try ohming out the injectors while it's in a no start condition. Also make sure the computer is actually trying to fire them.
if i remember correctly they should have about 1.3-1.5 Ohms

 

is there actaully a no start or does it just cut out and restart next key cycle?

 

It seems to be random. Wouldn't start at all today, again no fuel being sprayed. No start and stays dead lol

You want me to check resistance between the two connectors on the injector? And how do I check if the computer is firing them, do I get a 12v signal across the wires?

 

Man are you sure it's not the fuel pump. I'm getting the same problem on my blazer but it's just a bad ground or something at the fuel pump. Same symptoms as what you are describing.

 

By disconnecting the line right at the TBI I am able to turn the key and shoot a very healthy stream of gasoline from the line. I would say fuel pump is just fine. Could do a pressure test just in case but my gut tells me its healthy.

 

I just went through this with my 91 350 TBI. Fuel pressure should be between 9 and 12 psi, no higher than 15 psi.

If the injecters are not opening at all check the signal coming from the distributor magnetic pickup. The ECM won't enable the injectors till around 400 rpms during crank over. The pick up tells the ECM that it is trying to start.

 

How do I check the signal? What am I looking for?

 

are the injectors clogged up or simply bad? if its getting spark, fuel to the throttle body but not coming out of the injectors i would suggest the injectors are not working, trace the wires back to the ECM from the injector plugs, make sure nothing is melted or cut, try taking your injectors off your other tbi and see if they get it running. i know next to nothing about these tbi's but just from what have say its my deductive reasoning saying the injectors are working...

i would put a meter on them and have someone crank the motor... they should read something (not sure exactly the voltage) when trying to crank, if they arent getting any voltage than i would suggest wiring, or maybe a fuse if there is one that would have an effect on this system. if they are getting power then call a dealer maybe and see what voltage they need to pop open... and see if they are getting ENOUGH voltage, if not, check grounds... if they are getting proper voltage then i would suggest something the from the fuel inlet to the injectors is clogged...

if you take the return line off the back of the tb do you get fuel coming out of there at a good rate when you crank it? like the fuel is just bypassing the injectors due to a clog or something of that sort?

sorry if none of this helps, im a newb. just thought i'd give my .02...

 

1 Take the electical connector off of the injectors
2 Place your ohm meter across the two little prongs sticking up (each injector has two)
3 See what it say's should be around 1.5Ohms each
4 if not, replace injectors. FYI 350 and 305 injectors look the same but flow differnt amounts of fuel.

Next test.....
1 Go to your local parts store and buy a $2 noid light for GM throttle body.
2 Take the electical connector off of the injectors
3 install the noid light
4 crank and look for flash


all this should be done while its not starting. if this is over your head bring it to a shop, worst thing you can do is mess around with it and then bring it in. you'll just end up paying someone to fix what you took apart.

 

Not over my head at all, just don't know how to trouble shoot EFI (I'm still old school lol). Did some of this while working earlier. Didn't want to start at all when I went out to it, so dove right in.

Measured resistance across the pins on the injectors, both are the same at around 1.5, my meter is just a Canadian Tire special so it bounces around between 1.4 and 1.6 ohms. Sounds about right...?

I'll look into the "noid light", I've never seen one around here but never really had much reason to look. I'll be in NY tomorrow so maybe I'll drop in Autozone or something on my way thru.

I did actually put a volt meter across the wires on each but my results are rather inconclusive since I don't know what they should be. While cranking they bounced between .25V and .6V....

I had a loose computer (albeit from a pickup with 350/700R4, so different part number, but I would expect it to at least run if that was the problem) so I swapped it with no improvement. Not exactly a scientific test, also that computer had been out of service (stored in the house) for probably 10 years, so who even knows if its still good or died for some reason. Put the old one back in with no change at all.

