VintageMetalMaine
Newbie
Got a frustrating one here! We could really use some expert help figuring this out...
BACKGROUND:
We updated/replaced/performed the following with NO CHANGE to the original symptoms before finally diagnosing leaking intake manifold gaskets:
BACKGROUND:
- 1992 K2500 Sierra SLE 5.7l TBI w/147K miles
- Bought in 2017 for son and I to have as a work and "project" truck
- We have been repairing, upgrading, and restoring the usual - A/C compressor, alternator, pulleys, belts, bearings, shocks, brakes, bushings, catalytic converter, and complete tune up.
- ALWAYS started and ran strong given age, mileage, and obvious abuse by former owners
- We are barn/driveway guys with basic knowledge/skills and decent home shop setup/tools
- No, we do not own ALDL cable or use winALDL software, yet.
We updated/replaced/performed the following with NO CHANGE to the original symptoms before finally diagnosing leaking intake manifold gaskets:
- Vacuum was steady and at the low end of "normal" on our cheapo gauge - 17-18lbs?
- Replaced O2 sensor, CTS, MAP sensor, IAC, distributor, knock sensor, PCV valve, both valve cover bushings, all vacuum hoses and the EGR valve and EGR solenoid.
- Rebuilt TBI - She was really dirty/carboned up good. Good spray pattern before and after rebuild.
- Changed fuel filter as precaution. (Had swapped it when replacing fuel pump last summer)
- Replaced distributor after noticing timing not holding steady - it was very worn and had "slop" in it - basically fell apart when we started poking around
- Cleaned and/or replaced/reconnected all ground points and cables on entire truck - they were a mess - amazing it ran at all
- Sent Jet Stage II chip to be tested- all good
- Replaced ECM as a precautionary measure - the old one was very "crusty" but seemed functional
- Set base timing to 0 TDC with ECM bypassed - found it advanced 3-4 degrees BTDC
- While attempting to calibrate IAC and base idle, we discovered the engine would race when IAC disconnected - we could not set base idle
- Engine would start right up cold or hot, run rough, and get worse as it got hot - very little change from any of the upgrades/repairs listed here.
- This lead us to realize we had intake air issues, hence intake manifold gasket job
- RTV had failed terribly at rear of manifold
- Engine looked surprisingly clean for age and being worked hard by previous owners
- We replaced the oil pressure sensor while we were in there, just because it was ancient and easier to get at while apart
- Set timing by positioning the distributor where it came out - we had made marks and taken pictures to guide us
- Went back together great - without a hitch... UNTIL NOW!
- As our post indicates, motor will crank for days and almost start - sounds "flooded" - like a stuck choke
- Oil pressure gauge reads 25-30 while cranking - goes up a little when engine almost starts
- Good spray and pattern coming from both injectors - exhaust smells like gas after cranking a while
- Checked timing with ECM bypassed while cranking - adjusted slightly to get to 0 - was only half a degree or so out
- Produced a single loud backfire from exhaust while cranking when I tried WOT to clear fuel out
- Firing on all 8 plugs: #6 + #7 are notably wet/darker - #3 is clean and dry as a bone - remainder are wet/tan, not fouled, though
- Starting fluid does not help
- Triple-checked firing order with plug wires - it's correct with spark on all 8
- Pulled #1 spark plug and confirmed TDC compression stroke is correctly in position with distributor and #1 pin on cap - twice - it's perfect
- Unplugged MAP and cranked
- Unscrewed IAC a few turns to let air in during cranking
- Unplugged IAC and cranked
- Checked voltage to every sensor power wire- all getting required 4.8-5V
- Checked TPS voltage throughout range - 0.48V at closed to 4.8V at WOT - no fluctuation
- No codes except those produced when disconnecting sensors and cranking
- Reset ECM / cleared codes
- Let sit overnight and cranked it again
- Gave up and decided to ask for help here!