Author Topic: '78 Suburban 'chugging' on long grades  (Read 282 times)

Online RyanAK

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Re: '78 Suburban 'chugging' on long grades
« Reply #15 on: July 30, 2020, 06:51:18 AM »
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!

 

Offline tayto

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Re: '78 Suburban 'chugging' on long grades
« Reply #16 on: July 30, 2020, 08:49:09 AM »
the line that is hardest to get to is probably the problem, murphy's law. haha

Online RyanAK

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Re: '78 Suburban 'chugging' on long grades
« Reply #17 on: July 30, 2020, 04:55:28 PM »
Rigged up a soft line from the pump to carb with a clear plastic inline filter so I can see what

Online RyanAK

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Re: '78 Suburban 'chugging' on long grades
« Reply #18 on: July 30, 2020, 04:57:09 PM »
Posts keep getting clipped...

Rigged up a soft line from the pump to carb with a clear plastic inline filter so I can see what
« Last Edit: July 30, 2020, 05:07:40 PM by RyanAK »

Online RyanAK

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Re: '78 Suburban 'chugging' on long grades
« Reply #19 on: July 30, 2020, 05:08:07 PM »
AAAARRRRGGGGHHHH!!!...

Rigged up a soft line from the pump to the carb with a clear plastic inline filter so I can see what’s happening. Seem to get good fuel on first start on a cool engine. When I shut it down, it immediately drains back. After a 10 min wait on a hot engine, the carb will suck the filter dry before the pump can push fuel to the filter. Maybe 10 seconds from when the engine fires until fuel starts arriving in the filter.

I have a return line on the pump. Are these mechanical pumps supposed to allow fuel to drain back? Could something in the carb not be closing/sealing to allow the fuel to drain?
« Last Edit: July 30, 2020, 05:17:50 PM by RyanAK »

Offline hiy_po

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Re: '78 Suburban 'chugging' on long grades
« Reply #20 on: July 30, 2020, 08:36:59 PM »
I am trying how to understand how the carby can "suck" the filter dry
 I see a suck as a high pressure trying to get to a low pressure. so would need a pressurized line with a low in the carb to move fuel that way. Once pump has stopped and no pressure in line fuel shouldnt be going into carb unless you have heat causing a high pressure in the line. Is it possible your tank is vacuum locking and you are getting a huge low in the tank as the level drops causing movement of fuel back to the tank once pump is stopped?? This would also explain why it would need time to reprime. If you dont have an intank pump then you are rellying on gravity to get the fuel from tank pickup to first pump.

Online RyanAK

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Re: '78 Suburban 'chugging' on long grades
« Reply #21 on: July 30, 2020, 10:34:48 PM »
Thanks for chiming in, Hiy_Po. I

Online RyanAK

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Re: '78 Suburban 'chugging' on long grades
« Reply #22 on: July 30, 2020, 10:35:30 PM »
Thanks for chiming in, Hiy_Po. I’ll try to get video. The tank isn’t under vacuum. The vent is clear. It will do this with the gas cap removed... I checked. Pump is mechanical on the block. Not sure if it is supposed to have some sort of check in it to keep fuel from draining back... either through the supply line or the return line. But it drains as soon as the engine is cut. As for the fuel remaining in the filter after drain-back... after a few seconds after startup, it gets pulled from the filter to the carb. I’d guess the low vacuum in the intake manifold (20”) might be enough to pull the fuel from the filter to the carb. A few seconds after that the pump is pushing and refills the filter. If the engine were under load right after starting, the carb would get sucked dry before the pump could get fresh fuel to it and the engine would stall.

Offline Cliff Ruggles

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Re: '78 Suburban 'chugging' on long grades
« Reply #23 on: July 31, 2020, 03:36:20 AM »
I tried calling yesterday, then saw this morning where you called again.

Was out of the office most of the afternoon running parts and working on my shop and driveway.

I'll try again in a couple of hours, orders don't ship out till around noon......

Offline tayto

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Re: '78 Suburban 'chugging' on long grades
« Reply #24 on: July 31, 2020, 08:07:21 AM »
Posts keep getting clipped...

Rigged up a soft line from the pump to carb with a clear plastic inline filter so I can see what
trying pressing "Preview" button first then "Post"

Offline hiy_po

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Re: '78 Suburban 'chugging' on long grades
« Reply #25 on: July 31, 2020, 06:50:29 PM »
lol I think I may have read it wrong. I read it as the carb was emptying the fuel filter after shut down. With no manifold vacum or pressure in fuel line I couldnt see how that was happenning. I get it that the carb is running dry before the filter is primimg on start up.

Online RyanAK

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Re: '78 Suburban 'chugging' on long grades
« Reply #26 on: August 04, 2020, 07:35:44 AM »
I'm thinking that hot fuel / vapor lock is contributing here. I'll check the fuel return line is functioning from the fuel pump and explore ways to keep the pump and lines cool, along with the carb rebuild. Anyone have thoughts?

Online RyanAK

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Re: '78 Suburban 'chugging' on long grades
« Reply #27 on: August 04, 2020, 12:33:03 PM »
Fuel comes out the return nipple on the pump at idle. Stops when revving the engine. I'll need to check back at the soft line sections at the between the pump and tank to make sure fuel is getting all the way back and not turning this into a deadhead system. Lines were blown all the way back to the tank when I first had this issue to make sure they were clear, so I'm thinking it's not a return line issue.

Rigged up a fuel pressure gauge after the temporary clear filter between pump and carb. 6.5psi at idle. 6.5psi at 2500rpms. No load, sitting in the yard. Engine up to temp.

Next to rig the gauge so I can see it while driving. (short drives... this is all hillbilly rigged together for diagnostics only!!)

The soft supply line going into the pump from the hard line on the frame was resting on a heater hose. Moved that and put a split piece of fuel line on it in that area as an insulator.

Fuel pump gets HOT. Now I'm thinking the heat on the pump may be the culprit. How do you keep a block mounted fuel pump cool?

Online RyanAK

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Re: '78 Suburban 'chugging' on long grades
« Reply #28 on: August 04, 2020, 01:18:10 PM »
Well that was enlightening! Took the truck for a drive. 82* here today. Truck up to temp. Fuel pressure at idle and lazy acceleration stayed at 6.5psi. When I pulled onto the highway and accelerated the fuel pressure slowly and steadily dropped to under 2.0psi. Then the stumble and chug routine. Let off the throttle, pressure stayed low for quite some time until it began to rise again.

When the engine is turned off, pressure bleeds off immediately. I would have thought with the check valves in the pump that some pressure would hold in the line from the pump to the carb...

Pump is HOT.

So... Heat? Restriction? Wonky pump?

Hmmmm....

Offline tayto

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Re: '78 Suburban 'chugging' on long grades
« Reply #29 on: August 04, 2020, 10:00:36 PM »
is it starving from fuel or the new fuel pump defective? I think it's time to drop the tank, replace rubber hoses, inspect hardlines on sending unit and install a new pickup/sock.