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  1. #11
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    ok,, it is heat soaking when it sits for a few minutes at stops with no air or water circulation to help cool it. Before we give the ignition timing a free pass, put a light on it & see where it is & what it's doing. I can't think of why more heat would cause timing problems, but backfiring through intake & exhaust is telling me it has either valve, or ignition timing issues (or waaaay too much fuel is just pouring down your carb & into your exh. & muffler & lighting off there).

    As far as being heat induced, this could easily be a fuel problem or heat warpage causing an intake leak somewhere. Next time this happens on your way home I would leave it running in your driveway and spray some carb cleaner around the base of the carb, & around the intake manifold, & anywhere else you could have a vacuum leak. If the engine rpm picks up, you have found the leak(s)

    This might be easiest for you so do it first even though I listed it last----->
    For diagnosing this it would be great if you could put a vacuum guage on it and tell me what it reads when cold, when hot, at idle, and when it is having this problem! Put a long hose on it and have the guage inside the car so you can watch it while you are driving. Then when it starts running bad take note of what the guage is reading. It most likely will be bouncing back and forth wildly between 2 disparate numbers. Be accurate reading it, & write them down because there are about 15 different things that you can tell from vac guage readings.
    1988 Chevy C-3500 2wd (no pic)
    350 c.i. 5.7 L Stock Block, 4 Bolt Mains
    L-31 Vortec Heads, Edelbrock Cam & Intake,
    Holley 650, Flowtech Headers, Magnaflow exh.
    Jet Trans 700R4, B&M Ratchet, 4:10 gears,
    3" susp. lift kit "shadetree"
    No rev limiter, No speed limiter lol


  2. #12

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    Okay,You also mentioned some of the vacume lines could be replaced...How is the one going to the distributer?But the two month period that it went away for is odd and makes me think it has nothing to do with the distributer or timing.


    03 Chevy Silverado 1500 ext cab Z-71
    79 AMC Spirit GT
    77 Olds cutlass supreme

  3. #13

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    I will check that tomorrow as well and report back to you guys. Thanks alot!!!

  4. #14
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    Quote Originally Posted by Z-71 View Post
    But the two month period that it went away for is odd and makes me think it has nothing to do with the distributer or timing.
    I saw that too. My guess to that was that the old thermostat was stuck open
    & allowed better cooling of the engine, and when he put the new 195 degree thermo his problem started again. Also could have been related to his weather there at the time. Just too many questions to ask, and not enough room ha ha ha

  5. #15

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    Quote Originally Posted by stephan View Post
    I saw that too. My guess to that was that the old thermostat was stuck open
    & allowed better cooling of the engine, and when he put the new 195 degree thermo his problem started again. Also could have been related to his weather there at the time. Just too many questions to ask, and not enough room ha ha ha
    Very true That does make sense.

  6. #16

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    Yes! I took out the old thermostat and it was stuck open. When I drove home today it started to get hotter at a light and it backfired just before I got it on the 13 mile open stretch of road. With more airflow around the engine it didn't back fire all the way home. When I got home I let it run and sprayed carb cleaner around where vacuum leaks might be. There was no change in Idle speed. When it just sat getting hotter and I throttled it by hand it back fired like hell. Is it carburetion? Maybe a stuck float?

  7. #17
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    Yes it could be an intermittantly sticking float, if it is carbd. That would go along with the randomness of the prob. But I can't figure out why high temperatures would be affecting it because the carb/TBI is the coolest area of an engine because of the incoming air & fuel continuously cooling it. You need to put a timing light on it and watch & see what it's doing when you throttle it.

    Did you try the vacuum guage yet?

    Can you take pics & post them here? If yes, can you post a pic of the distributor with the cap & rotor removed, (take the pic from the passenger side & get close so we can clearly see it) & take a nice shot down the venturis so we can finally be sure if this is a carb or TBI. GM started putting TBIs' on in early '80s & if this is an "all stock 1987" it should not have the carb, it should be computer controlled TBI & distributor.

    ****edit
    If you can't take pics, then take the air cleaner off & tell us what you can see in the top of the carb/TBI
    also is there a small round metal pot on the lower front of the distributor that has a vacuum hose hooked to it?
    Last edited by stephan; 10-12-2010 at 04:48 PM.

