Odd condition, random misfire, no power under load, bucking, dying...

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Hey guys. Chasing an old condition that has now come back with a fury. The truck has a chronic low/no power condition on warm start, off idle. If I'm trying to exit a parking lot, or turn across an intersection, this can be a bit perilous.

Essentially, the truck has no power when applying throttle at low RPM. The more you apply, the more it bogs. When you let off however it bucks as though it has power. This causes me to have to feather the throttle until speed is achieved and over 2000rpm when it finally can move more freely. If I apply too much throttle, there is an audible thud which comes from under the truck.

Also, there is another weird associated condition when it's hot out. When cruising at low speeds (parking lot, taking off from red lights), I get a noticeable howl/woosh with reduced power. It's not a metallic sound, but sounds almost like rushing air. The truck will not shift into 2nd and when it does will do so very hard. As speed builds, the howl subsides and the truck has more power.

It is also beginning to die when shifting to reverse from drive.
 

east302

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Is the check engine light on?

It could be a bad throttle position sensor, idle air control valve, maybe a damaged catalytic converter...


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Check engine is definitely on. Random misfire, and speed sensor warnings
 

east302

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What are the code numbers? P0300 and ___ ?


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I'm getting the misfire(P0300), MAF absolute pressure (P0068), EVAP System (P0440).
 

east302

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P0068 is probably where you should start. Try cleaning the MAF sensor and see if you can spot any vacuum leaks. Could be a faulty MAF, MAP or TPS sensor or a connection issue at one of those. With the hard shifts, I'd think a TPS issue likely, but that'd be a guess.

GM has pinpoint diagnostics for each DTC in their service manual, but I'm not seeing that P0068 code on what I have for my 1998. Maybe it wasn't in their program for that year...not sure. Anyways, having that would really help track down the issue and keep you from having to throw parts at it.

Try alldatadiy.com. It's about $30/yr for a subscription and been well worth it for me. They have the factory diagnostic tables for each code.

The misfire code may resolve itself once the 0068 is fixed. I doubt that the evap system code is causing your issues.



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Hey, my apologies, I went back to the actual codes I had written down and I had some wrong... The codes it's displaying are:

P0101 - MAF Voltage/Circuit ??
P0300 - Random Misfire
P0440 - EVAP System Control Failure
 

east302

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Here are the tables for P0101, hope it helps...

MAF connector:


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b7a6c9ee6fd43e128397090fdb248f01.gif

a1d1b4b9e3e7551cbe56789527b82ddd.jpg

a78ed11f26b5a0c35b69a5ec2dd0b71a.gif

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a33c0915bc6bd3038b818f5e0b3cbdd5.gif

4feb8353c01ef995d1ac961ff43448e8.gif


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Cleaned MAF last night with MAF cleaner, disconnected battery and started re-learn process. Seems to shift a bit nicer but still got bucking off idle. Not sure if that is still the original problem or just how the truck behaves during the re-learn.

I'll move on to the TPS
 
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