Truck sputters and backfires when hot

[8thNote]

Hello, I am new to the forum and this is my first post. This forum seems like a great resource and I look forward to becoming an active member of the community.

I have a 94 C1500 extended cab short bed (Silverado trim) in Summit White. It has the 5.7L TBI engine with 196k miles on the original engine and trans. The truck is in immaculate condition both inside and out (had a respray in original color 2 yrs ago) and very original. The only mods are an open-element 14" air cleaner and a single glass pack muffler with a short turndown tail pipe.

Until recently, the truck ran great. A couple of months ago though, it started to sputter, hesitate, and backfire. Right before it starts to sputter, I can feel a noticible loss in power, like the engine is bogging down. It usually only does it under heavy throttle application or while cruising at 70+ mph. Also, it only happens when the engine is hot (180-200 degrees water temp). I have replaced the fuel filter, spark plugs, wires, distributor cap, and rotor button and the problem still persists. I noticed that a rubber fitting from the fuel tank vent line going into the charcoal canister was dry rotted and cracked, so I replaced that fitting as well. All the rest of the vacuum lines seem to be in good condition. All of the sensors and their wiring harnesses are attached and appear to be in good condition. There is no CEL light on. When the truck is running good, it runs great. Fuel milage is consistently in the 14-15 mpg range and power feels adequate 95% of the time.

I am at a loss as to what is causing this sputtering and backfiring. Does anyone have any thoughts or suggestions as to what I should try next? I greatly appreciate any and all advice and thank everyone in advance!!

 

[Gramps]

Check fuel pressure, could be a weak pump. Make sure timing is set correctly, and not jumping around(worn out chain)

 

[Eveready]

My bet is on the fuel pump. Sounds like one for sure. Welcome to the forum. If it isn't on here about the GMT-400 it likely isn't worth knowing.

 

[8thNote]

My bet is on the fuel pump. Sounds like one for sure. Welcome to the forum. If it isn't on here about the GMT-400 it likely isn't worth knowing.

Would a fuel pump or worn timing chain cause this only when the engine is warm? It never happens in the mornings on my way to work when I first start the truck. It only happens in the afternoons when the outside temp is in the mid 90s and Ive been driving 20-30 min +. Also, I cannot replicate the problem by revving the engine with the truck parked. It only happens while actually driving.

Thank you for the replies

 

[SAATR]

Cracked reluctor in the distributor could potentially be your problem. There are others on this board with TBI trucks that have reported similar power/running issues at operating temperatures and load conditions that could not be replicated at idle or when cold. You should be able to pull the cap and rotor and get the dust shiled off the reluctor to inspect it. Fuel pump is still on the table as higher ambient temperatures along with continuous highway operation will warm the fuel in the tank, which can bring out electrical issues.

 

[The Iron possum]

Heat/load misses seem to be a common topic on this forum. I’m currently chasing one of my own. If you google “Symptoms of failing (insert component here)”, nearly every hit you get will list a miss or poor engine performance as a symptom. MAP, Ignition Control Module, pickup coil, distributor, plug wires, ignition coil, blown cylinder to cylinder head gasket, etc.
First thing is to ensure the check engine light actually works. When you turn on the key, it should light up for a few seconds. If it doesn’t, you can’t be sure there are no codes. If truly there are no engine codes then you have to start testing the individual components. I don’t know if the address below posted as a link or not, but you can go to the website and drill down to troubleshooting of the GM 5.7 TBI motor. It’s a step by step how-to for testing the various components that provide inputs to the ECM to produce an output for timing. The Haynes manual for these trucks will show the same tests and more. With a miss, I always start out testing plug wires and inspecting plugs. I had a similar problem to yours in 2017 and it was a stuck open EGR valve, but it threw a code.

Good luck!



https://easyautodiagnostics.com/

 

[Schurkey]

Connect a scan tool, verify there's nothing goofy in the data stream. Drive the truck until it acts, up, look at the scan tool data to see what is out-of-range or otherwise questionable.

Assuming the scan tool can record data, you can have the scan tool record when the vehicle is running poorly, and review the data later.

The data stream is usually more helpful than "codes", but you could check for codes while the scan tool is connected.

 

[8thNote]

Thank you everyone for the replies. I was hoping (as always) that someone would be able to recommend a simple fix. It appears though, as is usually the case in life, it will require a more methodical approach and testing of many individual components. I guess I know what I'll be doing this weekend. I will update as I go.

 

[89_C3500_Larry]

Have you figured out what the cause of your problem? I am currently experiencing a similar issue with my 1990 7.4 tbi. seems to only misfire, bog, choke, sputter, and lack power after running for more than 10 minutes, and seems to be worse when its hot outside, and also only seems to do it in 2nd or 3rd gear. Runs fine in park. The block and heads were completely rebuilt recently, but most all of the other parts were put back on the engine, with the exception of: new distributor, coil, plugs, wires, new coolant temp sensor, new water pump and radiator. All of this was done at a local trusted shop. My next attemp at fixing this issue will be replacing the EGR valve and solenoid and then a new map sensor. It has no cats, and occasionally throws a lean or rich code. Any advice or ideas welcome.

 

[Schurkey]

I am currently experiencing a similar issue with my 1990 7.4 tbi. seems to only misfire, bog, choke, sputter, and lack power after running for more than 10 minutes, and seems to be worse when its hot outside, and also only seems to do it in 2nd or 3rd gear. Runs fine in park. The block and heads were completely rebuilt recently, but most all of the other parts were put back on the engine, with the exception of: new distributor, coil, plugs, wires, new coolant temp sensor, new water pump and radiator. All of this was done at a local trusted shop. My next attemp at fixing this issue will be replacing the EGR valve and solenoid and then a new map sensor. It has no cats, and occasionally throws a lean or rich code. Any advice or ideas welcome.

Stop throwing parts at it. See post #7.