96 Silverado - Miss at hwy speed

[SoCalSlaughter]

Hello all, I'm finally tired of my truck missing like this, its getting very annoying. Truck is a 1996 C1500 5.7 auto. The truck starts and runs fine. It can cruse around town all day with no problem. But once I get onto the freeway 50mph+ there is a noticeable, intermittent miss at speed. It will cruse smooth for a minute or so then just have a miss and the truck will shudder alittle bit. If I floor it to pass someone the truck is fine, it will fly down the freeway. It seems to happen more at a steady speed. There is no engine lights on, had a full tune up about 8 months ago, fuel pressure is 56/57 psi, MAS is about a year old, crank angle sensor is free and clear of any debris, ignition timing is dead on. The only thing I haven't checked is the fuel injection spider, it still has the old poppet style in there (I know this from doing the intake manifold gaskets about 2 years ago) Thanks in advance

[SoCalSlaughter]

I'd like to add, the truck has 175k miles, the truck is mostly stock besides a factory style K&N air filter. And the last recorded DTC was P0420, O2 sensor was replaced and code hasn't come back (This was mid 2017)

[speedygonzales]

If it truly has a miss, it will record a 300's code. So ?

[SoCalSlaughter]

A miss or stumble. Engine running at speed, there will be a miss, stumble, hick up, whatever you want to call it.

[speedygonzales]

My point in asking about 300's was, does it have any history codes?

[determined]

Good chance the gear on your distributor is almost gone.

[likaroc13]

I've noticed my '97 Tahoe doing something similar. Around town speeds it's never noticeable. It's only when I get up to steady highway speeds. It's also random with mine. Some days it does it, & others it does just fine. I've also been on longer road trips where it would start doing it & eventually go away. Sometimes it would come back later on, but other times it never did it again. I have no clue why it does it. I haven't tried to dig into it much. I tried the fuel filter with no change. In late 2016 the distributor was replaced, & shortly thereafter the fuel pump & sending unit were also replaced. The miss wasn't there before that work was done.

[SoCalSlaughter]

I tried to start the truck the other day, it was sitting for about a week. It would turn over but wouldnt fire. It would stumble and miss. It would try and back fire out the exhaust, sometimes intake. Smells and raw fuel. The truck has spark. Im thinking something is wrong with the spider. Have to be turning it over and feathering the throttle for it to finally fire up

[SoCalSlaughter]

Originally Posted by determined

Good chance the gear on your distributor is almost gone.

The gear is fine. Had the dizzy out about a month ago. Problem was there before and after.

[likaroc13]

Originally Posted by SoCalSlaughter

I tried to start the truck the other day, it was sitting for about a week. It would turn over but wouldnt fire. It would stumble and miss. It would try and back fire out the exhaust, sometimes intake. Smells and raw fuel. The truck has spark. Im thinking something is wrong with the spider. Have to be turning it over and feathering the throttle for it to finally fire up

Wow, yeah nothing extreme like that with mine. Hope you get it figured out. Be sure to update us if you do.

[SoCalSlaughter]

Originally Posted by likaroc13

I've noticed my '97 Tahoe doing something similar. Around town speeds it's never noticeable. It's only when I get up to steady highway speeds. It's also random with mine. Some days it does it, & others it does just fine. I've also been on longer road trips where it would start doing it & eventually go away. Sometimes it would come back later on, but other times it never did it again. I have no clue why it does it. I haven't tried to dig into it much. I tried the fuel filter with no change. In late 2016 the distributor was replaced, & shortly thereafter the fuel pump & sending unit were also replaced. The miss wasn't there before that work was done.

I'm thinking this is a fuel problem. Because my exhaust stinks of gas. I'm thinking its the fuel injection spider. Mine is the old style still. And if its leaking, just 1 drop of fuel could cause the engine to stumble

[SoCalSlaughter]

Originally Posted by likaroc13

Wow, yeah nothing extreme like that with mine. Hope you get it figured out. Be sure to update us if you do.

Well the Silverado is on the back burner now. Found alittle bit of water in my oil in my 90' Suburban, so have the top engine disassembled at the moment.

[Tom]

My 98 has done this for the last 100,000 miles. Its random and doesn't do it every drive. Never sets a code. Have pulled distributor to check gear, new fuel pump when old went out, tune ups etc. Have great fuel pressure. Never have figured it out. And yes mine is always at a steady rate not accelerating. Anyone else in the truck doesn't even notice it happen.

[Kudzupatch]

I had to replace plug wires once and the new set was bad. Took me MONTHS to finally find it and the symptoms were similar It would start missing mostly at idle or low load and sometimes at highway. I assumed it was something else since the wires were new. Coming back from a trip it started to get really bad and when I popped the hood I could hear it arcing or I would have never found it I don't think.

Not saying that is your problem, just something to keep in mind.

[likaroc13]

Originally Posted by Tom

My 98 has done this for the last 100,000 miles. Its random and doesn't do it every drive. Never sets a code. Have pulled distributor to check gear, new fuel pump when old went out, tune ups etc. Have great fuel pressure. Never have figured it out. And yes mine is always at a steady rate not accelerating. Anyone else in the truck doesn't even notice it happen.

Sounds exactly like mine. Weird thing is mine never did it before the distributor & fuel pump/sending unit were replaced. I tried watching the instrument cluster but couldn't see any fluctuations when it happened the last time.

[SoCalSlaughter]

I installed an Acdelco MFI unit and the truck runs great. Drove on the freeway for about 40 minutes and the truck didnt miss a beat. Passed its Calif smog today also.

[likaroc13]

Originally Posted by SoCalSlaughter

I installed an Acdelco MFI unit and the truck runs great. Drove on the freeway for about 40 minutes and the truck didnt miss a beat. Passed its Calif smog today also.

[Tom]

Originally Posted by SoCalSlaughter

I installed an Acdelco MFI unit and the truck runs great. Drove on the freeway for about 40 minutes and the truck didnt miss a beat. Passed its Calif smog today also.

I've been wanting to do this to mine as well. Thanks for the feedback!

[speedygonzales]

https://www.summitracing.com/parts/s...et/model/k1500

[SoCalSlaughter]

I bought a ACdelco unit from RockAuto for $238. Came with new plenum gaskets

[SoCalSlaughter]

Use this discount code at RockAuto under the How did you hear about us. Take 5% off orders. Its good til Aug 5th. (8464893476112266)

[likaroc13]

Was searching for something else in my subscribed threads when I saw this one & remembered I forgot to mention that I figured out my issue. One evening in early Fall, the miss got worse as I was coming home from a road trip to my old hometown in MS. It threw a code which said cylinder misfire. After having it looked at, turns out the distributor cap wasn't tight. It had apparently been this way the whole time, since I had the distributor replaced back in late 2016. Once that was tightened up, I haven't had an issue since!

[Palf70Step]

Glad the issue was found and solved