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  #1  
Old 07-14-2014, 08:28 PM
gordo2 gordo2 is offline
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backfire thru exhaust 81 ford 4.9

Been here several times with this POS, but need it to run for a couple more years if possible???? Last time here I had a fuel pump problem that we apparently got solved. However my last posting was something like engine dies "at WPO", which was a typo and should have been WOT. Well apparently I did not get that solved with a new fuel pump. Several days at about 50 mph the truck started running ragged backfired and actually exploded the muffler! I hobbled, lurching and jumping cutting in and out until I got it to the garage. Next day I started it up and it idled ok but w/o a muffler it roared a little. Again when I stomp the accelerator it will die unless I let up on it and ease the accelerator down and then it backfires. So it appears somehow I am getting an unburned gas build up flowing into the exhaust. I cannot find a vacuum leak, I reset the timing using a vacuum gauge, compression check low but within range on all cylinders. Now again I need some help. Vac reading is a vibrating 15-16 and I am in Denver at about 5,000 feet.
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  #2  
Old 07-14-2014, 08:50 PM
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It isn't a fuel problem but an ignition problem. When you push the throttle down you raise the pressure in the cylinder. More pressure takes a higher voltage spark to jump across the plug's gap. But you don't have enough voltage getting to the plug(s). And while there's no spark the gas is going down the exhaust. When you let up on the throttle the spark comes back and the hot exhaust gases ignite all the fuel that's gone down the exhaust system. (I learned about this at the age of three - in 1950 to be exact. I was standing by my father who was driving and I reached over and turned the key off. He immediately turned it back on, and it blew the muffler completely off.)

One possibility is bad plug wires. In that scenario the needed voltage goes through the sides of the wires to ground, and not through the plugs. Another is a bad rotor. Or a bad distributor cap. Or a bad coil. Or worn out plugs with a huge gap and dirty sides. But, probably not the ignition module as it usually fails completely.

I would replace what looks like it isn't new. A rotor and distributor cap might be first, followed by wires. Then maybe plugs. Or, do all of those if it hasn't had new ones in a while.
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  #3  
Old 07-15-2014, 06:14 AM
1986F150six 1986F150six is offline
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Good advice and explanation, Gary! I tried to rep you but was told to "spread the love!"
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Old 07-15-2014, 06:38 AM
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Thanks anyway, David. I have the same problem many times. But it is the thought that counts.
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Old 07-15-2014, 08:06 AM
gordo2 gordo2 is offline
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Thanks for the quick reply. Going down the wrong trail apparently. I was thinking maybe bad carb float. Will go thru and check everything you have mentioned and get back. Again thanks, you have probably saved me alot of time and headaches.
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Old 07-15-2014, 08:35 AM
gordo2 gordo2 is offline
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Just remembered that I recently had a "no start" that ended up being a bad ignition module so you are right about that. Makes me question the coil. Is there a way to check coil output w/o special equipment?
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Old 07-15-2014, 08:52 AM
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Yes, and no. The spark should be blue or white, and absolutely not yellow or red. So you can pull the coil wire from the dizzy and let it spark to ground on the engine - using the appropriate protection for yourself. Best to do that at least in the shade if not at night as it is hard to tell in lots of light.

But, if you have an ignition 'scope available it'll show the actual voltage, which is really the key.
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  #8  
Old 07-15-2014, 02:01 PM
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Why throw parts at it?
How many miles on this engine?

I would check the timing gears.
Valve and ignition timing would both be affected.
Your vacuum readings and lack of ability to accelerate plus backfire seem to point that way.

Try taking the cap off and rotating the engine first one way and then the other with a breaker bar on the front crank nut.
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  #9  
Old 07-15-2014, 03:38 PM
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Engine is a rebuilt Marshal with about 18,000-20,000 miles on it. The early 80's Ford 300 has no timing chain. I have changed the plugs, rotor, dist cap and wires with no change in performance. A couple of years ago I did have an overheat problem and burned out the head gasket. Installed a rebuilt head at that time and until recently have had no problems with backfire and engine choking out when stomping on the accelerator. I have not not put the new muffler on and do not know if that could make a difference?? Other wise any more thoughts. Also I thought a vac reading at 5,000 feet of 15-16 was within normal range?
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  #10  
Old 07-15-2014, 03:55 PM
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None of them have a chain.
I didn't say 'chain', I said gears...
(The cam gear is nylon from the factory)

With only 20k on it, gears shouldn't be an issue.

What caused you to overheat and warp a head on such a low mileage engine?

I meant the fact that the reading is jumping around, not that it is 16" hg
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  #11  
Old 07-15-2014, 03:57 PM
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I'm in the Denver 'burbs, my vacuum reading on my freshly-rebuilt 400 is around 17 IIRC.
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Old 07-15-2014, 04:04 PM
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I still think it is an ignition issue. So if the plugs, wires, cap, and rotor are relatively new then I'm thinking coil. However, I would open the hood at night and see if there are sparks dancing around. If so, you'll know where the problem is. If not, do you have another coil to try?

And, if it isn't ignition then you are running so lean that it dies when you give it throttle. A very low float level could do that, or a plugged filter. You said you don't have a vacuum leak, which would also do it. Are you sure your intake manifold isn't warped from the overheating?
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Old 07-15-2014, 04:54 PM
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...If it is running rich enough to blow the muffler to pieces I doubt he has a lean condition.

There is a new head, and yet the vacuum is not steady.
Not likely a bad intake valve.
I wonder if the pushrods are correct for an obviously decked cylinder head?

This '81 should be a DSII truck.
Coil resistance is the only question since it seems that the rest of the ignition has been thrown at it trying to 'solve' the problem.

Intermittent spark, worse under load, seems the only ignition issue that could cause this.
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Old 07-15-2014, 04:58 PM
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Good point. It would take a pretty weak coil to do that, though. But, what else could it be?
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Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads, Weiand intake, Street Demon 750/ZF5/3.50 gears w/Kevlar clutches
Worst fear: I die and my wife sells my trucks for what I've told her I have in them.
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Old 07-15-2014, 05:08 PM
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IDK

If the timing gears are good I'd wonder if the valve guides were too tight.


***Something caused the exhaust to fill with fuel and then an ignition event happened with the exhaust valve open.***

^^^
This much is FACT.

Add the crappy throttle response under load and fluctuating vacuum.
You tell me.
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Old 07-15-2014, 05:08 PM
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