78 Suburban bogs then dies on long grade

[RyanAK]

Hey, gang. My daily driver let me down a few weeks ago and put me dead on the side of the interstate 100 miles from home. I’ve methodically gone through diagnosis, fixing or replacing things and THINK I have it solved. I had been able to duplicate the fail a time or two close to home as I worked through my list, but as I completed the last few items I haven’t been able to repeat the condition that left me stranded. BUT... lack of evidence doesn’t mean I have it solved and frankly I’m a little nervous about my next 280 mile commute.

So... the truck: 1978 K10 Suburban pretty bone stock with a GM crate installed by the previous owner. About 18,000 miles on that motor now. Carter Quadrajet, TH350, NP203 (stock), 3.73 gears. The only ‘upgrade’ to the drivetrain would be 16” wagon wheels and 275/75R16 tires.

What happened? Well... Cruising home on I-80 from work near the Delaware Water Gap I hit the LONG grade up into the Poconos at Tannersville. This grade is a grind. Something like 8 miles of 4%. I’m doing 70, keeping up with traffic. Truck is working, but not struggling. Normal. Have done this drive several dozen times. This time however, I get a sudden series of surges and deaths. Sorts like running out of gas. Something like...

La la la going 70 up hill, all is right with the world, la la la.... booooog GO!!! boooooooog GO!! booooooooooog GO! boooooooooog Go. booooooog Go? boooooog go? no? dead. On the berm of 4 lanes at rush hour where two interstates diverge. I have tools and some ability, but I also have two young kids and a wife that like me pretty good. Tow truck.

Get towed to the next exit, Dad drives down to meet me. Fiddle around, get it started. Go for a slice of pizza with Dad. Talk about what we should do. Screw it, let’s try.

Dad chased me home. All good except for one long grade again where it started to do the same booooooog GO!, but not as sever this time. Got it home. Started in on it the next morning.

So... before I go into the long list of relatively small things I did to the truck, I’d love to hear any suggestions for what the cause might be. There are only two things left on my list to try, but as I said, I haven’t been able to replicate the symptoms again.

Thanks in advance. Hopefully I have this licked, but if not, I’m sure someone will suggest the best next move.

Ryan

[Wikid61]

Fuel filter checked recently? Steep grade no load bog?

[Dead Parrot]

Fuel filter
Fuel pump.

Suction hoses from tank to frame fuel pipes. Same from pipes to fuel pump. If original, they are EOL. Both cases often let air in but don't show a fuel leak while it is sitting.

Sender pickup sock rotted and plugged the intake.

Plugged fuel tank vent setup.

[kwmech]

Stop cross posting

[rpmerf]

Agree that it is somewhere in your fuel system. Where exactly is hard to determine without troubleshooting.

You can do a flow test - put the end of the fuel hose into a sauce jar, crank it, make sure it is putting out nice strong pulses. Should be minimum a pint in 30 seconds.

You can have the fuel pump pull from a gas can, rather than the tank and lines. If this significantly improves things, there is an issue with either the fuel sock or tank to pump hoses / lines.

Failing fuel pumps are worse when hot. I had one that wouldn't stay running with the engine hot, but would run fine with the engine cold.

Fuel hose for carbureted engines is cheap enough to just replace. You don't need fuel injection hose.

Your tank might have a bit of corrosion inside that is getting caught up in the fuel sock.

[RyanAK]

Thanks guys.

Wikid61 - fuel filter was the first thing I wanted to replace. Looked rough, but it seemed as though flow was still ok. Of course the parts store didn't have a filter in stock, so I had to wait to do this part. Then the wrong filters came in. So more waiting.

I wouldn't say this was happening on steep grades... just long ones. Felt like running out of fuel... or that the fuel system couldn't keep up on a prolonged pull up the hill. 3rd gear, 70 mph, dragging a heavy Suburban up the grade. No load other than the weight of the truck. It was also the first really warm day of the year. No rise in engine temp.

Hoses... I checked at the tank after I got towed and all seemed good. Then there's a short section of hose between steel lines on the frame. They seemed good as well. Then at the pump... return like looked good. SUCTION line did not. Extremely soft, tight radius bend so it was sorta flattened. Got the truck started and looked at the fuel pump suction hose again... it was pulsing like an artery. Seriously, it looked like a vein about to burst and was definitely going flat as the pump sucked. When we got the truck home, I replaced this hose with FI line with a longer radius bend. No more flattening, no more sucking closed. I intend to get to all the others to replace soon.

But I was still able to bog the truck out on long grades.

Other stuff I did while waiting on the filter to show up...

Replaced the divorced choke. The axle that the spring is wound on was spinning freely and not allowing the choke to operate correctly. Thought this could contribute if the vacuum in the carb pulled the choke valve closed. Replaced. Still able to bog it on long grades.

Checked fuel flow. Seems good. Have not checked pressure. I'm trying to get to that today.

No pressure or vacuum in the fuel tank.

While waiting on the CORRECT filter to come in, I did an ignition tuneup. Cap, rotor, coil, wires, plugs. Did not do module. Weights and springs looked fine. Still able to make it bog.

Also set the timing... but with a vacuum gauge. I have a light, and the balancer is marked for TDC... but the timing tab isn't. Don't have a piston stop or help to verify TDC so used my vacuum gauge. The truck would ping pretty good on 89 octane or less, so I set timing for max manifold vacuum (20"HG), less 1". Pinging went away, but seem to have lost a little pep. Will definitely establish TDC and time it with a light.

The fuel filter finally came in. The short one. But... without a check valve. Put it in two days ago. Since then have not been able to make it bog on a hill. But I'm still concerned that I missed something. Nervous about it coming back. I'm sure you guys know what I mean. Did I really get it?

