1979 Toyota 20r Distributor

20R engine in 1979 Celica

I was wondering why the mileage suddenly dropped from 24 to 15(!) mpg, with the engine barely able to idle.
I checked fuel pressure (good), but vacuum was only 15" (it was 20inches last I checked over the summer).

My first thought was a leaking intake manifold gasket. But today, I took the car on a long trip, and when I got into the garage, I heard a squeaking/scraping sound. It sounded like it came from the distributor, Sure enough, if I pressed on the cap, the sound got worse.

So: I removed the cap, and noticed what looked like carbon tracking. Cap was changed 21,000 miles ago. The Toyota FSM says to merely inspect the cap and rotor every 30,000 miles. I change them at 20,000 mi intervals anyway, keeping the old one in the trunk.

Am I looking at distributor problems? There is NO play when I try to shake the rotor shaft from side to side. The only problem it has is that one of the springs for the advance mechanism weights is weak (no longer available from Toyota)

I’m just wondering how a distributor cap could wear out so quickly.

Distributor is original, 247,000 miles


To be brutally honest, I’d first be concentrating on why your engine vacuum dropped down to a rather mediocre . . . in my opinion . . . 15"

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db4690: Last night, I swapped dist caps with an older one. Car idled better, with vacuum again at 20". Needle was steady, so I don’t think I have anything wrong with a particular cylinder, valves, or intake manifold/gasket. But I’ll be calling an auto electric shop to see if they can rebuild the distributor. Low vacuum and noise started at the same time.

The old distributor cap looked like new. This one was full of carbon deposits, with the rotor also looking worn out. They should last longer than 20,000 miles.

BTW: When I measured the 15" of vacuum, the car was idling at less than 200 rpm (versus the normal 800 rpm), about to stall out.
I may be wrong, but I wouldn’t expect to see normal vacuum at that speed.

db4690 (or others): An update:

Again this morning, vacuum was low (15"). I had the car running a few minutes, and it went back up to 21". In addition, the engine RPMs increased.
So, I’m looking at a vacuum leak in a vac hose? I’m thinking that a TVSV may be closing at normal temp, thereby blocking the path of a leaking hose.?

At any rate, the distributor is making noise, its spring for one of the advance weights is weak, it’s chewing up dist caps, so I’m still sending it out. RockAuto only sells rebuilds of customer’s own dizzy. Or should I deal with a local shop nearby? (there is one)

So I have two problems, not one. And I was apparently wrong. Very low RPM didn’t cause the low vacuum.

There are more vacuum hoses and valves on a 1979 20R engine than on anything else I’ve ever seen. I had one on a 4X4 pickup. Plenty of possible leaks. The one I had to deal with the most was the AAP (Auxiliary Accelerator Pump) diaphragm. It would develop a leak and suck excess fuel into the engine. Eventually I just blocked off the hose that led to the AAP housing. The engine started and ran fine without a functioning AAP system.

The fact that your problem seems to go with an engine cold - engine warm condition suggests you are on the right path, looking into switching valves and their hoses, etc. Good luck!

Not great, but I’ve seen worse:
EDIT - I had the wrong Toyota, here’s the Celica, lots of potential leaks:

I recall those Hondas with 2 multi function vacuum control modules attached to the firewall. It was best to unbolt them from the firewall and leave as many connections to the intake manifold connected and set the intake aside with everything in tact.

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Yeah, I was thinking of the last carbed Hondas, a real rat’s nest of vacuum hoses.


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Recalling the old distributor days I am thinking the carbon buildup on the cap was indicitve of a bad ground, don’t recall what ground, maybe the ignition coil. Buildup on the rotor indicated some other issue. Maybe some one here can recall.