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TBI to Carb...

stockk5

stockk5

1/2 ton status
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Well before everyone starts freaking out.. im just trying to weigh my options.. my truck has been down for over a month now. I installed a new engine.. drove it around town about 5miles.. came home and was going to button up all the final parts of it. I shut it off.. replace my TV cable to my 700 go to turn it on.. doesnt send a signal to the injectors to pulse to get the fuel spraying. I have replaced my ignition module, tried a new working TBI carb, had a computer diag. done and the computer is getting power, all codes are working properly, even swaped out my computer for my bro's working computer, no cuts in the wires anywhere and fuel pumping to the back of the carb. I even have had a professional former head mech. of a chevy dealership working on it and he is stumped. Both him and my dad say put a carb on it.. but i dont know.

My options are either to bring it to a dealer/shop and have them fix it.. prob cost me hundreds.. maybe a 1g? not really sure but trying to wiegh if i should do that, or just swap it all over to a carb set up. I just dont know whats involved with making it a carb set up. Mostly confused on what to do w/ the tranny since it does have those electronic plug ins and how would it work w/ a carb. Im just really lost on what to do.. either sell my truck, get another, swap to carb or get raped at the repair shop. :doah: :( Any advise would be very helpful...
 
Learning to fix it yourself is an option you didn't mention.

No reason you can't do what a shop can do, most are crooks and have no vested interest in anything other than taking your money anyways.

There are troubleshooting flow charts in threads here, ("crank no run" might be a good search string) you can fairly easily follow those, and any questions you have regarding how to proceed can always be asked here.

It's not too complex...ECM grounds the injectors based on signals from the ignition, one wire to each injector is 12V, and one is the ECM grounded one. Tough to troubleshoot sometimes, but when you are staring at a pile of spaghetti, it seems overwhelming. Just because someone supposedly more skilled than you couldn't figure it out, doesn't mean you can't. :)
 
I would just fix the TBI


Are you sure the injectors are not firing? You can run the rig by spraying carb cleaner in the TBI. If you are fast, you can do it by yourself... You can eliminate other problems by doing this test. For example, you can be sure the dist is firing.


I had a similar problem once. Turned out an orange wire was pinched and grounded. It ran on the driverside by the bellhousing. No codes thrown and everything seemed fine but the rig would not fire. It would run on carb cleaner but no fuel would pump. I can't recall what the orange wire went to but it was connected to the ECM.

I suspect the ECM turns off fuel if it detects anything out of the ordinary.

Try following the wires and make sure they are in good shape. I know you said you did that but that is where I would look if the injectors are not firing...

If you swapped out the parts to test like you said, i think you just have a simple wiring problem.


Not sure about getting the 700r4 to work without a computer. The ECM does connect to it... But the carb part is easy. just get the stuff and bolt it on. Intake will require slight bolt hole modification. Might need a different fuel pump setup too... Mech pump and new sender is pretty simple to install.
 
Expanding on what Dorian said -

There are several grounds for the ECM. I'll bet the injectors have their own ground wire. maybe something happened to that ground when you buttoned up the rig. I think there are a few grounds in the enging compartment alone for the ECM. Check em out by following the injector ground. Sounds like a good place to start.
 
stockk5 said:
I shut it off.. replace my TV cable to my 700 go to turn it on..
Click to expand...

Something happened during this timeframe. Everything ran right until you got under the hood. I'd guess you bumped something loose or installed your air cleaner and pinched the injector wire(s).

Might consider taking 24 hours away from it and try to come back with a fresh perspective. Sometimes the answer comes to you in your sleep.
 
yeah i've ran my truck on carb cleaner, starts up that way only for a quick second. I guess i can start from scratch. pull every wire off and go through it all again. When you guys talk about grounds for the injectors.. are they physical grounds i have to bolt down somewhere? sry for sounding so hopeless, i've just been trying to solve this for about 4weeks straight now on my own and its seriously starting to affect my mood, thinking oh this will fix it.. then again no it doesnt day after day. but thanks for the tips guys, ill try to get back at it again.
 
This might be a silly question, and this is probably a follow-on to what was posted earlier in the thread by Dorian and others, but have you tried the troubleshooting flowcharts from a GM factory shop manual (the real manual, not a Chilton book)?
 
stockk5 said:
When you guys talk about grounds for the injectors.. are they physical grounds i have to bolt down somewhere?
Click to expand...

