1995 Chev K2500 - 7.4L Misfire Under Load

[civicsi1279]

i have a miss when the truck is under load with a trailer on it. When the trailer is off the truck runs great, but with the trailer on it will miss and somtimes backfire on the interstate and won't go above 60mph. But with the trailer off the truck runs great.

Any suggestions?

[Rich]

How big is the trailer and load? Is this a 30 foot 5th wheel and a dozer? We'll help ya out. the guys and gals will want some more info though. Maybe fuel pressures with and without the trailer on. Misfire counters also?

Oh ya, welcome to the forums.

[civicsi1279]

it is probably 15 20 ft inclosed trailer. i drove it today and the only thing it did for me is mis when taking off from a light. when i got it on the interstate i went about 70 with no issues. but the customer said he has been having trouble with the backfiring and missing on the interstate but i couldnt duplicate it with him in the car. one thing i noticed is there is no heat shield on the left side of the engine for the spark plug wires. could it be there heating up to much. i checked fuel good has new wires and plugs ect.

[civicsi1279]

and the computer is not registering misfire codes

[steven kiser]

skip under a load is a symptom of a bad plug. truck is skipping and backfiring while pullig a trailer on the highway in overdrive. i would definaly replace plugs, wires, cap and rotor. the computer wil not detect a misfire at this level. it may read an 02 fault issue but not a misfire.

[Tim Martin]

Yes well. Even new plugs and or wires out of the box have been known to be faulty under load. I have seen this several times lately. Perhaps an ignition wire is not fully connected to plug or inserted in distributor completely. Coil may break down under heavy load, it likely is not fuel related. If a lab scope is available, it usually will pickup poor connections on the secondary ignition while sitting in the shop being tested. I know I have spotted several already that way.

Another possibility, although somewhat remote is a valve getting tight in the guide. I have also seen that already.

[civicsi1279]

problem fix. once i put some sleeves on the spark plug boots and advanced the timing a little the stumbling went away a customer was happy

[Tim Martin]

Thank you for the update. I'm not surprised at the fix.