L31 running odd/ unusual issues

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jimmyve2

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Sep 29, 2021
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Alright, so I rarely post anything in forums but this issue has us stumped.
99 suburban k1500 160,000 miles.
So I bought this truck about 2 years ago and it's always had some unusual runnability issues.
First it misfires if you lay into the throttle going up steep grades or floor it on the highway for an extended period of time, always cyls 3 and 5, never sets a cel when it does it.
It will carry on at full throttle, but after about 10-20 seconds it loses power and starts misfiring. It always clears up if you slow down a bit. Fuel pressure is at 55 psi at idle, hits about 58 when driving. Updated spider installed, made no difference. The oe distributor gear knife edged about 6 months ago. Brand new standard ignition unit was installed. Plugs and wires have about 5k on them and plugs look fine. Comp 160-170 across the board.
Installed a new blue streak coil and dyna mod icm. This made a noticeable difference in power, but it still misfires.

Second issue is, this thing gets about 8-10mpg typically, but randomly the economy will tank to 5-6 mpg and drink 40 gallons in 200 miles. It does this completely at random and seems to be a bit low on power and idles slightly rough when it does it. Typically when it does this it has an extended crank when hot. I always use Chevron gas, does not make a difference if I fill up at a different station than normal.

Both of these issues seem to be worse if the weather is hot. And like I said it is low on power on hot days. Any ideas?
 
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