Hi all,

So I have been driving my Blazer around in the last week with no problems including two 90 mile round trips recently.

However, this morning I went out to start it and it would not start. This was at 6:00 am because I was to go on a long trip and it was just a tad cooler (60'F) than it had been during the previous day ~85'F.

The engine would crank and then appear to "catch" as if there was ignition but then immediately die. This happened very quick with no running of the engine on its own.

So, jumping forward a couple hours I decided to do a fuel pressure test per the instructions of our member Captain_Hook. (https://blazerforum.com/forum/2nd-ge...eakdown-88305/)

But before I performed the pressure tests, I tried to start the truck and it did.

I then performed the pressure and leak-down tests. The results are below.

I used a the Fuel Pump Prime terminal in the engine compartment fuse/relay box to pressurize the system. I did the leak-down test three times and the maximum pressure test two times. By the time I performed the test, the temperature was about 65'F to 67'F.

Test Sequence 1: (For this one, I might not have bled all the air out of the pressure tester.)

• Max pressure with pump on: no data; forgot to record value
• Initial pressure with pump off: 52 psi
• Final pressure with pump off after 10 minutes: 49 psi

Test sequence 2:

• Max pressure with pump on: 58.5 psi
• Initial pressure with pump off: 57 psi
• Final pressure with pump off after 10 minutes: 55 psi

Test sequence 3:

• Max pressure with pump on: 58.5 psi
• Initial pressure with pump off: 56.5 psi
• Final pressure with pump off after 11 minutes: 56 psi

I then tried starting the truck and the battery was too low to start it. With my jump battery it did start.

Here are my interpretations of the results:

• The fuel pump and fuel delivery system do not pass the max pressure test because the maximum pressure was below the range of 60 to 66 psi
• The system perhaps passed the leak-down test because for the final two out of three tests, the pressure remained above 55 psi (although the pressure did drop by ~2 psi over ten minutes)

Questions:

• Are my interpretations correct?
  • If not, how would you interpret them?
• Can a partially discharged battery affect the maximum pressure reading?
  • I wonder about this because the battery was low after all my testing. I'll be recharging the battery and redoing the tests.
  • FYI: The battery is a 6-year Interstate one that is 5 years old.
• Have I caught my fuel pump just starting to fail?
• Should I replace it?
• What other tests should I perform?

Thanks

 

58 psi is normal max pressure for the pressure regulators on later spiders. You will need to dead end the pump to see if it has degraded. They normally put out a LOT of pressure (80+)

My Blazer did very similar to yours last Winter in the cold and would not start (tried to start just a little) after sitting for several days. Got pressure gauge and fuel seemed normal. Checked for RPM during cranking and scanner could read RPM, so crank sensor was good. Checked spark at coil output and seemed OK but nothing strong. Hooked coil wire back up and fired off first time. Never did it again. I attributed it to some sort of ignition module or coil glitch that was changed by my manipulation. I will probably see it again some time.

What you should do...... If it's starting and dead-ended fuel pressure is good there is nothing more you can do - other than just wait for the problem to reappear. Can't fix it if it isn't broken.

Only other thing I could see causing something like this is a faulty security function shutting off the injectors. But they typically start and then die when the security kicks in. I'm used to instant cold starts on mine.

Good luck!

 

Les,

This is super helpful. Thank you.

That is good to know about the proper pressure readings for the later regulators. I did put in a new, later model injection spider with its new regulator.

Now I guess I'll have to try to test the pressure at the filter. Dang, I hate dealing with the filter fittings.

I have a spark gap tester, where I can adjust the distance the spark has to jump. Do you know what distance is considered evidence of a strong spark? Is it 1.0"?

Thanks

 

I know when my pump started going out it would die when the temps started reaching triple digits outside along with hard starts and the occasional no start.

 

Les,

I finally got the parts together to test my fuel pump pressure directly at the outlet of the fuel filter. I plan to do it tomorrow.

n your post above you mentioned that the pump itself should be able to pressurize the system to at least 80 psi. Do you have any more specific values?

How about a leak-down test for the pump? Do you have values for such a test; i.e., change in pressure allowed over a given amount of time?

Thanks,

 

I think Captain's specs for dead ended fuel pump are quite valid. From the sticky...

Pressure must be 73psi to 108psi. Deactivate the fuel pump. Pressure must remain above 55psi for at least 10 minutes.

​​​​

 

I was able to to my direct fuel pump pressure test by measuring the pressure at the outlet of the fuel filter.

The short answer:
The maximum pressure it would provide was 94 psi.

Longer answer:
I de-pressurized the fuel system and installed my adapter hose to the fuel filter outlet and left the gas cap off.

1. I then pressurized the system using the fuel pump relay bypass terminal adjacent to the relay in the under-hood rely and fuse block. The initial pressure reading was 83 psi. I then repeated it but also flipped the key in the ignition from off to on and back to off. This time it pressurized to 90 psi. After 10 minutes, it dropped to 74 psi.
2. I then repeated the test with only the rely bypass power trick and recorded an initial and max pressure of 94 psi that then dropped after 10 minutes to 74 psi.
3. I used the ignition switch to pressurize the system and the maximum and initial pressure were 94 psi which after 10 minutes dropped to 91 psi.
4. Repeated test 3 by using the ignition switch and this time with the gas cap on and the max/initial pressure was 92 psi and it didn't drop over 10 minutes.

So, any thoughts on how to interpret these results?

The performance of it in terms of initial pressures and the amount of leak-down doesn't seem consistent but is within spec except for the very first of my tests at the Schrader valve at the intake manifold.

 

Here are some pictures of the adapter hose I created to connect my fuel pressure tester to the outlet of the fuel filter.

My fuel pressure tester came with an adapter to adapt the GM fuel system Shrader valve to a regular Shrader valve. This allowed me to create an adapter hose with a regular Shrader valve I found in the air-tool section of my local hardware store that had male pipe threads on it. This allowed me to connect it to a 3/8" ****** hose fitting. On the other end I used a quick fuel line connector (Dorman 800-086, but the straight version 800-082) would work too).


The full pressure tester connected to the outlet of the fuel filter. I made sure to get rid of any air in the hose.



The adapter hose itself. The adapter that came with my pressure tester is in the middle.

 

I think its good!!

 

Yeah, I'm reluctantly happy about it. I was not looking forward to doing a fuel pump replacement, but I bugs me that I don't know why it wouldn't start that morning.

I'll likely do a spark test just be sure of the ignition but I doubt I'll find anything.

Thanks for the help!