350tbi stalls for fuel cutoff

[geauxnow]

This 89 4x Burban w a 350tbi using oem tank fuel pump stalls after 5 minutes of idle. Not road worthy yet after reconstructing from heads up of the box of parts and bolts that came with the truck. Cranks but will not turnover until rested for another 5 minutes. Found that jumping the Test terminal at the fuel pump relay provides needed flow. What can I check?

 

[Edelbrock]

"Cranks but will not turnover"

Not trying to bust your chops, but... What?

 

[geauxnow]

The engine will crank but not start. It is a loss of fuel. I am looking for the culprit of what cuts out the relay.

 

[Ricko1966]

The relay needs a 12 volt power in which when the relay closes is provides power out to the pump. The relay gets a 12 v signal from the ecm to close the relay when the ecm has tach signal. The relay has a constant ground for the electro magnet side. I wrote this all up earlier today for someone else including drawing a diaghram of a how a relay works. Use the search function Key word relay by ricko1966. Read what I wrote to him. Look at the picture I drew him. Cranks and turn over mean the same thing to 99 percent of the automotive community. It cranks but doesn't start means the same thing as it turns over but doesn't start. The fuel pump circuit has redundant circuit also so ,you have 2 problems.

 

[geauxnow]

something I've always been confused about is I don't have a tachometer, but I still have a signal wire? I don't need lessons on relay logic, coils, relay closure. I was hoping to hear a "check your Kanuder valve" or "the blinker fluid is running low" for reasons why the circuit is interrupted, for a speedy repair. Looks like I need to dive into the wiring diagrams. It has to be on the coil side from the ECU; not sure what would signal the ecu to dropout the call for fuel at that point.

 

[Ricko1966]

something I've always been confused about is I don't have a tachometer, but I still have a signal wire? I don't need lessons on relay logic, coils, relay closure. I was hoping to hear a "check your Kanuder valve" or "the blinker fluid is running low" for reasons why the circuit is interrupted, for a speedy repair. Looks like I need to dive into the wiring diagrams. It has to be on the coil side from the ECU; not sure what would signal the ecu to dropout the call for fuel at that point.

But there is a second circuit. It is routed through a 2nd oil pressure switch, it is redundant and supposed to provide power to the fuel pump even if the relay or its circuit has failed,so you aren't stranded on the road. So both circuits have to fail to lose fuel. I would first make sure you have both circuits connected. I would also see if you lose spark at the same time you are losing fuel. ICM provides tach signal for ecm for ignition,and fuel pump relay. Will it restart immediately if you provide fuel or is there a waiting period. The fuel pump OP switch will not provide pump power if the engine doesn't have oil pressure,as in,key on engine off sering if tge pumps running it won't unless there is oil pressure.

 

[geauxnow]

Interesting on the 2nd circuit; I forgot about that. Yes, it starts immediately jumping that test circuit, and I forget the pin number. Remove the 12v and the engine dies. Let it sit for 3-5 minutes and the engine starts as normal.
Yes, so the 2nd circuit is also an issue, thank you for that. I'll find that as well. I didn't check spark because of the relay Test circuit proven.
Busy day today on work-work before I take three days off beginning tomorrow so I haven't delved into it other than this forum.

 

[Edelbrock]

If you want to try some simple stuff - Try turning the ignition on (not cranking over, just on - so that all the gauges and dash lights turn on). Turn it on for 3 seconds, then off for 5 seconds, then on for 3 seconds, then off for 5 second and so on. Do this about 5 to 10 times, and then see if it starts. This will prime the fuel system and also exorcise the relays, pump, etc. This is not a "fix", its just a diagnostic measure.

During the times that it wont start, does the fuel pump run? ( I assume not).

Also, try replacing the replay. They are super cheap on Amazon. If bypassing the replay solves the issues, then it stands to reason that the relay is bad. If that doesn't work, then I would check for 12+ at the relay signal or power wires. If the signal is bad, then what ever triggers that is bad.

Much of this was probably already covered by other posters. So just skim through all the posts and see what seems to make the most sense to move forward on the repair.

 

[geauxnow]

Not sure what you're diagnosing with the cycles of on/off. Everytime I key on, the pump primes and the engine starts. Except for these brief few minutes I explained above. So far I've only witnessed it at idle, about 4 times.
I've read some posts. Most are parts replacing, hoping for the best. My best recollection of the wire diagrams is there is nothing between the ECU and the relay and because so, something is triggering the ecu to drop call for fuel. Thanks for the help. I'll follow up on this post with new info.

 

[Ricko1966]

Not sure what you're diagnosing with the cycles of on/off. Everytime I key on, the pump primes and the engine starts. Except for these brief few minutes I explained above. So far I've only witnessed it at idle, about 4 times.
I've read some posts. Most are parts replacing, hoping for the best. My best recollection of the wire diagrams is there is nothing between the ECU and the relay and because so, something is triggering the ecu to drop call for fuel. Thanks for the help. I'll follow up on this post with new info.

The ecu gets it's signal from the ICM which also controls spark. So this is weird. If the ICM drops out you should lose spark and fuel,not just spark. The TBI sequence of things that are going on are so much more than what is just common sense or even used on other vehicles. For example the ICM controls spark and fuel below 400 rpm then it switches to ecm. That and the redundant power. Really your best bet would be follow the GM tbi flow chart. @gmbellew has the link memorized, I'm not good with finding the resources pages,so I tagged him hoping he'll put up a link for you.

 

[gmbellew]

https://www.gmsquarebody.com/threads/1973-1991-service-manuals.14499/

Stalling after 5min of idle could be when it hits closed loop and starts using the O2 sensor to fine-tune fueling. Maybe the O2 has an issue and it is either going rich or lean enough to stall out? Do you have any sort of ability to see the ECM data via a scan tool or ALDLDroid or WINALDL or HPTuners?

 

[geauxnow]

Do you have any sort of ability to see the ECM data

I do not but can look into it. The electric fans have cycled multiple times at 195deg by the time the fuel cuts off. I would say it hit closed loop long before that, but that's an interesting point. The O2 has less than 10 hours on it but who knows at this point.

 

[gmbellew]

I do not but can look into it. The electric fans have cycled multiple times at 195deg by the time the fuel cuts off. I would say it hit closed loop long before that, but that's an interesting point. The O2 has less than 10 hours on it but who knows at this point.

Closed loop is a function of temperature and also some minimum time threshold. If it stalls at nearly exactly 5min on a cold engine, it could be the time.

Next time it stalls, give it some pedal to the floor while you crank. That is clear flood mode, and slows the injectors down in case it is stalling bc it is flooding out.