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Old 08-09-2018, 07:38 PM   #1
C101971
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Loss of oil pressure

Hello all. So here's the story, I drove to work in the morning all was great. As well as most of the rest of the time since I have owned my truck. But a few times she's been fussy and I am really starting to wonder what is causing this. So on really really really hot days like today, it was 38C, a bit too hot for me to want to drive my truck. 1971 GMC c15 with a 350. So it will start up fine, idol fine but after let's say 10-15 min she will bog real hard, I blamed it on vapour lock cause it's a mechanical fuel pump and it's 37 out. But since I got an oil pressure gauge I noticed the oil pressure will drop quite a bit once it bogs. Hard to say if there is a leak cause in regular temperature it will not leak but when it's this bloody hot it will leak a little oil, but not enough to cause concern. I checked the fuel lines and filter and no bubbling. If I let her sit in the shade for a while she will start back up fine. I've done road trips with her all day with no problem. If it happens and I'm go on the highway, within a few she will putter and kick back into happy mode and all will be fine.

Anyone have an explanation as to why the loss of oil pressure and bogging?
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Old 08-09-2018, 07:51 PM   #2
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Re: Loss of oil pressure

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Originally Posted by C101971 View Post
Hello all. So here's the story, I drove to work in the morning all was great. As well as most of the rest of the time since I have owned my truck. But a few times she's been fussy and I am really starting to wonder what is causing this. So on really really really hot days like today, it was 38C, a bit too hot for me to want to drive my truck. 1971 GMC c15 with a 350. So it will start up fine, idol fine but after let's say 10-15 min she will bog real hard, I blamed it on vapour lock cause it's a mechanical fuel pump and it's 37 out. But since I got an oil pressure gauge I noticed the oil pressure will drop quite a bit once it bogs. Hard to say if there is a leak cause in regular temperature it will not leak but when it's this bloody hot it will leak a little oil, but not enough to cause concern. I checked the fuel lines and filter and no bubbling. If I let her sit in the shade for a while she will start back up fine. I've done road trips with her all day with no problem. If it happens and I'm go on the highway, within a few she will putter and kick back into happy mode and all will be fine.

Anyone have an explanation as to why the loss of oil pressure and bogging?

It may be the fuel boiling in the float bowls. Do you have a spacer between the carb and intake? If not try one of these.

https://www.summitracing.com/parts/e...SABEgLudfD_BwE

Get the phenolic one. Probably available at your local parts store..... And make sure it is correct for whatever carb you are using.

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Last edited by LockDoc; 08-09-2018 at 07:58 PM.
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Old 08-09-2018, 07:57 PM   #3
C101971
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Re: Loss of oil pressure

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It may be the fuel boiling in the float bowls. Do you have a spacer between the carb and intake? If not try one of these.

https://www.summitracing.com/parts/e...SABEgLudfD_BwE

Get the phenolic one. Probably available at your local parts store.....

LockDoc
Would that cause a loss of oil pressure though?
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Old 08-09-2018, 08:00 PM   #4
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Re: Loss of oil pressure

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Would that cause a loss of oil pressure though?
I edited my last post....

The bogging is what causes the loss of oil pressure, the fuel boiling in the float bowls is what causes the bogging.

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Old 08-09-2018, 09:54 PM   #5
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Re: Loss of oil pressure

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I edited my last post....

The bogging is what causes the loss of oil pressure, the fuel boiling in the float bowls is what causes the bogging.

LockDoc
Alright, I will have to try one of those out and see if that fixes, if not, helps the issue. I have the Holley street demon carb, heard it was supposed to be really good for hot weather because of the composite core. Guess it can quite handle that hot of weather. It's not normal for it to be that hot around here. This summer has been over 30C almost every day.
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Old 08-09-2018, 10:21 PM   #6
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Re: Loss of oil pressure

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Alright, I will have to try one of those out and see if that fixes, if not, helps the issue. I have the Holley street demon carb, heard it was supposed to be really good for hot weather because of the composite core. Guess it can quite handle that hot of weather. It's not normal for it to be that hot around here. This summer has been over 30C almost every day.
Keep us updated on what happens....

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Old 08-10-2018, 07:42 AM   #7
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Re: Loss of oil pressure

If oil pressure still low when hot I'd guess cam bearings wearing out.
If it continues after idle problem is resolved.
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Old 08-11-2018, 01:44 AM   #8
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Re: Loss of oil pressure

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Keep us updated on what happens....

LockDoc
Just bought the spacer. She runs great on the highway but when I started her up after cooling off it idles very rough, and same story driving. Had to pull over 4 times on the way home. I found out a few other issues though, water pump is making a very bad sound and the lack of fuel I am starting to think is my pump is going bad. Or might have something restricting it. Also a spark issue appeared, I will replace and troubleshoot what is causing the new problems and hopefully that will fix everything. Also noticed my harmonic balancer is a bit wobbly. So that's one more thing to fix now. Nice knowing you pay cheque....
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Old 08-11-2018, 01:46 AM   #9
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Re: Loss of oil pressure

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If oil pressure still low when hot I'd guess cam bearings wearing out.
If it continues after idle problem is resolved.
Oil pressure is fine till it starts to bog. After sitting for a bit she will run fine but then if I'm not on the highway it runs like ****. It's not normal for it to be this hot around where I live so the trick is not used to this
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Old 08-11-2018, 02:05 AM   #10
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Re: Loss of oil pressure

I had similer problems. raw fuel was getting into the oil and lowering the oil pressure. quick check is when it bogs again and pressure drops grab the dipstick and give it a sniff. if it smells like fuel or looks really thin and runny. it might be a carb/fuel problem putting raw gas into the oil.
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Old 08-11-2018, 10:44 AM   #11
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Re: Loss of oil pressure

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I had similer problems. raw fuel was getting into the oil and lowering the oil pressure. quick check is when it bogs again and pressure drops grab the dipstick and give it a sniff. if it smells like fuel or looks really thin and runny. it might be a carb/fuel problem putting raw gas into the oil.
If this is indeed the case, I have been told by people on this site that you really do not want to be running the engine with this diluted oil. Other damage to your motor could result. If you have fuel getting into the oil, fix that problem then change the oil ASAP. Just sharing what folks have warned me about.
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Old 08-11-2018, 09:41 PM   #12
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Re: Loss of oil pressure

Generally its not a bad thing because the fuel evaporates when the engine warms up. and in colder climates in aircraft it was common practice to pump some fuel into the oil a moment befor shut down so that the oil was thin enough to flow later on cold starts.

But thats intended and controlled. A malfunctioning fuel pump or carb is a different matter.
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