94 C3500 7.4L TBI Back Fires at WOT

[trcrumb]

Hi,
Does anyone know what would make my 94 GMC C3500 7.4L TBI Dually continuously back fire when going from idle to WOT when in park?
It also seems to have a loss of power, especially on freeway grades.
Just completed tune up of plugs, wires, distributer cap, rotor, PVC, Fuel Filter, Air filter & Oil change. Truck has 116,000 miles on it and has allways been well maintained. Plugs look good at change, no oil & no noticable smoke from exhaust.

PS. Truck is 100% Stock.

Thanks for any and all help that you may have.

TC

[jimss454]

WOT in Park? Ouch!! (Why do that?). It sounds like you maintain the truck well, all the stuff you changed is good. So, does it miss while you are driving, or just revving it up? When hot, cold, or always?
When you are noticing the power loss, is it during acceleration, (on-ramps), or just at increased load, (uphill grades at speed)? If the only time it misses is when flooring it in neutral or park, don't do that anymore. (Actually, there is never any reason to, and it can be harmful, especially to big-block engines!). Will try to help, need a bit more info.

[trcrumb]

Hi, Thanks for the reply!
Actually, It was a mechanic that did the WOT in park to check for loss of power. He suspected a clogged fuel filter. The engine never went past 2500 RPM when he punched it in park. It just kept backfiring like the engine was starving for fuel. He test drove it & said it was definitly lacking power. I replaced the fuel filter the next day & it does run better, but it still backfired when I did the same test as him, WOT in Park. Still backfired & stayed at 2500 rpm. He thought it may be some kind of rev limiter although he was not aware of GM doing that. The truck seems to lack power at all times Hot or cold when trying to accelerate entering freeway onramps & up hill grades. Truck runs great in regular traffic, it just seems to lack power under minimal load conditions. People pass me on the freeway going up hill unless I really put my foot into it, & i'm only talking 55 to 65 mph & then they still fly by me. I would think that a 454 should do better than that even with 116000 miles on it in good shape. Maybe fuel pressure?
What do you think?

Thanks

TC

[jimss454]

Hmmm. Possible fuel problem, maybe pump/pressure, or dirty injectors, but sounds more like electrical/ignition to me. Could be clogged Catalytic Converter, especially if problem set in gradually over time.
Have you had the truck for a while? Did the problem start before or after the tune-up? Did it begin suddenly, or gradually? What did you set the plug gaps to?
Try this: Remove air cleaner, start truck, climb up where you can see into the throttle body. (Be safe - Keep your face back as far as possible!). When you can see the spray of fuel coming out of the injectors, see if it is a nice, even spray pattern. Then put your hand on the throttle linkage and gradually increase the speed until the misfire begins. Note if the spray pattern changes, or becomes intermittent (starts/stops rapidly). Let us know what you find. -Dan.

[trcrumb]

Hi,
Problem has been setting in gradually over time. Problem was present before tune up.
I just noticed by doing your recommended test with the engine cold that it did not backfire. Injector spray pattern looked great on both injectors. No noticable pattern change. I am wondering what could be changing after the engine heats up? That seems to be when I really notice the power loss. When the truck is cold it seems to run much better.
I am thinking in your direction of electrical problem. Maybe distributer is not advancing properly after engine warms up? I noticed when i changed cap & rotor that there was alot of rusty powder inside. I even sprayed it with wd40. Do you know if there are cintrifigal (sp) weight mechanisms in these distributers that might be sticking? I was also thinking about the possibilty of plugged exhaust / cat problem as you mentioned, but does run good when the engine is cold. Maybe some heat related sensor somewhere? Thanks for all your help, what do you think?

Ted

[pickering_c]

It would appear that there are 3 of us with similar results...Our 454's are all losing power when loaded...Mine is a 96 with 150k miles, has the vortech tpi, 8 injectors to screw with. See post below, myself and this gentleman can't figure out anything usefull either.
http://www.automotiveforums.com/vbul...d.php?t=238530

I am really leaning towards the cat's (mine has 2 from GM) but really don't want to spend the bucks to replace if no good results. Dirty injectors are always a good possibility but how do you test? I have replaced the fuel filter, plugs, wires, cap/rotor, o2 sensors (all 4). It just doesn't get any better.....

[trcrumb]

I recently purchased a gm code reader to help diagnose the problem. I am getting a code 54 out of mine. Code 54 says that I am getting low voltage to the fuel pump relay, or fuel pump curcuit fault or mixture control solenoid fault. I replaced the fuel pump relay & after doing the recommended fuel pump circuit test, replaced the oil pressure switch as well. Still get code 54. Next, I am going to check fuel pressure at the TBI to see if my fuel pump is faulty. I will keep you all posted after I run the test.
I hate to lower the tank & replace the pump if I don't have to & like wise with the exhaust. We'll see what the test says. At least I am narrowing it down

[pickering_c]

An error code certainly is a start...I am going to track down a friend who has a code reader and see if mine gives the same code. I found one vendor who has cat's for the truck, $150 each, then you have to have someone install.
http://www.secureperformanceorder.co...e/powercat.cfm

I am not sure which will be worse though, dropping the tank to replace the fuel pump or having a muffler shop weld in some new cats.

Just for grins, have you replaced the gear oil in the rear recently? I did mine about 75k miles ago and replaced with castrol synthetic. I had someone ask and it made me wonder if the oil has broken down or something.

[trcrumb]

Havn't replaced the gear oil in the rear. I just get it checked & topped off if needed at oil changes. I have been seriously thinking of replacing Cat & muffler as well, but I want to know for sure that the power problem has been nailed down first.
The code reader for my truck was only $29.00 but for 96's & newer they jump to over $100.00.
It will be good to resolve this problem. It has been frustrating me for quite some time now. Gotta go to work now..............

[JoeBobE218]

I highly doubt this will help but i had a 350 doin the same exact thing and it turned out to be a loose rocker arm.

[JoeBobE218]

I highly doubt this will help but i had a 350 doin the same exact thing and it turned out to be a loose rocker arm.

[trcrumb]

I'm curious about the loose rocker arm. Did you hear any mechanical noise before you diagnosed the problem? I assume you have Hydraulic lifters & if so, how loose was the rocker arm? & thanks for your input.
PS. I ordered my fuel pressure guage from summit Racing today. Hopefully within a few days I will know if I need a new fuel pump or not

[broughy84]

My 350 did the exact same thing as what you are discribing, it was infact a faulty fuel pump. BTW, why don't you rip the cats and the mufflers off anyway.....man a 454 w/straights sound bad ass, my falther in law has a 454 in a 82 Chevy 1/2 ton 4x4.

[pickering_c]

I got a wild hair this morning and pulled all of the air filter and plumbing so that I could look at the throttle body. I was completely disgusted when I pulled the trhottlebody. The back side looked like the inside of an oil pan. Completely gunked up. I have never seen so much crud on the intake side. I removed all the sensors and cleaned with alcohol and q-tip, pulled the butterfly and cleaned the throttlebody and butterfly whit come carb cleaner. It seems to have more power and idles a lot smoother. We'll see how it behaves in the next week...

[trcrumb]

Just wanted to let you all know that I am having a hard time finding a fitting that will let me splice in to the fuel line at the TBI housing. They tell me it is a special thread. I finally found one on line at
http://www.tbispacers.com/gm_tbi_fuel_pressure.htm
I have ordered the adapter & will keep you posted on fuel pressure reading when I get it.

TC