Free 7 Day Trial RV GPS App RV Trip Planner Campground Reviews RV Maintenance Free 7 Day Trial ×


Reply
 
Thread Tools Display Modes
 
Old 10-27-2021, 05:50 PM   #1
Mini-Skoolie
 
Join Date: Nov 2019
Posts: 12
Coachwork: Thomas
Chassis: P30
Engine: 350 V8
Question Chevy P30 low/rough idle when tank is full

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?

yuletree is offline   Reply With Quote
Old 10-27-2021, 09:10 PM   #2
Mini-Skoolie
 
Join Date: Aug 2021
Location: Santa Fe
Posts: 22
Year: 2007
Coachwork: Thomas
Chassis: 31 ft. HDX
Engine: CATC7
Rated Cap: 36,300 GVW
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!
jimmythomas is online now   Reply With Quote
Old 11-16-2021, 07:06 AM   #3
Mini-Skoolie
 
Join Date: Nov 2021
Posts: 24
Quote:
Originally Posted by yuletree View Post
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!
JESUSBUS is offline   Reply With Quote
Old 11-16-2021, 09:48 AM   #4
Bus Nut
 
Tejon7's Avatar
 
Join Date: Mar 2020
Location: Missoula, MT
Posts: 273
Year: 1990
Chassis: Crown Supercoach
Engine: Detroit 6-71TA, 10 sp.
Rated Cap: 90 (40')
Quote:
Originally Posted by JESUSBUS View Post
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.
Tejon7 is offline   Reply With Quote
Old 11-16-2021, 11:32 AM   #5
Mini-Skoolie
 
Join Date: Nov 2021
Posts: 24
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.
JESUSBUS is offline   Reply With Quote
Old 11-16-2021, 11:38 AM   #6
Mini-Skoolie
 
Join Date: Aug 2021
Location: Santa Fe
Posts: 22
Year: 2007
Coachwork: Thomas
Chassis: 31 ft. HDX
Engine: CATC7
Rated Cap: 36,300 GVW
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**.
jimmythomas is online now   Reply With Quote
Old 11-16-2021, 11:57 AM   #7
Mini-Skoolie
 
Join Date: Nov 2021
Posts: 24
Quote:
Originally Posted by jimmythomas View Post
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 is offline   Reply With Quote
Old 11-16-2021, 12:08 PM   #8
Mini-Skoolie
 
Join Date: Nov 2021
Posts: 24
Quote:
Originally Posted by jimmythomas View Post
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.
JESUSBUS is offline   Reply With Quote
Old 11-16-2021, 12:25 PM   #9
Skoolie
 
Join Date: Mar 2021
Location: Central Kentucky
Posts: 128
Coachwork: Busless for now
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.
Kentucky Dreaming is offline   Reply With Quote
Old 11-16-2021, 12:34 PM   #10
Mini-Skoolie
 
Join Date: Nov 2021
Posts: 24
Quote:
Originally Posted by Kentucky Dreaming View Post
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.
JESUSBUS is offline   Reply With Quote
Old 11-16-2021, 12:44 PM   #11
Mini-Skoolie
 
Join Date: Aug 2020
Location: Alabama
Posts: 57
Year: 1996
Coachwork: BlueBird
Chassis: International 3800
Engine: DT 466 P-Pump
Rated Cap: 34
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 is offline   Reply With Quote
Old 11-16-2021, 12:46 PM   #12
Mini-Skoolie
 
Join Date: Aug 2020
Location: Alabama
Posts: 57
Year: 1996
Coachwork: BlueBird
Chassis: International 3800
Engine: DT 466 P-Pump
Rated Cap: 34
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.
PorchDog is offline   Reply With Quote
Old 11-16-2021, 01:24 PM   #13
Mini-Skoolie
 
Join Date: Nov 2021
Posts: 24
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.
JESUSBUS is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


» Featured Campgrounds

Reviews provided by

Powered by vBadvanced CMPS v3.2.3

All times are GMT -5. The time now is 07:41 PM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2021, vBulletin Solutions, Inc.