KFtransam

I can't get any help elsewhere. So I'm turning to the guys who've helped me the most.

96 k1500 pickup
5.7 vortec

Just replaced injectors with the "gp sorenson" set from autozone and now I'm having running issues.

Low RPM missfire, rough idle. When I give it gas it chugs and almost dies until you reach about 1200rpm and it clears right up And runs great up from there.

Sprayed the intake Down with starting fluid, no vacuum leaks. Removed the plenum and double checked the injector order. All is good.

Checked fuel pressure.. 50psi give or take from a brand new pump and regulator. I spliced a 1/4" plug valve into the return line, throttled the flow down a little to bump pressure up to 60 psi.
Runs a little better, but still has the same issue at low rpm

Checked the trouble codes, throwing a cylinder 5 miss. I changed cap, rotor, and number 5 plug and wire..

No more miss code but it Still runs like garbage at low rpm..

Any ideas what I may be dealing with?
Thanks guys!

JosephIV

iTrader: (14)

Aren't you supposed to have 60+ PSI constant? Sounds like low pressure at idle IMO. I'm no expert though.

Why did you change injectors / what problems if any, where you having prior to the new injectors? The easiest way to proceed is to rule out the new parts.

KFtransam

Your right, they say 60-65psi is best.

It seemed as if bumping the pressure didn't really make a difference after reaching that 60+ psi and I didn't push it. Especially considering the method I used to bump the pressure, I wasn't sure if I'd be causing any new problems.

I could try to bump it a little more though.

And my reason for changing injectors,
I had a hard start and a bit of lost power.
Which we're cleared up with no doubts..

But now I'm left scratching my head with new issues that I could only imagine can be directly related to the injectors or something I fiddled with in the process of changing them...

JosephIV

iTrader: (14)

I'd get the pressure up at idle and go from there. I'm not sure why a brand new pump and regulator (non-adjustable I'm assuming) wouldn't have the truck running at the proper 60 PSI or more to begin with. I'm not one to trust "parts store" parts so I'd be paranoid concerning the regulator. An AC Delco unit is about $45 online vs $25 for non-OEM.

My dad's pickup was the only Vortec small block I've had any experience with, but that got traded in 2000. Hope you get it figured out. Hopefully it's something simple like the fuel pressure or such. My friend was banging his head against a wall troubleshooting his "new to him" '93 Z28. New OPTI, plugs, coil, and Ignition Control Module and the symptoms kept changing. Turned out he had a bad coil wire. Sometimes it's that simple.

KFtransam

Rough part is that i had replaced the factory regulator with a new delco one last year, though the pressure still hovered around 50 or so.

I'm thinking my next step, aside from fiddling with fuel pressure, will be snagging a new coil to replace the parts store junk that's in there now.

I'm really hoping for a simple fix.

Just on the way to work this morning I noticed a stumble at lower rpm highway speeds.
Plugged my scanner In and was throwing a "MAF circuit A low voltage" code. But maf readings seemed okay. Fuel trims still floating around 0-10%

Whether or not 0-10% is normal.. I have no idea!

Either way I'll try cleaning the MAF after work

97lt1camaro

iTrader: (3)

I have a 98 vortec k1500 and I'm fairly sure my fuel pressure is no more then 50 or so. My truck runs like a champ.

Have the intake gaskets ever been done on the truck?

KFtransam

Yeah,just tried fuel pressure at about 70 and had no difference from 60 so I turned it back down. I'm starting to think maybe this MAF code might be a factor. I'll unplug it on my way in to work and see what results come.

And I did put freah intake gaskets in last year. That was a blast, chased the issue for a month or so before I realized I had vacuum at the oil fill. Fun stuff!

jimmyblue

iTrader: (11)

Different injectors have different "offsets". I have no
idea how these "sorensen" injectors relate to OE ones.
Offset differences affect low RPM light load operation
the most, they are a fixed fueling error.

KFtransam

It's the simple things... I unplug the MAF and it runs like butter this morning..
Now I can stop beating my head against the wall!

I do have a question though, how long can I run without the MAF? Do I have time to order an ebay one or do I need to go foot an arm and a leg at the parts store?

JosephIV

iTrader: (14)

I hope that you figured it out. Chinese MAF's are still over $125 aren't they? Used OEM ones go for $40 on Eb@y. You could buy two used ones from online and still save money or go to the junkyard. Nothing to lose if they have a 30 day or better warranty like my local yards have. I'd call a junkyard to see if they have one. Drive there, buy the MAF, swap it in the parking lot, and if it's good and runs right call it a day.

Just a thought, but did you make sure that you don't have a vacuum leak somewhere after the MAF? It could account for the rough running and also run better when the MAF is unplugged and the computer goes into Speed Density mode (if your truck does that).

hokeplaya05

iTrader: (3)

it does sound like a vacuum leak that would be causing it.

ive never heard of the "gp sorenson" unit before, was there a reason you didnt upgrade to the delphi mpfi unit? when my stock spider assy crapped out in my truck earlier this year, i did the upgrade to get away from the crappy stock style injection

KFtransam

Well it's not the MAF, a buddy of mine had 2 extra used ones lying around so we tried em in my truck to no avail.
Now I can't help but think I've narrowed my search by verifying that when the MAF is unplugged it runs great.

I've sprayed everything Down for vacuum leaks and given the intake assy. A look through checking for leaks, nothing jumps out at me.
The gp Sorensen set is sold at autozome. I originally wanted to foot the extra for the delco unit until autozone mailed me a 20% off coupon.

KFtransam

Even though the injectors are designed to be plug and play? Many reviews of other people with no issues, I can't help but feel like there's something else at play.. My factory injectors are in such rough shape and are physically broken at the poppet nosles. My scanner shows when the MAF is unplugged the fuel trims are at a .75% value. When I plug the MAF in, the values start there and rise until reaching 50%.. In not sure what it should be but the Map reading is 30kpa.

benny92

Check around the injector pod seal in the middle of the plenum, there should not be any gap between the lip on the plenum and the top of the injector assembly. On my brother's truck, the new mounting bracket was a little shorter(~1/8") and did not allow that oring to fully seat in to the plenum. We added a couple washers under the bracket to lift it and our problems went away.

benny92

That information in this application would be apples to oranges. The factory 96-99 vortec trucks used a large central injector through a hose spider and 8 poppet valve "injectors," like an injector pump and nozzles on a diesel. The replacement is an upgraded spider assembly with 8 individual micro injectors, similar to a conventional multiport gas setup. It is a direct plug and play, and common maintenance upgrade for vortec truck owners.