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Truck runs great with coolant sensor unplugged

Discussion in 'Truck & Equipment Repair' started by Moonlighter, Dec 3, 2013.

  1. Moonlighter

    Moonlighter Senior Member
    Messages: 643

    I need some fresh ideas, my main salt truck is a 91 Chevy 3500 1 ton 2wd with a 5.7L TBI motor. If I have the coolant sensor plugged in it idles at 11-1300 rpm, if i unplug the coolant sensor (the one in the head next to the goose neck) it will run at perfect rpm and purr. This is not supposed to be so, as it runs best when the computer thinks it's at -30. However the other day I had it running great with everything plugged in for about 30 minutes, until I shut it off and restarted it, It was also before I replaced IAC.

    Here is the list of what has been checked and or replaced.
    There are no codes being thrown at all unless I unplug the CTS.
    I checked all the vacuum lines 4 times, sprayed carb clean in and around anything that could possibly leak with no luck. New PVC valve, new IAC (due to me over tightening the original when I took it out to clean it). Warranted the CTS to make sure it was a new one. I looked down the throttle body to see if the base gasket was getting sucked in. I redid all the engine and harness grounds. I replaced vacuum lines as needed and ends that were cracked. Verified all sensor operations by a scanner computer. If I clear the code for the CTS while it's unplugged the truck will fall on it's face for a few seconds until the code pops again and it purrs again. There is no booster on this truck, has the hydro boost system in it.

    Tomorrow I am going to disconnect the battery for a half an hour and check the plugs on the computer for issues and check for any grounds under the dash that may have not been redone since I rebuilt the body. Aside from that I am out of places to go with it, I ran it tonight to fill it up at the gas station with the CTS unplugged seamed fine but it's just not supposed to run that well with it unplugged.

    I feel like I may be missing something, any ideas would be very helpful. P.S. I even threw in a picture of the truck shortly after I mounted the blade. Thanks in advance for any help it's driving me nuts!:dizzy:

    house salter 015.jpg
     
  2. Whiffyspark

    Whiffyspark 2000 Club Member
    from SOMD
    Messages: 2,402

    Ok first you need to get a vacuum garage on it and verify you do not have a leak. Carb cleaner doesn't always work. You can use the line that goes into the master cylinder

    Next, check your fuel pressure. That era is known for fuel pumps going out.

    Last, make sure your convertor isn't plugged.

    Your ect sensor should have a voltage range of .5-4.5. You want to verify that as well
     
  3. dieselss

    dieselss PlowSite Fanatic
    Messages: 11,001

    He's got hydro boost. No vacuum lines there
    Does the tk have a temp gauge on the dash?
     
  4. Whiffyspark

    Whiffyspark 2000 Club Member
    from SOMD
    Messages: 2,402

    Missed that. Theres a line on the back of the intake. I think theres one on throttle body too. Its been a few months since o worked on one. Doesn't really matter where you put the guage
     
  5. Moonlighter

    Moonlighter Senior Member
    Messages: 643

    I will check it with the vacuum gauge in the morning. I will try and find a place in the line to test the fuel pressure. The coolant sensor is new and the values checked out via scanner - cold and warmed up. I didn't think about the converter because the exhaust flows freely, but I will check it as well.
    I appreciate the suggestions I will report more findings tomorrow.


    Yes it does but I plugged a mechanical one into the side of the block for a more accurate reading.
     
  6. dieselss

    dieselss PlowSite Fanatic
    Messages: 11,001

    What does the scanner read for the numbers? Is it close to accurate?
     
