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Old 02-17-2024, 07:33 PM   #1
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1993 GMC 366 - Ignition

Hello,

Needing some help diagnosing an issue.
1993 gmc blue bird, 366 tonawanda gasoline carburetor engine.
Was running fine two weeks ago and won't catch now. Opened carb and there is no fuel coming out of injectors, not a single drop. Changed MAP sensor, no luck. Pulled a spark plug and noticed it wasn't sparking. Opened distributor and cleaned very minimal decay off connections, no change. All fuses are validated. Emergency doors are closed/unlocked. Dont believe it is ECM because all other electrical systems work fine, i can hear the fuel pump kick in with the key turning. I'm thinking of buying a new distributor, but want some help diagnosing. Has anyone ever had this type of issue?

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Old 02-17-2024, 07:34 PM   #2
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Oh, and it turns over good no issues there. All other electrical looks good. Full tank of gas only a couple months old.
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Old 02-18-2024, 06:54 AM   #3
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you can hear the fuel pump so i assume electric.
do you still have a points distributor?
full tank but not getting any fuel?
do you have a filter before the fuel pump?
can you tie your fuel pump into a five gallon can and see if you get fuel.
carb float hung up?
no spark?
ignition coil?
ignition control module?
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Old 02-18-2024, 07:24 AM   #4
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something is off here.. this is a 93 with a carb? thats either not factory or its not a 93... GMC converted to TBI fuel injection in 1990.. a TBI unit looks like a carb but isnt..



defimnitely by 93 theres no points distributor.. GMC converted in around 73 or 74 to HEI non points distributors..


there was a time in the 80s where GM used electronic carbs (yes it was a thing).. which had an ECM...



it sounds from the descriptiomn like you have a TBI system.. have you tested fuel pressure? I believe there was a test port on these near the throttle body (what you are calling a carb).. when you turn the key, fuel pump should kick on for 1-2 seconds and pressure should go to around 10 PSI... (they work asnywhere from 9-12 or so)..


when you turn the key to ON does your check-engine light come on and then go off? or come on and stay on?



if it doesnt then check the bulb and the fuses around your ECM.. once we can get the bulb working then you can in fact flash the codes out of these.. (they are OBD-I compliant)..


the biggest failures ive run into on these are the 4 pin module in the distributor goes bad.. you take the cap and rotor off and there is a module in the bottom of the distributor.. auto zone used to be able to test these.. often they die and a person installing a new one never used the heat sync compound on the new one which made a short life for it..



when that module goes bad, the engine has no sense of rotation and therefore wont fire the distributor.. it is possible the reluctor ring in the distributor is bad.. but I rarely ever saw that..


the 2 biggies for no-start were the fuel pump (runs but doesnt make pressure).. and the module in the distributor.


wow its been a LONG time since I worked on TBI.. I used to pull the EPROM chips and alther the fuel / timing tables on those , reburn the chips.. cheap and easy power-adding back in the day.. I remember building a digital dash display for my police impala out of an old thinkpad and a serial port interface connected to the ALDL port at a whole 160 Baud... I probably still have all the software for that on a floppy
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Old 02-18-2024, 11:53 AM   #5
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Cadillackid was spot on, I have been mistakenly calling this a carburetor system but it's a TBI. I think the notes on the fuel pump and distributor are still valid from jollyroger and I will be trying to validate the fuel filter as well.

Cadillackid, I hear the fuel pump come on when keying the ignition and I get the check engine light on and staying on. I had no idea I could flash the codes I will be trying to do that today before going the route of distributor or the 4 pin module inside.
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Old 02-18-2024, 12:52 PM   #6
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could also be the sock on the pick up tube in the tank.
thats why i suggested trying to run on a fresh five gallon can.
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Old 02-18-2024, 01:41 PM   #7
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Originally Posted by Jolly Roger bus 223 View Post
could also be the sock on the pick up tube in the tank.
thats why i suggested trying to run on a fresh five gallon can.

im not sure physically where the pump is on the busses.. I think I remember one of my buddies telling me it is on the rail just outside of the tank.. on the TBI cars the pump was often part of the sending unit in the tank. It was on my C body cadillacs and B-body chevrolets..



