Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
'78 Suburban 'chugging' on long grades
RyanAK:
New developments...
Cliff, I got jammed up at work so didn
von:
Trouble is with looking at the in-tank sock with a bore scope, is that when the engine isn't running under load the rust and sediment don't get sucked onto it and what was there has fallen off. Maybe you can see any stuff on the bottom of the tank though and general tank condition.
RyanAK:
My posts keep getting clipped for some reason... here's the cliff notes version of what I had typed above. (pun!)
It's super hot here. And humid. Truck still acting the same. Yesterday on the way home from work I stopped at a gas station for a soda and a cigar. Came out, started up, didn't get past the gas pumps before the truck shut off. Wouldn't restart.
Pulled the fuel filter. Clean and dry. And the check valve seemed stuck to the rubber gasket. Did a 'gatorade bottle test' to see if I was getting fuel. Took about 10 seconds of cranking, but eventually a good flow of fuel came from the pump. Buttoned up without the filter and got on my way.
Same starvation issues as before even without a filter. If I keep my foot in it too long it chugs like it's out of gas.
Stopped at the parts store for an alternator. Came out, started truck, same thing. Didn't get out of the parking lot before it shut off. Another 'gatorade bottle test'. 10 seconds of cranking before fuel flow but then it was strong. Fuel in the bottle looks very clean. Buttoned it up, cranked for a bit, got it started and headed for home.
What the heck!? Is the delay in fuel from the pump to the carb normal? Is underhood heat an issue?
This truck drove so well all winter and spring. 15,000 trouble-free miles. Then all of a sudden this started in June. Heat? Summer fuel? Hmmm...
I'm definitely going to rebuild the carb. Not sure that's all that's going on though.
Cliff, I got tied up at work yesterday but I'll be calling you shortly for an order. Book, kit, jets, accelerator pump, whatever else I'll need to get this sorted and tuned.
I appreciate everyone's input. Von, that's a good point about the sock and the truck not being under load.
Here's a few photos....
Cliff Ruggles:
Check all the lines and clamps before the mechanical fuel pump. Also check the routing of those lines to see if any are too close to the exhaust system.
I've seen these issues plenty of times in really hot weather and it is often caused by a restriction in the system or a leak in the system prior to the fuel pump. When the pump puts the fuel under suction and it's really hot outside and under the vehicle these problems car rear their ugly head.
I just went thru a LOT of "drama" with a customer that I built a carburetor for with similar issues. His issues were even worse and his entire fuel line was draining all the way back to the tank so he had to crank it for long periods of time before fuel returned to the carb after long periods of being shut down.
He was convinced the carb was draining down and wanted to send it back. I told him to check ALL the connections and replace all the rubber hoses before the fuel pump. He insured me that all of it was new when the vehicle was being restored over the last couple of years and that it got a new fuel tank.
So we go back and forth over a week and finally he goes under the vehicle and low and behold he finds a loose clamp on a metal line where it goes into the fuel pump on the engine. It was allowing the fuel pump to suck air but not loose enough to show a fuel leak after shut down as the air coming in was letting all the fuel return to the tank and empty the line.
That loose clamp caused him a LOT of issues including the carb going empty on a hard run, empty fuel line after sitting and LOTS of cranking to re-fill the carb, etc.
I've seen defected or old rubber hoses do the same thing, suck air but not leak fuel.......
RyanAK:
Hi, Cliff. Thanks for taking the time to respond. I went through the fuel lines yesterday. Sucking air and breaking the vacuum created by the pump makes perfect sense as to why it would drain back or suck air while pumping (sucking). More sense than the new pump is wonky.
Supply and return soft lines at the pump are new and clamps are tight.
Hard line from pump to carb is clear and I put rubber line around it to insulate it where it was near the block. Temp on that line (meat thermometer taped to line) just before the carb is 140* after running for a few hours.
Factory heat shield is in place where the exhaust maneuvers around the transfer case and gets near the frame where the fuel lines are.
Soft lines mid-frame are in good shape and clamps are tight. I'll replace anyway.
I can't get to the soft lines at the tank... dropping this tank will be a huge PITA. 40 gallons and there's a massive hitch welded in around it. I'm gonna see what I can do to maneuver myself into a position to replace that line.
I had a thought to try to check all of these with a stethoscope with the engine running to see if I can hear a leak where it might be sucking air.
Are the Quadrajets susceptible to heat soak/fuel boiling issues like some of the Eddy carbs are?
And I'll still rebuild the carb. Left you a message yesterday.
And... I'm trying to not be one of those 'drama' guys. Ha!
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