Only things left on my immediate list are the PCV valve (but that seems to be working fine...) and the fuel pump itself. I've had electric pumps fail, but no experience with symptoms on a mechanical pump going out. Is it a gradual death or POW, yer dead! Will a pressure check help diagnose? rpmerf - you mention failing fuel pumps are worse when hot. Is that for mechanical pumps?

I'm hoping I don't need to drop the tank. It's a 40 gallon and just about full!

Appreciate the responses.

R

[1976gmc20]

My experience is mechanical fuel pumps fail by getting a hole in the diaphragm and spitting gas out the vent hole.

After 42 years, who knows what the inside of that gas tank is like? They are a pain to remove and replace. I disconnected the big filler hose (which you have to do anyway) and siphoned out of that. I never could get a siphon hose down in there far enough by running through the filler.

Lacking a suitable jack, I hooked a tow strap to a crossmember above the axle, then under the tank and belayed to the trailer ball, with a helper to let out or take in slack.

[RyanAK]

I'm swapping out the pump. $20. My less than perfect test showed low flow and low psi, so why take a chance.

I'm a true 'shade tree' mechanic without a garage. White pine, actually. I'm trying to do everything before it becomes necessary to drop the tank and look at the sending unit. And honestly... this is something I might pay to have done.

I think we're on the right track. Would less-than-spec full delivery lean out the mixture at WOT and cause ping? I wonder if the ping with 89 and less gas was more a symptom of a lean mixture and not necessarily timing... hmmm....

R

[truckin 79]

I bet you got some carbon or something in the carb affecting needle and seat. It might actually be running rich and bogging out.

[Nick_R_23]

You might check that your accelerator pump is not leaking, and that the lobe on the camshaft or the rod that drives the fuel pump aren’t flattened or mushroomed, causing the pump lever to be short stroked.

[jayoldschool]

I went through this on my 65 Impala. I thought for sure the fuel bowls were emptying out. Did EVERYTHING fuel, pump/filter/etc, then ignition (plugs/wires/timing/electronic conversion. Then pulled carb to rebuild. When I did that, I found the cause immediately. Vacuum leak at front of carb base and intake. Rebuilt the carb anyways, cleaned the rusty spot, regasketed. Car has more power than any time in the past 25 years.

Look for a vacuum leak, just in case.

[RyanAK]

Thanks, guys. I’m not very familiar with carb internals. Everything on the outside... linkage, pull offs, idle mixture, high idle cam, choke... I’m good. The inside is still a mystery. I’m hoping this isn’t carb related. It’s functioned perfectly this past year in all weather and there’s only about 18,000 miles since it was rebuilt. Whoever put this crate motor in seems to have done an excellent job on everything else.... but maybe he isn’t a Quadrajet man. Any guidance, I’m all ears.

Nick, I’ll definitely have a look at the rod and cam. If I remember from the last time I did a fuel pump like this 30 years ago with my Pap, the rod is gonna fall in my lap when I take the pump off. Good suggestion.

Jay, I’ve listened and sprayed carb cleaner in all the suspected spots for a vacuum leak. Even around the hoses for the cruise module that isn’t there any more. Hoses all seem pretty new. Gauge on the passenger side vac port is dead steady at 20” at 700 rpm. I’m open to any suggestions or help in chasing a vac leak. Up until this very moment I felt pretty confident that everything was right in the vacuum department.

Also... I bogged again. This time I stopped at a convenience store, left the truck running while I ran in for a Coke, and then pulled out and normally accelerated to about 45. Lost power for maybe two seconds, then it caught itself and off we went. No hill, completely flat. Did not accelerate aggressively. Just moseying along. Didn’t happen the rest of the day.

Of note, and possibly related... today and the day I ended up on the side of the interstate have been exceptionally hot. Not sure if that could make symptoms appear or not...

[kwmech]

If you still have the old fuel filter, rip the input end off of the paper and separate the pleats and have a look. You may find that they are full of brown crud.

[Dleslie212]

I have a 77 K10 with a 350 and Edelbrock carb. When I first bought it, I had to drive it home on the freeway for about an hour. It kept doing the same thing - just felt like it was running out of gas. I did EVERYTHING to the fuel system - new pump, removed both tanks and replaced sending units, I mean everything. But it would still bog down at freeway speeds. The last thing I did was rebuild the carb - after taking it apart, I found a bunch of crap inside that was blocking off the flow. After cleaning that and rebuilding, it runs great on the freeway now

[RyanAK]

Replaced the fuel pump and soft fuel lines, about to go for a test drive.

KW - the filter was absolutely packed with red crud.

Is there a way to clean out any crud in the carb short of a full rebuild? I know that's asking for a shortcut to doing the 'right' thing, but I'm sort of intimidated by the thought of doing a rebuild... though I'm sure I'd get through it with time. Something I need to learn anyway. Just not a good time right now. Maybe a 'partial' tear down just to clean?

Appreciate all the help.

R

[RyanAK]

Things got better with the new fuel pump. Also sprayed down the carb really good and dumped a can of Sea Foam in the tank. Seemed to run much better and haven’t been able to reproduce the bog. But it’s been relatively cool here.

I also replaced springs and weights in the distributor and set timing. More improvement.

The biggest improvement came when I moved the vac advance to straight manifold vacuum. This truck has a temperature vacuum switch (TVS) that changes the vac advance between ported and manifold based on engine coolant temp. I couldn’t confirm it is working properly, so hooked up to straight manifold vacuum. Seems much more responsive.

If the TVS was wonky and basically eliminated the vacuum to the vac advance can, could that have caused the surging and bogging I experienced?