350 Tbi running bad

[87silveradok20]

I have 1987 k20 with a tbi 350 that has always ran great and started up right away. Well anyways it slowly started back firing and losing power. It eventually got to the point to where it would almost die if you gave it any throttle at all. Now it has gotten better but it still is hard to start and backfires like crazy when driving. Nothing on the engine has been messed with that I know of and I've checked spark plug wires and played with timing on distributer. What could be causing this?? Also the truck shifts hard.


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[HotRodPC]

Any clue on the mileage? Backfiring through the intake, exhaust or both? Do you know if the engine has been gotten into as far as any valvetrain work or timing chain?

 

[smoothandlow84]

Sounds like a fueling issue.

Have you checked the injectors ?

Changed the fuel filter (usually located on the passengers side frame rail)

Fuel pump (one in each tank)...either going out or the fuel filter sock on the in tank filter is clogged

 

[87silveradok20]

Any clue on the mileage? Backfiring through the intake, exhaust or both? Do you know if the engine has been gotten into as far as any valvetrain work or timing chain?

There is exactly 184253.9 miles on the engine and there hasn't been any work done on it recently. It's backfiring thru exhaust and intake.


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[87silveradok20]

Sounds like a fueling issue.

Have you checked the injectors ?

Changed the fuel filter (usually located on the passengers side frame rail)

Fuel pump (one in each tank)...either going out or the fuel filter sock on the in tank filter is clogged

I haven't checked injectors or filter. I'll replace fuel filter and see what happens


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[87silveradok20]

Also the engine is completely stock except headers and dual exhaust


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[HotRodPC]

That's kinda why I asked those questions. That many miles, probably a stock virgin engine if it's never been into before, and gradually losing power, now backfiring out intake and exhaust, then a look needs to be taken to see if it has jumped time at the timing chain. The losing power, hard to start, and backfiring out BOTH intake and exhaust suggest timing chain. If just back fired either or it could be something else, but usually when BOTH, its in the timing. Not always, but should be the first to check.

This is what makes me think so.. "Well anyways it slowly started back firing and losing power. It eventually got to the point to where it would almost die if you gave it any throttle at all. Now it has gotten better but it still is hard to start and backfires like crazy when driving. "

 

[87silveradok20]

That's kinda why I asked those questions. That many miles, probably a stock virgin engine if it's never been into before, and gradually losing power, now backfiring out intake and exhaust, then a look needs to be taken to see if it has jumped time at the timing chain.

This is what makes me think so.. "Well anyways it slowly started back firing and losing power. It eventually got to the point to where it would almost die if you gave it any throttle at all. Now it has gotten better but it still is hard to start and backfires like crazy when driving. "

Even with the fact that the engine hasn't been ran hard? This truck has had 315/75s on it for 20 years and the original transmission has never been touched to give you an idea of how this truck has been driven.


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[smoothandlow84]

Maybe its worth checking the plug wires again to make certain they are wired properly. I changed out my oem wires for MSD and I was absolutely certain that I wired them correctly. Backfiring and runnung poorly, I double checked the firing order...yep, it turned out that I had crossed two wires. Problem solved. Check the simple items first, its often those that get overlooked. It's still not a bad idea to replace the fuel filters. Todays fuel is horrible and it doesn't take much to clog a filter.

 

[87silveradok20]

Maybe its worth checking the plug wires again to make certain they are wired properly. I changed out my oem wires for MSD and I was absolutely certain that I wired them correctly. Backfiring and runnung poorly, I double checked the firing order...yep, it turned out that I had crossed two wires. Problem solved. Check the simple items first, its often those that get overlooked. It's still not a bad idea to replace the fuel filters. Todays fuel is horrible and it doesn't take much to clog a filter.

Checked the wires again. They are all good. I'll have to get some time to grab a fuel filter and put on it.


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[HotRodPC]

Even with the fact that the engine hasn't been ran hard? This truck has had 315/75s on it for 20 years and the original transmission has never been touched to give you an idea of how this truck has been driven.


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Sure, especially if GM was still using fiber or plastic timing cam gear in that late of a model engine. They become brittle just over time no matter the mileage or how hard they're driven. When they get brittle, they'll wear faster and or chip pieces off the gear and they chain is allowed to slip and jump time.
From your description, everything is saying timing IMO. It could even be ignition timing and not cam timing, but usually on ignition timing out, unless you've got the firing order wrong, it will either backfire exhaust or intake not both. So this also depends on what you're calling a backfire. Some call a spit and sputter a backfire, and that's not a backfire. This is why most do better at diagnosis by listening rather than reading a description. What also can happen, somewhat rare, but certainly possible, if you get alot of carbon dust in the dizzy cap like from the wearing of the button that rides the rotor cap, that carbon dust acts as a conductor of electricity and your spark coming off the tip of the rotor cap will run all around the dizzy cap like crazy following that carbon dust causing your ignition timing to be all over the place and in an extreme case, can cause both intake and exhaust backfire.

 

[87silveradok20]

Sure, especially if GM was still using fiber or plastic timing cam gear in that late of a model engine. They become brittle just over time no matter the mileage or how hard they're driven. When they get brittle, they'll wear faster and or chip pieces off the gear and they chain is allowed to slip and jump time.
From your description, everything is saying timing IMO. It could even be ignition timing and not cam timing, but usually on ignition timing out, unless you've got the firing order wrong, it will either backfire exhaust or intake not both. So this also depends on what you're calling a backfire. Some call a spit and sputter a backfire, and that's not a backfire. This is why most do better at diagnosis by listening rather than reading a description. What also can happen, somewhat rare, but certainly possible, if you get alot of carbon dust in the dizzy cap like from the wearing of the button that rides the rotor cap, that carbon dust acts as a conductor of electricity and your spark coming off the tip of the rotor cap will run all around the dizzy cap like crazy following that carbon dust causing your ignition timing to be all over the place and in an extreme case, can cause both intake and exhaust backfire.

Ok I'll try to get a video of what going on so you guys can see.


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[87silveradok20]

I messed around with the truck some more. When you shut the truck off there is smoke/ steam coming out of the throttle body. Is this normal?


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[HotRodPC]

A video or sound clip would be helpful. Try to get both the engine and a stand back so we can hear both the carb and the exhaust.


One quick way to tell if the chain has jumped time is, to set your harmonic balancer on the front of the engine to TDC (Top Dead Center) or at ZERO. Pop the distributor cap off and see where the rotor is pointing. If you're on the compression stroke, it should be pointing at #1 on the dizzy cap, if if you're on the exhaust stroke it should be pointing at #6. If the rotor is pointing well behind where it should be, that's a sign the chain has slipped and the cam is in the wrong place which is what drives the distributor.

 

[HotRodPC]

I messed around with the truck some more. When you shut the truck off there is smoke/ steam coming out of the throttle body. Is this normal?


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Ehhhh, yeah a little can be normal to a certain degree in some situations. If the injectors dribbled a little as you shut it off, and the intake is still hot, the fuel drops into the intake then it vaporizes quickly and comes up the TB as steam. Not sure you need to worry about that right now, unless it's just a massive amount.