95 Safari backfire under load

1995 Safari AWD 4.3 VorTec “W” (I think) 105,000 mi has recently started to backfire (through exhaust system) after pulling a horse trailer for about 30 minutes. The engine is not overheating and no service codes are shown. The backfire is extremely loud and causes the engine to shut down (safety issue while driving). Can be restarted in a couple of minutes, but will backfire again soon if continue trailer pulling. If allowed to cool longer, it will go longer before reoccurring.

The backfire never occurs when not pulling the trailer, even on long road trips pulling hard over mountain passes, etc. It has been more finicky to start lately however, sometimes needing to be cranked for several seconds, other times fires up instantly, doesn’t seem to be temperature related.

Could this be the EGR system? Your guidance is appreciated before I just start replacing parts in search of a solution.

Welcome to AF!

First, the condition you describe is technically "afterfire" instead of "backfire." Backfire occurs when unburned fuel/air charge is ignited in the intake. Afterfire occurs when fuel is accumulated in the exhaust and is ignited by a subsequent ignition cycle or hot component.

Afterfire is frequently due to excessively rich mixtures or misfiring ignition.

Although a properly functioning EGR system is going to help keep chamber temperatures lower, and potentially prevent preignition while the engine is more heavily loaded by the equines and their portable stable, it is less likely to have an effect on your symptoms.

Since the engine is not apparently overheating, I would not suspect excessive blowby gasses and a subsequent fuel and oil rich mixture to be the primary factors.

What is more likely is a weakening ignition system. Under heavier loads, chamber pressures are higher, and more energy is required to produce a plasma trail across the spark plug gap to allow the spark energy to discharge, thereby igniting the fuel mixture. Spark plugs which seem to "fire" just fine in open air or under light loads can fail to fire under higher cylinder pressures, or fire very weakly or late. Even though the PCM is not detecting a misfire condition and storing an error code, it would be best to start with the ignition system. Make sure the spark plugs are clean, have no insulator accumulation, and have no more gap than the specification. The spark energy required to fire across a gap increases exponentially with the size of the gap. Proper gap is extremely important, especially when load is increased, as you have experienced. If anything, err on the tight side of the gap of the gap specification, since the gap will increase with use and wear.

Use ONLY single side electrode spark plugs of the proper heat range. Splitfire plugs, NGK "U-Groove" plugs, Bosch "Dual electrode" or "Plus 4" plugs should never be used in an engine that is expected to actually run. Use Champion, NGK, or Delco dual platinum, single side electrode, RESISTOR plugs. If the makers of some of those "designer" type spark plugs are sponsors here, that's just too bad. Don't use them. Those things are made for one purpose - To sell to unsuspecting buyers who rely on junk science to validate their poor choices.

Use 8mm silicone jacketed resistor plug wires. Inspect them for insulation damage and terminal condition. Test continuity, and expect 500-1,500 ohms per foot of length. Repair any damaged terminals and insulation boots, or replace the damaged wires.

Inspect the distributor cap and rotor for wear, carbon streaks, cracks, or any other damage. After visual inspection, measure the megohm resistance between all the terminals on the cap. The bussing for each plug wire terminal is encased in an epoxy insulation material, and it can break down with multiple heating cycles and age. If in doubt, replace the cap and mating rotor with a quality replacement part. There are numerous anecdotal stories right here on AF regarding the use of "bargain" distributor caps on the "W" engines, and the problems experienced. If you want to get another 100,000 miles out of it, stick with what got you through the first 100,000 miles and get another AC/Delco cap and rotor.

Inspect the ignition coil for flashover damage, cracks, blistering, and test the secondary and primary winding resistances. Closely inspect the primary power connector for the coil and verify a good ground on the frame of the coil. It's not an easily accessible place on an Astro, but it is necessary for a complete tune-up.

Once the ignition system is reliable, any remaining problems will more likely be easier to identify.

Not to get too far off track, but isn't it ironic that about 100 years ago, we would have had the horses out front, and the human conveyance in the rear? Is that just backward?

Thanks for providing all the detail Blue Bowtie!

Yes, I do have Bosch plus 4's that have been in there for the last 35,000 miles, as well as Bosch replacement wires; not sure if I replaced the cap and rotor at that time. The afterfire condition just started, so maybe the condition has just degraded. Also, one of the wires got pinched quite a while back, located near the distributer. The insulation got a partial-depth slice, and yes, I just wrapped a bunch of electrical tape around it.

I'll install single-tipped plugs and other new ignition components, along with doing some investigation with the DMM.

Would copper-tipped plugs fire more reliably than platinum in this case?

We have 5 diverse vehicles and I am a member of several model-specific forums, but maybe now AF can be a one-stop forum to exchange iinformation on all of them. Thanks again!

One of the Bosch plus 4 plugs had considerable buildup along one of the ground electrodes. The rest look fairly OK, except I am having trouble getting the rear driver-side plug out. It continues to take a lot of force to turn after the normal 1/8 turn to break it loose. I've gone maybe another 1/4 turn or so, but am afraid that it is galled or cross-threaded and am worried what if the block threads are damaged or the plus siezes or breaks off. Do I just leave the old one in or should I press on removing it??

The cap inside surfaces have a whitish buildup and the rotor tip looks kind of bad too. The rotor center area has black powder around the center contact area. I was able to get Borg-Warner cap, rotor, and 7 mm wires (only 8mm wires were MSD cut-to-fit for $100, 8.5mm for $150 - is it worth it?), but no AC-Delco parts. I really need to get this back on the road before Monday.

Having trouble finding AC-Delco, Champion, 0r NGK double-platinum plugs in stock. Choices are AC-Delco Rapidfire platinum p/n 1, Champion non-plat truck p/n 4018, and (possibly) NGK Iridium p/n UR51X. Are Iridium plugs good or just hype? Would Autolite double-platimuns work well?

Thanks for the help!

OK - got the last plug out with the help of PB Blaster penetrant. The turning torque was maybe 20 ft-lb for at least one full revolution before it backed out more normally. The threads appear to have some crud in them, perhaps drug in during the last installation since visibility is so poor.

Found a set of AC-Delco Professional Platinum 41-803 plugs and will now proceed with the tune-up. Will follow up to confirm if this cures the afterfire while trailering problem.

Success!!!

Trailered horse on a couple hour trip on 90F+ afternoon without any problems.

The culprits of the afterfire problem appear to have been the Bosch Plus 4 plugs and nasty looking cap and rotor. The old Bosch wire resistances checked out fine, but I replaced the set anyway with Borg Warner wires (appeared pretty similar).

Thanks for the accurate diagnosis Blue Bowtie!