Lookin for a vacuum diagram

Discussion in 'Chevy Suburban Forum (GMC Yukon XL)' started by SuburbanSS, Jul 4, 2009.

  1. SuburbanSS

    SuburbanSS Member

    The following is in reference to a 1998 Chevy Suburban C1500 5.7 2WD

    So we got the olde girl fired up and driving and now it lacks power big time while accelerating.

    And now to the best of my conclusions I figure there is some sort of vacuum leak, I did notice there was a vacuum hose missing from a vacuum controller? behind the A/C accumulator that appears to have two hoses that run into it with a some sort of sensor and one is missing so I figured that is a good place to start.

    Are clogged cats known to cause this kind of performance or is it my guess of a vacuum leak?

    It is a little hesitant to start without some throttle but It will idle just about perfectly fine when it does fire and in park or neutral it will rev up fine, soon as you put it in gear it has some hesitation and barely wants to move even with full throttle (it bogs down) the fuel pump/filter is brand new so im assuming its not a fuel delivery problem.

    I'm currently looking for some sort of vacuum diagram that indicates some routing and what goes to what...


  2. L0sts0ul

    L0sts0ul Rockstar 100 Posts

    I ran into this problem before on another truck lemme look at my notes
  3. 2COR517

    2COR517 Epic Member 5+ Years 1000 Posts

    I can look at my 97. Basically the same motor.
  4. unplugged

    unplugged Epic Member 5+ Years 1000 Posts

    I use two methods to diagnose a vacuum leak.
    1. Pull the air cleaner off and with the engine running I cup my hands over the carb/tb opening progressively covering the air inlet opening. If the idle increases and smooths out it indicates a leak.
    2. To isolate the leak I spray water from a spray bottle around the base of the carb/tb around the intake manifold and any other common source of vacuum leaks. As the water temporarily seals the leak the same increase/smoothing of idle occurs.

    Not much info on diagrams out there. Maybe someone with the same vehicle can snap a pic of the vacuum diagram from their truck and post it.
  5. 2COR517

    2COR517 Epic Member 5+ Years 1000 Posts

    Looked pretty closely just now. I don't see anything behind the accumulator, or even on the firewall. The only vacuum lines I see at all are the Brake Booster hose, and what appears to be some sort of purge device for the vapor canister. That's it.

    This is the Alldata page for a 1998 C1500 Tahoe. Lucky you, it's one of their sample vehicles. You might look around here.

  6. SuburbanSS

    SuburbanSS Member

    Took a look around looks like its just a hose for the brake booster from what I can gather for hoses, so it appears everything is in order, I guess the next thing is to put a scanner to it and see what it says, at this point it could be anyone of the sensors MAF, MAP, O2, etc etc.

    Like I said its a bit hesitate to start off the bat without some throttle and after that it seems to idle/revs nicely in park/neutral, but as soon as its put into gear it bogs right down and lacks power, even under half+ throttle it barely wants to move out of its own way...

    Hopefully the reader can tell me something.
  7. 2COR517

    2COR517 Epic Member 5+ Years 1000 Posts

    It will only tell you something if there is a malfunctioning piece of equipment, which usually sets off the Check Engine light.

    A good place to start is a thorough tuneup. New Air Filter. Clean the MAF sensor, MAF cleaner only. Pull the throttle body and clean it and the Idle Air Control. It's a very easy job, did mine today. Suck a bottle of Sea Foam into the brake booster vacuum line while the engine is running. Dump another can of Sea Foam into the gas tank. Don't fill up till the tank is almost empty. New fuel filter.

    Full Ignition tune up with Genuine AC Delco plugs gapped at .040, AC Cap and Rotor. NAPA Premium wires or AC Delco. There is a breather in the bottom of your distributor. It probably has a screen in it, knock the screen out.

    Most people say to change the plugs after the Sea Foam has done it's thing. The plugs could have a huge impact. I was running Bosch+4. They have huge gaps, the truck was running terribly. Couldn't get out of it's own way, sloppy idle. Put in the Delcos gapped at .040, made a huge difference. Your sticker under the hood probably says to gap them at .060, but GM issued a notice years back to go with .040. The smaller gap takes a tremendous load off the ignition components, resulting in better driveability and longer component life.
  8. SuburbanSS

    SuburbanSS Member

    It currently has a brand new distributor, ignition coil, crankshaft position sensor, ignition control module, new fuel pump, fuel filter, bosch platinum plugs (gapped at .060 currently - would that be enough to explain its current performance?) Will definately also try cleaning all the stuff you mentioned...

    Thanks for the insight
  9. 2COR517

    2COR517 Epic Member 5+ Years 1000 Posts

    After doing the Seafoam, I would pull the Bosch plugs and replace with the AC Delcos gapped at .040. I know it sounds crazy that plugs would make that much difference, but I have talked to several lifelong GM mechanics. They all say to ONLY run the AC Delcos. Plain old boring single tip platinumgs. It made a huge difference for me. Also, when the distributor was installed, how did you time it?
  10. SuburbanSS

    SuburbanSS Member

    Well we put a compression tester to the #1 cylinder and burst cranked till we got it to the compression stroke and then tweaked the pulley till the timing mark lined up on the harmonic balancer then dropped the distributor so that the rotor pointed to the #1 cylinder terminal on the cap. After that as far as I know the timing is electronically controlled...

    All this was done with the assistance of a ex-mechanic.

Share This Page

Newest Gallery Photos