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1995 Chevy truck with 454 misses once warmed up

I have a 1995 Chevy C3500 crew cab with a 454 and an 4L80-E transmission. I bought the truck from the original owner in April 2014 with 136,000 miles. A few weeks after I bought the truck I filled up to go on a long road trip. The truck started missing and I thought I had filled up with bad gas. Well that gas is long gone and it still misses. Not when cold started but once it’s warmed up it misses like it has a bad spark plug wire. It still has plenty of power and will easily accelerate and cruise at highway speeds. I changed the fuel filter because the PO didn’t drive the truck much. In fact the tires on the truck had almost all their tread but were from 2002 and bone dry. He is in his 80’s. He told me the fuel pump is fairly new because it was hard to get started. It fires right up now on the first try every time. I’m not 100% sure this is an electrical problem because it runs fine the first few blocks after sitting all night. The check engine light is not on. The temp gage stays at 180, even in the triple digit heat we have here in Phoenix. Where should I start when trying to track down this miss fire?

My first thought is 180 is way too cold and do not know that truck specifically but would call napa and see what temp thermostat is recommended.

Barkydog nailed it.

The engine needs a thermostat.


There’s other possibilites if this turns out not to be related to the thermostat.

Considering the location, maybe someone in the past thought it would be a good idea to remove the thermostat completely and run it without one.

Not to hijack this thread, but just for my own knowledge, how would a thermostat be the culprit?If the truck runs fine at colder temps, how would a lower temp thermostat hinder combustion? This is such a common guess but, I would’ve thought crank or cam position sensor going faulty, mainly because heat is making the problem worse, and when its cold its fine.

Sorry for “thinking out loud” but I guess perhaps, the engine’s computer makes it run rich on a cold start to help with combustion, then it automatically leans it out some, but the engine heat isn’t there to aid in combustion due to the thermostat being too low of a temp.

Yeah, I’d be amazed if a thermostat fixes this (but I’ve been amazed many times in the past). Can we get more description of the miss? Is it a regular idle miss (i.e., running on 7)? Is it a stumble during gentle acceleration? The more details the better.

“. . .but once it’s warmed up it misses like it has a bad spark plug wire”.
Maybe it is a bad spark plug wire or a bad spark plug. When it is dark outside, warm up the engine and open the hood. Look for flashover from the spark plug wires to the block. This would let you know if a plug wire is really bad. As an alternative, if you have a shop around with an ignition scope, the problem should show up right away if it is in the ignition system. You might also think about treating the truck to new ignition wires and spark plugs.

I vote for a bad coil. They can be OK when cold and open when hot.

I also vote for an ignition problem, coil or plug wires. My '95 Suburban has a similar problem, turned out to be the plug wires. I don’t remember any temperature relationship, but it was most obvious on the freeway.