Don't remember how the injectors are wired but I am pretty sure there are at least three bolt down grounds under the hood. Might even be more. There are several ground wires in the harness that need to be hooked up. Again, I really can't remember since its been a year or two since I played with one but I think there was a firewall, head, manifold, and possibly a frame ground that all went to the ECM.

Also remember that some of the ECM wires go through the fuse box into the engine compartment, not just the loom by the heater. So check those too.


One other thing I did when working on TBI was to get a complete set of sensors from a junk yard that were good. I actually have a complete TBI system as test pieces to troubleshoot. When all else fails, I start swapping known good parts in the system until I find the one that is bad. Looks like you already did a little of that. You can get them cheap at junk yards and they are great to keep around...


You can keep the rig running on carb cleaner if you spray some in there, start it, then haul ass back to the TBI and spray some more before it dies. Hook up a test light to be sure the injectors are not firing... I guess you could do that cranking too. Are you sure the pump is running? test light it to be sure. Fuel at the back of the TBI may not be an indicator of a running fuel pump. Don't they hold pressure for some time?


Keep at it :) You'll get it solved. Just takes some thought and a good process of elimination
 
when i go to work tomarow morning i will show this to my 30+ year tech and aslo gm specialty tech. he might just know. and we dont call him wizard for the fun of it. if he cant figure it out we send it to the junk yard.
 
thanks a lot guys, and yes i have gone through a list of things that could be wrong with it. The fuel pump is def. working cuz when i take the lines off and turn my truck over TONS of gas spews out, you can hear the fuel pump turn on as well. I took my brothers computer and his tbi carb outta his truck and put it on mine and that didnt work, i also have the new ignition module, that doesnt work. There are 2 grounds on the back of the head that i bolted on. The ground that comes off the fire wall thats the braded steel line, and the ground that comes outta the wiring harness right as it goes down to the starter. Those are the only 2 i can think of other than the one to the frame near the battery. The map sensor wouldnt make a difference cuz thats emmisions right? Thanks for your help guys, its tough cuz i dont have the internet at my house, and can only get on here like 1time for a few mins every other day unless i go to a friends. You guys def. give me some hope though through this, thanks.
 
another thing, when i first put my engine in i made a heat shield that i just wraped around the solinoid of the starter and it went down over the starter wires to protect them from the headers. When this all started my dad thought i just blew up a fuseable link. There are a few near the starter but w/ all the test we've done with parts getting power it seems that isnt the case but could it still be?
 
IIRC the injectors are grounded thru the ECM. the injectors always have 12V to them when ign is on. ECM grounds them to supply fuel.

Have you checked your magenetic pick up coil in the dist? If it is bad the ECM will see no spark reference siginal and not allow the injectors to fire.
Do you have any trouble codes?
If the only code you get is a 12 and you have replaced the dist module. The pickup coil is the next thing you should check. Resistance between the wires should be 500-1500 ohms.If out of specs it is bad. Many times a bad pick up coil will also give a code 42 but not always.
Check all th wires in and to the dist.Be sure all the wires are plugged in
 
what about the grounds at the tstat housing.

and ya check the pickup coil ring like listed above.

also just cause fuel runs out dont mean its got pressure. it might have flow but little pressure.
 
yeah, i've checked the fuses and the ground at the tstat, im getting a computer tester today.. gotta go back and double check it all.
 
i was thinking, will the coil still send spark but not a signal to the computer? cuz when i spray carb cleaner in it starts... so that means the coil part of the coil is working but what about the connection between the coil and the distributor, the 2 wires that plug into the ignition module... is it possible for those just not to be sending a signal out?
 
stockk5 said:
i was thinking, will the coil still send spark but not a signal to the computer? cuz when i spray carb cleaner in it starts... so that means the coil part of the coil is working but what about the connection between the coil and the distributor, the 2 wires that plug into the ignition module... is it possible for those just not to be sending a signal out?
Click to expand...

Yes there is a magnetic pic up that sends the spark data to the computer. If the computer gets no signal of spark then it will never ground the injector therefore no fuel delivery. Mine did that exact same thing and it was the ECM inside the distributor. It has 2 weatherpac connectors on it.


Ira
 
Yes it is possible for the engine to run if the pick up coil is bad by spraying carb cleaner or dumping gas down the TB. It doesn't stop the spark if the pick up coil is out of specs. The ECM shuts off the fuel by not pulsing the injectors if it sees readings out of specs from the pick up coil or dist module.
It is a safety thing to keep the injectors from flooding the engine and possibly causing a fire if there is no spark or multiple spark related misfires.
 
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