jlhollowx13
Newbie
Posts: 8
Reg: 12-17-24
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12-17-24 11:58 AM - Post#2876330
Hi all, new member here looking for some help on, what I believe is an 89 year, sbc tbi 350. It is in an 80s land cruiser.
Recently my oil pressure started randomly dropping to zero. So I went in to replace the sensor and add some aftermarket guages as well, when I seemed to find that things are not hooked up how they should be. I have a port on the block on the filter side where my knock sensor was, but seems it's supposed to be on passenger side in a port in front of the starter. My problem is my headers are in the way of this port. I also found the oil pressure sensor feeding the guage is at the back of the block behind the distributor and I don't have a secondary one. It seems I should have 2, one for the low oil cutoff for the fuel pump and one for the guage.
So im looking for some guidance on what I should do here. I'm also wanting to add the aftermarket guages and keep my current guages working, so I'm wondering the best way to tap into the coolant and oil pressure. I appreciate any help.
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 Shepherd
Valued Contributor
Posts: 3656
Loc: Lake George, NY
Reg: 11-11-15
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12-17-24 06:47 PM - Post#2876335
In response to jlhollowx13
You can make up a "T" fitting at the port by the distributor, runs a lite and a guage, just did this in my shop. You can use one line for anything you want.
Edited by Shepherd on 12-17-24 06:48 PM. Reason for edit: No reason given.
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 someotherguy
Senior Moderator
Posts: 29835
Loc: Texas
Reg: 08-01-03
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12-17-24 08:01 PM - Post#2876337
In response to jlhollowx13
Hi all, new member here looking for some help on, what I believe is an 89 year, sbc tbi 350. It is in an 80s land cruiser.
Recently my oil pressure started randomly dropping to zero. So I went in to replace the sensor and add some aftermarket guages as well, when I seemed to find that things are not hooked up how they should be. I have a port on the block on the filter side where my knock sensor was, but seems it's supposed to be on passenger side in a port in front of the starter. My problem is my headers are in the way of this port. I also found the oil pressure sensor feeding the guage is at the back of the block behind the distributor and I don't have a secondary one. It seems I should have 2, one for the low oil cutoff for the fuel pump and one for the guage.
So im looking for some guidance on what I should do here. I'm also wanting to add the aftermarket guages and keep my current guages working, so I'm wondering the best way to tap into the coolant and oil pressure. I appreciate any help.
The early model TBI small blocks had separate oil pressure switch and sender - the switch at the top rear of the block behind the distributor isn't a cutoff switch, it's actually the opposite. It's redundant power for the fuel pump circuit that powers the pump once oil pressure gets a few psi above zero. The idea is that even if the fuel pump relay goes bad, you can still energize the circuit after a slightly longer cranking period builds oil pressure. (Which brings us to the troubleshooting tip of, if you experience long crank times before the engine will start, check your fuel pump relay.)
The sender for the oil pressure gauge would attach low on the block near the oil filter, which is where you can attach the sender for your aftermarket gauge.
(After the first couple years GM combined these functions into one sender/switch and it goes in the spot behind the distributor.)
The TBI small block knock sensor does usually live in the spot for the block drain on the passenger side of the block; I don't see why it couldn't occupy the same spot on the driver side if need be. Someone else might be able to give a reason why. There are some applications with a knock sensor on either side (Vortec big block for example) so I don't see a problem with using the driver instead of passenger side.
Coolant temperature sender for the gauge is the port between cylinders 1 and 3 on the driver side head. If you're only running aftermarket gauges since your application is an engine swap, I'd put the sender there in the head. The sensor at the front of the intake near the thermostat housing is the CTS which is the input for the ECM.
Richard
| 93 C3500 dually / 94 C1500 ecsb / 06 Silverado SS / 06 300C SRT8 |
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jlhollowx13
Newbie
Posts: 8
Reg: 12-17-24
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12-17-24 08:14 PM - Post#2876338
In response to someotherguy
Thanks for the info!
This is an old swap i purchased years ago. Just now realizing things are wrong.
The sensor behind the distributor is going to my stock toyota guage. The coolant sensor between 1 and 3 goes to my stock toyota temp sensor. I want to keep both of those as well as add more detailed guages.
I can't use the drivers side plug for the knock sensor either, headers are too close on that side as well.
So it seems i have no backup circuit for the fuel pump relay, as the only oil sensor I have is single wire and my guage goes to zero if that's unplugged. i don't see a plug for the dual sensor for that spot either, it's an Oval I believe as I have another, newer 350 that is somewhat similiar and has that sensor.
