4.3L miss and backfire under load

new wires? if not check the coil wire ends for corrosion and burning. If they are corroded put a new cap, rotor and wires on it.

New cap, rotor, wires all part of the tuneup. I saw no coil connection corrosion or arcing evidence.

Are u Sure you the ignition firing order correct? AC brand spark plugs will cause this

Yes I double checked the firing order. I installed Bosch Platinum Plus plugs this time.

A little more info is in order. It began some time back as a small hiccup that I thought was a downshift from OD to Drive going up a grade.

I then did the tuneup, filters, cleaned the IAS port.

This problem has gotten progressively worse.

I pulled the cap and positioned the rotor to point directly forward. I marked the crank and then turned the crank clockwise and counter clockwise - to check for a severly loose timing chain. The crank only moves 3/8" before the rotor moves. I've been told this is not out of the ordinary.

The miss begins right after I put it in drive, it doesn't miss while in park. If I floor it when I take off, it belches/backfires big time. Also when under any load.

Thanks, HB

is it carb or fuel injection?

Vortec?

It's fuel injection.

Water in GAs ( use DRY gas) Burnt Valve Valve lash too tight Worn CAM Worn lifter! injecter plugged ( put injection cleaner in the GAS tank)

if it is an 89 s10, unhook the brown wire just below the Heater core INSIDE the VEHICLE before timing the truck

As a last resort,, BAD COMPUTER !

BULLSHIT

Whitelightning

Seconded!

"Henry Bemis" wrote in message news:LG1Sf.21746$ snipped-for-privacy@newssvr24.news.prodigy.net...

Bosch plugs in a VW or Mercedes fine, in any GM is asking for problems. (I don't rate Bosch plugs much higher than champions) a good rule of thumb, if Autozone counter guys push something its best to avoid, and they love selling Bosch.

If it is backfiring through the intake the things that can cause it are burnt intake valves, incorrect valve timing , or incorrect ignition timing(too advanced).. That's the only way a source of ignition can get to the intake. a severely worn exhaust cam lobe or exhaust valve lifter can causing a "popping" in the intake If we're talking exhaust, we have the same, only burnt exhaust, incorrect valve timing, or severely retarded timing.. If the exhaust goes radically rich it can back fire in the exhaust, but that should trip a check engine light because the O2 sensor's readings is going to be way off the scale. Late timing can cause rich fuel mix in exhaust. You could have a pick up coil in the distributor going bad, or the magnet could be cracked. which can throw the timing off. The easiest way to find this problem is to have the truck on a diagnostic machine that can track both the primary and secondary voltages as well as the wave form of the pick up coil. I went nuts trying to find a similar problem on my 87 S-10 Blazer (that was the second S-10 I owned, I'm on my fourth now). In mine it got to where when hot it would start back firing intake and exhaust and then shut down till it cooled off an hour or so, then run fine till hot. It started out as a minor hic cup every now and then. also the bushings in the distributor could be worn, which lets the shaft move around, again messing timing up. What are the plugs looking like when you pull them? How bad was the rotor button when you replaced it? GM HEI ignitions are very hot, and love to burn holes down through the rotor. A lot of "red" dust in the distrib is an indicator this has happened, The dust is caused by the spark arcing of the inside of the distrib. Let go long enough and it cooks things like pick up coils and ignition modules, as well as the shaft and shaft bushings. On earlier distrib with advance weights it would wear the pivot pins and timing would bounce all over the place above idle. and most after market rotors don't seem to fit the shaft correctly on the

4.3 for some reason.

It would be nice if Snoman, Transurgeon, or Doc would jump in , they are a lot better at drivability issues than most

Whitelightning

It is the high voltage module in or around the distributor. I guarantee it.

[This followup was posted to alt.trucks.chevy and a c> 1993 4.3L V6, 4L60 misses bad and belches/backfires under a load when taking off

Well, I've already done the cap/rotor/wires.....still the same.

One fellow employee tells me it's the coil, another says timing chain slop.

Either way I have to buy parts to even try to get it going. I guess it's the coil first. The timing chain second.