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Help! Runs like crap after warmed up!!!**update**

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by ftn96, Mar 21, 2002.

  1. ftn96

    ftn96 1/2 ton status

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    Ok, like the other posts said, I have swapped in Thorley headers, replaced the fuel pump, sock, filter, plugs, O2 sensor, PCV and it runs like a champ while its cold. Soon as it warms up **poof**.... No power, hesitates, stumbles, causes the tranny to not shift right. etc etc etc.....
    Ideas? So if the ECM uses the Coolant sensor and the TPS while its cold then goes to the O2 sensor when hot, does this not point to the O2? But its brand new. So there in lies a few other options, MAP, EGR, TPS.....Where should I start next?
    Looks like all these sensors are about $65 so I'd like to get the right one in at least the 1st 2 or 3 tries if not the first.
     
  2. ftn96

    ftn96 1/2 ton status

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    Re: Help! Runs like crap after warmed up!!!

    Anyone wanna by a 90 Jimmy? /forums/images/icons/laugh.gif I've thought about it a time or 2 over the past week.
     
  3. Thunder

    Thunder 3/4 ton status

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    Re: Help! Runs like crap after warmed up!!!

    Have you checked to se if any trouble codes are on the computer? It will tell you if any sensors are not functioning.
    Even though the Check Engine Light does not come on there may still be trouble codes stored in the computer.
     
  4. ftn96

    ftn96 1/2 ton status

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    Re: Help! Runs like crap after warmed up!!!

    Yep, nothing but 12 12 12
     
  5. rlhenry

    rlhenry Registered Member

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    Re: Help! Runs like crap after warmed up!!!

    Check the injectors and the harnesses. A friend and I just got done working on his '91 K10. Went throught the EGR valve, TPS and new injectors. EGR needed to be replaced but the problems cleared up with new injectors and the harnesses. By harness, I mean the plug and pig tail that can be bought for $9.00 each. You have to splice them in (damn it) but they cleared up his problem.
     
  6. 6.2Blazer

    6.2Blazer 1/2 ton status

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    Re: Help! Runs like crap after warmed up!!!

    It really sounds like the O2 sensor, or related wiring, has an issue. Make sure the connector is secure and inspect the pins inside the connector to make sure they are not bent, loose, or pushed out. There is probably a procedure in a service manual for checking the O2 sensor and wiring harness.

    You could probably even just unhook the O2 and drive the truck and see if it runs any differently.

    If the TPS were bad, I would guess you would have issues show up when the engine was cold also.
     
  7. Blue85

    Blue85 Troll Premium Member

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    Re: Help! Runs like crap after warmed up!!!

    That's true. Check engine lights are only to indicate when emissions might be too high, they don't come on with all trouble codes. It seems like a bad O2 would log some codes, but it does make sense as the culprit since the ECM switches to closed loop only after the engine is warmed up and before that the O2 sensor isn't used.

    ftn,
    If the O2 was just replaced, did the trouble start about the same time? You could have a bad one. Also, it could be something else that is actually effected by temperature, like the ignition coil. I had a small break in my old accel coil and it would run good sometimes and bad at others. The thermal expansion of components could be making and breaking a connection.

    Good luck. Once you solve this, it won't seem like a big deal, so don't sell it! /forums/images/icons/smile.gif
     
  8. BigBad75K5

    BigBad75K5 1/2 ton status

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    Re: Help! Runs like crap after warmed up!!!

    The tps won't always give a code. Anouther thing to look for could be the fuel press reg. You could have an injector acting up. Did you make sure you cleaned ground wire and location it bolts to before you bolted it back down?
    I'd still change the plug wires and dist cap before I did anything else.
     
  9. 87GMCJimmy

    87GMCJimmy 1/2 ton status

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    Re: Help! Runs like crap after warmed up!!!

    I don't want to sound like a broken record but this is exactly how my truck ran until I got a heated o2 sensor with the headers. And no my truck was not setting any codes either. Get a cheap voltmeter and check to voltages on the single wire o2 sensor and you can see it. If you were in phx you could come over and I'd scan the truck for you and show you what's going on.

    Once again, Good Luck!!
    Mike
     
  10. 87GMCJimmy

    87GMCJimmy 1/2 ton status

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    Re: Help! Runs like crap after warmed up!!!

    Thinking about this some more you answered your own question.

    "So if the ECM uses the Coolant sensor and the TPS while its cold then goes to the O2 sensor when hot..."

    Yes! When the engine is warming up it's using the coolant temp/TPS and running fine. The computer is working in an open loop mode.

    "Help! Runs like crap after warmed up!!!"

    Once the engine warms up it, around 160 degrees, the computer uses the o2 sensor and runs like crap because the o2 sensor signal is crap because the it's not hot enough because of the headers. This is when the computer is running in closed loop mode.

    Hope this helps,
    Mike
     
  11. jimmy88

    jimmy88 1/2 ton status

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    Re: Help! Runs like crap after warmed up!!!

    Do yourself a favor, and rent an AutoXray or similar scanning tool. My local auto store lets you rent them free for one day and something like $10 each extra day. The AutoXray lets you monitor all the sensors in real time and you can store data for many seconds at a time and review it to see what was happenning. Its a PITA but write down the numbers and save them for future reference. Its best if you can get a good baseline reading when the truck is running good (but who rents the tool when everything is ok?)
     
