TBI trouble shooting?

[moturbopar]

Hi I have a 91 suburban 5.7 that is having weird no start problems. Anyways I will start from the beginning. I first noticed I was having problems about 2 to 3 months ago. While driving after 20-30 minutes the suburban would stumble, jerk, just like turning the ignition switch on and off real fast. i tried changing the fuel filter and it seemed to go away for a week or so. It never had any problems dieing or starting. always ran pretty good other than what as listed above. So I kept on driving it. So anyways a week or so after I changed the fuel filter it started acting up again, But this time after 20-30 mins sometimes when I would make a turn it would die or stumble really bad but would fire right back up. So I figured I will try a new fuel filter again, it didn't help a thing. So I figured I would drive it a couple more days to see if I can trouble shoot it. After a couple days the problem seemed to go away, So I started driving it again and had no problems for about a week. Then one day I went out to start it, It would fire then die. So I let it sit till the weekend I go to start it and it fired right up, and was running well, SO I drove it for a day then the next day During a test drive it died during a turn. This time when I went to restart it, it would fire but sounded like only half the cylinders were firing, and would die, It did this about 5 times then started and was able to get home. So today I went out to start it and it wont fire unless I prime It. I can hear the fuel pump run for about 2 seconds. Putting 12v to the fuel pump test wire I can hear the pump run. I am picking up a fuel pump adapter so I can check the fuel pressure, but I am not sure if this is the problem (i unhooked the fuel line and turned the key on and it shot plenty of fuel out, I guess volume is not pressure though). I noticed that no matter what no fuel comes out of the injectors, even when I prime it, Shouldn't (if enough fuel pressure) it start pulsing when the ECM receives an RPM signal? Does the ECM get its injector signal from the ignition module? If so can the Ignition module be bad but still control spark but not the injectors?

Sorry for the long post, but this thing is driving me nuts!

 

[4X4HIGH]

Sounds to me like a bad fuel pump. Same exact symptoms as when mine went bad.

 

[moturbopar]

Thanks or the reply. A bad fuel pump is what I am hoping for, since I have an extra one.

 

[Stomis]

Yeah you cant prime TBI. It wont throw fuel until the truck is on. I still remember my 5.0 TBI camaro it would take forever to crank and it would feel like it needed to be primed but if you turned the ignition on and gave the fuel a chance to pressureize to the TB it would start right up.

 

[Thunder]

Check your fuel pressure by bypassing the relay.. It should be a steady pressure between 9-12 PSI. If it pulsates or varies up and down the pump is bad.
If your FP is up to specs. Then I would strongly suspect the ignition module or magnetic pick up coil. It is possible for the ECM to not recieve a spark signal (which will shut off the injectors) but still be getting spark.
have you checked trouble codes?

 

[moturbopar]

Well I checked fuel pressure today but was only able to check by disconnecting the fuel line at the Filter and hooking the pressure Gage to the line coming from the tank so the fuel stopped at the gage. So what should the factory pump put out pressure wise without the bypass being hooked up? The gage showed 13 psi. Shouldn't the pressure be higher than this since the fuel pressure regulator isn't controlling the pressure? I didn't get any codes.
Thanks guys.

 

[moturbopar]

Well I checked to see if the injectors are getting a signal from the computer. From what I can tell they are not seeing any voltage. Shouldn't the light flash when cranking the engine over? I wonder if I should replace the ignition module first and See what happens?

 

[GNERGY]

That's all my stock fuel pump put out was 13 psi. 91 burb. I know when mine is running you can see the fuel spraying from the injectors. Look to see if you have fuel spraying when you try to start it. Just a thought?
Tarey

 

[Thunder]

i would go ahead and replace the ignition module. Cheap part easy to do. Beings you have 13 psi Fp, no trouble codes, and the injectors arnt working it sounds like your problem. Might also inspect the connections and wireing at the distributor.

 

[moturbopar]

Thanks guys I changed the ignition module and it now runs fine. Glad it as cheap and easy!!

 

[4X4HIGH]

Thanks guys I changed the ignition module and it now runs fine. Glad it as cheap and easy!!

Now go buy a spare module to keep in your trail box. like you said, it's easy to change and will same you some headache in the future if it goes bad on you in the middle of nowhere.