Backfires on load

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JRickey

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Apr 25, 2006
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I have a 27ft Itasca w/ a chev 454, throttle body fuel injection.  Took it out for our first long trip over mutliple passes.  Fully loaded with boat in tow. The engine runs fine on the flats but when I start to climb and open the throttle up it will bog down then backfire through the throttle body.  If I back off a bit it will eccellerate and go ok till I give it more throttle.  It feels like its at the point were the secondarys would kick in if it had them, right before it should down shift.  I'm thinking its a fuel delivery issue so I changed the fuel filter.  There is one in the middle of the chassi with both endes threaded.  I was told by a mechanic that that is the only one.  I also cleaned the contacts in the cap/rotor (they are new).  It has new plugs and new high heat wires.  Checked the timing.  It was advanced to about 10 degrees so I set it back to 4/6.  Thought all of this would fix it but it didn't.  I need help.  I have planned another week long trip in about two weeks so I gotta get it fixed.  thanks for all the help.
 

King

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Jan 30, 2006
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MA
Sounds like it's running way lean.  You may have a weak fuel pump.  It's easy to check, but you need the special gauge.  Time to see a good mechanic.
Art
 

John From Detroit

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Davison Michigan
In addition to the fuel filter there are injectors that can get dirty

and it could be a timing issue as well

Oh for the old days when I knew how to work on an engine

(Bad computer could do it as well)
 

Karl

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Sure sounds like a fuel delivery problem. How old is the fuel? Is the engine running hot? - could be detonation. With TBFI, there's only one injector per se, so you can probably rule that out, but not the computer if it's not adjusting the fuel flow for load, altitude, etc. Could be a bad oxygen sensor, air temp sensor, mass air flow sensor, coolant temp sensor - well the list goes on and on.  Reset the timing back to where it was; the computer adjusts that too. With modern engines and without a computer code reader and shop manual, you're pretty much shooting in the dark. I suggest you let a repair shop diagnose the problem. You've done just about everything you can.
 

JRickey

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Apr 25, 2006
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Well....I'm not there yet.  I'm goin to try to find a fuel pressure gauge and check the pressure.  If its good then I might roll over and send it in.  I've only taken two vehicles to mechanics and I am trying to resist but these darn computer things are getting to me. ;)
 

Bob Zambenini

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Mar 4, 2005
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Orange County California
I agree with these posts focusing on fuel delivery. But with 454 the wiring harness to plugs is frequently a problem as they are under a lot of heat and it degrades.

I had a backfiring in my old 454, about the same vintage as yours. It turned out to be the harness. That can be diagnosed but the dealer with good equipment. Big GM dealer in Canada finally found the problem and it ran like a top after.

Bob
 

JRickey

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Apr 25, 2006
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was it the harness or a bad connection at a plug?  I couldn't imagine following the wiring loom throughout the engine compartment.  The engine light has not come on.  Would it be worth checking it for any codes?
 

Jeff

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I also had a high alltitude, high load backfiring 454 that was solved with iginition harness, plugs, rotor cap and cover.
 

Karl

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Elkhart Lake, WI for the summer. Work at Road Amer
I'm goin to try to find a fuel pressure gauge and check the pressure.
Just remember that fuel pressure by itself doesn't tell the whole story. You need adequate fuel flow for the conditions, and the computer controls that in response to throttle position, engine speed, and many other things. It ain't simple anymore.
 

Dodgeman

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Jul 13, 2006
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17
JRickey,
You indicated you cleaned the cap and rotor contacts. I would replace both with OEM quality parts. Even new caps can have a problem with microscopic cracks.
One trick you can try is to open the engine compartment, wait until dark, and put the engine under load by putting it in gear with the brakes and parking brake on. Look for any light tracking or arcing along the plug wires, spark plug covers, etc.
Did you have an antifreeze leak in the engine compartment lately? Antifreeze residue can cause shorting and spark plug wire problems.
If it's not electrical, then I agree with others that it is a lean condition.
Good Luck!
Dodgeman
 

JRickey

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Apr 25, 2006
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Yea, I was rolling that around in my head last night.  The cap and rotor are new and was replaced by the previous owner so who knows were they came from.  It would be worth having a spare if its not the curlprit.  I'll try the night thing and no, there wasn't any anitfreez recently on the top of the motor. thanks for the input
 

JRickey

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Apr 25, 2006
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An update.  Picked up a new cap and rotor this morning.  Changed it out and looked at the old/new one.  I noted that the center stud for the coil was loose to the point that I could pull it out with little to no resistence.  The other ones would wiggle a little.  Also after a day of searching and actually reading the timing instruction on the air cleaner I realized that I needed to disconnect the tan/brown wire to the distributor prior to setting the timing.  This is why when I checked it it was about 12 degrees BTC.  Re-did the timing to 4 degrees with the wired disconnected.  All plug wires appear to be in new shape.  I don't notice any vaccum hoses off or leaking.  I'm going to button it up and take it for a drive.  I also stopped by the shop I have used inthe past and its going to cost around $150 to diagnos the problem.  If its the fuel pump it will be from $400-$500 to change.  The autoparts store has them for $50.
 

Dodgeman

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Jul 13, 2006
Posts
17
JRickey,
Hope your test drive went well.
If it didn't, another possibility of a vacuum leak might be the intake manifold. One way to check for that is to take your propane torch and attach a hose to it. Start the engine, let it idle, turn on the propane, (don't light it) and place the end of the hose at the intake manifold/engine block interface. Move the hose along the manifold connection, and if the engine idle increases in speed, you have a manifold gasket leak at that point.
Good Luck!
Dodgeman
 

JRickey

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Apr 25, 2006
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61
Well I took it out later in the day.  Went up a grade that the day before made it backfire.  No symptoms.  I tried my darndest but it just cruised up the grade.  I wasn't convinced so I went home and hooked up my boat.  Same grade but this time slowed to about 35-40 and romped on it just enought to keep it from down shifting.  Went all the way up the grade (about 1 mile) accelerating with no symptoms.  Dare I say it........I think I may have it fixed.  I told my wife I was going to hall my boat to the nearest pass and climb it for the true test.  She laughed and then realized I might be serious.  Thanks again for all the help.
 

Dodgeman

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Jul 13, 2006
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17
It sounds like congratulations are in order!
Don't you love it when a plan comes together?
You could fill the boat with water for a extreme test! (just kidding)
Good Luck, Regards,
Dodgeman
 

JRickey

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Apr 25, 2006
Posts
61
thanks,  its only took about of week of thinking, two days of working, a day of web surfing, a 15 dolllar filter, 30 dollar cap and rotor and 30 dollars worth of gas......a engine that (hopefully) will no longer backfire........priceless. ;D
 

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