So that is my conundrum. I don't seem to have a suitable port for the knock sensor (without new exhaust) and i don't seem to have the fuel pump relay backup. I'm not really sure what to do here. I'm guessing I could continue to run without the fuel pump relay backup, but the knock sensor issue seems like a big can of worms.
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jlhollowx13
Newbie
Posts: 8
Reg: 12-17-24
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12-17-24 08:15 PM - Post#2876339
In response to Shepherd
Thanks, I may utilize a tee fitting for the oil pressure, depending on how things pan out with the rest of it.
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someotherguy
Senior Moderator
Posts: 29835
Loc: Texas
Reg: 08-01-03
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12-17-24 08:17 PM - Post#2876340
In response to jlhollowx13
Knock sensor sounds tricky then due to the headers which I'm guessing are specialized SBC swap headers for your application. Maybe someone else can chime in here for an alternate location.
On the oil pressure, you can get a combo unit starting around probably 90 or 91 through 95, get the pigtail for it, and wire it for the gauge and fuel pump circuit.
The SBC heads are the same side to side so if you need an additional coolant temperature location just throw your aftermarket sender in the other head.
Richard
| 93 C3500 dually / 94 C1500 ecsb / 06 Silverado SS / 06 300C SRT8 |
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jlhollowx13
Newbie
Posts: 8
Reg: 12-17-24
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12-18-24 02:48 PM - Post#2876355
In response to someotherguy
Thanks! Hopefully someone has some insight on the knock sensor.
Is there a trick to get the plug out of the passenger side for the coolant sensor? It's a square and i don't want to mess it up trying to get it out.
I've got a 94? Parts truck, I'll have to check if it has the dual sensor. How would I wire that into the system?
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someotherguy
Senior Moderator
Posts: 29835
Loc: Texas
Reg: 08-01-03
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12-19-24 02:33 AM - Post#2876359
In response to jlhollowx13
The '94 will absolutely have the combo sensor. However, you'll want to buy a new one from a quality brand, because the sensors are known for going bad with age/use and begin leaking at the connector, so I wouldn't trust a used one. If the harness on your parts truck is already in bad shape you could consider chopping the pigtail off so you don't have to buy one - but a new one is only about $20 so if that harness is in good shape you should keep it intact as it's worth more to someone that needs it.
Wiring it in could be done by following the factory service manual for those years of trucks. If you don't have access to one you can download PDF scans of them for free; there's a link in the GMT400 dot com forums in the "Engines" section, sticky'd to the top of that list of posts. The manuals would be pretty handy to have anyway as they could aid you in troubleshooting issues with your engine swap.
Getting an old plug out of the head can be a little difficult, if it's a square head you'll want to get a square socket to go after it with. If you have room to work without damaging nearby components, a torch used to apply heat might help break it loose. I could be wrong but I don't think penetrating oils will do any good since the threads are a water-tight seal; I guess it wouldn't hurt to try anyway.
Richard
| 93 C3500 dually / 94 C1500 ecsb / 06 Silverado SS / 06 300C SRT8 |
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bobb
Super Senior Member
Posts: 6906
Loc: paradise
Reg: 09-05-03
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12-19-24 07:21 AM - Post#2876362
In response to jlhollowx13
not sure of how your block is drilled. i use the front port for oil pressure cause its the lowest pressure reading. that square plug could be a problem. as noted heat will be your freind. i would weld a nut on it.
| 70 L camino, grampa engine, g-force 5 spd, road rage suspension. Pray first before all else fails. |
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jlhollowx13
Newbie
Posts: 8
Reg: 12-17-24
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12-19-24 07:45 AM - Post#2876363
In response to someotherguy
Thanks! Yeah I guess I don't really want to mutilate the parts truck like that. Makes sense to keep that harness intact and the sensor will likely be the most expensive part of doing it. I might as well byy the connector too. I hope I don't find too much more wrong with this swap. I've been driving it as is for years and wouldn't have known of these issues if the oil guage didn't start being weird. Id just like to get it right as I'm sure the knock sensor not being in the right place might have made it run incorrectly.
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jlhollowx13
Newbie
Posts: 8
Reg: 12-17-24
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12-19-24 07:50 AM - Post#2876364
In response to bobb
Which front port is oil pressure on yours? I think the only 2 I have are by the distributor and by the oil filter. The square plug i mentioned is for the coolant and seems like the only place i can use for that at the moment, its accessible, but might be a pain removing it. My intake also has various plugs, mainly 4 around the throttle body, that I'm not sure of, but they are all very tight and I don't want to try and open them just to see what's there and potentially compromise things.
