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Cold start rough idle on a TBI 350

Discussion in 'The Garage' started by my88texastruck, Nov 13, 2006.

  1. my88texastruck

    my88texastruck Registered Member

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    Starts and runs/idles rough until you accellerate some the it smooths out and runs like it should. The exhaust smells "gassy" like it is running rich. NO codes or SES. Could it possibly be the temp sensor for the ECM - giving it a false reading and the computer is feeding it more gas than it needs? Seems to be doing it more/worse the colder it gets. I don't believe it is ignition in nature as I have replaced the cap, rotor, plugs and wires.
     
  2. Slapperbar

    Slapperbar Retired Navy NDT Examiner Premium Member

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    I think mine's doing the same thing. 'Sept off idle is stumbles, give it more and it clears up.
     
  3. Cricket

    Cricket 3/4 ton status

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    Is it idling up and down like it's hunting around for an idle speed or just missing altogether.
     
  4. Slapperbar

    Slapperbar Retired Navy NDT Examiner Premium Member

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    Not hunting, just ****ty.
     
  5. jays68yak

    jays68yak 1/2 ton status

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    Hmm my 350 does the same thing and has since I got it it 120000 miles ago. I know over the years I have replaced, motor, cap, rotor, plugs, wires, map, ignition module. If I advance the timing it will smooth out a bit, but wont pass smog.
     
  6. goldwing2000

    goldwing2000 Guest

    Check for vacuum leaks. Especially the TB base gasket and the gasket between the PCV ring and the TB.
     
  7. my88texastruck

    my88texastruck Registered Member

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    I don't quite think it is the TB base gasket as it has a new one on it - but I'll check it. Gasket between the PCV ring and TB???
     
  8. my88texastruck

    my88texastruck Registered Member

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    No, it starts up and fast idles like it should when it's cold, just "missing"
     
  9. 88Silverado

    88Silverado 1/2 ton status

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    Mine was doing a similar thing. Remove the air cleaner assy, on the back passenger side of the engine there is a vacuum hose from the intake manifold that goes to a black, plastic "Y-type" vacuum check valve behind the distributor. Remove and check this valve carefully. Mine was cracked where it was glued creating a vacuum leak. It would "miss" at idle and smell bad, then go away when you brought the RPMs up. Its a $1.99 part at Autozone in the "HELP" parts section. Took 3 years to find it.
     
  10. fwc

    fwc Registered Member Premium Member

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    Part number???

    EDIT: Looks like Dorman/Motormite/HELP #47150, anyone else?
     
    Last edited: Nov 14, 2006
  11. goldwing2000

    goldwing2000 Guest

    Yeah, that "stack" that's between the TB and the air cleaner has a gasket. If it's missing or torn, it'll cause all kinds of fresh hell.

    Be sure to check all the hoses, tees and connectors, too.
     
  12. 88Silverado

    88Silverado 1/2 ton status

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    Heres the valve I replaced. On the original one I removed, the vertical port on the left side was actually at a 45deg angle. The one I got from Autozone looks just like this and was in one of those little red "HELP" packages that hangs on the wall with all the little misc. parts. Their's is labeled as a "2-way" check valve and this one from NAPA is labeled as a "1-way" check valve. Mine was cracked right where the tapered cone mated to the large round assy. The cone side (right in this picture) connects to the intake manifold via hose. My valve was located back behind the distributor area. Follow the vacuum lines. Its worth checking out. My truck idles so smooth now I have a hard time telling its idling...except it still stinks :D
    [​IMG]
    http://napaonline.com/masterpages/NOLMaster.aspx?PageId=470&LineCode=BK&PartNumber=7301348&Description=Check+Valve+-+Universal+One+Way+Vacuum
     
    Last edited: Nov 14, 2006
  13. my88texastruck

    my88texastruck Registered Member

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    Gotcha, I just couldn't picture it. I'll check it out.
    Thanks!
     
  14. my88texastruck

    my88texastruck Registered Member

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    Thanks to you too! I'll do some checking in this area too. When I first got the truck, the PO had the vacuum hoses all ****ed up - EGR code with no vacuum at all - took me a while but I finally found the BB someone had stuck in the line to block it, lines with golf tee plugs, etc. I have since "re-plumbed" them but it has always had a "stumbly" idle - not too smooth - but certainly better than before.
     
  15. my88texastruck

    my88texastruck Registered Member

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    AH HA!!!:laugh: Not cracked but had a small hole in it and the little rubber diaphragm inside had seen better days. The part cost me $5.65 - up just a little, but we'll blame that on inflation - but she idles and runs like she should now!! Seems to accellerate better too. I've been looking for that leak for a long time now!! Amazing how big of a headache a little itty bitty P.O.S. $5.00 or so plastic part can cause! That takes care of one, now on to the next.
     
  16. 88Silverado

    88Silverado 1/2 ton status

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    Thats great!
    Like I said, took me 3 years to find that problem. After changing all the plugs, wires, cap, rotor, and just about everything else short of rebuilding the throttle body and pulling the heads to look at the valves and cam, it was that stupid valve. Actually found it was cracked by accident.

    Glad it worked out :waytogo:
     
  17. jays68yak

    jays68yak 1/2 ton status

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    Im going to go check mine at lunch. That would make me really happy if that is it.
     
  18. sfara002

    sfara002 Newbie

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    This is an ancient thread but I didn't want to start a new thread based on the same issue. I have a 91 GMC Jimmy which idles horribly (and it has even stalled) when it is cold outside (17* C or less) and it is the first start of the day. It idles rough for about 20-30 seconds (longer when colder) then smooths out to a normal nice smooth idle. I have been data logging and have a reference log if anyone is interested. Everything on the truck is in a stock configuration. At this point I have done a few things and have a few more in mind.

    Things I have done,
    1. Data logged, on the rough idle start I have noticed a couple things, the IAC is almost closed during the bad idle and when it begins to open the idle smooth out. I assume this acts like a choke and needs to be closed like this. Also when the truck is about to die the vacuum drops way down (14-15). I don't know if that means there is a leak or simply the truck is just not pulling enough vacuum at the time because its about to stall out.

    2. Coolant Temp Sensor reads good on all data logs

    3. Smoked the vacuum system for leaks, non found other than the seal above the TBI Throttle body and right below the filter housing do leak, I didn't think this should affect the vaccum. Also an EGR block was installed for troubleshooting and really didn't change anything.

    4. IAC was pulled, looked serviceable and ohms check was good right at 50.

    5. I hear the fuel pump with the key on and the spray patter from the injectors match each other.

    Things I was thinking of doing,

    1. Fuel Pressure gauge from the parts store to test for good PSI and no blocked return line, most of my data logging the BLM data is returning slightly rich numbers so I may be too rich open loop on the first start of the day.

    2. Pull spark plugs, I plan on getting to this but the entire ignition system is only 2,000 miles old and looks to be of reputable parts (this was installed before I bought the truck)

    I am open to checking any other suggestions, I haven't replaced any parts yet, I kind of wanted to confirm a bad part before I replace something. I will post more was time goes by. Thanks in advance!
     
  19. dyeager535

    dyeager535 1 ton status Premium Member

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    You say CTS reads good...as in before starting, it reads ambient, or very close?

    Not being a member, can you upload the datalog?
     

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