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I just got my 1980 Chrysler LeBaron 318 V8 back from the mechanic, who replaced the timing gears and chain, which were the originals and very worn.

Soon after leaving the mechanic, I noticed my vacuum pressure gauge was reporting high vacuum pressure on idle - 5 inches of vacuum pressure on idle. According to this article:

Idle vacuum for most engines is about 18 to 22 in.-Hg, but some may produce only 15 to 17 inches at idle.

This was indeed what I observed prior to this fix (closer to the 15 inch mark). Furthermore, it says:

Higher-than-normal vacuum at idle is a common clue to overly advanced ignition timing, while low vacuum can indicate retarded timing.

Can you confirm this statement? Is "ignition timing" managed by the timing chain that was just replaced? Was it timed incorrectly by the mechanic?

I also found this article about ignition timing which says:

To get the most power out of an engine you really need to give it as much advance as it will tolerate without preignition, or pinging, also more accurately called "spark knock"

So could the mechanic just have been trying to optimize the power of this old engine? It had been a bit sluggish, and after this repair it seems much more peppy. There does sometimes seem to be a little bit of an occasional knock that wasn't present before, which I'm postulating could be from it being a little too far advanced.

I am willing to consider that my vacuum pressure gauge is reading inaccurately, but until now it seemed to be providing consistently reasonable readings. Could the timing chain replacement have meddled with the vacuum pressure gauge inputs?

In short, should I be concerned that my gauge is reading 5 inches of vacuum at idle?

  • it sounds like the timing is off or you have a vacuum leak. what's barometric pressure in your area? i'd suggest getting a vacuum gauge and testing off the intake manifold. – Ben Mar 6 '17 at 12:31
  • Thanks, @Ben. I do not live in a high or low altitude area, so barometric pressure should be standard atmospheric pressure. – sehcheese Mar 6 '17 at 14:09
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First, verify the vacuum gauge and the port you are connecting to. 5in is very low vacuum and I suspect the car would idle like crap.

Vacuum is produced by the engine efficiently trying to pump air and the carburetor choking it off. Engine condition (piston, rings, heads), timing chain (advanced/retarded) affect how much vacuum it produces. Ignition timing is sort of a tuning issue. With more timing, the idle tends to be smoother and you can close the carburetor a bit more.

If you don't have enough ignition timing, you car could/would be hesitating quite a bit on acceleration.

I would recommend this. 1) Do a compression check - if low on all cylinders, the mechanic didn't install the timing chain correctly (too much retard, needs to advance the cam timing). 2) If compression is good, verify the ignition timing. Recommend you keep it within specs and not experiment. Putting a new timing chain in a car typically revitalizes the engine and everything should be smoother. And if you put a timing light on it, it should be rock steady.

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