Become a ltxtech.com member, Click here to register!
Lloyd Elliott

Results 1 to 8 of 8
  1. #1
    LTX Member


    Status
    Offline
    Join Date
    May 2017
    Name
    Jeff
    Vehicle
    1997 Firebird Formula
    Posts
    5

    Default Strange cold start

    On my 97 Formula with the auto tranny at 37000 miles, the first cold start of the day, I've been noticing an RPM stumble immediately after it fires up (just one time). It goes from 1300RPM right after starting up but rapidly drops to 800RPM for 1/2 second, then goes back to 1200RPM and stays there until warm where it drops to 700RPM. Warm starts are always perfect and the idle speed is steady both cold and warm. Car generally runs awesome otherwise, never got an SES light, and just recently passed emissions/OBD2 inspection.

    This started happening 5 years ago and can't seem to figure this one out. I had replaced the alternator at that time and am not sure if that is the culprit (reason I even suspect the alternator is because it started happening right around that time, and the volt gauge is higher at cold start than it was prior...on that line after 13 where it previously stuck between 13 and the next line). However, when I check with a digital volt meter it reads 14.65 cold idle, 14.1-14.2 warm/hot idle which I hear is perfectly normal.

    Had the fuel pump replaced 5 years ago to take care of another issue (the cold start issue happened before as well, and I thought the pump replacement would've taken care of it but it didn't). Also had the fuel injectors cleaned, throttle body serviced, and the fuel filter replaced at the dealer (all done when the fuel pump was replaced).

    I have the following mods: Hypertech PP+ and 160* stat, Moroso CAI, Airfoil, Spintech muffler. I tried removing hypertech and airfoil and nothing changed so I put them back in.

    Any idea what might be happening? Right now the car still runs great except for the cold start (which after 1 second it runs/idles normal).

  2. #2
    LTx Guru


    Status
    Offline
    Join Date
    Apr 2013
    Vehicle
    1994 Formula
    Location
    Central NJ
    Posts
    1,490

    Default

    Possibly a problem with the coolant temp sensor in the water pump. The PCM uses that to set cold start A/F enrichment and elevated idle speed. Could possibly be indicating the engine is warmer than it actually is. Off, but not enough to set a code and turn on the SES light.

    http://shbox.com/1/4th_gen_tech2.html#ect

    94 Formula A3+1: 381ci forged stroker - Callies Stealth, Oliver 5.85 billet rods, BME nitrous pistons / CNC LT4 heads / Comp Cams solid roller / TH400+GearVendors OD / 4.11's/Strange 12-bolt / 300-shot (dry) N2O / Spohn Suspension / 6-pt roll bar /AutoMeter instrumentation / MoTeC M48 Pro engine management system /a few other odds 'n ends.

  3. The Following User Says Thank You to Injuneer For This Post:


  4. #3
    LTX Member


    Status
    Offline
    Join Date
    May 2017
    Name
    Jeff
    Vehicle
    1997 Firebird Formula
    Posts
    5

    Default

    Thanks, I think this was suggested in another forum as well but still unsure how to check this. I just don't want to replace the part and wind up not resolving the issue.

  5. #4
    LTx Guru


    Status
    Offline
    Join Date
    Apr 2013
    Vehicle
    1994 Formula
    Location
    Central NJ
    Posts
    1,490

    Default

    Quote Originally Posted by formula97 View Post
    Thanks, I think this was suggested in another forum as well but still unsure how to check this. I just don't want to replace the part and wind up not resolving the issue.
    Did you read the link I posted? Explains in detail how to test the sensor and the circuit.

  6. #5
    LTX Member


    Status
    Offline
    Join Date
    May 2017
    Name
    Jeff
    Vehicle
    1997 Firebird Formula
    Posts
    5

    Default

    I read through it, and was actually wondering if a scanner can do this or do i need to use a volt meter.

    Also -- is this the same sensor that operates the cooling fans?

  7. #6
    LTX Member


    Status
    Offline
    Join Date
    May 2017
    Name
    Jeff
    Vehicle
    1997 Firebird Formula
    Posts
    5

    Default

    One last question -- is this also an "open loop" vs "closed loop" scenario? Reason I ask is because I've always had better throttle response in closed loop (warm engine) than open loop (cold engine).

  8. #7
    LTx Guru


    Status
    Offline
    Join Date
    Apr 2013
    Vehicle
    1994 Formula
    Location
    Central NJ
    Posts
    1,490

    Default

    A scanner would tell you what the sensor is reading. You would have to judge if the reading shown on the scanner is accurate or not. Compare to dash gauge, but those are not all that accurate. If it's a large error you might be able to judge it is inaccurate. If you determine it is inaccurate, then you have to follow the procedure to determine if the error is due to the sensor or the wiring, using a multi-meter.

    Again, the answer to your question about the fans is in the write up. The 2-wire sensor in the water pump is the only one that connects to the PCM. The PCM controls the fans based on the sensor reading. The other 1-wire sensor in the driver side head only connects to the dash gauge.

    Three things have to happen for the PCM to enter closed loop. 1) the pre-cat O2 sensors have to get hot enough (~600F) to work, setting the left and right O2 sensor ready flags (observable with a scanner); 2) the coolant temp has to reach a programmed temp that is typically in the 120-140F range; and 3) the timer from cold start, typically 206 seconds, has to time out. A faulty CLT sensor, reading lower than actual temp, could delay the transition to closed loop.

    94 Formula A3+1: 381ci forged stroker - Callies Stealth, Oliver 5.85 billet rods, BME nitrous pistons / CNC LT4 heads / Comp Cams solid roller / TH400+GearVendors OD / 4.11's/Strange 12-bolt / 300-shot (dry) N2O / Spohn Suspension / 6-pt roll bar /AutoMeter instrumentation / MoTeC M48 Pro engine management system /a few other odds 'n ends.

  9. #8
    LTX Member


    Status
    Offline
    Join Date
    May 2017
    Name
    Jeff
    Vehicle
    1997 Firebird Formula
    Posts
    5

    Default

    Thanks, I'll have to check the coolant temp sensor one of these days (today was a hot day in NJ at 90 degrees, and I still had that strange cold start).

    If that coolant sensor checks out, I'm wondering if the MAF sensor can cause this to happen. Again, I only have this happen when first starting up for the day (it idles fine, runs strong, etc).

    I also noticed a mechanical sound today during the cold start, sounding like it came from the brake master cylinder.

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •