TBI 454 Misfire

[Spareparts]

Yeah. forgot about intermittent.

 

[Road Trip]

And in order to provide a solid current return path for G104 & G105 on the radiator support,
you should also perfect the condition of G101 near the battery. IF you do all this and it still takes
noticeable time for the voltage to rise high enough to make your headlights come on normally,
then we need to take a look at the power side of things. (In my following reply.)

Cheers --

Re: Lights coming on a couple of seconds after turning on the headlight switch.

In my previous replies I was focused on the following grounds:

* G202 -- question about location of cab-related ground
* G104 & G105 -- exterior lighting circuit ground

OK, in this forum we do put a lot of emphasis on locating old, marginal
shared grounds on our aging platforms. With good reason, for the
electrical systems usually get a good leaving alone, and since they normally
go flaky/intermittent over a period of years, electrical issues don't get
dealt with until there are several issues to sort out.

****

When troubleshooting 12v vehicle systems, the 2 most important things to remember are:

1) We only have 1/10 the voltage to work with in a vehicle vs the normal (US) home wiring. (120v)
We have barely enough to work with when all is well, and can ill afford to lose
more than ~1/2 volt than what we started with while the electrical load is busy drawing power.
(Schurkey will often recommend to check the voltage across a load {fuel pump} when it's powered up.)

In English, losing a whole volt somewhere in the wiring harness is something to investigate.
Maybe on a circuit with a steady draw (like a power window) losing a volt just means a
slightly slower operation.

But on a electrical subsystem where heavy power is drawn for short periods of time
(think ignition system) ...a weak/marginal power feed can create weird, counterintuitive
symptoms. Easily seen with an oscilloscope, but maybe only 0.01% of the population is a
scope dope?

Given the above, when it comes to sorting out a sickly vehicle, I've learned the hard way
to first figure out just how good or bad the electrical system is. A robust 12V system
(~14.0-14.5v running, 12.6v off) is a mandatory foundation to start the troubleshooting
process.

2) The electrical load does not care if it is being undervoltaged because of a bad shared ground
-or- a weak 12v power feed. Or both. Full voltage is your friend.

Q: So, IF the grounds are all good, then what should be checked on the Power source side of the equation?

A: Since your vehicle is a '92, there is a "Always Hot" power bus bar at the top of
your firewall in your engine bay, where battery power is distributed to various areas
of your vehicle. (Including your fuse block.) This diagram shows this positive power Junction Block:

You must be registered for see images attach

And here's a video of a guy describing how he was chasing some electrical gremlins &
finally discovered loose electrical connections on this Junction Block:

xc_hide_links_from_guests_guests_error_hide_media

Just food for thought. If you methodically work through all the grounds discussed
above and inspect/clean/tighten your Junction Block power connections, eventually
your lights will come on like they are supposed to, instead of having to wait a couple
of seconds. And IF after all this the lights are still coming on slowly, then I'd be
looking for a switch out of the Treasure Yard for $. And if that solves the problem,
either stick with what works, or find a new switch that meets your purchase criteria.

And I know that slow lights are a lower priority than the engine bogging down under load.
But going back to the way that the Ignition system draws heavy power for short pulses,
sometimes if you fix the obvious, then suddenly the exotic is also cleared up.

Food for thought. But please note that in all of the above I never once tried to reset
expectations that a 32 year old truck can't run like new. Because I don't believe that
for a second. (!)

Best of luck!

PS:

Oh, and both shops seem to think that I am an idiot for thinking an online forum will help; according to them none of you are trustworthy or have any experience. Whenever I reference this forum they scoff and look down on me.

I cheerfully disagree with those fine, yet judgemental fellows. If they are under 50 years
old, then I've been working to make internal combustion things run for as long as they
have been alive. :0)

When somebody is giving me the business like that, I ask them what year they were born.
1979? Shoot, I was rebuilding Quadrajets and dialing them in on a Sun 1115 while you
were still filling diapers! All delivered with my very biggest, most cheerful grin. And I only
did cars & trucks for a living for a relatively short period.

There's plenty of talent in here that has swung wrenches in anger for their entire career.
They have 10-20x as much experience as yours truly. They have been successfully
troubleshooting remotely for years, while this troubleshooting in a closed phone booth
versus being elbows deep in the engine bay is still a new thing that I'm still learning
how to do.

There's literally tons of talent in this joint, and I enjoy watching the GMT400 hive mind work
on tough troubleshooting challenges. I am constantly learning the good stuff from the talent
that takes the time to share the wealth. (!) Methinks that both shops are guilty of thinking
that all forums on the internet are working at the same exact level. I don't share their
conclusion. So I'm going to have to agree to disagree with those pros. Especially when they
discredit themselves with statements like, "...I was unreasonable in thinking a 30 year old
truck could run right. "Its just old"."