I got irritated and hooked my crewcab up and pulled it up to the shop, where just for sh!ts n giggles hit the key, and didn't it start right up? :flipoff2:Friggin thing is annoying me. Ran fantastic for almost exactly one minute, and shut down again, same thing, no injectors.

Almost seems like a contact problem (towing/bouncing making it work), but then, why would it die out again after a min of idling every time?

 

TBI injectors should have a constant 12v applied to them when the ignition switch is on. As I recall, it's one wire from the ignition switch, divided into two inside the harness, each going to one injector. The other side of the injectors (two wires)go to the ECM, which places a ground on one side of each injector, to "fire" them in sequence (not at the same time). So check that you have "batt" voltage at both injectors, with engine off, key on. The voltage you measured sounds like what you might see on the ECM side of the injector as the engine is turned over; the "key" side should be constant.

 

Picked up a set of node lights for testing at Autozone while in NY today. I'll give them a try tomorrow if the weather is decent. $25 for a hardcase of I think 6 different types for different vehicles, its all they had.

 

Tbi injectors take power from the fuse box, then the ecm grounds them. I would run it until it stops, thencheck voltage at the injector. I would also find a pinout chart and ground the injectors at the ecm to see if the wiring is ok.

The ecm takes it's start signal from the starter, not the distributor.

I would also check the oil level, oil sensor, and fuel pump fuse and relay.

Is it giving any codes?

 

Well the node lights confirmed that when the truck refuses to run that there is no signal there (totally dark). So now I need to determine whether its a power supply problem or a grounding/ecm problem. My intent is to put the meter on the wires too look for the 12V signal... which my gut tells me I don't have. Does anyone happen to know what color the wires are of the four, which ones supply the 12V? Any reason why I can't feed power directly?

The truck is old enough to not have oil level sensors etc. Lots of oil, and pressure, and no codes. Fuel pump, fuse, relay all seem to be working fine.

 

Well it decided to run today so I tested the wires and found that (from the drivers seat) the two RH wires on the injectors are the 12V feed, so I ran it expecting the usual stall out to continue the test but that sucker ran for a good 20 mins without stalling out for me.... it stumbled a few times like it was going to quit but changed its mind and kept right on going...

Leaving for night shift, I'll try again over the weekend.

 

I'm wondering if maybe the truck hasn't run for so long it forgot how. :laughing:

 

There's a fuseable link that runs from the starter to a junction block on the firewall on the drivers side check it had the same problem fuseable link was bad

 

If a fuseable link was bad, wouldn't it just not run at all?

 

The wire that is suppose to be hot (12vdc) is the pink wire. The ECM provides the ground to operate the injectors. The Pick wire should always be hot with the key in the run position and start.

 

If you have voltage on one of the injector wires when its doing its no start thing, then unplug the 4 wire connector on the distributor. With the key on ground a test light and touch it to the pin on the 4 wire connector that goes to the purple/white wire. That's the rpm signal wire from the module to the ecm. Each time you touch that test light probe to the pin it will complete the ground giving the ecm an rpm signal. Every time you touch that wire you should hear the fuel pump run a second and one of the injectors will fire or your noid light will flash. If you do it repeatedly the injectors will alternate firing.

If the injectors fire when grounding the purple/white wire at the distributor then you have a bad ignition module if it is still sparking the coil or could have a bad pickup or module if it doesnt spark the coil. The module can fire the coil during cranking (at base timing, it wont switch to computerized timing until the rpm hits 400 rpm) but not be sending a signal to the computer so it doesnt know the engine is turning over, therefore it wont spray fuel.

If the injectors dont fire when grounding the purple/white wire then that wire could be bad somewhere in the harness, or you have a bad computer. We had a Buick Century at work the other day that was doing the same thing as your truck. It had the 4 cylinder with the 1 barrel TBI unit on it. It would run then cut off and wouldnt restart for a while. Computer was bad on it.

 

Days and days of rain here, no progress... but hey if I'm lucky maybe it will float away!!! :grinpimp: