78 Suburban surging then dies on long grade

[RyanAK]

Hey, Michael. Appreciate the input. I'm getting clean gas to the carb (I temporarily installed two clear inline filters for observation purposes... one on the frame rail and one between the pump and carb). When I shut down the hot engine the fuel disappears from the line between the pump and carb. Also, I did clean all surfaces and use the heat sink paste when I installed the new module in the distributor.

We now have all new rubber lines between the tank and pump. Cannot locate any issue with the steel lines.

This is a complete mystery. I have a suspicion that heat is playing a part in going lean/dry when the engine is under load, but that could just be a coincidence.

Carb rebuild kit from Cliff should be here today. But I'm not sure this is a carb issue. We shall see!

[geezer#99]

Curious!
Did you try wiring the secondary air door shut?

[RyanAK]

Yup. Thought maybe it wasn't "not enough gas" but "too much air". Same issue. If it was the air door opening too early I should have been able to accelerate through it, right? When the chugging starts, the only cure is to let up on the throttle.

Wired the choke open. Same issue.

Also replaced the choke pull off since the one on it wasn't pulling in consistently. Same issue.

Rigged up a temp 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 after getting up to operating temp, fuel immediately drains back. After a 10 min wait and restart 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?

Vapor lock when engine is hot and working under load causing a lean condition or fuel starvation?

I'm hoping a rebuild of the q-jet solves this, but I'm not confident something else isn't going on.

[geezer#99]

Who knows!
You could try plugging off the return line.

I learned to ignore what I saw in the filter. If it keeps running, it’s good.

I ran into a similar problem on a coworkers ford 302. Turned out the intake was cracked (corroded)and when it warmed up the problem would occur. The splash shield had filled with corrosive crap and rotted a hole in the intake. Weird stuff happens.

How’s your hot idle vacuum reading?
Using any oil?

You should post your video you mentioned.

[RyanAK]

I'll see what I can do about posting videos... looks like I can't post directly...

Idle vac when hot is steady 20". No apparent vac leaks and I have a gauge in the cab to watch it while driving.

Not using any oil now that I have the PCV sorted.

So curious! Frustrating, but I'm still having fun.


GM crate motor with 17,000 miles so I'm hoping it isn't something internal to the manifold!

[geezer#99]

There’s a bunch of possibilities listed here on your carb woes. Some things to watch for when you rebuild the carb.

https://www.digitalcorvettes.com/thr...by-lars.88376/

[Richard]

Seeing as the truck was running well after the pump was replaced I would check that out again. Would guess there is a good chance it is failing if reman or knock off. Even if a GM Delco pump, who knows where they are made now. Unfortunately I have had new parts go bad more than I want to say.

[RyanAK]

Just took the truck for a drive. Still chugs at highway speeds when engine is working enough to drop vacuum to <9". Just like it's starved for gas.

Pulled back in the drive way and the filter between the pump and the carb is dry. Like... a minuscule amount of fuel in the bottom of the filter and no apparent effort by the pump. What... The... F....?!?

Ordered another new fuel pump. We'll try Delphi this time.

geezer - I've gone through that doc from Lars, but it's been a while. Thanks for reminding me. I don't think a proper q-jet rebuild is going to solve all of these problems.

[RyanAK]

Hey, Richard. The new pump I put on a few months ago was a NAPA. Just got them to order in a Delphi. Who knows where any of these are made. Might be the only difference between NAPA, Delphi, Carter, Delco pumps is the box it's shipped in...

It seems like a "ticka-ticka-ticka" has showed up with the NAPA pump that I can't remember being there before. No fuel coming out the weep holes.

Wondering if the return line has anything to do with this... or the associated check valves in the pump. My understanding is that unused fuel from the pump body returns to the tank via the return line and that fuel that has made it to the carb line should stay there due to the check valve in the outlet. Maybe the fuel that the carb needs is ending up going down the return line....

Hmmm....

Hope the carb rebuild kit is in today's mail! But I can't see how the carb could be contributing to this.

