idles fine but back fires when under load ?

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markmitchinnh
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idles fine but back fires when under load ?

Post by markmitchinnh »

I just got my van back from a shop and drove it 15 miles home, it starts great, idles great, but when I give it gas to move in traffic it backfires through the exhaust. It seems to get worse after it warmed up. The TBI is rebuilt with rebuilt injectors, all tune up stuff is new, and I did the WD40 check for vacuum leaks and din not find anything. No codes either. Anyone have an idea what I should look for next? Oh and serious lack of power too.
Recently bought a 1989 Astro RS from Florida. Replaced stock engine witha 1997 5.7 vortec with TBI and replaced transmission with rebuilt 700R4 with mild shift kit.
Bought on Monday the 16th of April 2012
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1Gary
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Re: idles fine but back fires when under load ?

Post by 1Gary »

Setting here shaking my head.Maybe I'm wrong,but sure would have made things tons simpler if you have transfer everything over from the 95.

That said,I'll take a shot at it.It does sound like the ignition isn't advancing correctly.
1979 Malibu drag race only car
1999 Sonoma 4.3 5 speed-Rufus
1989 Astro-Ole Yellar cancelled-still selling off parts
1985 Astro-shop van R.I.P. my friend
1994 Astro LT RWD W4.3 rod knock RIP
1982 Winnebago single rear wheel-Chevy 350 Scraped 1/28/13-broken dreams......


Rochester,NY

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markmitchinnh
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Re: idles fine but back fires when under load ?

Post by markmitchinnh »

isn't that computer controlled? and wouldn't that cause a code to show up? just curious
Recently bought a 1989 Astro RS from Florida. Replaced stock engine witha 1997 5.7 vortec with TBI and replaced transmission with rebuilt 700R4 with mild shift kit.
Bought on Monday the 16th of April 2012
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doyoulikeithere
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Re: idles fine but back fires when under load ?

Post by doyoulikeithere »

1Gary wrote:Setting here shaking my head. Maybe I'm wrong, but...
Yes, you are. (shaking my head at Gary, again).
FYI - It would not have been easier, In fact, it would have been 5 times more difficult and complicated Gary...

I had the exact same problem after my first few start ups. It Idled, but backfied under load...
I re-checked a bunch of stuff like timing, and looking for wires that were arking, etc. made a few real minor adjustments to the timing and route of the plug wires.
I was also concerned that my TBI might not be atomizing very well, and instead, just dumping drops of fuel. (looking at fuel spray under a timing light shows it up)
So, I also ran some injector cleaner they the gas. Yes, they were brand new re-conditioned-matched injectors.
Then, it seems like it just went away after the computer must have adjusted itself, or the injectors were flowing cleaner, or both, and it ran fine afterwards, for a long time..
Last edited by doyoulikeithere on Thu Dec 20, 2012 12:31 pm, edited 1 time in total.
2021 Yamaha Tenere 700 - Blue.
2016 Yamaha XSR900 - Silver.
1990 Astro - AWD - Brown.
1987 Astro - With 5 Speed & 4wd - Silver & Blue.
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Smiliesafari
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Re: idles fine but back fires when under load ?

Post by Smiliesafari »

Yes it is. BUT the timing has to be set correctly initially for the ECU to have a base to work from. My second thought is maybe there are some weak valve springs.
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Re: idles fine but back fires when under load ?

Post by doyoulikeithere »

can you explain to us the procedure you use to set your timing?
It has to be done right or it just wont be right.
2021 Yamaha Tenere 700 - Blue.
2016 Yamaha XSR900 - Silver.
1990 Astro - AWD - Brown.
1987 Astro - With 5 Speed & 4wd - Silver & Blue.

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markmitchinnh
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Re: idles fine but back fires when under load ?

Post by markmitchinnh »

The timing was set by a shop so I will ask them in the morning and reply with the answer they give me.
Or you can tell me the correct way and I can try it.
Recently bought a 1989 Astro RS from Florida. Replaced stock engine witha 1997 5.7 vortec with TBI and replaced transmission with rebuilt 700R4 with mild shift kit.
Bought on Monday the 16th of April 2012
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Re: idles fine but back fires when under load ?

Post by doyoulikeithere »

Basically, you just need to unplug the connector in the distributor wire bundle before you set it with a light.
If you didnt unplug it, your timing set will be wrong.
I like to disconnect the battery to clear the computer first too.
It probably explains that and what setting to use (0 degrees probly) on a sticker under the hood.
2021 Yamaha Tenere 700 - Blue.
2016 Yamaha XSR900 - Silver.
1990 Astro - AWD - Brown.
1987 Astro - With 5 Speed & 4wd - Silver & Blue.

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Re: idles fine but back fires when under load ?

Post by NotDadsW41 »

Doesn't backfiring under load possibly indicate a lean condition? Thus low fuel pressure?

Maybe that's more of a WOT thing vs. just load...
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markmitchinnh
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Re: idles fine but back fires when under load ?

Post by markmitchinnh »

could it be the fuel pressure regulator? I ask because the pump is the same as with old engine but the pressure regulator is not.
Recently bought a 1989 Astro RS from Florida. Replaced stock engine witha 1997 5.7 vortec with TBI and replaced transmission with rebuilt 700R4 with mild shift kit.
Bought on Monday the 16th of April 2012
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1Gary
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Re: idles fine but back fires when under load ?

