Low fuel pressure in first gear.

Supercharged111 · Apr 7, 2020

How long ago did you rebuild the TBI? How rusty is your frame? The ground wire screws directly into the frame. Then again, since you already did the pump, you're likely aware.

PlayingWithTBI · Apr 7, 2020

Doughnuts said:
I'm guessing the signal for the fuel pump come directly from the ecm?

Also there's an oil pressure sensor on the back of the engine near the distributor that will turn on the fuel pump when it senses pressure, somewhere above 5 PSI IIRC. So you have 2 avenues to turn on the fuel pump. Here's a diagram from an 88.

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PlayingWithTBI · Apr 7, 2020

I don't know how I got a double post of the diagram in the same post?

Supercharged111 · Apr 7, 2020

PlayingWithTBI said:
I don't know how I got a double post of the diagram in the same post?

Site's been a little weird for me too. I hit post twice yesterday because it was taking forever and I got 4. I deleted one and 3 disappeared.

Doughnuts · Apr 7, 2020

Talked to my buddy, who's a Lexus mechanic, and showed him the wiring diagram. And what he told me the probably a wiring problem like
L31MaxExpress said it was. Only problem is that you have to be going 20 mph for it to happen and I'm not too keen in testing voltages as I'm driving. I am interested in knowing what a common failure areas for the wiring or even if there is any. still open for any suggestions as I probe through it tomorrow.

Supercharged111 I rebuilt the TBI 2 years ago, so maybe it could have something to do with the fuel pressure regulator. Is there I can test that without taking it all apart? would the best thing to do is rebuild it again?

Supercharged111 · Apr 8, 2020

The best thing is to rebuild it with an adjustable fuel pressure regulator and set it around 13psi. The off the shelf kit I used had me at 11.

Doughnuts · Apr 8, 2020

Okay iv'e heard about the about the adjustable fuel pressure regulator. is there a certain brand or someone who sells them that I should check out?

I'll probably take it apart this weekend and rebuild it and see if that helps. if nothing else it'll probably help my mpgs a hair.

Supercharged111 · Apr 8, 2020

Not sure if this was already posted here or somewhere else.

http://rvmorsemachine.com/product.htm

Kenny posted a thumbwheel too, but could have been in anothrr thread.

PlayingWithTBI · Apr 8, 2020

Supercharged111 said:
Not sure if this was already posted here or somewhere else.

http://rvmorsemachine.com/product.htm

I did the same thing, here's how it looks - you have to drill an offset hole through the plate and bend the MAP sensor vacuum tube a little to make it fit.

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Here's how it looks on the engine, I had a 30LB spring running ~28PSI at the time this was taken. The only issue is you have to unbolt the Throttle Body and lift it up when you want to adjust your pressure but, you can leave the fuel lines hooked up and adjust by turning the key on, engine off (of course), the fuel pump should run for ~2 seconds and build pressure so you can see where you're at, HTH.

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Schurkey · Apr 9, 2020

Doughnuts said:
It has a tendency to, when warm, bog down heavily in first gear at wide open throttle.
I know it is a fuel related issue due to hooking up a fuel testing gauge and seeing it plunge to nothing then bouncing back up to 10 p.s.i.
It only happens around the 2-3 thousand rpm mark and only lasts a second. after that it'll run through all the other gears just fine without stuttering or bogging down.

How much fuel is in the tank?

G-forces and low fuel level could cause the fuel pickup sock to suck air.

Would be worth your time to connect a scan tool and look for anomolies in the data stream at the point where the engine stumbles. Do you lose RPM signal when the fuel pressure falls?

Low fuel pressure in first gear.

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