Should my TBI's fuel pressure act like this?

[bucket]

This would be an interesting test. But I returned the fuel pressure test kit to Auto Zone yesterday - having already made up my mind that I should get a new fuel pump.
If I'm still having issues after the new FP is installed, I'll circle back to this post and try your test.

You can still do this test without a pressure gauge. If you disconnect the oil sender with several wires (not the one with a single wire) and it no longer starts, then you know the issue is with the fuel pump relay or circuit.

 

[Rusty Nail]

Yep thats whatcha want - one of them adjustable deals. I dont know how it works but when ppl get to "tuning" on their tbi after making more power OR looking for more power..that is often the answer.
They get them gooder sprangs.
It's sure where mine was headed...needed more gas.
I know about changing the fuel pump ground wire AND I have chased a loose fitting hose inside the tank too. Similar symptoms.

Worm gear clamps are your friend.

 

[gmbellew]

the pressure regulator controls the pressure between the pump and the regulator that feeds the injectors. it bleeds off any excess pressure into the return line. so you can measure pressure just after the fuel filter at the flex line (where I have my adapter installed) or at the TBI on the supply line (have to bend the hard line to get adapter in) or anywhere else you could between the pump discharge and the regulator at the TBI and the pressure will read the same.

and if the pump primes with key on, the fuel pump wiring and relay are likely good. disconnecting the oil pressure switch is a good check. but i bet it starts the same. i dont think 2 or 3 sec of cranking is building enough oil pressure to start. I'd be looking at trying to figure out if the power is somehow getting cutoff to the fuel pump circuit during cranking momentarily and verifying that the fuel pressure is really going to 0 during initial cranking.

 

[RanchWelder]

o-rings everywhere.
Don't forget the OEM fuel filler hose and vent hose are probably dry rotten and leaking from rock hard rubber in the filler lines.

 

[dsteelejr]

For full disclosure I’m a carb guy through and through and my knowledge of TBI is limited, but I have an ‘88 K5 with 350/700R4 that’s 100% stock. I bought it from my uncle who is the original owner, he’s a retired mechanic and does all his own work. When he sold me the Blazer he gave me a heads up that it takes extra cranking time to fire up, not drastic but like a carbureted engine waiting for fuel to pump to the carb. He said it’s because there’s a check valve in the fuel line (I believe it’s on the pump but not 100% sure) that’s supposed to keep the fuel line charged with pressure for instant starts. He said through ordinary wear and tear the check valve has wore out. It hasn’t completely crapped the bed, but fuel slowly bleeds past it back into the tank. So if it’s not sitting for long it fires right up, but if it sits for hours or overnight or longer the fuel system loses pressure and takes a few seconds longer than it should to prime and get fuel to the TBI.

I never bothered to pressure check the system to see if you and I are experiencing the same sort of pressure drop because at this point it’s not a big deal for me. A few extra seconds of cranking, but when it fires up it runs fine. I’ll ask my uncle about your issue next time I see him.

 

[gmbellew]

my 350tbi (1990 burb) fires in 2sec, maybe 3 sec, on a cold start every time. it fires instantaneously on a hot start every time. fuel pump is newish, and it did the same before and after the pump swap. I don't think the TBI system retains any residual pressure. I always just chalked it up to how quickly the fuel fires off in a hot engine vs a cold engine for this fuel delivery method, for the difference in start times.

 

[dsteelejr]

I don't think the TBI system retains any residual pressure.

Again, I’m not super knowledgeable on TBI, but with my uncle being a retired mechanic he always had the factory service manuals for every vehicle he owned and he gave them to me when he sold me his ‘88 Blazer so I had to look it up. I assume not much changed between OP’s ‘87 and 1988. Some TBI did automatically relieve pressure and some did not. For you and the OP:

 

[dsteelejr]

Anyone??? Please!
Is fuel pressure supposed to dip to zero right after key on?
And it's falling quickly to zero after shut off normal? (Or a sign of a weak FP?)

The 1988 Factory Service Manual says in one of the diagnostic tests to note the fuel pressure in the first two seconds and the flow chart goes from there with other things to check. 9-13 psi is the correct operating range of pressure, so it seems your 12 psi of pressure means your fuel pump is probably good unless you go through the tests and find out otherwise. If it was less than 9psi it most likely wouldn’t be drivable. Here’s some bathroom material. Hope this helps. Good luck.

 

[gmbellew]

Again, I’m not super knowledgeable on TBI, but with my uncle being a retired mechanic he always had the factory service manuals for every vehicle he owned and he gave them to me when he sold me his ‘88 Blazer so I had to look it up. I assume not much changed between OP’s ‘87 and 1988. Some TBI did automatically relieve pressure and some did not. For you and the OP:

thanks for that info. I have a L05 engine, so never looked into that for the other engines.

All the GM manuals are available in the reference library on this forum. they are a GREAT resource!

 

[bucket]

The 9-13psi spec is correct, but ideally you want 12-13psi because it will run noticeably better than at 9 or 10 psi. Some newer cheapo pumps are only capable of around 10psi right out of the box.

Also, they will actually run at 6 or 7 psi too, just terribly lol.

 

[Rusty Nail]

I can fix this.
WE can. I'm gonna help stupervize.

Even got pixz mess around.

I used to have an 88 Blazer
- r.i.p Bonnie Blazer.


Simple fix after you drop the tank. Lemme know when you're ready...im at work.