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Vortec 350 backfiring on highway

GsxrMike

GsxrMike

1/2 ton status
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Feb 12, 2008
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Location
Tonganoxie, KS
So when I installed longtubes and full exhaust on my truck it started backfiring all the time and really really bad while slightly decelerating driving down the highway. I mostly eliminated that problem when I got a re-tuned computer from black bear performance but now it has seemed to come back after some modifications. Basically what happens is when I am cruising at a steady pace and let off on the gas just a tiny bit to maybe slow down 1-2 mph my truck will repeatedly backfire. Can anybody tell me the cause and possibly how to eliminate it? It isn't a real loud popping but my wife can hear it while driving behind me on the highway. I think it may be timing related since that is the only thing I can think of that would have eliminated it with a re-tune but let me know what you think. Also, why would it start doing it (and really bad) after installing longtubes and full exhaust? Thanks in advance for the help guys!
 
Probably not it but could you have gotten #5-7 plug wires on backwards?

You might be able to get rid of it by putting a crossover or "X" pipe in between the pipes if it is full duals.
 
O2 sensors are heated and brand new, exhaust leaks are possible because the rear most flange bolts are broke off in the head, I'll check to see if the leak. Plug wires are correct.

As for the crossover pipe, how close does it need to be to the headers? I'm not sure if there was room the way the exhaust was run and I didn't stress the installation of one because I have a 2 in 2 out flowmaster 50 series muffler although it is was down the line. (crewcab long bed and its toward the rear of the truck)
 
Crossover wouldn't change that if it is a true backfire. Did you change anything else while swapping out the computer other than exhaust? Check the cam retard as the dizzy is just a high voltage switch. Needs to be -2, 0, +2. Anything more and crossfiring will occur.
 
Exhaust leaks caused backfires on my Suburban. The O2 sensor read the excessive air and told the computer the engine was running lean, so it overfueled it. I fixed the exhaust leaks, and the backfires went away.

Martin
 
Cornfield creations said:
Crossover wouldn't change that if it is a true backfire. Did you change anything else while swapping out the computer other than exhaust? Check the cam retard as the dizzy is just a high voltage switch. Needs to be -2, 0, +2. Anything more and crossfiring will occur.
Click to expand...

How do I check the cam timing?

Don't I have to unplug something to check the ignition timing? I'll do this first because I have never done it and it ought to be done anyways.
 
Is this what you are talking about?



Below is the text from GM Service manual for 1997 Chevrolet C Pickup 5.0L, 5.7L, and 7.4L
Accordind to this you don't set the timing but the timing offset.
You need a "Scan Tool" that plugs into the truck and can read the data from the trucks computer.
Besides from that, it looks like a normal timing procedure.
You might be able to borrow a scan tool from a parts store, or at least use their tool and do it in their parking lot.

Setting Timing
Camshaft Retard Offset Test
The ignition timing cannot be adjusted. The distributor may need adjusting to prevent crossfire. To insure proper alignment of the distributor, perform the following:

With the ignition OFF, install a scan tool to the DLC.
Start the engine and bring to normal operating temperature.

Important
Cam Retard Offset reading will not be accurate below 1000 RPM


Increase engine speed to 1000 RPM.
Monitor the Cam Retard Offset.
If the Cam Retard indicates a value of 0° ± 2°, the distributor is properly adjusted.
If the Cam Retard does not indicate 0° ± 2°, the distributor must be adjusted.
Adjusting Camshaft Retard Offset




With the engine OFF, slightly loosen the distributor hold down bolt.
Important: Cam Retard Offset reading will not be accurate below 1000 RPM


Start the engine and raise engine speed to 1000 RPM.
Using a scan tool monitor Cam Retard Offset.
Rotate the distributor as follows:
To compensate for a negative reading, rotate the distributor in the counterclockwise direction.
To compensate for a positive reading, rotate the distributor in the clockwise direction.
Repeat step 4 until 0° ±2° is obtained.
Turn the ignition OFF.
Tighten the distributor hold-down bolt to 3N·m(25 lb. ft.).
Start the engine, raise engine speed to 1000 RPM and recheck Camshaft Retard Offset.
​
 
Yes there is a code for it. I've seen them off -14* and never throw a code for 6 months. But once set it on "0" it throws a P0300 misfire code. strange things happen
 
Egr is blocked off and code is deleted. I'll have my mechanic next door at work check the timing and stuff.
 
82355 said:
Exhaust leaks caused backfires on my Suburban. The O2 sensor read the excessive air and told the computer the engine was running lean, so it overfueled it. I fixed the exhaust leaks, and the backfires went away.

Martin
Click to expand...

Right on the money! I put a Banks longtube exhaust on my motorhome and and it did the same thing. Backfiring out the exhaust (actuly called afterfiring, backfiring is out the intake) Banks had me check for exhaust leaks first and check the evap canister for fuel saturation. It seems that if the evap canister is loaded with fuel it pulls the vapor into the engine on decel.
 
Cam retard offset was at -4 so I adjusted it to +1. I'll let you know if it helped anything, thanks for the help guys!
 
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