1999 K1500 Suburban Mis-fire

[MASTERBrian]

This truck has a Vortec 5.7L and it seems to have a slight mis-fire or something similar going on. Several years ago I had the spider injectors replaced with supposedly the updated version but I've been having issues on/off for a while now. Several years ago, while towing our boat, with the boat and truck loaded with people and camping gear, getting on the highway it started bucking really badly, but once I leveled out at speed all was good. That was under heavy acceleration and after that I took it easy and all was good, except one time on a steeper boat launch, when pulling my 18' Bass Boat from the water.

I took it to two shops when I got back and one shop said spider injection issue, the other (which had replaced the spider about a year prior) said something different like maybe intake manifold. From that point forward, we only really used it for town driving and never much more issues that I noticed, so we left it alone. My wife mainly drove it and did occasionally complain that under really heavy acceleration it acted up a bit and occasionally threw a few mis-fire codes, once or twice even a tranny code. When I looked into the tranny code, I was told that pointed to an issue where without a rebuild I would eventually loose OD, then D, then 2, etc....that was a year or two ago and the tranny still shifts fine and I haven't gotten the code for some time now. I figure I'll cross that bridge when/if I have to....not fixing what isn't totally broken if either way requires a rebuild that's the same cost.

Back to the stutter/mis-fire I'm having I thought maybe a bad plug, I had a total tune up last year I believe and I've even replaced the distributor once in this truck. #7 cylinder plug was a bit corroded, I cleaned it and the problem was better. Cap/rotor look fine and I see no electrical sparks from wires.

The stutter/mis-fire feels like a 2-stroke boat motor does when it gets a bit of water into the intake and with some gas it cleans out and runs....not sure if that makes sense, but that's how it feels. Years ago I road stand up jet-ski's and would do sub-marine tricks and such were they'd take on water, run a bit rough, clear out and go again....this feels about the same way.

Upon reading online it seems intake manifolds are a common issue on these as are the spider injections (notoriously needing replaced every 50k+ miles). Intake apparently is fluid loss, this truck does run low on anti-freeze a lot, BUT it seems to have a lot of cold leaks on the lines going to the rear heater core. I recently had to re-tighten a line and it held anti-freeze for several days, then it lost some and I noticed another leak on the fitting under driver door, so I need to cut the factory crimp out and put a hose clamp on. Because of this, I can't tell if the manifold is leaking or just the hoses....I was starting to think mani, but after fixing it for a few days again, I'm 2nd guessing that.

I rented a fuel pressure gauge and tested that. I seem to have mis-placed my notes on this, but from memory....
With Key on-not start pressure was 56-58psi and it didn't drop rapidly with key off...in fact seemed to hold for a minute. At times that reading of 56-58 fluttered a bit before settling to one or the other.
Engine on at idle was about 48psi, blurping the gas rose the pressure back to 58.
I then replaced the fuel filter and reading rose a bit at idle. I think it went to 52psi and blurping the gas it still went to 58.

From what I understand, anything under 55psi can cause drive-ability issues on these. I ran some BG fuel injector cleaner, that seemed to help some. Had to get gas, put in about 10gallons of fuel and the problem seems about the same again....any chance a rich mixture of sea-foam will help?

Where do I go from here? Some things say the fuel pressure regulator never leaks on these, some say they do. I believe a test for this is pinching off the return line....

What might cause #7 to be the dirtiest of the plugs? To clean it I used one of those plug sand blasters, which usually buys some time on plugs like my boat plugs that tend to run rich. The plugs in the burb all looked pretty good mixture wise.