R

[RyanAK]

I'm thinking this might be a vapor lock / boiling fuel issue. I'm going to check that the fuel return is working and look at ways to keep the fuel pump cool. Any thoughts?

[RyanAK]

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?

[RyanAK]

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....

[v30crewcab]

have you tried leaving the fuel cap loose? might be sucking a vacuum on the tank.

[v30crewcab]

sounds like you either have a plugged vent on the fuel cap, or the sock is getting plugged from junk in the tank under higher load. also, did you replace the rubber fuel line thats about half way back? not sure if the 70's had it but mine has a rubber flex line kinda under the passenger door area in the middle of the metal fuel line.

[RyanAK]

Does it with the fuel cap off. Never had a vacuum release on the cap when removing it. But it does seem like that could be a cause.

Rubber line at pump, halfway back the frame (currently with temporary clear plastic filter for diagnostic purposes...), and at the tank are all replaced. (The one on top of the tank was a challenge!!)

Wonder if I could borescope the tank pickup sock...

[v30crewcab]

you've got some trash floating in the tank or something.
My dad had trouble with his truck like this, I drove 2 1/2 hours to pick him up with a trailer. Got it home, pulled the tank and there was green grass clippings floating in the tank somehow. They kept jamming up the fuel sock. no idea how they got in there.

[garyd1961]

Didn't read the whole thread but did read the last page. Sounds like a leak in th he fuel line between the tank and pump sucking air into the line or the rubber line collapsing and shutting off the flow.

[RyanAK]

Alright, fellas. For any of you still reading this saga, thanks for sticking around!

Rebuilt the carb with Cliff's kit - my first solo rebuild. Went very well. This thing was pretty damn clean inside. Just a few issues that I ran into...

As a reminder - 4MV 17058213 on a '78 Suburban, 350 GM 190hp crate, TH350, NP203, 3.73 gears, stock, stock, stock.

This 4MV doesn't have an "Idle Vent Valve" as shown on the instruction sheet. Not sure if this is an issue... What the heck is it?

Also... the "Idle Compensator" on the back of the carb is missing. Ditto... what the heck is it?

I didn't get the APT screw exposed because I wanted to get this buttoned up... but I will eventually do that.

Also didn't replace the Secondary Air Valve Cam because I wasn't sure how to remove the staked screws on the air valve shaft without buggering something up.

As I said, this unit pretty clean and everything seemed to be correct. The accelerator pump didn't have the skirt spring in it, and the Secondary Air Valve tension spring was a bit loose, but everything seemed "right" otherwise.

Some specs that are a mystery to me and I could use some input on:
Primary Rods - 40B
Jets - 64C
Power Piston - arms level and even, Cliff's orange spring
Secondary Rods - DP
Hanger - C
N/S - Cliff's .135" non-window, no clip on needle
Accelerator Pump - Cliff's
Float - was A67, now Cliff's A61 set to stock 15/32"
All linkage adjustments in spec.

Need to re-tune idle rpms, idle f/a mixture, etc. Now that the carb isn't an issue, I can go back through my timing. I had it slightly retarded to 8* initial to limit the pinging that apparently was my carb going lean from fuel starvation and not a detonation issue.

It was fun! My only worry for future rebuilds now is that Cliff will retire and the good parts won't be around!

All that said... the carb rebuild didn't fix my issue. Same starvation/chug when under load.

What DID fix my issue after everything I went through... was replacing the 2-month-old NAPA fuel pump! Ordered in a Delphi unit from NAPA and after some struggles getting the fuel line flare to seat on the pump and not leak, I have fuel pressure and flow to the carb. The NAPA pump fell apart in my hands when I pulled it off the block. Spring fell out, lever fell out, and there was a black rubber seal of some sort chewed up and jammed near the pump piston. UGH!!

