Author Topic: 390 saga continues BACKFIRES  (Read 13743 times)

Offline Matt1968

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390 saga continues BACKFIRES
« on: June 29, 2015, 10:47:34 PM »
I have my 390 running now BUT,  I am getting backfires out of my exhaust.  At idle it's popping and backfiring.  I know if it backfires at the carb its a specific issue and if you backfire at the exhaust its a different issue.

Can someone educate me please.  Are there any FE specific items I need to look at?

Matt

Offline 67gtasanjose

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Re: 390 saga continues BACKFIRES
« Reply #1 on: June 30, 2015, 09:13:42 AM »
ALWAYS, with such a problem, begin with the basics. Not really an "FE ONLY" sort of a thing.
1.) Check and double check Firing Order. This is it 90% of the time.
2.) Make sure the dwell angle of your points is correct and NOT moving around. You need a dwell meter to accomplish this while it is running. (assuming you have points ignition).
3.) lastly, if all above is OK you have valve(s) leaking some. A compression test would then be in order to determine the cause.
Richard Urch

1967 (11/2/66, S.J.) GTA Luxury Coupe, 289-4V w/Thermactor Emissions, C-4, Int./Ext. Decor +many options

2005 (04/05) GT Premium Convertible, Windveil Blue, Parchment Top w/Med. Parchment interior,  Roush Body Appointments

Offline WT8095

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Re: 390 saga continues BACKFIRES
« Reply #2 on: June 30, 2015, 09:27:03 AM »
More basics:
  • Check the plug wires to make sure they are securely attached on both ends.
  • Check the condition of the plug wires.
  • Check the routing of the plug wires (crossfire).
  • Check the inside of the distributor cap for moisture or other foreign material.

Beyond basics:
  • Check the timing chain for excessive play (have to remove the fuel pump from the timing cover).
Dave Z.

'68 fastback, S-code + C6. Special Paint (Rainbow promotion), DSO 710784. Actual build date 2/7/1968, San Jose.
'69 Cougar convertible, 351W-2V + FMX, Meadowlark Yellow.

Offline cobrajetchris

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Re: 390 saga continues BACKFIRES
« Reply #3 on: June 30, 2015, 10:14:05 AM »
I had a problem similar to this on my FE and it about drove me crazy. I checked ALL of the basics as listed above and many more only to find out it was the mufflers causing the problem that disguised itself as a backfire and was not an engine problem at all. The noise had the identical sound. While the engine is idling in part take a rubber mallet and bang on the mufflers and see if that changes anything. The mufflers looked like new on the outside but the guts where all rusted out causing the problem. I disconnected the mufflers and problem solved.
CHRIS KNOBBE
69 MUSTANG COUPE, DEARBORN BUILT 06/10/69 OWNED SINCE 1978
70 BOSS 302 MUSTANG, DEARBORN BUILT 10/24/69 OWNED SINCE 1987
69 R CODE MACH1 AUTO, DEARBORN BUILT 10/10/68 OWNED SINCE 2006
69 R CODE MACH1 4 SPEED (factory black) SAN JOSE BUILT 12/30/68 OWNED SINCE 2007

Offline 67gtasanjose

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Re: 390 saga continues BACKFIRES
« Reply #4 on: June 30, 2015, 12:59:26 PM »
What Chris just posted made me think of another scenario:

WITH a THERMACTOR SYSTEM, the Smog Pump could be injecting AIR into the exhaust if things are not connected or working correctly. The AIR should only be entering the exhaust ONLY DURING ACCELERATION, if at other times (Idle or deccelerating) it will produce a backfire effect in the exhaust system, MOST PRONOUNCED BACKFIRING WHEN DECELERATING, to the point where it can blow up the exhaust system (usually the mufflers).
The AIR should be diverted away from the downstream (exhaust) when connected and operating correctly. The backfiring in this scenario would be caused from the addition of AIR which helps (any) unburned fuel to detonate. BOOM-BOOM BOOM
« Last Edit: June 30, 2015, 01:06:06 PM by 67gtasanjose »
Richard Urch

1967 (11/2/66, S.J.) GTA Luxury Coupe, 289-4V w/Thermactor Emissions, C-4, Int./Ext. Decor +many options

2005 (04/05) GT Premium Convertible, Windveil Blue, Parchment Top w/Med. Parchment interior,  Roush Body Appointments

Offline J_Speegle

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Re: 390 saga continues BACKFIRES
« Reply #5 on: June 30, 2015, 04:00:37 PM »
Guessing this is a 68 390?

