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Fuel Delivery Advice

How low are you on fuel when it starts to stumble?

My truck can run like crap when I am down below 1/4 tank in hot weather. I figured out the problem to be the fuel is to hot. The Walbro pump will pump WAY more gas then the TPI needs so it cycles threw the tank over and over through the warm fuel pump & hot fuel rail on the motor. The fuel gets hot and makes the truck run bad.

Last time I had this problem I stopped at the gas station and put in 10 gallons and the problem was totally gone.

I knew when I did my TPI swap the truck would always need to have at least a 1/4 tank of fuel in it especially if I was offroad. I try never to go below 1/2 tank offroad & 1/4 tank onroad.

Harley
 
Interesting, I thought it might be fuel temp, and will try again, but last time this happened, the issues was there whether the truck was hot, or just starting out after sitting for a few days. In both cases, teh tank was at most 2 gallons short of full.

Could it be the actula pump getting too hot?
 
If it'll do it at idle, you could put a fuel pressure gauge on it and see what's going on. Also do a good visual, make sure no vacuum lines, plug wires, etc are screwed up somehow.

Also double check to make sure all your fuel fittings are tight. (just don't strip them)
 
OK, I checked the truck while it was sitting in the drive way and it looks like 46 or 47 (hard to see the dial from inside the truck) at idle and it goes up a few pounds to about 50psi at 3000rpm...no hiccups or nothing.

Also, I noticed that it does the surging much more in second or third as opposed to first gear, and more in third than second. (SM465). I have to change the oil because, gulp, my air cleaner fell off...but then I will take it for a test drive.

Any thoughts or feedback is welcome...
 
That fuel pressure actually sounds a little high. Do you have an adjustable fuel pressure regulator?

Do you have a code light on your truck? If not you might try and read the codes when it is acting up or if you can hook it up to a laptop with the scanning software to see what it is doing real time is the best option.

Harley
 
should be running 38 PSI with the regulator disconnected, and 34 - 36 with vaccum applied, if I recall correctly...
 
I have a code light, but the light is not on...it came on and went off a few times on the trail, but I think it was a 24, at least that was what it was last time... I do not have a pressure regulator...
 
Isn't the shaffer (sp?) valve before the regulator? In other words, that is my line pressure in teh rail before it goes into the regulator which is controlled by vacuum..
 
Schrader, or however it's spelled, just in case someone can't figure out what you mean. :)

If you are dumping too much fuel from too much pressure, and the ECM simply can't compensate, you'd (hopefully) throw a rich O2 error. Wouldn't bank on it, but theoretically.

Why the code 24?
 
The pressure you get at the sharader (no idea how to spell it either, lol) valve is the same pressure that the injectors are getting. The regulator does it's regulation of the rail pressure by letting more or less fuel pass through it and back into the tank.
 
35to37 p.s.i. sounds about right if we are talking about the L-98 TPI set-up. Hope you didn't go too radical on your cam selection. This will have an effect on the ECM. Lower vacium pressures will cause some strange surging.:rolleyes:
 
Stock cam so that should be good...

A code 24 is becasue the VSS isn't hooked up....

I threw a "too lean condition" a few weeks ago, but it never came back after I changed the tank and fuel filter...which was in part due to the stumbling issue, I was afriad there was ismply too much sediment in my tank...but now with the new tank, it sounds like I have too much pressure adn am stumbling again...

Is the high pressure and the stumbling likely related?
 
If you are dumping too much fuel at the wrong time you could be.

Hook VSS up, not that hard. :)
 
The sad part is that the VSS module is attached to the transfer case and I just need to run a wire to 12v and to the pin...I will hook that up this week, what to do about the high fuel pressure is still escaping me...:haha:
 
Make sure you've got vacuum to your regulator, and that it's good. (holds vacuum)

The pressure you are running is roughly what pressure is, unregulated.
 
Good news adn bad news. First the good news. I checked the vacuum and it is about 20hg. More good news, the regulator appears to be working and reduces the fuel pressure about 10 PSI...the bad news is that with the key on and the engine off, I show 60 psi...the regulator gets me to 48-50...so, is the regulator not strong enough? busted?
 
What you've found is normal on a TPI set-up. The vacuum signal will reduce the fuel pressure to whatever it's set at once the engine is running.
 
Now you're going to get mad, but if my regulator is doing what it is supposed to, but my fuel pressure is still too high, I assume that I need to either adjust the regulator down to about 40psi, or if it is not adjustable, get a new one?

Generally speaking do regulators say "reduce by 10psi" or "go to X psi?"...in other words, if my pressuer was higher, would the regulator take me to the same place, or will it just take off 11psi from whatever it receives..

100-11=89
60-11=48
 
To simply answer your question, if your fuel pressure is too high and you don't already have an adjustable regulator then you need to buy one. For the record though, a TPI should be running somewhere between 45-48 PSI on a 350 engine and between 42-45 on a 305 engine.
 
According to ChevyThunder,
The only differences being the TBI pump is a low pressure (15psi) compared to the tpi's 45psi.


If I am shooting for 45-48, then I am golden and won't waste tiem chasing a gremlin in teh fuel pressure arena...

As usual, thanks...if this keeps up, I am gonna have to add most of you guys to my Christmas card list...:D
 
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