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Holy Backfire batman!

Discussion in 'The Garage' started by Russell, May 15, 2006.

  1. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    Man, just went out to try and start my truck, it was flooded out yesterday, and wouldn't fire for me. So I went out just now to try and start it, and dang did it ever give off a HECK of a backfire!

    I mean, I felt the concussion wave from the thing, I just sat there like this: :eek1: for a few minutes, and my father came running out of the house to see who shot me, LOL

    I gotta figure out whats causing that! I don't know if it was an intake or exhaust backfire, but DANG, I've neve heard a backfire that loud in my life!
     
  2. 78Suburban

    78Suburban 1/2 ton status

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    I've heard of people winding their truck up to 5 grand, shutting the ignition off, flooring the gas pedal, and firing it back up to get that affect, to make people pee in their pants. I bet those jammed open injectors poured a bunch of gas into your exhaust manifolds.

    Now go change your underpants and get back out there and do some troubleshooting :haha: :haha: :haha:

    I think I would have sharted if I had a backfire like that :eek1:
     
  3. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    I'm still not convinced that my balancer is the correct one for my truck.

    I just pulled the balancer and timing tab off my 89, which I KNOW was accurate beforehand, and just manually checked TDC, and found that my timing mark was STILL at 12:00 when the hissing from cylinder 1 stopped, even though it was at 3:00 on the 89.

    Do different cranks have different key positions on them or something?!

    And to make things just that much better, I just laterally tossed my knee out sitting down at my computer, so I can barely walk anymore!

    Talk about a bad day!
     
  4. Wishbone

    Wishbone 1/2 ton status

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    Cleaning Pepsi off the keyboard:haha:
     
  5. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    Thanks for the emotional support! You guys rock :rolleyes:


    :haha:
     
  6. ZooMad75

    ZooMad75 1/2 ton status Premium Member GMOTM Winner

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  7. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    Well, I decided that a torn knee ligament wasn't gonna ruin my day of figuring stuff out, so I limped my butt back out to the truck, and kept at it.

    Using a little dentist's teeth poker tool thinger I found the other day, I was able to confirm the timing mark's accuracy in relation to the piston's physical movement.

    When you reach about 12 - 14 degrees BTDC the piston seems to stop, then at about 12 - 14 degrees ATDC, it begins to decend. Since the stopping and starting distance appear to be pretty much the same, I can only assume that the balancer and tab is infact accurate.

    However, the reason why I felt it may be inaccurate was due to the compression seeming to stop when I reached about 12:00 on the balancer.

    So, I figured that if the engine timing mark is accurate, then I'd midaswell go back to the basics, and check my compression.

    I found the following:

    1 - 125 PSI
    3 - 125 PSI
    5 - 130 PSI
    7 - 125 PSI

    2 - 140 PSI
    4 - 125 PSI
    6 - 125 PSI
    8 - 125 PSI

    Now, this is all on a basically stock bottom ended motor. I've got a Comp Cams eXtreme Energy XE262-H cam shaft (218 in. / 224 ex. Duration @.050, .462 in. / .469 ex. Valve Lift with 110 LSA), but otherwise I have stock heads 74cc (less upgraded valve springs for the cam), with a .020 overbore, and dished pistons.

    Is 125 PSI what I should be aiming for, as most of my cylinders are running that, or are all of them low, and I should be aiming for the 140 PSI? Would that be a valve adjustment issue? I did my compression test with all of the plugs out of the engine, dry. Should I try putting a shot of oil into each cylinder to see if my compression increases?

    Unfortunately, my plugs are of no use trying to diagnose anything, becuase they were totally gas fouled. Soaking wet with liquid gas, every last one of them. I knew my injectors were bad, but I thought I had one or two reasonably good ones. Apparantly not!

    Though, there is a mild possiblity that it may be caused by the 02 sensor being hooked up. It may be detecting the valve overlap at an idle, and pouring tons of extra fuel into the intake. I'll see what happens if I unplug the 02 sensor.

    Anyways, once I get my new injectors, I'll also buy myself some new AC-Delco spark plugs (I've got 7 autolites, and one AC-Delco in there right now, so they gotta be replaced anyways) and we'll see if we can't get this pig running...
     
  8. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    Ok, and now the problem deepens further yet. I went ahead and took the whole intake apart so I could physically look at how bad the injectors leak. Turn the key, and absolutely nothing came out of any of the injectors. I must be loosing all my pressure back into the check valve in the fuel pump, or in the connection between the pump and the hard line.

    That puts me straight back to square one with what is wrong. I think I can rule out any major mechanical problems. So there must be something electrically messed up. Which sucks big time :(
     
  9. ZooMad75

    ZooMad75 1/2 ton status Premium Member GMOTM Winner

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    Dude, You might be overthinking this one just a bit. You did the right thing to go back and check the basics. Compression looks ok. You are not looking for one high number to compare against, just what is within 10% of the average of all cylinders. I havent run the math, but it looks ok to me.

