     
  20. lookforjoe

    lookforjoe True Classic

    Did you play with the base timing with the vacuum disconnected? If it's idling too fast & the idle stop is essentially closed, then probably too much advance at idle, or still have a (intake/vacuum) leak - OR (most likely) the cam timing is off, IMO.....
     

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