Frustrated 87 GMC owner!

[OldBlueDually]

Ohhhhh....now I see why you are toying with an engine swap!

Well, GM just introduced that beautiful 632 with 1004hp on pump gas

 

[75gmck25]

The wiring for the ignition switch does not run all the way up the column to the key. All the wires are connected to a wide flat connector lower down on the column, about where it goes through the dash, and a rod and rack attached to the key moves the electrical switch that is plugged into the flat connector. If something under the dash (brake pedal?) is moving those wires the wrong way it could cause a complete loss of power, just like turning off the key.

Its also common for the rack on the end of the ignition switch (the rack pulls the rod up and down) to fracture and/or break completely. This throws off the switch positions and can cause a complete power loss.

Since your voltage dropped from 12 to 8 when you stepped on the brake pedal, I would guess that it made some part of the brake circuit short to ground. There may have been enough resistance in the brake lights to prevent a dead short that would blow the fuse, but something was definitely grounding out.

 

[AyWoSch Motors]

@ animal​

@ Dave M​

@ SirRobyn0​

@ AyWoSch Motors

@ 1973c10

I hope you all get a chance to read over the novel I wrote with the issue at hand Above. Wanted to be sure to tell you all that has been done in two weeks fixing one problem to now have this one come up. Any thoughts would be appreciated and I will go lookin' at them one by one. Seems electrical to me since I have lost power now when it died even to the radio. But not sure where to look.
WHen I hit the brake pedal the volt gauge goes down to 8. When I let off it rises slowly to about 10. Then it will go up NEAR but not completely to 13 but I have checked it and the battery under load etc. I wonder if there is an ignition wire under the dash the pedal hits or something? When it died at first I had power still to radio but then even that died while it was still in run position. Also when I hit the brake pedal in the drive when I got it home I noticed the stereo lost power briefly. Is there a single wire that will kill the engine under the dash? I am the worst with electrical on a vehicle.

Thanks guys and Thanks for the Welcome!

Took me a bit to read threw all that, haha, but I have a few thoughts.
Couple from experience, couple just basic ideas.

First of all, I'm gonna say you have electrical gremlins. What in particular, I dont know, but id start with looking at the wiring above your pedals. Look around the brake pedal arm, the gas pedal, clutch of it has one (but I'm assuming it's an auto), look around the steering shaft. I'd guess something somewhere is rubbing and shorting out.

Might be something stupid simple like you have a loose battery cables or a bad connection at the battery or the computer.
Maybe faulty injectors or bad wiring there abouts.

As far as the cutting out and popping and spuddering, I've had 3 different things happen to me that sound similar:
1.) I was working on a friends 87 k20, and its was driving me crazy, it would run great one day, and not the next, die randomly. I threw parts at it trying to fix it, ended up being the fuel pump. I even tested it, and it tested good, but it was the pump. Could be you need a new one.
2.) I took my 92 suburban on a roadtrip the other day (early 350 tbi). Drove all day with no issues, until I got back to town. Was cutting in and out like it was out of gas but it wasnt. Turns out it was the fuel line the goes into the back of the TBI. It was so loose, it was about a quarter turn from falling out completely. Was loosing about a shot glass of gas ever couple seconds. Could be that fitting is loose, and it's not getting sufficient gas, or maybe the line down the way has a loose fitting or pinhole leak.
3.) A while back my 93 yukon started doing that thing where it would rev super high, and if you shut it off and turn it back on it would sometimes rev super low. That was the throttle position sensor.
That's just things I have experience with.

I'd say dont sell it, it can be a good truck with some more tinkering, and dont bother with a new engine, that one is still healthy.
If it were me personally I would totally swap all that computer crap out for a carb, but thats just me. Might not be the best thing for you. I'd try to fix it first, then see about changing things. But if you do carb swap it, and simplify it, I think it can be done alot cheaper than what your proposing. A junkyard 4bbl dual plane, Rochester, and a HEi, you could be in and out in and hour for 50$.

Just a few thoughts of mine. I'm no expert, but I've worked on chevys my whole life.

 

[mtbadbob]

Maybe the ECM is bad. That could cause both the electrical & fuel issues

 

[AyWoSch Motors]

Maybe the ECM is bad. That could cause both the electrical & fuel issues

I was thinking that too, but didn't mention it. Possible, especially the first gen ECMs.