    Now for the fuel problem. What could cause that much fuel to be run through *without* leaky injectors? 2 things come to my mind. 1. Way too much fuel pressure, either the regulator itself went bad or the adjustment is too much. Did you check what kind of pressure you have? 2. Controls. Meaning, what is the PCM seeing to need to react by adding fuel? Lean o2 readings come to the top of my mind. If the PCM *thinks* it is running lean it will keep adding fuel until the readings change. Given that this is a batch fire setup, it's going to dump fuel in all 8, 4 at a time. If you have a dead o2 sensor, high resistance in the wiring or a bad grounds it could be throwing off the 5v reference. That skew's what the PCM sees and the PCM is just dumb enough to react to it. Do you have anything that you can monitor the o2 sensors with? A tech 1 would be ideal, but most aftermarket scanners that can read data should be able to show what the live readings are. Keep this in mind, this is an OBD1 system, it won't set codes for a sensor that is just reading out of range like an OBD2, it's only going to set a code for a dead sensor.

    I'm thinking some time spent with some type of scan tool should shed some light on your problem. It's harder than hell to diagnose a drivability problem on a computer controled engine without one.
     
  10. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    Fuel pressure is 43 PSI, right on spec for these injectors. Drops to 38 PSI with vaccum applied to the regulator. I know that the system is not leaking anywhere outside of the fuel tank, so the leak has to be in the fuel tank. If thats where the leak is, I am not concerned about it.

    And I agree with you 100% -- If I had a scanner, I'd probally have it running correctly by now. My problem is that I have no way of getting the scanner software onto my laptop, and even though I did think about it, setting my desktop up in there with a generator in the back doesn't quite make sense, LOL

    I'm currently hunting down a cheap laptop that has a USB port so I can use my USB stick to move data logs around, and even to burn chips for the truck.

    Which is why I was mad it wound up being electrical / computer related, cause I am poor have don't have a laptop to fit the bill at the moment, lol
     
  11. gmcman7202

    gmcman7202 1/2 ton status

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    what about a cd-r or howabout serial transfer cable? if the laptop has a slot on the side u can get a lan card to do a lan transfer of files.
     
  12. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    My current laptop is an ancient old Pentium I machine. Something like 770 MHz or so, lol

    I had a CD-ROM drive for it, but since I last used it on my TBI truck last year, it has vanished without a trace. Not having the USB port isn't such a big deal, and the machine is good enough for running the software (even if it takes 20 minutes to boot up, LOL) but I just don't have a way to get the software onto the machine.

    Unfortunately, no little slots on the sides for extra PCI cards. The Serial transfer sounds like a possiblity. What is involved with sending data that way? I've never tried before...
     
  13. dyeager535

    dyeager535 1 ton status Premium Member

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    Not to say this is the right path for troubleshooting, but I don't see how the above is relevant, unless it's just not clear what you mean...without the engine cranking, unless the injectors leak you won't get any fuel out of them. So what pressure problem is there?

    While not a thorough test of an injectors, you can check them with a multimeter to see if they ohm out to spec. (13-16 IIRC)
     
  14. gmcman7202

    gmcman7202 1/2 ton status

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    get a cable, cheap at radio shack then just need to set up the protocal, its easy, but i have forgotten how to exactly to do it since its been about 10 yrs since the last time i had to do shuch a thing, i think googleing the topic will yeild great results.
     
  15. smalltruckbigcid

    smalltruckbigcid 1/2 ton status

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    Isn't that cam LSA spec a little narrow for a tbi setup. I'm not a tbi expert at all but most FI cams are about a 112 or wider LSA, most tuners like a 114 or wider. Just trying learn more here is all.
    George
     
  16. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    Yeah, I know the cam isn't exactly good for TPI. I ran the engine in my truck carbed for a few weeks before I decided to switch over to TPI. At the time, I was quite a bit younger, and far less informed, or I'd have gotten a new cam back when I had more money than brains :p

    Now I've got more brains, but no money, so I'll have to use my brains to tune around the cam's rich idle issues. Basic plan is to use my sense of smell to adjust the idle A/F target ratio, then eventually put some money down on dyno tuning with a wideband 02 sensor to get it perfect.

    dyeager535: I originally thought my problem was with the injectors leaking into the intake after I shut the engine down. I knew it was insanely rich, rich enough to foul plugs, and wash cylinder walls, but as I found out yesterday, its the ECM that is making everything rich, not leaking injectors. I thought it was the injectors due to the fuel rail quickly loosing fuel pressure after priming it. I now know that all the fuel pressure is leaking back into the tank, which is fine, it won't affect anything.

    However, I do have some very good news! A buddy of mine told me that he has a laptop I can borrow for as long as I need :D I'll have it this afternoon, so I can finally get things figured out :D
     
  17. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    Well, I've got the laptop all rigged up. I can scan the truck, change chips, and burn chips all from the comfort of my driver's seat now :D

    But, the ALDL data all looks to be within spec, but I have got a LOT of work ahead of me as far as tuning goes :S
     

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