maroguy1

My daily driver is a 93 s10 pickup with a 5 speed and a 4.3 tbi. 92k on the clock. Well anyways did a tune up about a year ago and it runs fine but seems a little slow for a standard cab short bed 2wd. I get beet by full size suv's. The thing is if I rev it up with the air cleaner off it will sometimes pop or backfire thru the tb. Sometimes especially when cold it will hesitate real bad taking off from a stop lite. It also burns oil on a cold startup. I have changed the timing, checked tps, complete tuneup, installed new maf and ecu. Could it be timing chain or bad camshaft?? I had an old 305 that backfired thru the carb and the camshaft was shot. But I believe this is a roller motor. Please help if you have any ideas. Thanks in advance

dimented24x7

iTrader: (2)

Sounds like the timing is retarded. With the EST bypass connector disconnected, set the base timing to something like 0-4 degrees BTDC. Check that first and see if it helps.

JPrevost

Replace fuel filter and then fuel pump. It's lean if it's backfiring worse when it's cold. You've got a fuel delivery issue.

maroguy1

I replaced fuel filter about a year ago and I will have to check fuel pressure this weekend when it is cold compare to warm. It will still hesitate from a stop when it is hot though. I will check the timing on it to be sure. Thanks, Mark

JPrevost

Well I know it's not your o2 sensor. Those fail the other direction, you'll flood the motor and it won't restrart hot and your milage will be crap. Do this to test if it's lean;
Accelerate hard enough that it doesn't backfire and quickly turn off the car and coast over to the side of the road. Now pull a spark plug and take a look. If it's white chaucky (can be wet from misfire) then it's lean. If it's ALL tan it's okay, if it's black and looks like crap then it's really bad oil and/or way too rich. Your car won't backfire through the TBI when it's really rich. Also check your base timing, too much can cause the wrong cylinder to fire, always check that the plug wires are wired correctly because it WILL run in weird orders. Also, make certain both TBI injectors are flowing the same amount of fuel. It's pretty easy to see. Also, as a quick check for fuel pressure starvation, pull the air cleaner off and blip the throttle. Pay close attention to the fuel spray. With low pressure the cone will get narrow and large dropplets of fuel instead of a spray.

maroguy1

The thing is the truck runs great. I have been driving it every day for years. I just noticed it a few weeks ago when I have it running and the air cleaner off. That is good info though I will have to check the fuel spray. Fuel milage isn't that bad around 20. Thanks

RednGold86Z

iTrader: (1)

The air cleaner being off will really throw the AE (accel enrichment) tune out of whack on a TBI. It's really hard to get enough fuel in as fast as that first rush of air, and with an Open element, or no air cleaner at all, it'll lean pop much easier (don't look into the TB while doing this).

JPrevost

I never had that happen on a stock f-body tbi, never happened on a 454 truck tbi either. Sounds like a clogged fuel filter if you got it to backfire just from removing the air cleaner.
When there is very little load on a tbi engine and you rev it. I do know however that at the right RPM you can get it so that the AE overlaps the sync pulsewidth timer and it won't carry over, it'll just be overlapped resulting in really annoying lean spikes. I believe it was 800rpm and 1200rpm.

maroguy1

You might be on to something there because it will backfire in that rpm range. Doubt it is the fuel filter it was changed about 5000 miles ago.