flash codes were easy.. 93 is a later year for TBI.. I mostly dealt with the 80s cars.. by the 90s cars i had, they were running TPI or MFI whuch was a different animal..



the 80s cars you turned the key to ON then put a paper clip from the A to B terminals and the car flashed you its codes.. a single flash then a pause is the digit 1, 2 flashes and a pauses is the digit 2.. after the second digit is a longer pause before it goes to the next code or repeats..



code 12 ironically is always there and nfortunately its also a code that tells you there is no pulses from the distributor reluctor ring.. so its always there as you never take codes with the engine running and you never try to start your engine with pins A and B jumpered..



you also wont see low fuel pressure in the code list as theres no transducer.. you have to test for that manually with a pressure test gauge..



since I have a whole damn garage full of tools, my progression here would be as follows:


1. check for fuel pressure at the test port - if none theres your troubleshooting point. some of the things roger brings up like fuel filters, clogged sock, bad pump come into play at this point.



2. if you have pressure the next likely point is to check the distributor module, the module not only provides spark timing but also pulses to the ECM so it knows when to fire the injectors.. each injector pulses a little fuel into the manifold based on the timing of when a cylinder is beginning its intake stroke.. its a bit of a voodoo since its just firing fuel into that bank but never the less ther pulses alternate and are timed..



a somewhat risky poor-mans test is while the engine is COLD you can pour a bit of gasoline down into each hole in the throttle body.. just a drizzle.. absolutely dont pour a bunch.. then SCREW THE AIR CLEANER LID ON!! with the filter in.. and see if you can start the engine.. if it runs and dies for a few seconds then you know the distributor is working and providing signal, timing and spark.. look to your fuel.. if it doesnt even try to start then you likely have electronics in play.. at that point im full on suspect of that module..
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Old 02-18-2024, 02:47 PM   #8
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if you are getting fuel pressure also pull the dizzy cap and have someone crank the engine to make sure it is spinning. i had one shear the gear pin off at the bottom of the dizzy.
that was fun to troubleshoot on the side of the road at 4am?
oh and that was a 71 ford with a 400. not a chebby.
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Old 02-18-2024, 03:07 PM   #9
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Originally Posted by Jolly Roger bus 223 View Post
if you are getting fuel pressure also pull the dizzy cap and have someone crank the engine to make sure it is spinning. i had one shear the gear pin off at the bottom of the dizzy.
that was fun to troubleshoot on the side of the road at 4am?
oh and that was a 71 ford with a 400. not a chebby.

good call!


ha! i remember shearing a dizzy gear off once.. in a street race.. 2 of us going neck a neck down a 2 lane freeway ramp.. im pulling ahead of the friend in a vintage built-up roadrunner. (I was in my G-body monte carlo with a 387 stroker).. 6500 RPM and all the sudden from under the hood.. pop-de-pop-pop and then nothing! ..that sinking feeling that I just blew my engine. the tach went straight to zero and im coasting...



I look in the mirror no smoke.. but definitely no throttle response.. and 0 tach.. but wait.. my A/C is getting cold again(I hada wide-open throttle A/C cutoff)..and my oil pressure is still up... how could that be if i just wiped out my engine.. I had the revelation and a big whew! that my engine was still spinning and not making any ratatata noises... just no spark.. LOL...
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Old 02-24-2024, 10:20 PM   #10
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Just closing this up, changed the ignition module and she fired right up! Had checked all the other various items, and couldn't get any ignition with a little high octane dribble direct into the throttle body so the culprit seemed pretty obvious by then.

Thanks for all the help!
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Old 02-25-2024, 07:11 AM   #11
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Originally Posted by Rocky303 View Post
Just closing this up, changed the ignition module and she fired right up! Had checked all the other various items, and couldn't get any ignition with a little high octane dribble direct into the throttle body so the culprit seemed pretty obvious by then.

Thanks for all the help!

sweet!! be sure to use the heat sink compound on it.. and get a spare to toss in the glove box.. those modules were / are a staple of any GM TBI / even earlier HEI to keep in the toolbox.. they give you zero warning in most cases... they just quit..
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Old 02-25-2024, 11:05 AM   #12
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Just a note
ECMs in many brands will not turn on the injectors If the ecm does not see a signal at the coil ( OR at a crank sensor if so equipped)
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