Strange fuel issues with 1990 GMC truck....

I have a 1990 GMC (210K miles, 350, auto, 2wd).

I have two fuel issues that I haven’t figured out, partly because I’m not a mechanic.

First, the valves rattle when accelerating. I’ve done a decarb job with Seafoam, straight water (about a pint, both sides of the intake), half quart of ATF. Not sure what else to try to decarb lol. But the problem definitely seems to be preignition of some sort. When I put in a fuel injector cleaner (boosting the octane) the valve clatter totally goes away (or when I put in higher octane fuel).

Main problem, probably unrelated.

My fuel gauge is about 1/3 tank out of calibration. When full (35g) it shows just under 3/4 tank. When it’s down to around 8 gallons, it says it’s stark empty.

No big deal, until you hit that empty mark. Then, the truck starts shuddering (like a how a truck acts with a clogged fuel filter), and the truck will NOT start easily. Starter has to wind the motor a good 45-60 seconds before it will fire. You can drive for 20 miles like this once you get it started, and it will shudder/stammer only when accelerating more than a little.

Give it ONE gallon of gas and all that goes away until the gas hand gets below the E mark.

Neither of these problems seem to be a huge issue so long as I keep fuel in it, but I would like to have a clue what’s going on. Suggestions? I need to limp this old beast along as far as possible. It’s been a good truck to the family and I sure do need a truck. Can’t afford a replacement at this point.


Saying “I am offended” is telling everyone else that you cannot control your own emotions, and thus you need everyone else to do it for you.

Check your base timing. High timing will cause detonation with low octain fuel.

Not sure on your low fuel issue. But I have seen a few issues with the small hose that contects the in-tank fuel pump to the metal supply line. I’ve done three in the last couple of months that were just a gooey mess. You can hear the pump running but no fuel pressure at the throttle body.

Fuel Pump Getting ready to quit…if its in the tank?

Woodfloats?

If your pump goes, do what I did and just cut a flap out of the bed. Pull back, remove pump, put in new one and screw it back down using some sheet metal tabs. If it needs another pump down the road, repeat. P.S. I had a bed liner and toolbox on the back, so the “flap” was under both in front of left rear tire. Saved me a few hundred bucks the second time I had to change it.

it is easier to beg forgiveness than ask permission

14’’ McKee/30 'rude
“Shenanigans”

quote:
Originally posted by Fred67

Check your base timing. High timing will cause detonation with low octain fuel.

Not sure on your low fuel issue. But I have seen a few issues with the small hose that contects the in-tank fuel pump to the metal supply line. I’ve done three in the last couple of months that were just a gooey mess. You can hear the pump running but no fuel pressure at the throttle body.


OK, I will look into the timing issue. That’s one thing I haven’t had to do before on a vehicle so I’ll have to read up on it. Thanks!

quote:
Originally posted by BSB98

Fuel Pump Getting ready to quit…if its in the tank?

Woodfloats?


I hope that’s not the issue… it’s done this all summer if I let it get low. I’ll use Shenanigan’s trick if I need to.


Saying “I am offended” is telling everyone else that you cannot control your own emotions, and thus you need everyone else to do it for you.