Post by 1Gary »

doyoulikeithere wrote:
1Gary wrote:Setting here shaking my head. Maybe I'm wrong, but...
Yes, you are. (shaking my head at Gary, again).
FYI - It would not have been easier, In fact, it would have been 5 times more difficult and complicated Gary...

I had the exact same problem after my first few start ups. It Idled, but backfied under load...
I re-checked a bunch of stuff like timing, and looking for wires that were arking, etc. made a few real minor adjustments to the timing and route of the plug wires.
I was also concerned that my TBI might not be atomizing very well, and instead, just dumping drops of fuel. (looking at fuel spray under a timing light shows it up)
So, I also ran some injector cleaner they the gas. Yes, they were brand new re-conditioned-matched injectors.
Then, it seems like it just went away after the computer must have adjusted itself, or the injectors were flowing cleaner, or both, and it ran fine afterwards, for a long time..

The answer man huh.Well...................... :butthead: :butthead:

Go ahead.Take the reins.Just remember it can't always be the single occurrence that happened to you. Takes some diagnostics.
1979 Malibu drag race only car
1999 Sonoma 4.3 5 speed-Rufus
1989 Astro-Ole Yellar cancelled-still selling off parts
1985 Astro-shop van R.I.P. my friend
1994 Astro LT RWD W4.3 rod knock RIP
1982 Winnebago single rear wheel-Chevy 350 Scraped 1/28/13-broken dreams......


Rochester,NY
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Re: idles fine but back fires when under load ?

Post by doyoulikeithere »

Gary, Lets see if I have this straight.... :poke:
You are telling us that it is easier to rewire a whole engine compartment and install a completely different fuel system and computer, rather than use the stock system that is in the van already. I really dont see how converting a TBI van to be a CPI van is easier than using the TBI. :poke: All he did is replace the longblock. Big deal if the block was a different year? I still dont understand the logic of how that is easier, sorry, but thats it. So, explain Just How is that, Please? :poke: It isnt rocket science is it? :poke: I can only share what I've experienced, I'm not claiming to know the answers. Just sharing my experiences.Plus, I really dont want any peek at your wrinkly old ....... Tell us the answer to that before you BA me, K? Hows ol yeller runnin?
:smurf:
All that aside, I would say if you paid some shop to tune it up and it backfires you should bring it back.
So, we will see tomorrow what they say I hope....
2021 Yamaha Tenere 700 - Blue.
2016 Yamaha XSR900 - Silver.
1990 Astro - AWD - Brown.
1987 Astro - With 5 Speed & 4wd - Silver & Blue.
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1Gary
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Re: idles fine but back fires when under load ?

Post by 1Gary »

doyoulikeithere wrote:
1Gary wrote:Setting here shaking my head. Maybe I'm wrong, but...
Yes, you are. (shaking my head at Gary, again).
FYI - It would not have been easier, In fact, it would have been 5 times more difficult and complicated Gary...

I had the exact same problem after my first few start ups. It Idled, but backfied under load...
I re-checked a bunch of stuff like timing, and looking for wires that were arking, etc. made a few real minor adjustments to the timing and route of the plug wires.
I was also concerned that my TBI might not be atomizing very well, and instead, just dumping drops of fuel. (looking at fuel spray under a timing light shows it up)
So, I also ran some injector cleaner they the gas. Yes, they were brand new re-conditioned-matched injectors.
Then, it seems like it just went away after the computer must have adjusted itself, or the injectors were flowing cleaner, or both, and it ran fine afterwards, for a long time..

Naw-with this post to what me??.Show us what ya got. :poke: :poke:
1979 Malibu drag race only car
1999 Sonoma 4.3 5 speed-Rufus
1989 Astro-Ole Yellar cancelled-still selling off parts
1985 Astro-shop van R.I.P. my friend
1994 Astro LT RWD W4.3 rod knock RIP
1982 Winnebago single rear wheel-Chevy 350 Scraped 1/28/13-broken dreams......


Rochester,NY

Topic author
markmitchinnh
I sleep in my van
Posts: 589
Joined: Wed Oct 13, 2010 2:07 pm
Location: Barrington NH

Re: idles fine but back fires when under load ?

Post by markmitchinnh »

I called the shop they told me they timed it as best as they could (not a good shop I am now thinking) SO my plan is this: I am in the following steps gonna do this: 1. Check the timing according to the manuals. 2. buy all new hoses for the vacuum and replace old ones. 3. add some injection cleaner with new gas. and if thats not enough I have another fuel pressure regulator to swap into it.

I already started it in the dark and looked for any sparks coming from wires etc. and I sprayed WD 40 in areas where they may be a vacuum leak and found nothing. I am open to ideas if anyone has any.
Recently bought a 1989 Astro RS from Florida. Replaced stock engine witha 1997 5.7 vortec with TBI and replaced transmission with rebuilt 700R4 with mild shift kit.
Bought on Monday the 16th of April 2012

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Re: idles fine but back fires when under load ?

Post by blacknome »

first thing is timing, second place I would look would be the cat convertor. If its partially plugged the engine will idle fine but run like poop under load with no power because of all the back-pressure.

Cool trick I learned on a sealed / welded system is to Sawsall the cat off and get some Stainless band clamps when replacing it. They are sold at performance shops or heavy truck parts for appx $10-$15 ea

good luck
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