454 Backfires under load

[darsben]

So I left Phoenix in 100 degree heat and as I was pulling uphill the engine overheated. I pulled over and replensiished the coolant level The radiator cap had gone bad apparently and allowed the coolant to be purged out of the system. So that problem was cured however the exhaust was sounding loud and apparently the overheating had caused the collars for the exhaust header to exhaust pipe to become loosened (from the heat?). Got that fixed and all was fine for a little while. Now however when I get lead foot the motor backfires and eventually the check engine light comes on. I cannot get a code from the OBD I  system. Turning of the engine and then restarting 5 minutes or so later and I am good to go.
If one relatively flat highway I can and have gone all day without the check engine light coming on. However on a grade if I have to give her the gas heavily (lead foot) the light will come on and the engine will back fire and run like crap.

Any thoughts an subject
Professionally tuned and timed about 3 months ago with high temp racing wires

Carb rebuilt  in March

Could it be the O2 sensors

 

[PJ Stough]

Spark plug wires, and/or spark plugs would be my first guesses.

 

[8Muddypaws]

Bad exhaust valve or valves.

Do a compression check.

 

[Tom]

The Chevy 454 was notorious for having ignition wires go bad (actually, the insulation on the wires went bad), and arcing would occur between the wires. If one opened the dog house in low light while pulling a grade, you'd see the fireworks. The same thing happened on boats with that Chevy 454, especially when they were the boat was 'coming out of the hole' (trying to get on plane) i.e. under high load.

 

[TheBar]

I worked as a mechanic 45 years ago while in college but engines haven't changed all that much. I've run into this a bunch of times. Unfortunately there are multiple causes. Are you sure the engine isn't still overheating? As mentioned above backfires can be caused by faulty spark wires and this is a cheap easy fix. They could have been damaged when the engine overheated. Your sensors could have been damaged also. Backfires can also be caused by timing too far advanced, also as mentioned above sticking valves, vacuum leak, or carbon deposits in the cylinders.  With any of these you'd generally hear a backfire in the intake manifold. If too much gas is passing through to the muffler it eventually explodes like a loud gunshot.  I had a sticky lifter in my 78 Firebird for 20 years. Didn't happen all the time so it wasn't worth taking the head off.  Tried all kinds of miracle products then read about Seafoam, link below. It fixed my sticky valve three years ago and the problem never came back. It also cleans fuel injectors and carb jets. Your carb may need to be rebuilt. I've read it also helps eliminate carbon deposits. You might try that but at your own risk. It says it is safe for all sensors but I haven't tried it in a modern engine.

https://www.walmart.com/ip/Sea-Foam-Motor-Treatment/16664932

 

[Arch Hoagland]

I've never had good luck with aftermarket ignition parts. Never.

I'd start by replacing the spark plug wires.

I suggest you buy a OBD1 scanner for a GM and find out what the code is.

 

[dktool]

I suggest you buy a OBD1 scanner for a GM and find out what the code is.

This is how to do it for FREE ........

https://www.obd-codes.com/faq/read-gm-2-digit-obd-codes-free.php

 

[Broke Boater]

You can get a obd 1 jumper at oriellys for cheap. It takes a couple times to learn the cadence of the engine light blinking to understand the code number then look it up on a website. You tube has vids to help you. That Gen motor is a gas thirsty finicky motor. I hated the one I had in a 1 Ton Dually. I'm going to guess your overheat was worse then just a exhaust donut. Easy to check how things are inside the motor with a compression check and also check intake manifold pressure with a vacuum gage. Then go to electrical after you know that you have a sound motor, good luck my friend,,,gregg

 

[darsben]

Update. Will start tearing into it next week.

 

[dktool]

What year model is this chassis ? Keep in mind it may be a year older than the what you think as the couch builders buy chassis then build on them later, check the vin.

 

[darsben]

chassis is an 89 P32

 

[tote]

Replace your distributor.
I have a ford 460 that would backfire for no reason.
Someone mentioned they had a similar problem and it was a loose wire in the distributor arcing causing a misfire.
After trying everything else I replaced the distributor.
Not a single backfire in over a year.

 

[darsben]

SO as I mentioned earlier the check engine light would come on but I could not get a code on the reader.
I took the rig to a mechanic. We had an appointment for today after I got the check engine light to activate. There is no code showing on his OBD1 reader.
However the service manager had this happen before on another rig and it was the EGR valve that was bad intermittently.

So that is were we will start looking.
Otherwise the engine runs great  and it took about 45 minutes for me to get the check engine light to illuminate today.

 

[Charlie 5320]

Change the distributor cap and rotor. The cap gets a glaze on the terminals inside the cap and the spark will jump to another post and cause the back fire. When replacing the cap and rotor try and find one with brass terminals instead of aluminum. They last longer before they get glazed over.

 

[darsben]

The shop I go to could not get a reading either from the OBD I reader even though  the check engine light was on.

However  it was determined the fuel pump on the motor was failing and starving the engine under load.
All seems well but I will not be testing it for a few days.

 

[spdracr39]

Headers got hot and cook a plug wire or two. Time for new ones.

 

[darsben]

No the wires are fine it was the fuel pump