Backfire while coasting

Carbs & F.I.

Moderators: Amskeptic, Sluggo

User avatar
Mulcheese
Getting Hooked!
Location: Maple Grove, Mn
Status: Offline

Backfire while coasting

Post by Mulcheese » Fri Jul 10, 2015 6:08 am

I have originally created a post that stated I had a rattle while coasting. Update. I now have determined that it is a backfire out of the exhaust, while I am coasting, a sputter. I was able to hear it this last week when I was in the mtns and coasting down some grades. The echo helps to hear the sputter. If I am on the throttle It is not present but if I coast, off gas, it occurs. I feel that I am running rich at idle. I have done some AFM adjustments recently ( Richen dynamic, leaned static, and leaned idle. Was running lean at open throttle) and It is still there. I have tried leaning at idle and still there,

Where do I look first? More lean idle?
"attending to things in the moment with curiosity and acceptance."
____________________
82 Vanagon Westy - aka: Honey Badger - "cause she just dont give a ...."

User avatar
the miz
Addicted!
Location: Minnesota
Status: Offline

Re: Backfire while coasting

Post by the miz » Fri Jul 10, 2015 6:10 am

...it sounds all too familiar! How's your catalytic converter? :blackeye:
miz
1982 Westy- Vana White

User avatar
hercdriver
Getting Hooked!
Location: Beaver, PA
Status: Offline

Re: Backfire while coasting

Post by hercdriver » Fri Jul 10, 2015 6:26 am

I just helped a friend chase down a "popping through the exhaust" during deceleration. It turned out to be several exhaust leaks at the unions of the 180 degree elbows of the newer style 75 and up exhaust.
66 Beetle
75 Westy

Remember that there is nothing stable in human affairs; therefore avoid undue elation in prosperity, or undue depression in adversity. -Socrates

User avatar
Amskeptic
IAC "Help Desk"
IAC "Help Desk"
Status: Offline

Re: Backfire while coasting

Post by Amskeptic » Fri Jul 10, 2015 8:57 am

Mulcheese wrote:I have originally created a post that stated I had a rattle while coasting. Update. I now have determined that it is a backfire out of the exhaust, while I am coasting, a sputter. I was able to hear it this last week when I was in the mtns and coasting down some grades. The echo helps to hear the sputter. If I am on the throttle It is not present but if I coast, off gas, it occurs. I feel that I am running rich at idle. I have done some AFM adjustments recently ( Richen dynamic, leaned static, and leaned idle. Was running lean at open throttle) and It is still there. I have tried leaning at idle and still there,

Where do I look first? More lean idle?
Paradoxically, it is too lean that causes backfires on overrun. If you have an exhaust leak, you will niot be able to get rid of the sound of backfiring, because it is actually normal behavior that an intact exhaust can dampen out.
Colin
BobD - 1978 Bus . . . . . . . . . . .111,130 miles
Chloe - 1970 bus . . . . . . . . . . . 206,787 miles
Naranja - 1977 Westfalia . . . . . 93,996 miles
Pluck - 1973 Squareback . . . . . 55,510 miles
Alexus - 1991 Lexus LS400 . . . 72,113 miles

User avatar
Mulcheese
Getting Hooked!
Location: Maple Grove, Mn
Status: Offline

Re: Backfire while coasting

Post by Mulcheese » Fri Jul 10, 2015 10:16 am

I just completed re-checking all adjustments.

Idle good
Timing good
AFM adjustments good (point to a bit lean)

As a result of the lean running condition and more than likely an exhaust leak or two I will have to live with the putter, until I plug the leak.
the miz wrote:...it sounds all too familiar! How's your catalytic converter? :blackeye:
miz
Cat?? I have what resembles a cat but it is hallowed out and only there as a spacer. Could that be of issue?
"attending to things in the moment with curiosity and acceptance."
____________________
82 Vanagon Westy - aka: Honey Badger - "cause she just dont give a ...."

User avatar
the miz
Addicted!
Location: Minnesota
Status: Offline

Re: Backfire while coasting

Post by the miz » Fri Jul 10, 2015 10:30 am

Mulcheese wrote: Cat?? I have what resembles a cat but it is hallowed out and only there as a spacer. Could that be of issue?
...probably not. I was just thinking that exhaust restriction might be a culprit, as exhaust as leak might. If your cat has already been hollowed, then it is unlikely to be restricting. I'd echo what Colin said about lean mixture causing back fire or "spitting back" at idle/low throttle, I definitely experienced that while at altitude...dumping fuel into the system via throttle ended the back firing, though.
It was like this: up the pass=All "Quiet" on the Western Front --down the pass=like the Gunfight at the OK Corral.
miz
1982 Westy- Vana White

User avatar
Randy in Maine
IAC Addict!
Location: Old Orchard Beach, Maine
Status: Offline

Re: Backfire while coasting

Post by Randy in Maine » Fri Jul 10, 2015 11:31 am

If you have one, does the decel valve work?

