Chevy P30 low/rough idle when tank is full

yuletree · 10-27-2021, 04:50 PM

Hiya!

I've encountered a strange issue in my 86 Thomas Chevy P30, Rochester Quadrajet, 5.7 SBC 350.

When the gas tank is full, regardless if the engine is warm, hot engine RPM drops to ~400-500. It causes the power steering to be sluggish, lights to dim, but doesn't ever stall.

I first encountered this problem during a fillup pit-stop. My starter was having a heat soaking issue that has since been remedied, so instead of turning the bus off, I filled the tank up. A few minutes into the drive at a stop sign, I noticed the issues mentioned above. It went away after about an hour or so of driving

Continuing to troubleshoot a few months later, I noticed that if the tank is full after a cold start, and allowing the engine to warm up a bit, the low idle reoccurs, at a stop sign or stoplight.

The issue appears in neutral, drive, reverse; the second I give it gas it runs fine. I do not recall the symptoms when it is in park.

If the tank is 5/6 or less full, the problem does not appear, regardless of cold, warm or hot engine. If the problem appears, and I drive for about an hour, when the tank is 'less full' the problem goes away.

The issue seems more related to the amount of fuel in the tank than any other driving or engine behavior.

Some details about the bus
-New Engine (4000 miles now)
-New mechanical fuel pump
-New fuel filter
-Carb cleaned ~5000 miles / Dec 2019 ago
-idle screws set by a competent mechanic
-occurs at sea level / <4000 ft, bus is primarily driven at low elevation and carb was tuned for that area. Haven't taken the bus to a higher elevation yet.

The only thing that hasn't been replaced is the fuel pump in the tank

I've considered vapor lock, but as far as I can tell, this 86 has the appropriate filler neck configuration with breather tube and it has not exhibited other vapor lock symptoms; as long as the tank isn't completely full, the bus runs great! even when the issue is occuring, as long as I'm using the throttle, no other issues occur. No struggling uphill (aside from the limitations of a 350 / th400 moving a 10000lb brick), no misfire, no stalling, nada.

And again, if the tank is less than full, the bus performs normally; normal idle in all scenarios at all engine temperatures.

Has anyone else ever encountered this issue?

jimmythomas · 10-27-2021, 08:10 PM

Does your bus have a charcoal canister and a fuel tank evap system with a purge valve?
Classic symptoms of a stuck open purge valve.
Google is your friend!

JESUSBUS · 11-16-2021, 06:06 AM

Quote:

Originally Posted by yuletree

Hiya!
I've encountered a strange issue in my 86 Thomas Chevy P30, Rochester Quadrajet, 5.7 SBC 350.

When the gas tank is full, regardless if the engine is warm, hot engine RPM drops to ~400-500. It causes the power steering to be sluggish, lights to dim, but doesn't ever stall.
The only thing that hasn't been replaced is the fuel pump in the tank

Well, here is the thing scout, if you have a Quadrajet on a Chevrolet 350, then it has a mechanical fuel pump. There is no reason to believe that you have issues with an electric powered fuel pump in the gas tank on an 86 era vehicle. It just didn't happen then. If your Quadrajet is messing up your quality of your ride, then rebuild it. The first clue is that the metering rods have small little caps inside the carb that fail and makes the engine run rich, the carb needs rebuilt by a qualified mechanic who is 50 years old or above, or just buy a TBI system to replace the carb and intake.

I may be wrong, but fuel pumps inside the gas tanks did not start to happen until after 1988 or 89

Sorry if I come off like an asshole, but that's just how I am. I wanna help ya!

Tejon7 · 11-16-2021, 08:48 AM

Quote:

Originally Posted by JESUSBUS

Well, here is the thing scout, if you have a Quadrajet on a Chevrolet 350, then it has a mechanical fuel pump. There is no reason to believe that you have issues with an electric powered fuel pump in the gas tank on an 86 era vehicle. It just didn't happen then. If your Quadrajet is messing up your quality of your ride, then rebuild it. The first clue is that the metering rods have small little caps inside the carb that fail and makes the engine run rich, the carb needs rebuilt by a qualified mechanic who is 50 years old or above, or just buy a TBI system to replace the carb and intake.

