Weird oil pressure gage issue......

[89GMCJOHN]

1989 oil pressure gage issue .... start the truck and the gage goes to 70 psi , new short block ,melling m55a , short gm oil filter (non truck) .....as I rev it the the oil pressure goes down to 45 psi ???? Is it possible the short oil filter is going into bypass mode ? I also put a new Bosch sending unit on it as the other one was shot .....any ideas ?

 

[89GMCJOHN]

Well I changed the filter tonight suspecting the filter was operating in bypass mode as the M55A hi pressure standard volume pump spun up ...I went from a Delco PF454 (short filter )at 3.2 Inch length to a Fram PH5 filter at a ( long) 5.172 length and no change.....as the revs go up the pressure goes down ..so I guess I will put the mechanical gage back on it and see if it tracks what "NEW" sending unit is sending to the dash gage ....uuuugh what a pain .....

Beyond that I am reading on the net that if your oil pickup depth is off in regards to the bottom of the pan it can do this ......

 

[L31MaxExpress]

Well I changed the filter tonight suspecting the filter was operating in bypass mode as the M55A hi pressure standard volume pump spun up ...I went from a Delco PF454 (short filter )at 3.2 Inch length to a Fram PH5 filter at a ( long) 5.172 length and no change.....as the revs go up the pressure goes down ..so I guess I will put the mechanical gage back on it and see if it tracks what new sending unit is sending to the dash gage ....uuuugh what a pain ..... Beyond that I am reading on the net that if your oil pickup depth is off in regards to the bottom of the pan it can do this ......

My Express van was doing that. Ended up being a combination of having the wrong windage tray for the pan in use and lifters pumping up from floating springs causing excess oiling to the rockers and upper end. Actually came close to spinning the number 1 rod bearing from it but luckily caught it before I damaged the crank.

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[89GMCJOHN]

Interesting L31.....was that a new rebuilt engine ? I am running a summit pan and the small baffle it had in it - not running a full or add on windage tray per se' and an M55A pump ....so it shouldnt be too close to the bottom of the pan ? I did notice when I got the new sending unit it had a threaded post on the end of it instead of a single spade terminal like my original but I didnt think it would make any difference.... I have read EURO gages senders run backwards of ours in the U.S. possibly ????? So I will hook the mechanical gage back up and check it again ...as I recall I didnt notice any pressure fluctuations like this with the mechanical gage before ......... head scratcher here for sure ....

 

[L31MaxExpress]

Interesting L31.....was that a new rebuilt engine ? I am running a summit pan and the small baffle it had in it - not running a full or add on windage tray per se' and an M55A pump ....so it shouldnt be too close to the bottom of the pan ? I did notice when I got the new sending unit it had a threaded post on the end of it instead of a single spade terminal like my original but I didnt think it would make any difference.... I will hook the mechanical gage back up and check it again ...as I recall I didnt notice any pressure fluctuations like this with the mechanical gage before ......... head scratcher here for sure ....

50K mile GM crate engine that I opened up the top end on to swap heads and swapped cams. While it was out I swapped in a GM 5qt truck/marine pan not realizing they used a longer windage tray on those L31s.

 

[89GMCJOHN]

Update reinstalled the mechanical oil pressure Gage and it's a steady 70 Psi (slightly edges up with the M55A hi pressure pump) when revving.... So something is DEFINITELY wrong with that new Bosch electrical sending unit I just bought to fix the old original broken one .....so good news !!!

 

[89GMCJOHN]

Ok so the final solution on my 1989 GMC Sierra 5.7 dash oil gage problem was another 1 wire oil pressure sending unit from Oreilly ....I used

BWD Part #: S4049 BWD 1 Terminal Oil Pressure Sender With Gauge ($42 yikes) ....the replacement sensor that DID NOT work on my 89 and made my gage

literally run backwards when revving it was a Bosch SP0F000011 Oil Pressure Sender I got off of Amazon on the cheap for less than $20 delivered ....I guess you

get what you pay for ........ All is well .