  7. Moonlighter

    Moonlighter Senior Member
    Messages: 643

    Yes the scanner is accurate, all the sensor numbers are within spec. I can look at almost everything with it, o2 crosscounts, IAC fwd - rev and voltage, TPS voltage reads smooth from closed to wide open, CTS unplugged reads -36 plugged in reads right and climbs with engine temp rising, MAP sensor values read right, shows me 14.5 volts on fuel pump but no pressure reading, also shows injector pulse, batt volatge, ect. There is plenty of comunication between scanner and computer. I had a van last year that wouldn't read the computer & ran like crap, I cleaned all the grounds on the motor and body and cured it right up. I could write down all the numbers off it while truck is running good if that would help. I didn't get a chance to disconnect the battery again and check the plugs to the computer I am doing that first thing later today. Just really puzzled that I had it running good until I shut it down and restarted it, without doing anything of consequence.
     
  8. Whiffyspark

    Whiffyspark 2000 Club Member
    from SOMD
    Messages: 2,402

    Need to check fuel pressure and vacuum. I doubt this is an electrical problem.
     
  9. Moonlighter

    Moonlighter Senior Member
    Messages: 643

    I checked vacuum today and it reads 19 inch pounds, at the pcv port. Fuel pressure will be a little challenging but I will check it as soon as possible. Thanks Brian
     
  10. Whiffyspark

    Whiffyspark 2000 Club Member
    from SOMD
    Messages: 2,402

    Was it steady?
     
  11. Moonlighter

    Moonlighter Senior Member
    Messages: 643

    Yes it was steady 19.
     
  12. rjigto4oje

    rjigto4oje PlowSite.com Addict
    Messages: 1,297

    Does it use a knock sensor I had one on a motor home give me trouble once
     
  13. Whiffyspark

    Whiffyspark 2000 Club Member
    from SOMD
    Messages: 2,402

    Most likely Fuel Pump. Do you have a tester? They have an adapter for your motor. I have one, but I'm no where near you
     
  14. Moonlighter

    Moonlighter Senior Member
    Messages: 643

    No knock sensor I think, scanner had no reading for it, I think sometime after 91 they started using them not sure.
    That would kinda suck if it was, I replaced the tank when I rebuilt the truck this summer, would have been so easy then. I will have to buy an adapter or a whole kit new one for this truck so it will be a few days, doing a rebuild on the hydroboost tonight while it's warm.
     
  15. dieselss

    dieselss PlowSite Fanatic
    Messages: 11,001

    not understanding how the f.p. could be causing this???
     
  16. maxwellp

    maxwellp PlowSite.com Addict
    Messages: 1,112

    There is something going on in closed loop. When you unplug the coolant sensor it runs in open loop off of a set of predetermined values. When it is warm and plugged in it goes in to closed loop and reads all the sensors. Keep looking there one or more that is making it do this. Did you check the knock sensor? Are there two temp sensors on this?
     
  17. Moonlighter

    Moonlighter Senior Member
    Messages: 643

    You are correct in your assumptions that's why it is puzzling me. Even when it's warm and plugged in it high idles, kinda leading to getting a computer out of a yard and see if it cures it. It won't go into closed loop I forgot to mention that, It hasn't since I started testing and trying to trace down the issue. However all sensors tested out properly both high idle and normal idle with the coolant sensor unplugged. Yes there are two sensors one in the head, is just the gauge sensor, I have a mechanical gauge in it, and the new one I put in the intake next to the goose neck is the computer sender.
     
  18. rjigto4oje

    rjigto4oje PlowSite.com Addict
    Messages: 1,297

    DOES IT have a power steering sensor im the pressure line also if it does not go into closed loop maybe the engine is not getting hot enough
     
  19. maxwellp

    maxwellp PlowSite.com Addict
    Messages: 1,112

    The computer was my first thought but I convinced myself that was just trying for an easy fix. But if you are thinking that also I would try one if you can come up with a bone yard one. I have spares for my older trucks I got off fleaBay for $50 because it also controls that alternator in a Dodge.
    One other thing could be that you have a wiring problem and the sensor signal is not getting back to the ECM.
     
  20. Whiffyspark

    Whiffyspark 2000 Club Member
    from SOMD
    Messages: 2,402

    Take it to a shop and have them check fuel pressure. This is a VERY common 5.7 problem. No need to reinvent the wheel and piss money away