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My teenage son just bought his first car, an '82 280zx that is backfiring out the intake. When we bought it, it ran poorly and backfired out the exhaust when cold, but improved when warmed up. Since then, we fixed a broken wire to the oxygen sensor and replaced: plugs, dist. cap, rotor, fuel filter, PCV valve, all vacuum and fuel lines, and all electrical connectors to the fuel injectors and other sensors. We also set the idle speed & timing (8 degrees BTDC). Now it idles smoothly and doesn't backfire out the exhaust any more but it does have slight hesitation and backfires out the intake upon rapid acceleration. Compression is somewhat low (about 125 psi) but pretty consistent between all cyllinders. It has a bad exhaust leak at the exhaust manifold to head-pipe joint, but I can't see how this could cause the backfiring on the intake side. Electronic and vacuum spark advance both seem to be working, judging from the timing changes at idle vs higher RPM. Any suggestions as to what to try next? Should I suspect a bad timing chain, cam, sticking valve? I'll probably pull the valve cover and inspect the cam, but don't know how to check the timing chain without having to pull out the radiator, etc (yuck). Any advice is appreciated.
 

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i read an tech article on adjusting valves on worn engines on zhome.com
and it made sense. they adjust valves a little tighter than spec because of the tolerances in the valve train are a little sloppy when its a high mile engine.

an exhaust leak can send wrong signals to the oxygen sensor since
oxygen can be entering into the exhaust thru the leak. that would be the
first thing i would fix.

did u check the distributor vacuum pot for leakage? that can cause a big
vacuum leak.

ive heard of people timing their engine with a vacuum gauge. they
say to turn the distributor until you get the highest reading and
back it off 1 inch of vacuum. im wondering if anyone times their
z car that way. i have yet to try it since its been pouring rain here for 2 freaking
weeks
 

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Discussion Starter #3
Bakachan - I checked the vacuum advance, and sure enough it had a pinhole leak in the diaphram! Not only was it causing a vacuum leak, but it wasn't advancing the timing properly either. I have a new one on order through the local dealer, so haven't been able to verify yet that the backfiring is fixed, but that's probably the culprit. I really appreciate your advice. Give yourself a pat on the back for this one!
 
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