'92 K2500 5.7 TBI - Crank for days and no start after intake gaskets...

[VintageMetalMaine]

Got a frustrating one here! We could really use some expert help figuring this out...

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.

FORMER ISSUE: Began running rough/low power/poor fuel economy with intermittent check engine light at speed. The loss of power and backfiring at speed finally got so bad we began tearing into it.

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

Tore it down and performed the intake manifold 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!

CURRENT ISSUE/STATUS:

• 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

ATTEMPTED SOLUTIONS:

• 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!

We really appreciate any guidance or advice!

 

[Schurkey]

You say you have both spark and fuel, you say you have proper ignition timing. About all that's left is fouled plugs, because you wouldn't have changed the cranking compression or the exhaust flow with the work you did.

Remove spark plugs, clean the firing end in a propane torch flame. (NOT MAP-Pro, NOT Acetylene...use PROPANE) When the yellow is gone and all you see is orange flame, you're about done. The porcelain should be white when you finish. Check the gap, adjust as needed. Or just replace the plugs if they're worn.

Spark at the plug wires is not spark across the plug gap. Spark across the plug gap is what's important.

 

[smdk2500]

You said that your checked firing order but is it going in the correct rotation direction? It should be in a clock wise direction.

 

[Hipster]

Checked for timing chain slack? compression test? Sounds like it's got some serious miles on it. How did the cam look while you were in there?

 

[PlayingWithTBI]

About all that's left is fouled plugs,

X2

Random thoughts from your post...

No, we do not own ALDL cable or use winALDL software, yet.

winALDL is old school, try TunerPro RT v5 instead.

While attempting to calibrate IAC and base idle, we discovered the engine would race when IAC disconnected

Did you follow this procedure?
http://www.gmcmidwestclassics.org/Web pages/Tuning the TBI.pdf

Replaced O2 sensor, CTS, MAP sensor, IAC, distributor,

What brand distributor, what about your coil?

Tore it down and performed the intake manifold job

Check all of your ground connections including the back of the head and at the T-Stat snorkel.

Changed fuel filter as precaution. (Had swapped it when replacing fuel pump last summer)

Check your fuel pressure at key on, no crank s/b 11 -13 (factory says 9 - 13)

As always, you need to connect a scanner or TunerPro RT, data log and see what mat be your issue.

 

[VintageMetalMaine]

You say you have both spark and fuel, you say you have proper ignition timing. About all that's left is fouled plugs, because you wouldn't have changed the cranking compression or the exhaust flow with the work you did.

Remove spark plugs, clean the firing end in a propane torch flame. (NOT MAP-Pro, NOT Acetylene...use PROPANE) When the yellow is gone and all you see is orange flame, you're about done. The porcelain should be white when you finish. Check the gap, adjust as needed. Or just replace the plugs if they're worn.

Spark at the plug wires is not spark across the plug gap. Spark across the plug gap is what's important.

THANK YOU! Great suggestion. Will do. Plugs were replaced as part of the process a few months ago and definitely look darker/wetter than "normal for low miles on them.

NOTE: Old plugs had about 12K miles on them and were mostly tan/brown and a just little "burnt" at the electrode tip when we replaced them. Nothing unusual or noteworthy. Even the ancient plugs we took out when we bought it didn't look bad after God-knows-how-long they were in there, which gave us confidence in the overall health of the old 5.7.

 

[VintageMetalMaine]

You said that your checked firing order but is it going in the correct rotation direction? It should be in a clock wise direction.

Thanks! Yes! Marked wires and cap before taking it out... Dummy proof!

 

[VintageMetalMaine]

Checked for timing chain slack? compression test? Sounds like it's got some serious miles on it. How did the cam look while you were in there?

Thanks! No. Only did intake manifold gaskets at this time. Did not go deeper into motor as no other symptoms were present.

Timing light is dead on the mark and does not jump around at all... Ran great right up until the problems described here, before the gasket job.

Thoughts??

 

[evilunclegrimace]

Just a thought. Did you put rags in the intake ports while cleaning the heads to keep debris out of the intake ports and forget to remove them? I've seen it happen.

 

[evilunclegrimace]

You say you have both spark and fuel, you say you have proper ignition timing. About all that's left is fouled plugs, because you wouldn't have changed the cranking compression or the exhaust flow with the work you did.

Remove spark plugs, clean the firing end in a propane torch flame. (NOT MAP-Pro, NOT Acetylene...use PROPANE) When the yellow is gone and all you see is orange flame, you're about done. The porcelain should be white when you finish. Check the gap, adjust as needed. Or just replace the plugs if they're worn.

Spark at the plug wires is not spark across the plug gap. Spark across the plug gap is what's important.

Grab yourself one of these to test your spark with.
https://www.bing.com/aclk?ld=e3YLhb...=9c606f41f84f1840299f326025c011b2&adlt=strict