New Delphi pump... pressure seems a bit high, but no flooding yet and no starved condition. At hard acceleration I was getting <2psi out of the NAPA pump. Now at WOT it holds a steady at 6psi with the new Delphi pump. Only thing that makes me nervous is at idle and cruise the pressure can be 10psi or a smidge higher. Am I looking at trouble with that pressure and this particular pump? As I said, zero issues so far. And no starvation/stumble/chug.

Appreciate any further comments on the pump pressure or the way the carb is set up. I learned a lot and had fun doing it!

[53burb]

awesome!! glad u got it taken care of....I don't think you will have an issue with that pressure...if it was an Eddy carb, then yes you would, but not on that carb.
I believe the idle compensator attaches to your idle linkage & is an electric solenoid/plunger that pushes the idle linkage up to increase your idle when the A/C is kicked on...usually has a wire attached to it which is attached to your a/c compressor or binary/trinary switch which is inline on one of your a/c lines...I THINK
Did you have the same problem before you replaced the pump and the pump didn't solve it OR problem happen after you R/R the pump?

https://www.summitracing.com/parts/s...yABEgJE9vD_BwE

[RyanAK]

Thanks, man! This was a two month struggle on my daily (only!) driver. Let me sit on the interstate. Did a bunch of things including replacing the mechanical fuel pump. Seemed to be ok for a while, then symptoms came back. Did more things, including the carb rebuild and rigging up a pressure gauge to monitor while I drive. At wits end I decided to swap in ANOTHER new fuel pump. Problem seems to be solved. Good to know I’m likely good with the PSI!!

Lesson learned. Don’t assume your new parts actually work!! Diagnose, diagnose, diagnose!!!

I know about the solenoid. Mine doesn’t have that either. The “idle compensator” on this 4MV is some sort of widget and a cover that goes on the back of the casting on the carb. According to the exploded view in the rebuild instructions. There seems to be some sort of air passage that may or may not be blocked off by the air horn gasket...

[RyanAK]

As way of an update...truck is running well down the highway. No fuel starvation issues. Can't believe it was a bonked out new fuel pump!

New issue though... when the truck is at operating temp and idling - whether in Park or in gear - the engine will occasionally stumble hard and then recover. Idle in P is ~900RPM and it'll stumble down to under 500 and then come back. In gear were around 700 and it'll stumble to almost stalling out before smoothing back out.

I popped the air cleaner so I could watch. I can see the fuel level increase in the bowl by looking down the secondary rods. Up it comes...stumble... down it goes... smooths back out. Does it just once occasionally, or it'll start cycling like this every 20 seconds.

I have a video but can't seem to post.

Float is set at 15/32. New pump is pushing ~8psi. Non-check-valve filter. Zero issues while driving.

Thoughts?

[geezer#99]

Sell that pile!
Buy a ford.
Just kidding!
What’s your float drop setting?

[RyanAK]

Ha! Can’t sell it. Love it. So do the kids. Coming up on one year with it and put almost 20,000 miles on it. She’ll be ok. I know it. It ain’t the rig, it’s the crappy auto parts!

Fords... ugh. Though I wouldn’t mind a ‘67 Fairlane. (Take a Merc Comet over the Ford though...) Bowtie trucks, but my eyes do wander when it comes to mid-‘60s cars.

Float drop.... uh... remind me what drop is. Height is stock 15/32. This is my first time through the inside of a carb.

[geezer#99]

I think I miss spoke.
Float drop is not an adjustment on the quadrajet.
Was thinking eddy carb.
However your fuel pressure could be suspect.
Is there a piece of rubber hose on your fuel line you can squeeze a bit to see if the excess fuel is overwhelming your float and needle/seat?

[RyanAK]

Alas... I reinstalled the stock steel line from the pump to carb. And I had a heck of a time getting the fitting to not leak at the pump, so I hesitate to hook up my Rube Goldberg rubber line with filter and gauge.

My rebuild kit is from Cliff Ruggles. Cliff thought this carb would be fine with 8psi fuel inlet. But it does seem like it could be pushing through the needle, overfilling the bowl, and the stumble is the motor almost flooding out...

Hmmm...