Another hot idle oil pressure thread

I know, I know, this topic may have been beaten to death, and I have read the many discussions on this, but wanted to get a bit of advice from the experts here on my particular issue. I have a 1977 308, so a carb car, and am currently running VR1 20W50 oil. So what is the issue that I am having? Well, I recently went through and did a full sync and tune on my carbs, and got the engine singing beautifully. Because of this, I was able to set my idle down to 900 RPM from my previous 1000-1050 RPM where I was before. Before the tune, I did not have any oil pressure issues. Now, on cold start everything is good. I see about 85 PSI on the gauge and no oil light. When warm and idling no issues either. However, once fully hot, when the oil temp gauge reads 210, and the car is at 900 RPM, the gauge reads 0 PSI, and the oil light comes on. If I even slightly touch the throttle and get the revs back to 1000 RPM, the gauge comes off of zero and the light goes away. If it was just the gauge, I would not worry, as I know how inaccurate this can be. However since I get the gauge and the light, which are two separate senders I am a bit worried. When I changed my oil last, I did not see any glitter or signs of metal, so I am not too worried about this being a bearing issue, but I guess it can't be ruled out.

So here is my series of questions. The first is, now that I am at 900 RPM, is my idle simply too low, and I need to bump it up to 1000 RPM again? I know typically you expect to see 10 PSI per 1000 RPM, so at 900 RPM, I would expect 8-9 PSI, would this be low enough to trip the light? In terms of resolving this, what is the best course of action? Should I bump my idle up, or should I try switching oils from VR1 20W50 to something like 25W60 to give a bit more pressure at hot temps and low revs. Or is this not even something I should worry about at all. At hot idle, I did remove the oil filler cap, and can see oil circulating on the valve train. It wasn't spraying, but it was moving. Thanks in advance for the advice on this. I know there are some people here who have decades of experience with these cars and their nuances, so it is always a wealth of knowledge in here.

 

Thanks! This is extremely helpful. I will bump the idle back up to 1000. I have a digital inductance style rev counter to use instead of the dash tach. After getting it adjusted I will check OP at 6k revs. I didn’t realize there was a spec there. Makes sense though since there is a high RPM timing spec. This is why I love this group. So much knowledge. Thanks again.


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Thanks! I will check OP at 6k RPM. I will report back here on what I find. I definitely appreciate the help. Always so much great knowledge in this group.


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So here is an update. Steve you were absolutely correct about the idle being too low. I checked the idle with a digital inductance type tach, and at hot idle I was reading about 740 RPM even though the dash tach was indicating about 900 RPM. Gotta love the Vegalia gauges. So I set the hot idle to right around 950-960 RPM. I noticed some fluctuation up to 1000 RPM from there, so I thought that could be a good place to leave it. With the idle set there, with an oil temp of 210, there was no more oil light, and my gauge was reading about 10-15 PSI or so. So I was pretty happy with that. One thing checked off. Then I tried to check pressure at 6k RPM. When I did this, the gauge was sluggish and did not get past about 35-40 PSI or so. By the way it is behaving I think it is a gauge issue and not an engine one, although I have not been able to connect a mechanical gauge to verify. I was able to do some checking on the dash gauge though. I unplugged the sender and grounded out the wire, and the gauge went to full pressure as it should. I did the same with the oil light, and the light came on. However, when I checked voltage to both senders, I saw 12V to the oil light sender, and only 6V to the sender for the gauge. That is leading me to believe there is an issue with the gauge or the wiring to the gauge. I would think that if the gauge is only running at half the voltage then a reading of the same resistance from the sender would cause half the increase on the gauge. So 35 PSI at 6V would be 70 PSI at 12V, but that is just my fuzzy guess at this point. I have not pulled the gauge yet to check the wiring, but I suppose that should be my next project.

 

Ok, I wanted to give everyone a little update. I think I may have figured out the issue with the oil pressure gauge, along with some electrical gremlins, and an idle issue that I had. I as trying to resolve an issue that I had at idle where it would idle nicely at 950-1000rpm, and then drop down to 500rpm. It seemed like it was dropping an entire bank, but giving just a hair of throttle would resolve the issue. Which I found very weird. Well all the issues came down to my Pertronix setup. I had replaced the points with an Ignitor II and also replaced the coils with the Pertronix coils as well. At the time of install I rebuilt both advances and set the timing for both banks using the front distributor but was able to check that they were both at 34 degrees advance at 5k rpm. With the Pertronix, it uses both dizzys to distribute spark but the pickups for the coils are only in the front one. So once installed it is not really easy to phase the back distributor, and I really didn't think I needed to since it was only distributing spark.

Well I was trying to track down this issue, and noticed a purple glow inside the back dizzy if you take the boot off of it and look through the hole where the points connected. This led me to believe that the rotor was arcing but seemingly only at idle. I adjusted the phase of the back distributor just a bit, and the arcing went away, it stopped dropping the back bank of cylinders at idle and my oil pressure gauge started working, along with a few other gremlins going away. I guess the Pertronix coil was strong enough to arc with the dizzy out a bit, but this extra draw was causing some electrical issues. The car idles and starts a lot better, nd no change in the upper rpms with timing, so I think hopefully all should be good. I probably should have just left the points but now that its set, hopefully I can have miles of trouble free driving with it.

 

Thanks. I had setup the distributor as you described but my issue was the adjustment rotating the slave distributor for what would be advance. It seemed like it was just missing contact and arcing at idle as I had originally fussed with it at 5k with the timing light. I’m using the old style rotors though. I bet if I swapped to the QV style ones it never would have been an issue at all. I may pick up a set when I order my next parts from Superfotmance.


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