Starting problem

[texas tough]

overpressuring these systems can cause rich running problems.. the fuel pressure goes DIRECTLY to the injectors first, and will provide more fuel than needed, whether you want it or not. the systems are sized, by the injector openings, pump pressure, line sizes ect, to give the engine a very narrow pressure band parameter. if u goose up the fuel pressure past the stock 9 to 13, you are forcing more fuel into the engine than the computer can manage.. same if its not enough, the computer cannot manage it.. I worked on an engine that had been surging at idle, and pre ignition problems.. it was a 5.7 with 4.3 injectors in it.. ran like crap, until it got the correct injectors in it.. I would advise against making and fuel related mods unless u know how to reprogram the computers, or change the proms to adapt to what your trying to do. The computer block learns will never be happy, and the truck will constantly trying to lean out the fuel mixture, when you over pressure the fuel system. the running conditions will be all over the place. when the computer cant pull things into an acceptable operating parameter, it can default back to factory block programming,, and run ok for a bit, then back to problems. I have a friend whos a , he has a snap on verus, a bosch, and all the best equitptment,, he worked on a 94 chevy that had a 4.3 and was replaced with a 5.7.. has a matching computer.. the guy put headers on it, and it never ran right from the beginning.. after 5 days working on the truck, it turned out it was a aftermarket 18 pound pressure regulator spring that was keeping the truck in a slightly rich condition and the computer just could not manage it. he put a stock spring in it and it ran great! Timing is another thing folks tend to want to mess with,, leave it alone.. its ran by the computer.. what folks need to understand is these ARE NOT 1970s carbureted, vacuum advanced engines.. !!!!

 

[delta_p]

Is there a good book I could get? And thanks for the info, much appreciated.

Everything a '94 would need and then some thanks to Hatzie.

https://www.gmt400.com/threads/88-95-service-manuals.43575/

 

[dave s]

I got my tbi buttoned up awhile ago. I installed the adjustable regulator at first but after getting it together didn't like that I don't know where the pressures are going to be. I tried comparing the stock setup vs the CFM one then decided to just swap the stock one back in at least until I figure out the starting issue. I put the little piece from CFM on the back that holds a gauge so I can see my fuel pressure from now on. I kinda like that little thing. I was going to put it together but noticed a lot of oil around that little pipe with a brass or bronze fitting on it and a sensor screwed into that and it's all right sort of under the distributor cap. I'm guessing oil pressure sender? So with the tbi off, it's much easier to get to that so I ran out of time for today. I'll pull that sensor and pipe out and try to get it all cleaned up and sealed good while tbi's off. From reading everyones posts I'm glad I just used the stock regulator for now. It actually ran really well once started so with everything cleaned and new gaskets and stuff it should be even better. I also have new valve cover gaskets so there's more room right now to do all that too. It's going slow but nice it's here at home now to do stuff to. I will try starting tomorrow with the fuel lines in a container and see if there's fuel flow while cranking. Thanks for everyones help.

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[dave s]

Everything a '94 would need and then some thanks to Hatzie.

https://www.gmt400.com/threads/88-95-service-manuals.43575/

Oh man! There is so much information there! Now all this old guy needs to do is figure it all out. I'll give it a go for sure. Thanks for sending me this link!

 

[Schurkey]

My 94 with a 5.0 is only starting with a shot of starting fluid. New pump, filter... ...I got CFM Technologies pieces to put gauge inline and check pressures.

REALLY IMPORTANT: Do you have fuel pressure for two seconds after turning the key from "Off" to "Run" or "Crank" with the engine NOT RUNNING? If the fuel pump won't prime, the problem is in the fuel pump relay, the relay wire harness including the fuse(s), or the ECM. Maybe the ignition module, but probably not.

I read a lot about the pickup coil and module in distributor curing this issue as well as relays, oil pressure switches, thermostat temp sensor or something like that. ?

It might be in the ignition module, but that seems unlikely since the engine runs good once it starts. The ignition module sends a signal to the ECM to turn on the fuel pump via the fuel pump relay. So far as I know, if the ECM is not getting that signal from the ignition module, the ECM won't advance the ignition timing, and the engine runs bad and sets at least one code.

Can't be the pickup coil, since the engine runs.

Shouldn't be the temp sensor, since the engine runs good once it starts.

the other scenario, is these trucks have 2 paths to engage the fuel pump,, thru a circuit in the oil pressure switch, and the fuel pump relay.. if the fuel pump relay isnt working the engine has to spin at 400 rmp or higher, then the switch will energize the pump.. so your fuel pump relay may not be working, and when you shoot some gas in, and it starts, the oil pressure switch lets it run.

An oil pressure switch doesn't sense engine RPM. The oil pressure switch energizes the fuel pump when it senses oil pressure. Has nothing to do with "400 rpm"; it has to do with enough oil pressure to close the contacts.

