TBI 454 Misfire

[Rock Hard Concrete]

My 1992 K3500 454 has a terrible misfire. It is worst on cold start up, but it is still there when warm. All spark components have been replaced at least once. The misfire seems electrical and not air/fuel related. I am no expert, I make that assumption because the problem is intermittent, and the misfire is hard, like an ignition cut.

I have been searching various threads on the internet for months, and no one has exactly the symptoms I have had, and the ones that were similar were all fixed by things I have already replaced.

The misfire is worst at low throttle, 2000ish rpm. The truck will buck, backfire, misfire, stumble, surge etc.

Sometimes, the truck will run perfectly fine, although rarely.

Two separate shops have told me theres nothing wrong with my truck and they don't want to work on it anymore. One being what used to be the best independent shop in town, the other being a dealer.

The misfire used to be accompanied by a jumping tachometer but the last shop replaced the cap, rotor, plugs, and wires (for the 3rd time) and the jump in the techometer went away, but the misfire is still present.

New Parts (related to how the truck runs at least):

Vortec fuel pump
Fuel filter
Distributor
Cap and rotor 3x
ICM 3x
Plugs and wires 3x
Coil
All temp sensors
All grounds cleaned/replaced and added extra just to be sure.
High flow cat
5&7 plug wires not crossed
I'm sure there's more that I'm forgetting

I am sure the intake gaskets need replacing, but I am not at all convinced that is causing the problem. That explains a high idle but not a hard, intermittent misfire.

The only things I can think of that might cause this issue are:

TPS
EGR
FPR



I managed to drive the truck without misfiring one day and got almost 12mpg. But most of the time it gets 5.5 mpg just to give you an idea of how bad it is.

Anyone else have this issue?

 

[Spareparts]

Thats a TBI, correct?
Why would you use a Vortec pump. Way more pressure than you would need. If i remember right a TBI wants about 15PSI.
A Vortec pump can put out much higher pressure and flow. Your FPR may not be able to compensate.

 

[RichLo]

What do the fuel cones look like coming out of the injectors? Any large droplets or odd pattern compared to the other side?

And yea, with that pump you really need to check fuel pressure at the TB. TB's arent that hard to rebuild and the 'maybe' parts you listed are pretty cheap considering all of the other parts you shotgunned at it.

Edit, add o2 sensor to your maybe list. While your replacing it, upgrade to a 3-wire heated sensor.

 

[GoToGuy]

Your a big block. Some big blocks what year not sure, check the service manual your year, operate with higher fuel pressure, two fold reason, larger throttle body, larger injectors, and higher underwood temps, higher pressure avoids vapor lock.

 

[Rock Hard Concrete]

Yes its a tbi. Went with vortec fuel pump as that is a very common upgrade. Fuel pressure was 9psi before and 13 psi after according to the shop. The stock regulators are supposed to be fine as long as theyre in good condition. Thats one of the things on my list I was thinking may be the issue. Fuel pattern at injectors is good. I will try a fpr next.

As far as shotgunning parts, I have paid 2 separate shops to diagnose and repair this problem, they are the ones shotgunning parts and I'm still left with fixing the issue myself.

 

[RichLo]

TBI's are super simple, there's no reason a shop should have replaced ignition parts 3x and still haven't touched the TB.

 

[Rock Hard Concrete]

TBI's are super simple, there's no reason a shop should have replaced ignition parts 3x and still haven't touched the TB.

I completely agree. Seems if a shop can't plug a scanner in to see the issue they arent interested in fixing your stuff either. Lots of shops wont work on my truck because its obd1.

 

[tayto]

making a block off plate for the EGR is a good idea just to eliminate it for now. have you tried opening the hood in a completely dark area and seeing if there is any arching going on? have found many ignition problems this way.

 

[Carlaisle]

That vortec fuel pump is delivering 4X the pressure required for your fuel delivery system. Even if your FPR appears to be handling it fine for now this should be corrected just to eliminate the potential for it be(com)ing a contributing factor to the main problem.

The fact that you're getting half the mileage you should be is a clue. Something is telling the computer it needs to deliver twice as much fuel as is actually needed. My first suspect would be the oxygen sensor(s).

Any chance you kept all of those parts that you were charged for that may or may not have been replaced? That stash could prove valuable to your troubleshooting efforts. Have you personally verified they actually were replaced? You wouldn't be the first person charged for parts that were never installed.

Take Tayto's advice about looking for the lightning show in the dark. It's completely free and very easy. I found a headache with this same technique on more than one occasion. Bad wires/grounds every time.

How many miles are on this vehicle and what transmission is in it? Which of the parts that were replaced are actually installed on it now? i.e. Accel coil 12345, etc. Not all parts are created equal. I received multiple sets of new plug wires with dead wires out of the box.

Have you personally checked the firing order to make sure the correct wires are leaving the correct spot on the cap and arriving at the correct plug?

Nothing to gain from replacing the FPR unnecessarily. Use a fuel pressure gauge and see if it is working correctly. Someone here can tell you the correct range for your engine. If not, I think the FSM is linked here on the site under a sticky.

 

[Rock Hard Concrete]

I will have to double check but I think the o2 sensor was replaced. The plug wires are correct. Issue was here before the fuel pump was replaced, and other than the misfire not going away, the truck definitely runs better than with the previous pump. I only have extra parts that I replaced myself which is only 1 icm and the coil, which I just did today. Checking for light in the dark is a good idea I will give that a go.

170k miles and 4L80E