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Does overheating cause oil pressure light to come on?
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caden564
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PostPosted: Wed Jul 11, 2018 7:58 pm    Post subject: Does overheating cause oil pressure light to come on? Reply with quote

I have a 1978 FI Bus with a GE engine. I have noticed that the oil pressure light comes on only after i have been driving around for a while and the engine is hot. Does an overheating engine cause the light to come on? I can grab the dipstick handle and hold it, and i can touch the bottom of it where the two lines are only for a second without burning my fingers. It has a brand new oil pressure switch and I've tested the pressure to be at 10psi at idle. There are a few small oil leaks near the oil cooler and driver's side valve cover, but they are not very bad. The engine is running 20W-50 oil. Please help
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PostPosted: Wed Jul 11, 2018 8:26 pm    Post subject: Re: Does overheating cause oil pressure light to come on? Reply with quote

It it just coming on or flickering at hot idle after a highway run?
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PostPosted: Wed Jul 11, 2018 9:04 pm    Post subject: Re: Does overheating cause oil pressure light to come on? Reply with quote

It only comes on when hot after a highway run when the engine is at idle. If i rev it a little it goes away, then comes back on at idle.
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PostPosted: Wed Jul 11, 2018 9:10 pm    Post subject: Re: Does overheating cause oil pressure light to come on? Reply with quote

caden564 wrote:
It only comes on when hot after a highway run when the engine is at idle. If i rev it a little it goes away, then comes back on at idle.


Perfectly normal. See the last paragraph before the grey box here:

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If it bothers you, remove the pressure relief valve screw plug and place a 3-4 mm thick appropriate diameter washer as a shim under the spring. This should prevent this.
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PostPosted: Wed Jul 11, 2018 9:19 pm    Post subject: Re: Does overheating cause oil pressure light to come on? Reply with quote

Okay that makes me feel a lot better. What about the dipstick test? Does that sound like it could be overheating? I cant hold it for more than a second or two
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PostPosted: Wed Jul 11, 2018 9:22 pm    Post subject: Re: Does overheating cause oil pressure light to come on? Reply with quote

caden564 wrote:
Okay that makes me feel a lot better. What about the dipstick test? Does that sound like it could be overheating? I cant hold it for more than a second or two


It might be running a bit hot, yes, but what's the ambient temperature where you are? What distributor are you running, and how is your timing set?
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PostPosted: Wed Jul 11, 2018 9:42 pm    Post subject: Re: Does overheating cause oil pressure light to come on? Reply with quote

I am in Idaho, the temperature is usually 86-90 degrees in the summer. It has the stock Distributer with one vacuum advance tube. I have been having a hard time with the timing. Its been set at 7.5 degrees BTDC. I think the Bentley manual says its to be set at 5 degrees ATDC but i haven't been able to get it to that.
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PostPosted: Wed Jul 11, 2018 9:59 pm    Post subject: Re: Does overheating cause oil pressure light to come on? Reply with quote

Do you have a timing light and a tach?
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PostPosted: Wed Jul 11, 2018 10:09 pm    Post subject: Re: Does overheating cause oil pressure light to come on? Reply with quote

Yes, I have timing light and a tach/dwell meter.
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PostPosted: Wed Jul 11, 2018 10:36 pm    Post subject: Re: Does overheating cause oil pressure light to come on? Reply with quote

You do not want to time your engine at idle, that was all fine and good when your distributor still exactly met all the factory specs, which it likely no longer does. Instead you want to time your engine at full mechanical advance, 28 BTDC @3500+ rpms, hose(s) off and plugged. This will simulate high speed full throttle operation when you are demanding the most from your engine.

You oil temps are likely at 200F or above, so one wouldn't expect to be able to grab the dipstick or touch its end.

Buy an IR gun if you are worried about your engine temperatures.


Last edited by Wildthings on Thu Jul 12, 2018 7:56 pm; edited 1 time in total
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PostPosted: Wed Jul 11, 2018 11:15 pm    Post subject: Re: Does overheating cause oil pressure light to come on? Reply with quote

I borrowed my friend's temp gun and got these temps around the engine. I'm not really familiar with what they should be. I read somewhere that the oil shouldn't get over 235 degrees, but I wasn't sure how different the temp outside the engine is.

This was taken right after the oil light came on. Engine was off.

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PostPosted: Wed Jul 11, 2018 11:44 pm    Post subject: Re: Does overheating cause oil pressure light to come on? Reply with quote

The one valve covers seems to be significantly hotter than the other. That could indicate a problem. Maybe pull the valve cover on the hot side and see if the valves are overly loose in their guides.
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PostPosted: Thu Jul 12, 2018 3:10 am    Post subject: Re: Does overheating cause oil pressure light to come on? Reply with quote

Exhaust leak on that hot side
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PostPosted: Thu Jul 12, 2018 9:55 am    Post subject: Re: Does overheating cause oil pressure light to come on? Reply with quote

Wildthings wrote:
You do not want to time your engine at idle, that was all fine and good when your distributor still exactly met all the factory specs, which it likely no longer does. Instead you want to time your engine at full mechanical advance, 28 BTDC @3500+ rpms, hose(s) off and plugged. This will simulate high speed full throttle operation when you are demanding the most from your engine.

You oil temps are likely at 200F or above, so one would expect to be able to grab the dipstick or touch its end.

Buy an IR gun if you are worried about your engine temperatures.


Time it like Wild says, and do that before you do anything else.

Then road test it and do your readings again with the engine idling, just to make sure that your one side hot temp reading isn't a weird outlier due to heat soak.

Report back.
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PostPosted: Thu Jul 12, 2018 4:50 pm    Post subject: Re: Does overheating cause oil pressure light to come on? Reply with quote

delete

Last edited by Wildthings on Thu Jul 12, 2018 7:56 pm; edited 1 time in total
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PostPosted: Thu Jul 12, 2018 6:25 pm    Post subject: Re: Does overheating cause oil pressure light to come on? Reply with quote

UPDATE:

Okay, so I got the timing to 28 degrees BTDC at 3500 RPMs like you guys said, engine runs better now. I took the temps again in the same places and there was less of a difference between the two valve covers. Also I found a deep fat fryer thermometer and stuck it where the dipstick goes just to get an idea of the oil temp. The temps are a little higher because i just let the bus sit to warm up instead of driving it. The valve cover that was hotter was the drivers side. I took off the hose that goes from the heater blower motor into the drivers heat exchanger and noticed that there was what seemed to be exhaust gases coming from it. There was no exhaust gases on the passenger side though. Is my driver's side heat exchanger leaking causing the warmer head? One last thing to add, that darn oil pressure light came on and started flickering like crazy so i quickly turned the motor off.

Driver side:
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Passenger side:
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PostPosted: Thu Jul 12, 2018 8:08 pm    Post subject: Re: Does overheating cause oil pressure light to come on? Reply with quote

If you were getting exhaust gases entering the heater air flow, you would have gotten carbon monoxide poisoning by now, so I doubt that is happening. Both sides should leak heated fresh air when the hoses for the booster fan are removed not just one. I would guess that one of the worthless flapper valves on the cooling fan outlets into the heat exchangers has failed blocking the air flow through the one heat exchanger.

It might be time to pull the plungers from the relief valves and see if they move freely in their bores and if the springs are the correct lengths. Note: you may only have one relief valve.
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