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Thread: 1989 c1500 rough idle issue..HELP!

  1. #1
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    1989 c1500 rough idle issue..HELP!

    So it has finally come to this. I need some advice on an issue i've been chasing for months now. I have a 1989 chevy c1500 350 tbi. When you first start it up the engine will idle high and then drop down to regular idle speed. If i leave it for about 5 minutes or so, the idle will slowly start to drop and stumble like it wants to die but it doesnt. The idle will come back up again and then go back down. I put it in drive and the same thing will start to happen. If i'm sitting at a stop light it will slowly start to hesitate and the idle will drop down to a point where it sounds like it wants to die, come back up again, and go back down. But when i'm driving it drives very smooth except for a small hesitation when you first hit the gas. It doesnt feel like it has lost any power, there is no smoke coming out of my tail pipe at all either. The timing is set to 0 degrees (done with that one wire unplugged), i have replaced the TPS, IAC (reset the IAC when i replaced it), plugs, wires, distributor cap and rotor, EGR, O-rings around the injectors, fixed the vacuum leak around my TBI spacer plate, ran seafoam through it, replaced all the vacuum lines that go to the TBI unit itself. Im not getting a check engine light at all. It also smells like its running a rich when i have the windows down and i'm stopped. I've checked the injector spray pattern and it seems fine. I've also unplugged the MAP sensor and CTS and it runs even worse so i'm assuming those sensors are fine. The only other thing i can think of is that the fuel pressure regulator spring could be broken or the ground that comes off of the negative side of the battery has bad wiring (it looks like the connector someone used to crimp it with is burnt) and needs to be replaced. I'm going to check those out this weekend. Anyone have any ideas? I'm running out of them and i just want to get my baby running normal again

  2. #2
    Senior Member semocoachworks's Avatar
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    I would check the map sensor. It may be telling the ecu that it needs more fuel than it does. Easy to check.

    1) On the map sensor, there is a red, black, and green wire. Put a paperclip in the green terminal.

    2) Set a voltmeter to DC volts, hook up the negative lead on the voltmeter to a ground(battery).

    3) Put positive lead to paperclip hooked to green wire(backprobe).

    4) Have a vacuum pump available and unhook the vacuum line on the MAP sensor. Hook up the vacuum pump to the vacuum stud on the sensor. Turn the key to run but do not start the engine. With no vacuum, you should have 4.3-5.0 volts. pump the vacuum gauge up to 5 in. of vacuum. The voltmeter should read 3.3-4.0 volts. For every 5in. of vacuum, the voltmeter should drop around 1 volt. At 20 in. of vacuum, you should have around 1 volt. If this checks out, check your line going from the sensor to the tbi. If you have no voltage or erratic voltage, chances are MAP is bad. It will give symptoms you describe.
    "Why is it that when you drop a tool while working on your truck, there is always a drain pan full of oil to catch it for you?"

  3. #3
    Senior Member gmctrucks's Avatar
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    Did you disconnect the wire by the fire wall when you set the timing? Also make sure that you bought good replacement parts and no cheap Auto Zone trash. Say guys could his fuel pump be going out and that's why it's acting weird?? When you replace your pump buy a Delphi those cheap off brands pumps don't last long.
    http://i961.photobucket.com/albums/ae98/falken86/dadnew-1.jpg

  4. #4
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    I'll check out the MAP this weekend. I did unplug that wire before I set the timing as well. Weird thing happened today, while the truck was running I decided to unplug the coolant temp sensor and take a drive. As soon as I unplugged it, the idle went up a little but when I drove around it drove like a champ. I plugged it back in and it started idling horrible again at stop lights. Does this mean my CTS is bad?

  5. #5
    Senior Member 87c10slammer's Avatar
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    that would probley be your problem. i had to do plugs and wires on a 01 firebird LS motor and i forgot to plug the cts back in. i couldnt figure out for the life of my why it ran like ****. i hooked up my scanner to it to see what it was doing and as soon as i saw the cts didnt have any voltage i smacked myself in the head.

  6. #6
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    So a little update. I replaced the CTS and the MAP just for the heck of it. I'm at a loss, its still running funny. It idles fine for a little in drive but then the longer it sits there, the lower the idle goes. I'm running out of ideas. Is it getting too much fuel that's why it sounds like its going to die? I can give it some gas and it'll get up and drive normal.

  7. #7
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    Ever find the solution?

    Did you ever find the correct solution?
    I just purchased an '89 C1500 350 with the sane issue. I haven't replaced as much as you have (yet), so I'm curious if you found the real issue, so I can head in that direction.

    Thanks,
    joshuncc

  8. #8
    Senior Moderator chevychase's Avatar
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    From what i read in this old thread... sounds like either plugged cat... or 02 sensor.

  9. #9
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    Had the Same problem

    Quote Originally Posted by chevychase View Post
    From what i read in this old thread... sounds like either plugged cat... or 02 sensor.
    I had the same issue on my 92 Z71 after putting in a rebuilt engine. I bought new sensors from A-Z and replaced all before attempting to start it. New plugs, wires, cap, bug ignition module. The only part that was original was the TBI. When I started the engine it ran rough but I thought the ECM just needed to reset itself, so I drove it for a few days nothing changed. So I messed with it for another week or so, Finally realizing I had not changed the EGR valve. Still no change. After everything was changed and replaced with High quality parts I was still at at a lose. No body could point me in a right direction to fix my problem,( so I went to putting my old parts on since they worked before I put a new engine in.) I got to the point I was ready to give up un junk my baby since it would not straighten up. I put old everything old back on and was left with plugs, wires, and cap to replace. Thinking it was NOT one of them I held off. Big mistake. Finally did plus and cap still no change. Last thing to change was the Wires. Well guess what that solved the whole problem and she runs like a top. So just a note, That because something is new does not mean it is always a good part, so keep that in mind. So you may look at your plug wires even if you put on new ones. Hope this helps.
    96silverado likes this.

  10. #10
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    its probably and hopefully the FUEL PUMP!
    This surging/rolling idle on the TBI engine is usually caused by lean mixture, a failing Fuel pump or a bad fuel pressure regulator!

    NOTE: Install a fuel pressure gauge and check for fuel pressure to be 11-13 PSI, If pressure is low, block the fuel return line to see if pressure comes up or if still low.
    If pressure comes up, replace the pressure regulator and if still low then replace the pump. (A failing Fuel pump can still start the vehicle but not be able to produce the needed fuel pressure to have the engine running with the proper fuel mixture)

    ALOT of people are doing everything you did just to find out it was a fairly cheap easy fuel pump.

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