Negative, Ghostrider.

 

[Rock Hard Concrete]

New Developments:

Used truck all last week for work. Ran horribly on Monday and Tuesday, but ran great for most of Wednesday. Back to running like **** for Thursday/Friday. The intermittent nature of the problem makes this really difficult.

Went to check spark plugs, and every single lead pulled out of the boot. I guess spark plug wires are a one and done kinda thing, they were new just a few months ago. I checked the cap and rotor, the rotor was missing a chunk of plastic and the cap was an el cheapo aluminum contact pos.

So naturally, the cap, rotor, and wires have been replaced 4x now.

I also did the tps since it was cheap and one of the few electrical components that had not been replaced.

I just ordered a throttle body rebuild kit
will be here in a few hours.

I checked and cleaned the connections at the junction block. I think my delayed light issue is from the switch in the cab and nothing else.

I will report back after rebuilding the throttle body.

 

[Schurkey]

Went to check spark plugs, and every single lead pulled out of the boot. I guess spark plug wires are a one and done kinda thing, they were new just a few months ago.

Don't pull on the cable. Plug wires are meant to be removed by pulling on the boot. Twisting the boot can help break the bond between boot and porcelain.

Modern silicone boots will practically weld themselves to the porcelain of the spark plug. Thus the recommendation to coat the plug, and perhaps the inside of the boot with some dielectric grease before cramming the boot/wire into place.

 

[Rock Hard Concrete]

I pulled the boot, not the wire. I twisted the boot to release it from the plug. The steel clip that holds the lead to the plugs were so tight I had to use pliers to remove them after they pulled out of the boot.


After rebuilding the TB the truck seemed to be running better, but that was just the intermittent nature of the beast leading me to believe it was fixed. Issue is still here.

*Edit* the original nature of the problem is now reversed; the truck used to only misfire and stumble at low throttle, now the low throttle response is good with no stumbling or misfires, but the issue now shows up at half throttle to wot.

 

[udidwht]

Did you thoroughly check the throttle shaft ends for vacuum leaks?

 

[McDermott]

I have had that same problem and chases my ass around trying to find out what the hell is going on. I have a 92 C3500 , BB 5 speed dually. In my case I had a TPS going bad causing the problems . I would start there . TPS errors should turn on the service engine light , but in my case it was intermittent and enough to leave the light off .
I have also had my distributor get a touch of moisture and freeze the rotor to the cap and turn just enough to change timing , so retiming it would be a good idea especially since someone threw a cap and rotor on it .

We have to sadly remember today’s techs arnt that knowledgeable on 32 year old tech . I bought my c3500 about 10 years ago from a dodge dealer that took it in on trade . The service engine light was on when I test drove it and asked them to scan it to let me know what it was before driving it about 75 mile home. A tech with ASE patches up and down his arm who looked to be almost able to shave popped the hood and said that’s why the light it on … I looked under the hood and just couldn’t visually see what was causing the service engine light … and he said “your air cleaner cover is flipped upside down” I just said I’ll check it when I get home and closed the hood …

 

[Rock Hard Concrete]

New Developments:

The hard misfire is gone completely. The truck runs great at low throttle positions.

It shifts sooner, and smoother than before. The revs don't hang up waiting for an upshift.

I noticed the wiring connector for the water neck grounds had poor continuity, so I replaced them and cleaned the connections again. Much better continuity.

The entire engine is extremely well grounded, less than 1 ohm of resistance anywhere I test.

The hesitation and stuttering are still here. They show up at greater than half throttle.

I drove the truck up a 5% freeway grade. I had to go WOT just to maintain 60 mph. It used to only take half throttle or so to maintain 60. The truck did not have any obvious hesitation or stuttering, just very low power.

When I rebuilt the throttle body, the spring was in good shape. Not broken, kinked or soft.

The only 'performance' modifications are a throttle body spacer that a po installed, and a high flow cat. Would these increase the airflow enough to require greater than stock fuel pressure? I don't think they would.

When I floor it from a dead stop and pull through first gear it will hesitate, run okay, hesitate, run okay, hesitate run okay. Usually if I hold the petal to the floor and keep pulling in second gear it will clear up and not hesitate, just not have much power.

And again, sometimes the truck will run perfectly with a "Wow!" amount of power. It only runs like this for short periods of time, typically. Sometimes, it will run great for hours at a time.

 

[Schurkey]

I'd just love to see what's shown on a scan tool when it runs bad...and when it runs good.

Also the voltage at the fuel pump (and fuel pressure.)

 

[Carlaisle]

With the latest week of use on it how was your mileage?

Live input from a fuel pressure gauge plumbed in as near to the injectors as possible would be valuable to figuring out why you need twice as much throttle now vs. in the past.