94 C3500 7.4L TBI Back Fires at WOT

[pickering_c]

I ran mine through a decent analyzer. I have dual cat's and 4 O2 sensors. It is running extremely rich out of the back O2 sensor on the #1 side (I would have to assume that would be the drivers side). To compound the problem, the #1 side is adding extra fuel at the injector (long term fuel trim). That seems to explain why my mileage had dropped off.
We are going to start swapping O2 sensors around to see if the problem follows the O2 sensor; starting with the rear ones. The bummer of the whole deal is that it is a one tyr per day project because of the heat under the cats.

[trcrumb]

Well, It's been quite a while.
I changed the Fuel Pump, No Difference. I had a new flowmaster exhaust put on just a few days ago, no difference. I have a new High Performance distributer & coil arriving in a few days to change the stock units out.
If that still doesn't fix things, I'm taking it to a diagnostic guru I just found out about the other day. I am suspecting my torque converter or transmission sensors may not be working correctly. But at this spoint surely I don't really know.
It's been a while since I updated this post, I was in the hospital a couple of times for some surgery. That slowed me down a tad

[trcrumb]

The case has been solved! This will be my last post on this subject.
I just finished replacing the distributer with a procomp late model HEI Hi Performance model that I found on Ebay for $85.00 New. I also replaced the coil with a MSD Hi performance coil, also from Ebay new at $30.00.
The truck runs like a raped ape now! I couldn't believe how much power I had lost!
There was definitly something wrong with the stock items. I am guessing that the pickup in the stock distributor was working but not at full capacity.
In any event, I hope this helps some of you out there that may still be having problems with big block power issues.
Thanks for all the feed back & happy Motoring

Ted

[rjrhoades]

Did anyone find a solution to this?
I have been having similar problems. Fuel pump actually failed and replaced but did not cure problem.
Took catalytic converter off and it helped but didn't eliminate.
I think I can also detect spark knock.
this usually starts to occur only when engine is good and warmed up and under moderate to heavy acceleration.
It has been getting worse over time, initially noticing only when pulling a trailer. Will now do it when truck is unloaded.
Local mechanic has spent lots of time.
Replaced mass airflow sensor, improved idle problem but not this problem.
Also noticed water temperature oscillating slowly through about 25 degrees over a cycle time of a few minutes.
I'm stumped. Can't use truck for hauling.
92 Chevy c3500, 454.

[trcrumb]

If you look at my last post before yours, you will see that I solved my problem by replacing my distributor and coil. If I were you, I would replace the pickup coil inside the distributor, or replace the entire distributor. The pickup coils seem to go out frequently on high milage vehicles like ours. I think that is the main culprit for loss of power.
Let me know if that helps.

TC

[rjrhoades]

Thanks! I guess my eyes were pretty red and unseeing by the time I read this thread. I had a difficult time finding a forum that was discussing this problem.

[trcrumb]

Oops, I called it a pickup coil. It is actually called an Ignition Module. My mistake, sorry.
Here is a picture of what it should look like. At least on mine, a 94 it looks similar to this one here.



Yours may look different, but I think you will get the idea.
Here is an auto parts web site that you can look at for you to determine which part you need.

http://www.partsamerica.com/SelectPa...ectParts%2Easp

Ted

[RSTYCVY]

I have a similiar problem in a non vortec 4.3. Checked all fuel, ignition, and sensors.Then for shits and giggles I did a lobe lift check on the cam and BINGO, 2 lobes on the came were worn.The valves were not opening fully.

[rjrhoades]

I printed this out and gave to my mechanic. He "accidently" got shocked by the ignition wires while doing some testing and looked further. It seems the last tune up was done with Champion plugs and inferior plug wires/cap and were leaking badly, the plugs were the wrong temperature range. He has replaced those items and I am now testing to see if the problem is solved. Will see soon. Have to wait for enough snow to get off the roads to tow the trailer....

[Fireplug]

go to Snap-on web pages you will find anything and very thing you need for fuel testing

[jveik]

i hear that a clogged pcv valve can cause backfiring through a carburetor i dunno if ur new ones even have that vavle tho?