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someotherguy
Senior Moderator
Posts: 29835
Loc: Texas
Reg: 08-01-03
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12-19-24 08:11 AM - Post#2876367
In response to jlhollowx13
Which front port is oil pressure on yours? I think the only 2 I have are by the distributor and by the oil filter. The square plug i mentioned is for the coolant and seems like the only place i can use for that at the moment, its accessible, but might be a pain removing it. My intake also has various plugs, mainly 4 around the throttle body, that I'm not sure of, but they are all very tight and I don't want to try and open them just to see what's there and potentially compromise things.
I think those 3 apparent ports on the TBI intake are just casting plugs, the large allen socket plugs around the TBI itself. The square plug on the tall rear runner is a vacuum port not used in most applications.
About the only ones used in most setups are the coolant temperature sensor at the very front next to the thermostat housing, heater core connector at the rear, and of course the vacuum fitting off the side of the TBI mount for the brake booster.
Richard
| 93 C3500 dually / 94 C1500 ecsb / 06 Silverado SS / 06 300C SRT8 |
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jlhollowx13
Newbie
Posts: 8
Reg: 12-17-24
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12-19-24 02:13 PM - Post#2876378
In response to someotherguy
Thanks! I was looking again and want to see if you can help me confirm something.
The oil port near the oil filter, how close is it to the filter? I was looking again, it's very tight in there, but it seems like I have a plug DIRECTLY above the oil filter, pretty much on the filter housing. It's hard to see in there to be sure, but I think I can see an Allen plug. Then, I have the hole where my knock sensor was, it's catty corner from my filter, exactly between the rear 2 cylinders about a half inch to inch below the line of spark plugs. Is this common? I'm wondering if the plugged one above my filter is an oil port, and maybe they drilled and tapped the one where my knock sensor was to relocate that sensor? It's on a little flat half round spot and that spot is on all corners of the engine between the cylinders, but isn't tapped anywhere else. I need to figure out adding photos here but hopefully the explanation makes sense.
While probably not ideal, I'm thinking I'll need to keep my knock sensor in place, if the other port is an oil port I can tap in there for the aftermarket guage and make an attempt at the coolant port on the passenger side.
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someotherguy
Senior Moderator
Posts: 29835
Loc: Texas
Reg: 08-01-03
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12-19-24 02:28 PM - Post#2876379
In response to jlhollowx13
Yes, the plug just above the filter is for the oil pressure sending unit (the old location before they combined them as I described.) You can see one of the combo style units screwed straight in without an elbow, at that location on the engine in this picture.
In this pic borrowed from thirdgen.org, I wonder if the indicated "probably best option" the poster is asking about, is where your knock sensor is currently located? If so, that's probably fine. That should be the spot for the old mechanical clutch linkage. Don't do what the poster was asking though and drill through it.
Richard
| 93 C3500 dually / 94 C1500 ecsb / 06 Silverado SS / 06 300C SRT8 |
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jlhollowx13
Newbie
Posts: 8
Reg: 12-17-24
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12-19-24 03:10 PM - Post#2876380
In response to someotherguy
Awesome thank you!!! Yes, that one they mention as tapping is where my knock sensor is currently, so i guess that's what they decided to do. Makes me feel a bit better about the swap. I did buy it from a well known retired Land Cruiser mechanic/customization shop owner and it was one of his mechanics previous to that, so i assumed they did things 'right'. The normal spot in front of the starter and potentially other option pointed out in the photo on the other side were not options, so looks like they went with that location. Doesn't seem like they drilled through either, nothing comes out when the knock sensor is removed.
Thanks so much for the help! I think I'll leave the knock sensor in that location, tap into the port directly above the filter for aftermarket oil pressure and tap into the coolant between the exhaust. I'll have to look into the combo setup some more, but can at least confidently drive it once again once the guages are sorted. My fingers are crossed that my fluctuating oil pressure is in fact the sensor and not the pump, and finally can get the original issue i started with dealt with!
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bobb
Super Senior Member
Posts: 6906
Loc: paradise
Reg: 09-05-03
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12-21-24 09:56 AM - Post#2876433
In response to jlhollowx13
above the timing chain cover 1/8 pipe.
| 70 L camino, grampa engine, g-force 5 spd, road rage suspension. Pray first before all else fails. |
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