Aside from that, you're right. Lots of "cranks a long time before starting" issues are a failed fuel pump relay, or relay wire harness including fuse(s), or a failed ECM that doesn't turn on the relay--and the vehicle has to crank long enough to build oil pressure before the fuel pump will run.

 

[dave s]

REALLY IMPORTANT: Do you have fuel pressure for two seconds after turning the key from "Off" to "Run" or "Crank" with the engine NOT RUNNING? If the fuel pump won't prime, the problem is in the fuel pump relay, the relay wire harness including the fuse(s), or the ECM. Maybe the ignition module, but probably not.


It might be in the ignition module, but that seems unlikely since the engine runs good once it starts. The ignition module sends a signal to the ECM to turn on the fuel pump via the fuel pump relay. So far as I know, if the ECM is not getting that signal from the ignition module, the ECM won't advance the ignition timing, and the engine runs bad and sets at least one code.

Can't be the pickup coil, since the engine runs.

Shouldn't be the temp sensor, since the engine runs good once it starts.


An oil pressure switch doesn't sense engine RPM. The oil pressure switch energizes the fuel pump when it senses oil pressure. Has nothing to do with "400 rpm"; it has to do with enough oil pressure to close the contacts.

Aside from that, you're right. Lots of "cranks a long time before starting" issues are a failed fuel pump relay, or relay wire harness including fuse(s), or a failed ECM that doesn't turn on the relay--and the vehicle has to crank long enough to build oil pressure before the fuel pump will run.

Thank you for that information. That sounds like the exact issue I'm having. I'm kind of kicking myself as I replaced the fuel pump before rebuilding my tbi and adding the gauge, which would have shown me good fuel pressure possibly. But now I know it's a new pump and the tank was absolutely spotless inside. Just as soon as I get the oil pressure sending unit sender and pipe all resealed back there and valve cover gaskets replaced, I'll jump right on this troubleshooting that relay correctly now. I downloaded that good manual from online but I'm a bit slow understanding it all yet. I need to read stuff a few times before it starts making good sense to me usually. Also, is the ECM the box behind the glove compartment?

 

[aaron33]

I chased my tail for years with what I thought was the similar issues on my 98 5.7. Turned out to not be the fuel pump, FPR, Spider, etc., but the ignition wire in the steering column. The part that plugs into where the key goes fails. It used to crank and crank until the battery would die. I'm not saying that's your problem, but just a different avenue.
Best of luck!

 

[PlayingWithTBI]

Also, is the ECM the box behind the glove compartment?

Yes it is.

GM spec for fuel pressure is 9 - 13PSI as mentioned previously but these trucks work better if it's 11 - 13. Low fuel pressure will cause hard starting. As @Schurkey said you should see pressure for 2 seconds when you turn the key on. That will show that your pump relay is working.

 

[Yamaben]

I'm kind of kicking myself as I replaced the fuel pump before rebuilding my tbi and adding the gauge, which would have shown me good fuel pressure possibly.

That thing is 26 years old. I would say you are smart to replace things as you go. Good on you.

 

[texas tough]

not

REALLY IMPORTANT: Do you have fuel pressure for two seconds after turning the key from "Off" to "Run" or "Crank" with the engine NOT RUNNING? If the fuel pump won't prime, the problem is in the fuel pump relay, the relay wire harness including the fuse(s), or the ECM. Maybe the ignition module, but probably not.


It might be in the ignition module, but that seems unlikely since the engine runs good once it starts. The ignition module sends a signal to the ECM to turn on the fuel pump via the fuel pump relay. So far as I know, if the ECM is not getting that signal from the ignition module, the ECM won't advance the ignition timing, and the engine runs bad and sets at least one code.

Can't be the pickup coil, since the engine runs.

Shouldn't be the temp sensor, since the engine runs good once it starts.


An oil pressure switch doesn't sense engine RPM. The oil pressure switch energizes the fuel pump when it senses oil pressure. Has nothing to do with "400 rpm"; it has to do with enough oil pressure to close the contacts.

Aside from that, you're right. Lots of "cranks a long time before starting" issues are a failed fuel pump relay, or relay wire harness including fuse(s), or a failed ECM that doesn't turn on the relay--and the vehicle has to crank long enough to build oil pressure before the fuel pump will run.

not exactly.. the oil pressure switch does not sense pressure and energize the relay.. heres how it works.. in the fuel pump relay there is a normal closed path going to the oil pressure switch, and a normal open position going to the fuel pump.. if the relay fails to energize and close the contacts to the fuel pump, when the ignition cranks, the path goes thru the already normally closed side of the fuel pump relay. the oil pressure switch must be getting pressure and close contacts, to allow the reluctor wheel in the distributor to tell the computer that the engine is spinning at least 400 rpm.. the engine has no idea how fast the engine is spinning based on the oil pressure switch.