Just recently picked up this 1995 chevy c1500 5.0, and i've been struggling with this backfiring issue i cant seem to pin point the issue. I've put a new distributer in and timed it. Any help would be appreciated
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That's an interesting video. Steady popping back through the throttle body like that
suggests something along the lines of a leaky intake valve allowing a small part of a
cylinder's compression stroke to blow back into the intake manifold > back through the throttle body?
Here's a series of questions I would want to answer in order to further the diagnosis:
1) Is the computer throwing any codes? If so, what are they?
2) Is the idle quality smooth, as in all 8 cylinders firing equally? Or is it more like 7 cylinders
+ 1 weaker cylinder?
3) If it was me, I would definitely pull all 8 plugs, keep them in order, and take a 'family photo'
so that we can see if there is an outlier, especially if it correlates to a cylinder with a weaker
reading than the other cylinders during a compression test?
4) While all 8 plugs are out, perform a compression test & see what we have to work with?
If all are within 5-10% of each other, record the data, stop here, and either reinstall the
existing plugs/or fresh plugs. (your call)
5) IF one or more cylinders are much lower than the others, then perform a leakdown test
on the offending cylinder(s). If you actually had a bent/leaky intake valve, the leakdown test
would confirm it by the sound of escaping compressed air not from the exhaust, or the
oil fill opening, but from the throttle body itself.
6) Another easy 'old school' troubleshooting tool is a vacuum gauge. On a healthy engine
the vacuum gauge would give you a steady reading at a warm idle from ~14-19". (Depending on cam,
altitude, ignition timing, etc.) On the engine in the video above, I would expect the vacuum gauge to
be 'jumpy', and a good (fast) gauge would jump in time to the kick back through the throttle body.
7) Tightening/correlating the diagnosis: It may be just the angle that the video was taken, but
it seems that the driver's side throttle body opening is the one with mixture blowing back? IF
this is correct, then we've got the bad actor cylinder narrowed down to 1 of the 4 that is fed
by that side of the intake manifold? IF the blowback is from both left & right throttles, then
then not so much.
In a perfect world, step #7 would correlate with a bad cylinder found on the compression test
in step #4, and furthermore the leakdown test in step #5 would be hissing out of the throttle
body.
Disclaimer:
I jumped right into testing the engine for mechanical integrity based upon the
assumption
that you have already verified that the engine is in a good state of tune. (Fresh plugs, wires,
distributor cap, etc. In other words, all the normal steps to make a healthy engine run smooth
have already been accomplished. Then again, when the intake charge is being kicked back at
a steady pace like what's shown in your video usually means that 7 cylinders are working
properly, but 1 cylinder is periodically pressurizing the intake during it's compression stroke.
Looking forward to your observations on as many of the questions above as you can answer.
Great video, BTW -- if we can diagnose/solve your problem this would be a great read for others
experiencing similar symptoms with their TBI small blocks.
Happy hunting -- good luck!
