93 Suburban runs rough and rpm fluctuations.

Discussion in 'Chevy Suburban Forum (GMC Yukon XL)' started by SMCZ28, Apr 29, 2013.

  1. SMCZ28

    SMCZ28 New Member

    Hello everyone,

    I've been messing with my 93 Suburban for the past 2 days and I just can't pin point where the problem is.

    1993 Chevrolet Suburban
    7.4L 454 BB

    On Saturday it wouldn't start so I took a look and found out that the injectors weren't working so I took out the ICM and had it tested, they said it was fine so I went back home and put it back in and than the injectors worked but than it would die.

    I went back over every wire and hose I messed with until finally it would stay at the point of idling but only at about 100 or 200 RPMs it would backfire but run, when I gave it gas it would shake like an earth quake was going on until it hit 3000 or more RPMs it would be fine but have some backfires.

    While it was running I jiggled more wires and now it will idle at 100 rpms and than a couple of seconds later it will jump up to 1100 rpms and than die back down to 100 rpms, it will keep doing this until I turn it off. I thought I found the problem when I jiggled the 2 pronged wire coming from the coil to the ICM but it's hard to say since it dies and picks back up randomly, I started it this afternoon and it ran fine for a couple of minutes and than it went down to 100 rpms and stayed there and than it will fluctuate, for as long as I've had it I have had to hold my foot on the gas a little to be able to start it, it wouldn't start on it's own unless it was warmed up.

    I have already checked when I was trying to figure out the injector part, the fuel pump and fuel filter "those were both changed a year ago" tested the ICM 5 times. Distributor cap and rotor could use replacing but I took some 1000 grit sand paper to the points and cleaned out the cap and cleaned off the rotor, spark plugs and wires were replaced the same time the fuel pump and filter were changed. I went back to make sure all the wires were in the right location and to make sure the wires were plugged in nice and tight as well the wires for the ICM and anything else I touched.


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    Alright I went back out there and started it, it is fine for 25-30 seconds after that it runs like crap, I checked the plugs and they are all carbon fouled so I went through them and cleaned them off but still it runs like crap and sometimes it will run fine, it will pop up to 1000 RPMs like there isn't a single thing wrong with it and than it will drop back down to 100 or 200 RPMs, I put it in drive and it's all over the place between 100-1000 RPMs. I tried to rev it up gently to 1000 and sometimes I'll get intake backfire as well as exhaust backfire. I unplugged the TPS and made sure the connection was clean as well, I went back and checked over all the hoses, they are all fine and no cracks. The injectors appear to be spraying fine.
  2. Trevs Yukhoe

    Trevs Yukhoe New Member

    If the injectors are spraying fine then your problem could lie with the idle air controller or your tps. Those two could cause the fluctuation in rpms.....but you add the backfire and carbon fouled plugs and that would point me toward the pickup coil in the distributor, timing issue or electronic spark controller. Did you just visually check the vacuum hoses or check them while running? Alot of little variables and sensors that cause these problems and sometimes no cel gets tripped. I'd go back to the basics old school timing and ignition, check the grounds, look for play in the distributer shaft. Let us know if you make any progress. Last thing though, I would yank the valve covers off and make sure you havent bent a pushrod or cracked a rocker or have a rocker off, that would also cause a backfire....
  3. SMCZ28

    SMCZ28 New Member

    I was going to try and take off the TPS and the sensor next to it and clean them but I don't have anything that will fit to take them off which is shocking, the next thing I was going to do was take off the coil and test that, I would say the whole thing is stock, the ICM still said GM on it which is surprising. It ran just fine until that morning the injectors didn't work and I had to take out the ICM, that's when it started running crappy when I put it back in.
  4. Trevs Yukhoe

    Trevs Yukhoe New Member

    Did you replace the ICM or just pull it. This is the ignition control module under the cap right? Did you put a good bed of silicon under the icm before you put it back on? If not those suckers won't last long. If you dont think thats the problem then I would cosider rebuilding the throttle body. I did mine last year because I had a dead spot on my TPS and I decided to put a new TPS, IAC and replace the regulator. The kit came with the gaskets, injector o-rings and filters for the injectors. Did you check your egr valve when you were troubleshooting?
  5. SMCZ28

    SMCZ28 New Member

    I pulled it off to take it to autozone and they tested it 5 times so it can warm up. He said they normally will show the fail at a higher temp so that's why he did it 5 times, so I just installed it back, I think some silicon was left on the dizzy so I just stuffed it back on there and the injectors worked. I really didn't do anything to the egr except take the hose off to inspect it. I am wondering if the K&N filter has anything to do with it, when I bought it it was on there and haven't had any issues with it.

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    Let me ask this, is it possible to plug the pickup coil in backwards into the ICM....?

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    I cracked the dizzy open again and looked at the pick up coil wires I switched them around for fun and of course it didn't turn over so I went back in unplugged it and looked at the pick up coil wires more, I broke off the rest plastic covering the wires since when I first undid it most of it crumbled in my hands. I put the wires back to where they needed to be and wrapped them in tape, put everything back together and it runs fine now. I let it sit for 5 minutes and it ran just fine. I turned it off and back on to check it stayed below 1000 RPMs around 800 so it works now so I'm happy.

    Thanks Trevs Yukhoe for the helpful suggestions.
    Last edited: Apr 30, 2013
  6. Trevs Yukhoe

    Trevs Yukhoe New Member

    sweet, hope that solved it for ya.

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