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Hesitation when warm outside...

Discussion in 'General Chevy & GM Tech Questions' started by moogvo, Apr 9, 2012.

  1. Strino78

    Strino78 Member

    Quick question for you Tom: is your '95 an OBD-I or OBD-II diagnostic computer system? On the C/K series trucks the two major sensors for controlling the idle are the Idle Air Controller, and the Coolant Temp Sensor (the one in the intake manifold). Some food for thought, a failing IAC doesn't make the computer throw any codes, nor will a sloppy but somewhat functional Coolant Temp Sensor.

    Try unplugging the CTS sensor's plug. You should get a Check Engine, but if your idle improves -- you've discovered the culprit.
    It may also be the IAC sensor is slow/sloppy. They're allowed to have some "wiggle" to them, but shouldn't move. They carbon up quite regularly. You mentioned that you cleaned the TB. Did you remove the IAC sensor and give it a good scrub?

    If you have an OBD-I system you can do a basic IAC reset to fully closed position to allow it to re-learn. To do this, disconnect the NEG battery post for a minute. Reconnect the battery. Under the dash jumper pins A+B on the diagnostic connector. Turn the key to ON, but do not start. Let it sit in ON for ~1 minute. Turn key off, remove jumper pins and then start. OBD-II systems will self calibrate as long as you let it sit for a minute in the ON position before starting after connecting the battery.

    I've had to chase a few of these gremelins down myself, and it ultimately ends up having us check each and every single sensor, hose and fitting under the damn hood!


    Hope this helps out.
  2. moogvo

    moogvo Epic Member 5+ Years 1000 Posts

    Hi Strino.

    95 is pre-OBDII. I replaced the CTS and the data stream shows that it is reading normally. It doesn't have any problems idling, so that leaves out the IAC Valve. Thank you though. I will take any ideas I can get!
  3. moogvo

    moogvo Epic Member 5+ Years 1000 Posts

    I am coming pretty close to listing this truck on Craigslist!

    So today, I get in the truck to run some errands. I drove about 10 miles to the next town. On my way, I turned the AC on. It was cycling BAD! I have lost almost all of my refrigerant after 2 weeks. I just replaced the line seals on the back of the compressor. Turns out that the switch is bad on the back of the compressor. No biggie really, but while inside the auto parts store, my water pump let go and started squirting from the weep hole. Add this together with the hesitation and jerky driving and I am just about at my breaking point with it.

    I have replaced everything short of the fuel lines and fuel pump. I am ready to take this truck to the WOODS and beat on it!
  4. osgprometheus

    osgprometheus Member 2 Years 100 Posts

    How was the port hole under the EGR valve? Did you clean it out while you had the valve off?
  5. moogvo

    moogvo Epic Member 5+ Years 1000 Posts

    Yes. The passages are all clear. Thanks for the idea!
  6. osgprometheus

    osgprometheus Member 2 Years 100 Posts

    Everything seems to be ruled out but fuel. Check the pressure.
  7. moogvo

    moogvo Epic Member 5+ Years 1000 Posts

    I have an adapter to add a schrader valve in line, but it won't fit because I can't move the lines enough to make it work. I will work on that one this weekend too. I am thinking that I might have a bad ISM. I replaced it already but they are commonly bad outta the box. Maybe a bad pickup coil...
  8. Strino78

    Strino78 Member


    Sorry about that... I re-read.. and you did say that it didn't have idle issues. Only outside air temp, and engine temp related issues triggger your problem. When the engine comes up to temp, it then has issues.. and feels sluggish.

    Hrmm.. I'm thinking the air temp sensor. Particularly with the ambient temperature factoring into the equation. When its above 60, maybe its just warm enough that the warm engine heats the air enough to really skew with the air/fuel mixture. Below 60 degrees.. the air is "cool" enough for it to run right until it warms up under the hood.
  9. moogvo

    moogvo Epic Member 5+ Years 1000 Posts

    Ok... so after many months of cursing and threatening to put the truck on Craigslist... I think I have fixed my problem... Almost.

    I replaced the distributor this afternoon with a new unit I picked up at Rock Auto. It was a breeze, really. So I started the truck and took it for a spin. Mind you, I didn't check the base timing, but all of the marks I made on the distributor lined up perfectly. Most of the hesitation is gone and the power level has returned seemingly to normal. Hopefully a few minutes with a timing light will make it perfect!

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