Go Back   iRV2 Forums > iRV2.com COMMUNITY FORUMS > Vintage RV's
Click Here to Login
Join iRV2 Today

Mission Statement: Supporting thoughtful exchange of knowledge, values and experience among RV enthusiasts.
Reply
 
Thread Tools Search this Thread Display Modes
 
Old 10-15-2011, 11:05 AM   #1
Senior Member
 
Senior Chief's Avatar


 
Vintage RV Owners Club
Fleetwood Owners Club
Join Date: Jun 2009
Location: Somewhere in the woods in Belfair, WA, WA
Posts: 1,250
backfire/vaccum problem 454 chevy

Two probs, maybe related. At 45-50 mph my vacuum reads slightly lower than normal (14 vs 20). When accelerating, vacuum goes HIGHER to 25, instead of dropping! Just started this.
When at idle, it drops to 0, and quits running. ?????????

Also, it has (1 year or so, since we bought it) backfired through the carb when taking off from a stop, or climbing a hill. ???????

Any ideas? Any advice gladly received!!

Senior Chief
__________________

__________________
Life rocks when your house rolls
Senior Chief
& the Cheese Queen


Senior Chief is offline   Reply With Quote
Join the #1 RV Forum Today - It's Totally Free!

iRV2.com RV Community - Are you about to start a new improvement on your RV or need some help with some maintenance? Do you need advice on what products to buy? Or maybe you can give others some advice? No matter where you fit in you'll find that iRV2 is a great community to join. Best of all it's totally FREE!

You are currently viewing our boards as a guest so you have limited access to our community. Please take the time to register and you will gain a lot of great new features including; the ability to participate in discussions, network with other RV owners, see fewer ads, upload photographs, create an RV blog, send private messages and so much, much more!

Old 10-15-2011, 01:01 PM   #2
IC2
Senior Member
 
IC2's Avatar


 
Join Date: Jun 2010
Location: Upstate New York
Posts: 1,880
Quote:
Originally Posted by Senior Chief View Post
Two probs, maybe related. At 45-50 mph my vacuum reads slightly lower than normal (14 vs 20). When accelerating, vacuum goes HIGHER to 25, instead of dropping! Just started this.
When at idle, it drops to 0, and quits running. ?????????

Also, it has (1 year or so, since we bought it) backfired through the carb when taking off from a stop, or climbing a hill. ???????

Any ideas? Any advice gladly received!!




Senior Chief
Nasty problems to diagnose from afar

- Check the spark plug wiring routing first of all - two of them on the wrong distributor terminals will cause this

- Check the condition of the plug wires. Cross firing between wires can be difficult to diagnose. If those wires are 5-6 years old or an "off brand" they can be 'toast'

- Check the inside of the distributor cap for carbon tracking. Also, check the rotor for damage or wear. Is the initial timing correct? Are both springs under the rotor attached? Does the advance plate work easily?

- Is the distributor vacuum advance ruptured? Is the hose hooked to intake vacuum (below the carb butterflies - I assume it's a carbureted engine)

- Check for vacuum leaks anywhere along any vacuum hose including the master cylinder and PCV hoses

- Do a compression check. A burned intake valve will cause a carb backfire.

- Then - there is that big block engine, especially Chevy, age old problem - a camshaft that has lost a lobe. You need to pull the valve covers to do this check.

Let us know what you find
__________________

__________________
Dave W along with my DW, Susan and our poodlepups, Callie & Molly,2011 Ford F250 6.7 CCLB, 5er Hitch Option w/B&W Hitch,,Ride Rite air bags, 2014 Montana High Country 343RL (38')
IC2 is offline   Reply With Quote
Old 10-15-2011, 01:09 PM   #3
Registered User
 
Vintage RV Owners Club
Gulf Streamers Club
Join Date: Oct 2010
Location: Indiana
Posts: 4,954
If a Holley, theres also the Power Valve.. A single backfire could blow it.
__________________
Midniteoyl is offline   Reply With Quote
Old 10-15-2011, 01:44 PM   #4
Registered User
 
Join Date: Dec 2010
Posts: 336
Sounds like a vacum leak. Pull a hose that has vacum all the time when running and see if the rpm changes. Could be a bad hose anywhere or even an air leak where the carb bolts to the intake.
__________________
jimbo16720 is offline   Reply With Quote
Old 10-16-2011, 01:53 AM   #5
Senior Member
 
Kurmudgeon's Avatar
 
Monaco Owners Club
Vintage RV Owners Club
Holiday Rambler Owners Club
Join Date: Mar 2008
Location: Klamath County, Oregon
Posts: 245
Vacuum dropping to 0 at idle definitely sounds like a massive vacuum leak. That's when vacuum should be highest. Two things that could cause that big a leak are the power brake booster and the EGR valve.