  8. #18

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    Quote Originally Posted by stephan View Post
    Yes it could be an intermittantly sticking float, if it is carbd. That would go along with the randomness of the prob. But I can't figure out why high temperatures would be affecting it because the carb/TBI is the coolest area of an engine because of the incoming air & fuel continuously cooling it. You need to put a timing light on it and watch & see what it's doing when you throttle it.

    Did you try the vacuum guage yet?

    Can you take pics & post them here? If yes, can you post a pic of the distributor with the cap & rotor removed, (take the pic from the passenger side & get close so we can clearly see it) & take a nice shot down the venturis so we can finally be sure if this is a carb or TBI. GM started putting TBIs' on in early '80s & if this is an "all stock 1987" it should not have the carb, it should be computer controlled TBI & distributor.

    ****edit
    If you can't take pics, then take the air cleaner off & tell us what you can see in the top of the carb/TBI
    also is there a small round metal pot on the lower front of the distributor that has a vacuum hose hooked to it?
    My camera is on the fritz. I have one at work that I can use tomorrow but I loked at the carb again and it says Rochester Quadrajet on it. Looks like the one I had on my 68 GTO. There is no vacuum line going to the distributor and the coil is not attached to the distributor. There is a pot shaped piece below the distributor but it is attached to the intake and it has an electrical lead coming from it.
    I haven't tried the vacuum gauge yet.

  9. #19
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    OK, it's nice that you've got the quad. The only bad thing about those is that it's almost impossible to get the top off to check, set, or fix/replace the float without tearing the gasket, & it's almost impossible to find someone who sells a single gasket ie: "the one single gasket" that will fit your quad. Edelbrock sells a bowl cover gasket set #1987 that has a half dozen of the wrong ones and the 1 right one, ha ha.. The pkg. sells for $20. around here, which is really spendy for ONE gasket, and 6 extraxs that don't fit!!

    Take the air cleaner off, & get the eng hot & make sure the choke plate is fully open! It should be straight up & down when hot.
    Next with the eng running hot, AT IDLE, (do not throttle the carb with your face over it ha ha) look down in the carb with a flashlight & see if it's flooding. If there is any fuel coming out of the boost venturis or leaking down the walls from the gasket, this is not normal. The only fuel that should be going in the mani is through the idle circuits which are under the throttle plates and are not visible with the plates closed.

    If you see no leaking fuel that is good but there is one more thing to check and that is the secondary throttle valve/plate. With your face a safe distance from the carb top, snap the throttle 1/2 way open quickly and then close it just as quick & see if the eng backfires & IF the secondary valve/plate opened or tried to open during this time!
    It should not open, but if it did this is part of your problem. If it doesn't open at 1/2 throttle, then try it at full throttle for just a half second, and see if it opens. If it is opening for either of these tests, then it is at least part of your problem!

    Whether or not the secondary has passed or failed the throttle tests, take a small piece of wire, & wire the valve lever that is attached to the secondary valve/plate (it is on the passenger side of the carb) CLOSED. Be sure you wire the lever and not the rod. Be sure and check that you have it wired in the closed position. Push down on the BACK EDGE of the plate and make sure it doesn't open. Don't push down on the front edge of the plate to check, as it means nothing other than that it is against the stop.
    This will eliminate the sec. from the system, and allow you to test drive it for a few days.
    It will be a dog at anything over 1/2 throttle but if you drive it like this for a few days and get it hot, and it hasn't exuded any of the backfiring and dieing that has been the norm, then you have found the problem, or at least one of them..

    FYI the Edelbrock gasket kit part# 1987 has nothing to do with the year of your car lol, it is just a coincidence.

  10. #20

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    'm guessing a stuck valve, It it were mine, in my opinion the easiest and cheapest solution would be to throw a quart of mystery oil in it and run it. 9 out of ten times the mystery oil will free the valve. I have bought seized engines for cars,trucks,and it has freed them every time and they continue to run fine for many,many miles. It will also work on seized lawn equipment. It's less than 4 bucks a quart at walmart and great stuff.. Good Luck

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