I may be wrong, but fuel pumps inside the gas tanks did not start to happen until after 1988 or 89

Sorry if I come off like an asshole, but that's just how I am. I wanna help ya!

This sounds like a really helpful reply. Makes me wonder, though... if you know you come off as an A**hole, why not just... don't.

JESUSBUS · 11-16-2021, 10:32 AM

Because that's what I am good at. It doesn't mean I don't love my fellow man, it is just the way I am. Remember well friend that anything done in love is worthy of the Lord. When I say something, even if I sound like an idiot, and who doesn't these days? I say it with my heart and spirit. One thing we have in common is that a forum is an abstract meeting place and is no substitute for people communicating face to face out in the open.

jimmythomas · 11-16-2021, 10:38 AM

The OP said the problem only occurs with a full tank. That would rule out problems with the Quadrajet. No need to be a A**.

JESUSBUS · 11-16-2021, 10:57 AM

Quote:

Originally Posted by jimmythomas

The OP said the problem only occurs with a full tank. That would rule out problems with the Quadrajet. No need to be a A**.

What I am saying is a carb can be combined with an electric fuel pump, but I have no knowledge of this guys bus. Normally a carburated engine would have no need for an electric fuel pump in 1986. In 1986 it might have been possible that School Busses were equipped with an in tank pump, but based upon the small block Chevrolet Engine, a 350, the common fuel pump on cars and vans and trucks was a mechanical fuel pump that was attached to the engine block on the front passenger side. The camshaft had a portion that was eccentric to the centerline of the cam, and pushed a hardened steel rod in and out towards the fuel pump, which was mounted fore of the 350 engine block. If it was done away with, then this guy might have an electric fuel pump in 1980-something. Just giving my advice, Senator.

JESUSBUS · 11-16-2021, 11:08 AM

Quote:

Originally Posted by jimmythomas

The OP said the problem only occurs with a full tank. That would rule out problems with the Quadrajet. No need to be a A**.

Jimmy this is absolutely the time to be an ass, because everyone will read this. A Carburetor is not a fuel injected system.

Don't get rid of me just because I am gonna say something out loud.

Now, does it occur to you that maybe the OP does not have a lot of knowledge about engines, or schoolbuses, and are in fact amateurs who are only seeking good advice? Perhaps, you are skilled only in taunting a newby, so you improve your position on the forum? That's exactly how it works.

Now, I digress, because my time has more value than being heated with you. Cheers, and drink some Jim Beam, maybe if you can.

Kentucky Dreaming · 11-16-2021, 11:25 AM

I'll try to break the trend and stick to the topic...

My brother had a 1985 Southwinds motor home on A p30 chassis. It had a 454 with Quadrajet carb (I rebuilt it on a campground picnic table). It had the standard mechanical fuel pump. (I helped him replace it on the side of the road.) I also believe, but am not certain, that it had an electric pump in the tank to push the fuel the long distance from tank to mechanical fuel pump. Look for a fuse labeled as fuel pump fuse

I agree with the first reply by Jimmy Thomas that your prob is more likely the Evap system more than anything else.

Seems that the electric pump would be more likely to get weak when fuel is low.

Good luck solving your problem.

JESUSBUS · 11-16-2021, 11:34 AM

Quote:

Originally Posted by Kentucky Dreaming

I also believe, but am not certain, that it had an electric pump in the tank to push the fuel the long distance from tank to mechanical fuel pump. Look for a fuse labeled as fuel pump fuse

I agree with the first reply by Jimmy Thomas that your prob is more likely the Evap system more than anything else.

Seems that the electric pump would be more likely to get weak when fuel is low.

Good luck solving your problem.

Thank you KY Dreaming. You collected all our thoughts and gathered them into one basket. See, I did not know that your bus had also a fuel pump in the gas tank. This was unheard of in late 1980's. However, the Quadrajet Carb is very notorious for leaking gasoline into it's few ventriciles. The Quad was a work in progress so far as carbs go. My advice is to replace it with TBI or a Holley carb.