The vacuum booster is the easiest to check, so you might want to check that first. If you have one, that is. You may have Hydro-boost brakes, which means you would not have a vacuum booster.

When an EGR valve gets old it will frequently stick and quit functioning. Since it spends its life exposed to hot exhaust gases it can get pretty grungy and corroded. Depending on what position it's in when it sticks, it is a potentially massive vacuum leak.

There are certainly other possibilities, but these would be the most common. Unless someone has changed it, you almost certainly have a Quadrajet carburetor, so there is no power valve issue like the Holleys have. But even with a blown power valve, I wouldn't expect to see vacuum drop all the way to 0 at idle.

Vacuum leaks in the engine bay are pretty easy to locate. But some of the heating/AC controls behind the dash are vacuum operated, and leaks there can be a real bear to find and deal with.

Good luck.
__________________
Jim Price
curmudgeon: noun; a crusty, ill-tempered, irascible, cantankerous old person . . . .
79 27' Holiday Rambler Statesman, 78 32' HR Imperial, 85 36' HR Imperial 5th Wheel
Kurmudgeon is offline   Reply With Quote
Old 10-16-2011, 08:35 AM   #6
Senior Member
 
Ragingbull's Avatar
 
Vintage RV Owners Club
Texas Boomers Club
Fleetwood Owners Club
Join Date: May 2011
Location: Cherokee, Texas
Posts: 406
How many miles are on your motor at this time? Its very possible that you have had a timing chain jump a tooth or more. This could cause any or most of the symptoms you stated.
__________________
1979 "Vintage" Pace Arrow 28 Ft,
Ragingbull is offline   Reply With Quote
Old 10-16-2011, 06:44 PM   #7
Senior Member
 
Join Date: Jul 2011
Posts: 148
Hey Chief! I would start with the basics and start looking for a vacuum leak.
__________________
bicklebok is offline   Reply With Quote
Old 10-18-2011, 09:50 PM   #8
Senior Member
 
Join Date: Oct 2010
Posts: 421
A few years age I had a Chevy 454 in a MH that would backfire at highway speeds when going up a steep hill. Turms out the RV shop that tuned it up set the timing at 6 degrees after TDC when it should have been 8 degrees before TDC (14 degrees difference). If you have electronic spark advance instead of vaccum make sure you disconnect the brown wire to the distributor before setting the timing. Also, there are two timing marks, one on the top (difficult to access) and one on the bottom. Make sure you use the correct spark plug wire for the timing mark you use. Unfortunately I don't remember which one is which anymore.
__________________
Poptop is offline   Reply With Quote
Old 10-19-2011, 09:12 AM   #9
Registered User
 
Iron Man's Avatar
 
Newmar Owners Club
Join Date: Jul 2006
Posts: 1,051
look at this on how to use a vacuum guage
How to Use and Interpret a Vacuum Gauge
__________________
John, Pam, Nicholas, Aria
NKK 16073L
2007 Essex 4502
2015 GMC Seirra
Iron Man is online now   Reply With Quote
Old 10-19-2011, 09:42 AM   #10
Senior Member
 
Ford Super Duty Owner
Join Date: Dec 2009
Location: Mcdonough, Ga.
Posts: 4,383
Almost for sure a vacuum leak. Check the large line to the accumulator first. These crack at the ends very often. Check the accumulator for cracks. These are plastic and tend to crack.If these are OK then go to the rest of the hoses.
__________________
1998 Pace Arrow 35 ft. F53 Ford V10 2012 Honda Civic toad
32 years mechanic at Delta Air Lines 15 year motorhome service manager. 3 popups....2 travel trailers....5 motorhomes....loved them all.
ga traveler is offline   Reply With Quote
Old 10-19-2011, 09:49 AM   #11
Senior Member
 