Also would check the vacuum hose routing since the diaphragm could get the wrong signal/vacuum at the wrong time

Basics and one thing at a time

I'm guessing your using the original point style of ignition. Before I rebuilt my Boss I had a bad backfiring issue once the car warmed up to the point of it just shutting down. Cool down and it would fire right up.  Issue was the Pertronics
Jeff Speegle

Anything worth doing is worth doing concours ;)

Offline Matt1968

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Re: 390 saga continues BACKFIRES
« Reply #6 on: June 30, 2015, 09:10:06 PM »
What Chris just posted made me think of another scenario:

WITH a THERMACTOR SYSTEM, the Smog Pump could be injecting AIR into the exhaust if things are not connected or working correctly. The AIR should only be entering the exhaust ONLY DURING ACCELERATION, if at other times (Idle or deccelerating) it will produce a backfire effect in the exhaust system, MOST PRONOUNCED BACKFIRING WHEN DECELERATING, to the point where it can blow up the exhaust system (usually the mufflers).
The AIR should be diverted away from the downstream (exhaust) when connected and operating correctly. The backfiring in this scenario would be caused from the addition of AIR which helps (any) unburned fuel to detonate. BOOM-BOOM BOOM

No thermactor on my engine..

Offline Matt1968

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Re: 390 saga continues BACKFIRES
« Reply #7 on: June 30, 2015, 09:12:28 PM »
Guessing this is a 68 390?

Also would check the vacuum hose routing since the diaphragm could get the wrong signal/vacuum at the wrong time

Basics and one thing at a time

I'm guessing your using the original point style of ignition. Before I rebuilt my Boss I had a bad backfiring issue once the car warmed up to the point of it just shutting down. Cool down and it would fire right up.  Issue was the Pertronics

Yes 68.....yes single points

Offline Bob Gaines

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Re: 390 saga continues BACKFIRES
« Reply #8 on: June 30, 2015, 09:57:31 PM »
Did you have the distributor out?
Bob Gaines,Shelby enthusiast, Shelby collector , Shelby concours judge SAAC,MCA,Mid America Shelby

Offline Matt1968

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Re: 390 saga continues BACKFIRES
« Reply #9 on: June 30, 2015, 10:32:42 PM »
http://www.concoursmustang.com/forum/index.php?topic=11315.0

Yes dist was out recently.  Please see original post.

3 years ago I parked it.  At that time it ran incredibly well.   Took the dist out to oil prime the engine.

Now backfiring every 5 seconds.

Offline WT8095

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Re: 390 saga continues BACKFIRES
« Reply #10 on: June 30, 2015, 10:35:17 PM »
http://www.concoursmustang.com/forum/index.php?topic=11315.0

Yes dist was out recently.  Please see original post.

3 years ago I parked it.  At that time it ran incredibly well.   Took the dist out to oil prime the engine.

Now backfiring every 5 seconds.

Did you cover the tailpipes? Do you have any cats on patrol where the car was stored?  ;)
Dave Z.

'68 fastback, S-code + C6. Special Paint (Rainbow promotion), DSO 710784. Actual build date 2/7/1968, San Jose.
'69 Cougar convertible, 351W-2V + FMX, Meadowlark Yellow.

Offline Matt1968

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Re: 390 saga continues BACKFIRES
« Reply #11 on: June 30, 2015, 11:07:01 PM »
Anyone in the Toledo OH want to check this thing out?

Offline Bob Gaines

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Re: 390 saga continues BACKFIRES
« Reply #12 on: June 30, 2015, 11:36:35 PM »
http://www.concoursmustang.com/forum/index.php?topic=11315.0

Yes dist was out recently.  Please see original post.

3 years ago I parked it.  At that time it ran incredibly well.   Took the dist out to oil prime the engine.

Now backfiring every 5 seconds.
Logically if it ran well and then after removing and reinstalling the distributor it started backfireing , the distributor is the source of the problem and in this scenario the most likely thing is that the distributor has been re installed incorrectly.
Bob Gaines,Shelby enthusiast, Shelby collector , Shelby concours judge SAAC,MCA,Mid America Shelby

Offline 67gtasanjose

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Re: 390 saga continues BACKFIRES
« Reply #13 on: July 01, 2015, 08:22:40 AM »
Anyone in the Toledo OH want to check this thing out?

I'm 2 hours away in NE Ohio, do you have a trailer?
Richard Urch

1967 (11/2/66, S.J.) GTA Luxury Coupe, 289-4V w/Thermactor Emissions, C-4, Int./Ext. Decor +many options

2005 (04/05) GT Premium Convertible, Windveil Blue, Parchment Top w/Med. Parchment interior,  Roush Body Appointments

Offline svo2scj

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Re: 390 saga continues BACKFIRES
« Reply #14 on: July 01, 2015, 11:38:41 AM »
HI Matt

Give me a call 517 861 7630  (North of Ann Arbor)  It sounds like you know car pretty well -do you think you need a second set of eyes, work together and just confirm correct?

Can you take the time to work on car in one day if planned (or plan a visit and a return if you want to work on it)
Would you need timing light, fresh plugs - have floor jack and stands (clutch) ?
Let me know if you can only work weekends OR if you are avail weekdays (stays light until 9:30).   

Mark Haas
P.S.  I'm not a certified tech, but never met an engine I couldn't start !No cost no obligation - other than a follow up post to inform all.  (good or bad) LOL
« Last Edit: July 01, 2015, 12:28:57 PM by svo2scj »
1969 R Code , Sportsroof (non Mach) W Axle
AB , Standard Interior  San Jose built 4/22/1969