  12. ftn96

    ftn96 1/2 ton status

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    Re: Help! Runs like crap after warmed up!!!

    what ground wire? The one on the fuel pump assembly? If so yes!
     
  13. ftn96

    ftn96 1/2 ton status

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    Re: Help! Runs like crap after warmed up!!!

    <blockquote><font class="small">In reply to:</font><hr>

    tn,
    If the O2 was just replaced, did the trouble start about the same time? You could have a bad one. Also, it could be something else that is actually effected by temperature, like the ignition coil. I had a small break in my old accel coil and it would run good sometimes and bad at others. The thermal expansion of components could be making and breaking a connection.

    <hr></blockquote>
    The problem came about after installing new gas tank, headers and tranny.
    SO I started with the fuel pump, then the filter then the O2. Still doing it. So Im looking more towards the Engine compartment now. I will double check the O2 wire connection.
     
  14. ftn96

    ftn96 1/2 ton status

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    Re: Help! Runs like crap after warmed up!!!

    87 I understand. But I'm not totally keen on how all this works. So what your saying is that the heat of the "headers" is basically masscarading the O2 sensor with all the extra heat? Am I getting you correctly here? If so I sorta understand. But on the flip side. If the engine runs great when its cold, ie the O2 would be cold too, seems like keeping it cold would keep me where I want to be insted of heating it up to 600 degrees.
     
  15. HarryH3

    HarryH3 1 ton status Author

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    Re: Help! Runs like crap after warmed up!!!

    The O2 sensor doesn't start to produce voltage until it reaches about 600 degrees F. Older GM EFI systems would completely ignore the O2 sensor until the engine temp came up to 160 degrees or so. By this time the O2 sensor should also be up to its operating temp. On newer vehicles, they use an electrically heated O2 sensor, so it is at or above 600 degrees just a few seconds after the engine starts. This reduces cold-start emissions and also helps with cold driveability.

    In your case, headers lose a lot of heat into the engine bay. A cast iron manifold retains the heat and sends it down the exhaust pipe much better. The heat loss can mean that your O2 sensor is not getting up to the 600 degree mark before the ECM decides that it is time to go into closed loop mode. This is where using a heated O2 sensor can help. The heater will get the sensor hot enough to work properly. Your ECM will still ignore it when the engine is cold, but when it switches to closed loop, then the O2 sensor will be providing the correct output.
     
  16. Grim-Reaper

    Grim-Reaper 3/4 ton status Author

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    Re: Help! Runs like crap after warmed up!!!

    I agree with 87. Sounds like a O2 problem. when first started the computer runs on a preset fuel map and ignores the O2 sensor. Hence the runs fine till warmed up. At that point the computer tries to go to closed loop and refferences the O2 sensor to adjust the fuel mixture. Bet it's running rich.
    Where was the factory O2 sensor located? where is it located now? My guess is it was located in the factroy cross over pipe and now it's in a single header and not getting enough exhaust running past it to get hot enough. If the truck were running lean it would have excessive heat on the O2. Running rich would be cold to the O2. Take a sniff and see if the exhaust smells like it had about a gallon of gas poured though it.
     
  17. dyeager535

    dyeager535 1 ton status Premium Member

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    Re: Help! Runs like crap after warmed up!!!

    What hes saying is that the O2 sensor is more likely to NOT be in the correct position to reach the temperature it needs to be, to work correctly.

    *Typically* an O2 sensor not quite close enough to the engine, and thus not being kept warm all the time, I believe it will drop the engine out of "closed loop" which can be seen with a scan tool...again typically, if this is the case, then when you get moving again, and the exhaust temp raises, the O2 sensor can "work" again, because proper temp is achieved. Hence the heated O2 sensor, especially with headers.

    I've heard the same thing about aftermarket O2 sensors..problems with the O2 sensor getting cold at idle, then ok at cruise, but not a problem with a Delco one.
     
  18. ftn96

    ftn96 1/2 ton status

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    Re: Help! Runs like crap after warmed up!!!

    The O2 is in the same location on the headers as it would be if the y-pipe were still in place. And no it doesn't smell rich. The only thing I can smell is the nickle plating on the headers baking in!
     
  19. dyeager535

    dyeager535 1 ton status Premium Member

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    Re: Help! Runs like crap after warmed up!!!

    Man, I wish you were close by, I'd loan you my autoxray, it would take a lot of the guesswork out of this. If the headers are coated, (better heat retention apparently) and the O2 sensor is the same distance as stock, AND seeing as much exhaust gas (stock was probably before the y-pipes merged right, so it only "sees" 4 cylinders, right?) if it's an O2 problem, it's probably going to be a defective one or something. But it *almost* doesn't sound like a heated O2 would help...
     
  20. TopOff

    TopOff 1/2 ton status

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    Re: Help! Runs like crap after warmed up!!!

    Disconnect the O2 and see if it runs better when it warms up.

    You should get a "check engine soon" light. But it is easier than renting or buying a tool.
    Of course then you will need to clear the computer code.
     

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