bigskymt's Avatar
 
Newmar Owners Club
Join Date: Apr 2010
Posts: 462
Unless your plug wires were recently replaced I'd start with them. You may also inspect the inside of the distributor cap for a carbon trail between two towers. Failing plug wires are most evident under load. HEI is 30-50kV. An open wire or high resistance will allow the spark off the rotor to search for a connection to ground. If it jumps to another tower on a cylinder that has an open intake valve, you have a backfire.
If your vacuum drops to zero at idle that would be main, not ported vacuum leak. On the rear of the intake manifold behind the carb is a 3/8" or so pipe with several lines running from it, there would be the first place to look. The carb base plate is another source of main vacuum. The carb body is ported vacuum. We used to spray WD-40 at a suspected vacuum leak and listen for RPM change. Now we use something that's not so combustible. Water sprayed at vacuum leak will provide a noticeable change in RPMs.
Quadrajet carbs have an accelerator pump with a leather or neoprene seal thats prone to leakage. Failure of the accelerator pump to operate properly will result in a hesitation, not a miss, when trying to accelerate.
__________________
bigskymt is offline   Reply With Quote
Old 10-19-2011, 11:40 AM   #12
Senior Member
 
Workhorse Chassis Owner
Join Date: Dec 2006
Location: La Quinta California
Posts: 523
Quote:
Originally Posted by Iron Man View Post
look at this on how to use a vacuum guage
How to Use and Interpret a Vacuum Gauge
Iron man.....That is indeed a great chart to be used constantly and thats why a small 2 1/8 gauge installed in your dash is such a perfect instrument to use......

Jim
__________________
JimElliott is offline   Reply With Quote
Old 10-19-2011, 11:49 AM   #13
Senior Member
 
Workhorse Chassis Owner
Join Date: Dec 2006
Location: La Quinta California
Posts: 523
Bigsky.....
Sometimes I will use unlit propane IF the air leak is "tuff" to find....

Mapp gas is really good.

Jim
__________________
JimElliott is offline   Reply With Quote
Old 10-20-2011, 11:25 AM   #14
Senior Member
 
Senior Chief's Avatar


 
Vintage RV Owners Club
Fleetwood Owners Club
Join Date: Jun 2009
Location: Somewhere in the woods in Belfair, WA, WA
Posts: 1,250
Thanks a million!! You hit the perverbial nail on the head I, found the vacuum leak(actually leakS). But, since you seem to be very good at this, let me ask further. The backfire has been with us since we bought this critter 2 years ago, but we've really only been driving it for about a year.
Plug wires, plugs, HEI coil all brand new high quality stuff. Firing order right, no wires crossed.
At random times, when you turn off engine, a column of gas spray erupts from the top of the carb, Not very often, just sometimes.
There are no valve or rocker noises heard with mechanics stethoscope on valve covers. The 'rotational' noise of the valve train is a lot noisier on one end of one cover than all other places, but can't be heard with out the stethoscope.
At first, and sometimes now the 'backfire' is just a little cough. While I still had the vacuum leak, it was actually an explosion. A real backfire in the carb.
Can't think of anything else.
Any ideas?
Again, thank you so much for curing my vac.problem.

Bill
__________________

__________________
Life rocks when your house rolls
Senior Chief
& the Cheese Queen


Senior Chief is offline   Reply With Quote
Reply



Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools Search this Thread
Search this Thread:

Advanced Search
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


Similar Threads
Thread Thread Starter Forum Replies Last Post
Hazard Flasher Problem Ms. Jean Thor Industries Owner's Forum 17 08-28-2014 05:54 PM
Oil Change on Adventurer P30 Chevy 454 (Funnel help!) oneydee Winnebago Industries Owner's Forum 3 10-14-2011 07:18 AM
Cluster problem TXredfish Workhorse and Chevrolet Chassis Motorhome Forum 5 04-24-2009 07:00 PM
Chevy 454 Running rich ddwillis Vintage RV's 5 11-09-2008 12:02 PM
Dip Stick problem ... 454 in 86 Holiday Rambler Hemi Vintage RV's 5 07-17-2007 01:10 PM

» Virginia Campgrounds

Reviews provided by


Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -6. The time now is 10:19 PM.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.