Question about 1996 Chevrolet G10

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1996 Chevy Express 1500 w/5.0 bogging only when warm.

I have a 1996 Chevy Express 1500 conversion van with a 5.0 vortech equipped. When cold (first start of the day or after sitting long enough to cool completely down from it's normal operating temperature) it idles fine and does not bog down at all. If you drive it from cold, it will warm up and run perfectly with no bogging or hesitation. Shut it off, wait about 5 minutes to half an hour and start it up again, then drive and it will bog down on gradual acceleration (but not hard acceleration) for the first minute to two minutes. (Acts almost as if it's flooding while running). I've replaced the plugs (checked the gap to verify correct), rotor, distributor cap, fuel filter and plug wires. I have gotten two codes: Cylinder 7 misfire (occured only once) and P0420 Bank 1 Catalyst below threshold. However, the problem started about a month ago and the errors only happened yesterday and today. At my wit's end here. Please help with suggestions? (I'm going to try to get a compression test tonight and praying it's not a valve/head gasket or cam issue)

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  • Randall LaClaire Jan 13, 2009

    Thanks. That's similar to the theory I was explaining to my wife, just much better explained technically. I'm going to do the compression test tomorrow since it won't cost me a dime and will help eliminate several possibilities. As you state, I already figured the misfire was symptom rather than cause. I did however forget to mention in my original post that I have "vanishing coolant" issues. No obvious external leakage or odor of leaking antifreeze/coolant, yet the level drops fairly steadily. Hence part of my concern with the possible head gasket. I'll know more tomorrow. If it is the converter I want to be sure I've covered all my other bases to prevent damaging a new converter if there is another issue that's hiding from me!

  • Randall LaClaire Jan 30, 2009

    Well I did the compression test today, but let me back up a bit. Three days ago I found a vacuum line with about a half in long hole in it. It connects to what looks like a thermal vacuum switch on the driver side of the intake manifold. This switch supplies vacuum to the PCV valve and to the damage line which routes up to the front driver side and from there I can't tell where it goes without tearing down quite a few things. After replacing the line the problem with bogging seemed to more or less disappear. I did a vacuum test and the vacuum results were within normal range and responded fairly quickly on letting off the throttle. Today when I did the compression test, compression ranged from 155 to 165 in all cylinders. There were no drops in compression. Unfortunately I realized later this evening that I forgot to block open the throttle. Also, during the compression test I realized during cranking for cylinder #5 that there was a bare wire which was cut off at some point prior to me owning the vehicle. The coil wire was arcing to that bare wire. On reassembling everything I initially got rough idle and a "random multiple misfire" code. It smoothed out after idling a while and I assumed it was likely that the plugs got damp from working in the low temperatures here. Now, however, the van runs rough constantly and misfires frequently. Is it possilbe that either leaving the throttle closed may have damaged something or perhaps the arcing might have damaged the ignition module? The bogging issue is still gone so I'm guessing the vacuum was the problem there the whole time, but now there is this puzzler!

  • Randall LaClaire Jan 30, 2009

    Also, to clear up the obvious: I checked all plug wires 3 times to be sure connections were tight and in the right order. I checked the rotor and cap. Other than a little whitish oxidization on the electrodes they seemed in good shape. (The wires, cap, rotor and plugs are all new as well installed within the last month.) I removed the fuel pump relay again (did so for the compression test) and checked to be sure the contacts were clean, then reinserted it to be sure it was seated well. I'm wondering if the ignition module or coil might have been damaged. That arcing wire, by the way, was something someone had spliced into the grey wire which I'm guessing is the ground on the top of the coil.

  • Randall LaClaire Feb 01, 2009

    Ok. Got this issue taken care of. As it turns out it was a direct intake manifold vacuum leak. While getting ready to run a vacuum test I found a damaged vacuum hose that goes to a vacuum switch which also feeds the PCV valve. Once replaced everything smoothed out. After a few days the number of P0420 errors gradually reduced in frequency and have stopped completely. No more missing either, until after my compression test. That, fortunately, turned out to be a dead plug at #2 which must've been damaged during replacement.

  • Randall LaClaire Feb 06, 2009

    Problem solved.

  • Randall LaClaire Feb 06, 2009

    And thanks, 3Mac. It was an adventure to say the least ;)

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I have a 1996 chevy express conversion van 5.7 liter I have misfire in all cylinder after the truck is warmed up I have new plugs cap rotor wires new o2 sensor. o2 sensor has been changed 3 times in 3 months I was told by my mechanic it was my catalytic convertor dose that sound right please let me know thanks Vince

Posted on Oct 27, 2011

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Van runs in park, but put it in gear and hit the gas it stalls everytime? im thinking map sensor. any advice. its a 1996 g10 conversion

Posted on Apr 16, 2010

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The cylinder 7 misfire is likely a symptom of the problem, not the problem itself.

Typically, the catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream 02 sensor reading should be fairly steady. Typically the P0420 code triggers the check engine light if the readings of the two sensors are similar. This is indicative of (among other things - such as faulty sensors in the "emissions loop") a converter that is not working as efficiently as it should be.

I do not believe that it is a problem with a valve/head gasket or cam.

Posted on Jan 13, 2009

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  • Chris Hall
    Chris Hall Jan 14, 2009

    Please keep me posted on the results of the test. I'll be curious to see if you've got a problem there and/or if it is at all related to this issue.



    Thanks and good luck!

  • Chris Hall
    Chris Hall Feb 02, 2009

    Randall, sorry I didn't get back to you sooner (was out of town for a bit with family). Wow, you had a heck of a puzzle going there. Great job on your description and diagnostics. I'm glad you got it taken care of. If you need anything, hauler back.



    Thanks

  • Chris Hall
    Chris Hall Feb 06, 2009

    Wow, that is an adventure. I'm glad you got it squared away. Take care, 3MAC

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