TBI 454 Help

[F85Cutlass]

Hello, my name's Ryan, new user with my first square body. 89' V3500 TBI 454, TH400. 66k on the odometer.

Been on quite the gremlin hunt going on a month. When I first got the truck, it had been sitting for 7-8 years. Gave it a tune up, and it ran beautifully, which was my go ahead to begin going through the brakes and some other projects on the truck. Of coarse, after a month or so of runnign well, and being yard driven, I register it, and then it starts acting up.

While Idling, I am topping off the trans fluid because I had the lines off, and it quite instantly starts to idle in a rythmic surging manner. Thinking that it couldnt be a coincidence that this began, I went about testing vaccum to the transmission pressure transducer. With good vaccum there, I proceeded to replace the transducer and do a transmission fluid/ filter service. The issue remained.

I did eventually get it to throw a len mixture code, which at least gave me something to go off of.

I proceeded to to check and replace vaccum lines. The only difference came from caping off the MAP sensor, which to my understanding puts it into a kind of rich mixture default. With the map capped off it ran well enough to chug in and out of the garage, but that was about it, no guts whatsoever.

My understanding is that this rich default can mask poor fuel pressure, so I turned my attention to the fuel system.

As many of you will know, GM did not give us a schrader valve to check pressure. To make a long story short, I got an adapter off of summit, which was garbage, leaked from the swivel, and did not give me a read out, so I just went about a fuel pump replacment, bc I had a hunch. Turns out the hose between the pump and sending unit was split, which I belive was resulting in just enough fuel to run, but not enough to run well.

So, I replace the pump, hose, strainer, ect. Now truck will not start, but now it appears to be dumping excess fuel. Will almost catch if I hold throttle wide open. Not sure if theres some kind of ecm relearn process, but battery was disconnected during the fuel pump replacement, so I would think it would start from a default fuel trim at this point anyway.

I did end up replacing the throttle body with a remanufactured unit before I got to the fuel pump because while attempting to test the IAC valve and TPS, the screws just about disolved when i touched them, and rather than drill them out a chase threads, a rebuilt TBI with new IAC and TPS cost about the sum off all the parts, so I took the easy route. After the fuel pump, I thought maybe the reman throttle body had a bad regulator, so I tried putting my old unit back on, but the very rich over fueling symptom continued.

In order to make this look more like a forum post than a novel, im going to bullet point a few other thigs ive checked along the way.

-Engine temp sensor, ohmed, appeared to be good, replaced just in case
-Map sensor, checked for internal vaccum leak and ohmed, appeared good.
swapped with another good map on loan to check, no change
-Gasket underneath the TBI adapter to the manifold was replaced.

Any advice would be greatly apretiated, im loosing my mind with this one. I have checked a number of threads regarding TBI issuse, but I havnt come across one that mirrored my transition from lean before fuel pump to rich following. Unfortunately all the little thigs that occured between the initial issues and the fuel pump replacement have created more variables than I would have liked to, but it it what it is now.

-Ryan

[DieselSJ]

Did you end up with a later fuel pump? You need the lower pressure pump. First you need to check fuel pressure.

Use this pressure adapter - it installs in place of the fuel filter - https://www.amazon.com/Tool-Aid-3765...CRD7CA4PM6KBGP

Also change your fuel filter after checking the pressure.

When cranking can you see fuel spraying into throttle bores? Should be a nice fine cone with no streams or drips.

[F85Cutlass]

Thanks for the reply DieselSJ,

The fuel pump should be the correct one. Its a delphi unit that lists for the truck, rated for 12 psi which should be appropriate for the truck. I'll have to track down that adapter, I originally went for the on throttle body adapter b/c I liked the idea of having a perminent schrader valve, but that didnt go so well.

We've got a nice even conical mist, but it's heavy, to the point where fuel is splashing back a bit.

Another though that occured to me was the possibility of a blockage in the return line, maybe at the tank selector valve. Thinking that maybe backpressure in the line could cause a problem at the regulator, not allowing it to send the excess fuel back to the tank. I may play with bypassing the valve and giving that a try. I could have even kinked that hose coming over the frame rail when I put the tank back in.

So thanks again for the feedback, I'll let you know what I find out when I play with that a bit, and once I get the fuel filter swap test adapter.

-Ryan

[Nodnarb76]

Just a stab in the dark here.
Have you swapped the oxygen sensor?
If it’s bad, maybe it’s causing the ECU run way too rich?

[cadillac_al]

Your spark plugs may be soaked with gas by now. If you are seeing a fuel spray it should be trying to fire.

[F85Cutlass]

Thanks everyone, did a bit of tinkering last night, and think I've jst about figured it out!

Removed the return line from the engine side of the tank selector valve. Fuel spray was noticably reduced, but still a fine conical mist, and after a few revolutions she fired up!

One thing that really surprised me was the volume of fuel running from that return line. Over the 15 seconds or so i let it run it almost filled the 1 gallon gas can I directed the return hose into.

Proceeded to connect that hose directly the the return hose to the tank, and fired up and ran just as well. So my conclusion is there is a restricton or failure in the tank selector valve. The resulting pressure was fighting the pressure regulator, and some, if not all of the excess fuel was being forced out the injectors resulting in my rich/flooding condition.

Im guessing this problem in the valve was present before I replaced the pump, but because I was not recieving enough pressure and running lean, there wasnt enough pressure for the restriction in the return to be a problem.

So I changed the oil, bc there was definate fuel in the oil, and ill let it run a little bit next time before I chance or clean the plugs, bc im sure theyre fuel fouled.

I'll replace the selector valve later in the week and let you guys know. Thanks for all of the replies and feedback!

-Ryan

[SCOTI]

Originally Posted by F85Cutlass

Thanks everyone, did a bit of tinkering last night, and think I've jst about figured it out!

Removed the return line from the engine side of the tank selector valve. Fuel spray was noticably reduced, but still a fine conical mist, and after a few revolutions she fired up!

One thing that really surprised me was the volume of fuel running from that return line. Over the 15 seconds or so i let it run it almost filled the 1 gallon gas can I directed the return hose into.

Proceeded to connect that hose directly the the return hose to the tank, and fired up and ran just as well. So my conclusion is there is a restricton or failure in the tank selector valve. The resulting pressure was fighting the pressure regulator, and some, if not all of the excess fuel was being forced out the injectors resulting in my rich/flooding condition.

Im guessing this problem in the valve was present before I replaced the pump, but because I was not recieving enough pressure and running lean, there wasnt enough pressure for the restriction in the return to be a problem.

So I changed the oil, bc there was definate fuel in the oil, and ill let it run a little bit next time before I chance or clean the plugs, bc im sure theyre fuel fouled.

I'll replace the selector valve later in the week and let you guys know. Thanks for all of the replies and feedback!

-Ryan

Nice investigation & thanks for sharing the details.

[DieselSJ]

Great work figuring it out!

[F85Cutlass]

Thanks for the feedback! Hopefully this proves helpfull to the next person with this issue, its a relativly cheap diag. Would have more info if I checked fuel pressure with guage before and after, but either way to look at the issue, bc i was hung up on thinking it was an issue with the throttle body or ecm.