Could it be from an exhaust leak? Have you done the wet rag shoved in the tailpipe while you are looking at the exhaust joints?
79 VW Bus

cegammel
Addicted!
Location: Thomasville Georgia
Status: Offline

Re: Backfire while coasting

Post by cegammel » Fri Jul 10, 2015 5:25 pm

I just learned a cool trick for exhaust leaks: run a shop vac on "blow"...as opposed to "suck"....attached to the tailpipe, then spray with soapy water like searching for a leak in a tire. Do this cold, by the way...

Instead of a shop vac, I bought a foot spa thing at the local goodwill. I didn't want to risk killing my vac.

User avatar
airkooledchris
IAC Addict!
Location: Eureka, California
Contact:
Status: Offline

Re: Backfire while coasting

Post by airkooledchris » Sat Jul 11, 2015 6:38 pm

when my vanagon was running a crappy EMPI exhaust, and it was on it's very last lets, it started doing the same thing. somehow when it failed inside it would trap the fumes in such a way that I had backfiring on decel.
1979 California Transporter

User avatar
Mulcheese
Getting Hooked!
Location: Maple Grove, Mn
Status: Offline

Re: Backfire while coasting

Post by Mulcheese » Mon Jul 13, 2015 5:36 am

Did some more cheecking yesterday. I pulled the plugs and noticed that they are maybe a bit on the white (lean) side.

Mine are tan/grey. Here is a pic but it shows WAY whiter than it is. Bad pic.

Image

Can someone show a pick of a correct plug color?


I decided to attack the AFM and adj the dynamic one notch to the rich. In relation to the lean running the head temps are running about 300-340 at 65-70mph so Im not sure why the plugs show lean.

I have done some modifying to the sender. At one tiime the ring broke off so I drilled the end. Not sure what to do in the moment and I felt brave so I tapped the head (shallow) and mounted it just outside of the plug hole. Been there for years and seems to work great. Now I dont have to deal with it when I pull the plug.

Image

Maybe that is giving me some false readings.


I feel that the from the responses that an exhaust leak it to blame. I know for a fact that I have a leak at the extension between cat and the muffler. There is a build up of black soot. I am not about to replace any exhaust components currently so I am wondering if some exhaust repair (paste) will hold for temporary. There may be more but have not found them.
"attending to things in the moment with curiosity and acceptance."
____________________
82 Vanagon Westy - aka: Honey Badger - "cause she just dont give a ...."

User avatar
Randy in Maine
IAC Addict!
Location: Old Orchard Beach, Maine
Status: Offline

Re: Backfire while coasting

Post by Randy in Maine » Mon Jul 13, 2015 7:20 am

You could try running with the O2 sensor disconnected to see if that makes any change. Short term only.

I have never had any luck with "the paste", but it would not hurt to try it.

Perhaps a visit to a welder guy to fix the leaky exhaust?
79 VW Bus

User avatar
Bleyseng
IAC Addict!
Location: Seattle again
Contact:
Status: Offline

Re: Backfire while coasting

Post by Bleyseng » Mon Jul 13, 2015 8:23 am

If the CHT sender ring is gone and you are just using the tip screwed into the head I think you are getting false low readings. I'd invest in a new CHT sender...
Geoff
77 Sage Green Westy- CS 2.0L-160,000 miles
70 Ghia vert, black, stock 1600SP,- 139,000 miles,
76 914 2.1L-Nepal Orange- 160,000+ miles
http://bleysengaway.blogspot.com/

User avatar
Mulcheese
Getting Hooked!
Location: Maple Grove, Mn
Status: Offline

Re: Backfire while coasting

Post by Mulcheese » Mon Jul 13, 2015 9:40 am

Bleyseng wrote:If the CHT sender ring is gone and you are just using the tip screwed into the head I think you are getting false low readings. I'd invest in a new CHT sender...
The ring is just a conductive material. The end of the ring is still attched to the wire which is what I am connected to. Any conductivity lost or gained should be very minimal as to not effect readings. I have read of people switching out the ring terminal for different ones and so on. The ring is just a means to attach the end and I have done that. I am concerned about the location. When we talk about head temps, is the target temp relative to that specific spot on the head (under the plug) or is it a "entire" head temp surrounding the combustion chamber?

Randy in Maine wrote:You could try running with the O2 sensor disconnected to see if that makes any change. Short term only.

I have never had any luck with "the paste", but it would not hurt to try it.

Perhaps a visit to a welder guy to fix the leaky exhaust?
Sorry Randy but no O2 here. I do have a welder and have thought of that but then I am permentely fixing tow pieces together. I may be doing this but I will have to weigh all options.
"attending to things in the moment with curiosity and acceptance."
____________________
82 Vanagon Westy - aka: Honey Badger - "cause she just dont give a ...."

User avatar
airkooledchris
IAC Addict!
Location: Eureka, California
Contact:
Status: Offline

Re: Backfire while coasting

Post by airkooledchris » Mon Jul 13, 2015 7:57 pm

The idea behind using the same ring that everyone else does is so you can compare your numbers to theirs, not necessarily that one location is more accurate than the other.
1979 California Transporter

cegammel
Addicted!
Location: Thomasville Georgia
Status: Offline

Re: Backfire while coasting

Post by cegammel » Tue Jul 14, 2015 3:43 am

I initially connected my cht sender under my TS2. The CHT worked fine, but it did screw up the TS2. There was about a 50° difference, but it was just as consistent.

Post Reply