97 Bravada P0300 engine misfire - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 09-28-2015, 12:35 PM   #1
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Angry 97 Bravada P0300 engine misfire

So this is going to be a long one. So my dad and I do all the work on our cars. My 97 Bravada was originally put in our garage for a dead spot. It would fall on it'* face during hard acceleration after a cold start and overall power would drop out after getting hot. Originally confirmed not a transmission issue with a local tranny shop. Well, while under the wrench, we found that the intake manifold was being eaten away due to bad seals. So we replaced it and put felpro seals and tossed it all back together to find another issue. This is when the Misfire takes place. I have an obdII diag tool that also had PIDs for reading cylinder misfire. The MIL light came on and sure enough, P0300 engine misfire. After reading on the issue and doing some testing, it was determined that the 4.3l vortec v6 has troubled many with this p0300 code. Many throw parts at it and often never find the answer. Even some shops haven't been able to repair the issue. So far, we have found little things that needed to be done anyways that could effect the misfire. Found that the sfi injectors were faulty due to gm service bulletin recommending a multiport style injector. So far we have replace the following:
fuel injector pcv valve making noise
EGR Valve was leaking a bit
spark plugs
Distributor, Cap, and Rotor
fuel filter was clogged
post catalytic converter 02 sensor heater circuit was bad.
Tested plug wires
The fuel pump is whining now. Its not a bad whine but fuel presser checks out all the way through the system and no leak downs. The rest of the o2 sensors read like they should. Mass airflow sensor is good but did show that it was reading 1 gram per second higher than the factory manual range that was given. Honestly, this engine has never run this smooth in its life. The fuel injectors gave this some nice pep that it'* never had. I did notice that the misfires occur the most when decelerating from speeds of 40mph or more. But they do still occur in general driving. Counts when decelerating were as high as 90 counts off on cylinders 1 and 4 while others generally never reach more than 20-30 counts off. Only things we have not checked yet are the timing chain and the cam shaft. I was thinking about buying the pro comp upgrade kit for the camshaft and timing chain anyways.
I desperately need help and a direction on this guys. It'* been out of commission for two months due to me only being able to work on it on the weekends. This work is out of most of the local shops abilities except for maybe a dealer shop. I cant afford labor rates which is why my dad (mechanic of nearly 30 years) and myself are tackling this issue our selves.
P.*. I forgot to mention there is bad vibration on hard acceleration when nearing the top of each gear. Its most notable in 2nd and 3rd gear. I replaced the front differential about a year and a half ago due to the previous owner running 2 different sized tires on an AWD vehicle. I will mention that I have installed a 2" body lift and 31X10.50x15r tires. This is a 2" size up from stock.

Thanks for your time everyone!
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Old 09-30-2015, 07:21 PM   #2
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Can your obd scan tool display a graph of the TPS sweep?
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Old 09-30-2015, 09:15 PM   #3
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Quote:
Originally Posted by 1.8TTony View Post
Can your obd scan tool display a graph of the TPS sweep?
It cannot. But I got a lead that is promising. The plug wires are low resistance wires. They check out at around 1k ohms a foot when it should be 3k ohms a foot. My father found that low resistance wires can fry stock ignition systems. So we are gonna try replacing them, the coil, and the plugs with double platinum and see what happens.
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