Questions on TBI

[Mike Harrington]

Good morning everyone!! I've got a 1989 K3500 that I've recently put a new 454 in that came out ofa 1995 K2500. I left the TBI with the '95 injectors in it and I upgraded my fuel pump to the 1996 version. I've noticed it's been running rather rich and will backfire out the exhaust so loud you'll just about crap your pants!!! No joke!!! It only does it occasionally so I put a pressure gauge on the fuel line right before it goes into the throttle body. It read 90 PSI!!!! How much does the regulator in these TBI units reduce the pressure and how can you even tell what psi your actually running unless the test is after the regulator? It really makes no sense to test pressure before a regulator. Would a gauge on the return line give you the pressure that the tbi unit is actually running on?
Next question....on my injectors I have the high flow ones from the '95 installed with that high pressure pump but I also have my injectors from the original engine which would be from the 1989 454 as well as that original fuel pump which puts out about 25-30 psi. Would I be better off using the original setup for that truck or the newer one from the '95?
Last question....if it comes down to it I will probably get an adjustable FPR but not the one that goes in the TBI unit...but one that goes inline before the tbi unit. If I do this do I need to remove the spring from the regulator in the tbi unit or do I leave in in there.
Thanks for any info in advance.

 

[skysurfer]

I'm not familiar enough with the GMT400 tbi systems to answer all of your questions but I can help with the last one.

Last question....if it comes down to it I will probably get an adjustable FPR but not the one that goes in the TBI unit...but one that goes inline before the tbi unit. If I do this do I need to remove the spring from the regulator in the tbi unit or do I leave in in there.

I'm running an Aeromotive afpr and it's installed on the return line. The tbi systems build pressure by restriction, meaning that I didn't have to disable or modify the stock regulator on the throttle body at all. The Aeromotive unit is installed AFTER the stock fpr and is set to a higher pressure setting than stock so it acts the the choke point to increase pressure. The stock fpr upstream senses the pressure increase and goes wide open in a (futile) attempt to reduce pressure. I've been running this setup for a dozen years or so and it works perfectly.

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[Mike Harrington]

Is the 90 PSI too much for my stock regulator?

 

[skysurfer]

The stock setup was two 74lb injectors fed by a pump producing around 10-13psi. You installed 1995 injectors, I believe they are rated at 90lb/hr @ 30 psi. Now throw the '96 pump into the mix, it's my understanding they run at about 60 psi. How you're getting 90 psi is anyone's guess, but it's possible that the pump is simply putting out so much fuel that the return line can't handle the volume. In effect, the return line is acting as the restriction in the system because it was sized for 13psi and creating runaway fuel pressure. Assuming you've done nothing with the oem prom which controls the fuel delivery, it's no wonder you're having issues, it still thinks it's controlling smaller injectors at lower pressure.

Some speculation on my part here, waiting for more knowledgeable members to chime in.

 

[1987 GMC Jimmy]

I think you should have met it in the middle and installed the corresponding 94-95 454 fuel pump. I believe it runs at 30 psi.

 

[Mike Harrington]

Yeah I don't know what's up with that pump and why it pumps such a high psi. And I've checked it with 3 different gauges.
So I guess the regulator in the TBI can't take that high of a pressure?
Should I go back with the original setup with the 30 psi and the 74lb injectors?