PorchDog · 11-16-2021, 11:44 AM

I am going to agree with some sort of issue with the vapor recovery system. When you fill the tank full-- if there is a ruptured diaphragm, or faulty control solenoids, you can suck raw gas into the intake, and it will make the engine run rich, or stall and die. This is more noticeable at idle-- at highway speeds it might go undetected.

PorchDog · 11-16-2021, 11:46 AM

Find the line from the vapor cannister (if it has one) to the intake, and pinch it off- and see if the problem goes away. That will not cost anything to do-- and will rule that out pretty quickly so you can move onto something else.

JESUSBUS · 11-16-2021, 12:24 PM

For your purposes bro, the factory emissions don't mean anything unless you are a hardcore vegan or tree hugger. The Internal Combustion engine can work without any electronics or a computer. If your ride has a small or big block chevy you only need an old school HEI ignition, which was the predecessor to multiple coils and extra gadgets which cost you money. The bottom line is money after isn't it? No one can refurbish a school bus without a great and substantial source of wealth. To the old guys, I say, that you should invest your wisdom in the father who has given you the gospel.

craigify · 12-19-2021, 01:19 PM

A little late here but this is my first thought as well...checking the canister and checking if it is sucking up raw fuel.

The EVAP systems work like this:

- There is a vacuum line that runs from the engine to the fuel tank through a diaphram of some sort that is usually mounted to the charcoal canister.

- I can't remember exactly, but I _think_ this is designed to be ported vacuum. This means no vaccum at idle. This is an important point, so read on.

- The vacuum from the engine bypasses the coarcoal canister, and its designed to suck vapors straight from the tank into the engine when running. If my memory serves me properly, not at idle (hence ported vacuum).

- When the engine is off, and there is no vacuum, the tank is designed to vent to the atmosphere, but it does this by venting through the charcoal filter so that you don't smell fuel vapors everywhere and not vent raw fuel vapor into the atmosphere, which is a pollutant.

- The gas cap, when on, seals the tank. Therefore the only way to allow for the movement of atmosphere in our out is through the EVAP tubing and canister, or into the engine.

As I first mentioned, it could be sucking raw fuel from the tank. The canister should be mounted higher than the top of the fuel tank, and from the factory usually is. Check for that. Also, though I am not 100% sure on this, to try moving to ported vacuum. If the air passage to the outside is blocked (such as raw fuel in the canister, or somebody blocked the suction port), and/or you have a manifold vacuum as port (which means strong vacuum at idle), you'll be creating a slight negative pressure inside the tank which could cause an older style mechanical fuel pump to have trouble getting fuel out of that tank at low rpm.

yuletree · 09-24-2023, 06:39 PM

Few days late and a few dollars short, but thank you all for your responses. It was the EVAP. It was full of sludge and gunk sticking the solenoid. The EVAP canister is above the tank and engine in the upper driverside cowl in the engine compartment.

JESUSBUS, the 'scout' comment coulda been missed, but I appreciated the assistance

Regarding the carb, I did have it rebuilt when I purchased the bus, the original owner let an amateur work on it who put the needles in the wrong places. I had a boat shop do the rebuild, but I found out this year they were off the mark a little when doing a tuneup with a good shop. Ironically, the second mechanic's work has made it a bit harder to start, but appears to run better.

Albatross · 09-26-2023, 10:18 AM

The Quadrajet was probably one of the most incredible machines ever built by man.

A properly-tuned (even tuned to a particular driver/driving-style)Q-jet with maybe some jet modifications was essentially capable of getting a '70s hotrod to run as lean as modern equivalent vehicles while also generating about as much power as you can get out of an equivalent engine these days.

Sadly, what it took to achieve something like that was usually some 50 y/o retired mechanic with a two-day shop that was usually full of shelves of different -sized and -aged Q-jets along with at least one wall full of tackle-box -sized shelves of nothing but various jets and needle valves. And all of these guys are dead or demented now.

It's kind of sad because you could do things with a Q-jet that no Holley or Edelbrock carb could ever compete with, but those carbs are so simple that pretty much anyone with a basic understanding of carbs can work on them or rebuild them at home.

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