 

[Dave M]

@ animal​

@ Dave M​

@ SirRobyn0​

@ AyWoSch Motors

@ 1973c10

I hope you all get a chance to read over the novel I wrote with the issue at hand Above. Wanted to be sure to tell you all that has been done in two weeks fixing one problem to now have this one come up. Any thoughts would be appreciated and I will go lookin' at them one by one. Seems electrical to me since I have lost power now when it died even to the radio. But not sure where to look.
WHen I hit the brake pedal the volt gauge goes down to 8. When I let off it rises slowly to about 10. Then it will go up NEAR but not completely to 13 but I have checked it and the battery under load etc. I wonder if there is an ignition wire under the dash the pedal hits or something? When it died at first I had power still to radio but then even that died while it was still in run position. Also when I hit the brake pedal in the drive when I got it home I noticed the stereo lost power briefly. Is there a single wire that will kill the engine under the dash? I am the worst with electrical on a vehicle.

Thanks guys and Thanks for the Welcome!

Not too familiar with the later trucks, but does yours have the fusible links in the wiring loom. These things can break down internally and you can't see it.
Also when you tagged others in , it didn't work. (Don't know much about that one)

 

[eric 87]

The wiring for the ignition switch does not run all the way up the column to the key. All the wires are connected to a wide flat connector lower down on the column, about where it goes through the dash, and a rod and rack attached to the key moves the electrical switch that is plugged into the flat connector. If something under the dash (brake pedal?) is moving those wires the wrong way it could cause a complete loss of power, just like turning off the key.

Its also common for the rack on the end of the ignition switch (the rack pulls the rod up and down) to fracture and/or break completely. This throws off the switch positions and can cause a complete power loss.

Since your voltage dropped from 12 to 8 when you stepped on the brake pedal, I would guess that it made some part of the brake circuit short to ground. There may have been enough resistance in the brake lights to prevent a dead short that would blow the fuse, but something was definitely grounding out.

Learned something. It is super cold this morning but I am going out in the garage and going to search high and low for wiring issues. If nothing else I will at least see if there is a wiring nest. There are two toggle switches on the bottom of the dash the previous owner prior to the flipper guy that didn't even know they were there....they do nothing as far as I know but have to trace them to find out where they go. Thanks though for that lesson on ignition.

Not too familiar with the later trucks, but does yours have the fusible links in the wiring loom. These things can break down internally and you can't see it.
Also when you tagged others in , it didn't work. (Don't know much about that one)

I am not super sure about fusable links either. I mean I know what they are and had a vehicle before with them. THERE is what appears to be a big monster blue wire that has a clear (fuse) inline about a foot from the battery. It runs along the inner fender to the passenger side and into the compartment but I lose the trace. Not sure where it goes to. BIG thick wire though. Man when I figure this stuff out I will have one huge post to update.

Maybe the ECM is bad. That could cause both the electrical & fuel issues

ECM was replaced. Not that the new couldn't be bad. If anything I wonder if all the work done by myself or the shop has left something loose.
Odd thing I lost power. However upon coasting to stop putting in park it cranked. And as I mentioned you aren't supposed to pump a THROTTLE body but it would not start until I did. It should have not lost pressure. But the shop did rebuild the throttle body and regulator. New diaphragm etc. When I pumped it I had power obviously and it started. So it makes me think it is a short of something is going out. If a coil goes out (factory type and may be original) wouldn't it act up and not start back when hot? But it was not hot yet as I was 3 blocks from home. Maybe ECM plug is loose? I will post results but any other thoughts you have are welcome.

Took me a bit to read threw all that, haha, but I have a few thoughts.
Couple from experience, couple just basic ideas.

First of all, I'm gonna say you have electrical gremlins. What in particular, I dont know, but id start with looking at the wiring above your pedals. Look around the brake pedal arm, the gas pedal, clutch of it has one (but I'm assuming it's an auto), look around the steering shaft. I'd guess something somewhere is rubbing and shorting out.

Might be something stupid simple like you have a loose battery cables or a bad connection at the battery or the computer.
Maybe faulty injectors or bad wiring there abouts.

As far as the cutting out and popping and spuddering, I've had 3 different things happen to me that sound similar:
1.) I was working on a friends 87 k20, and its was driving me crazy, it would run great one day, and not the next, die randomly. I threw parts at it trying to fix it, ended up being the fuel pump. I even tested it, and it tested good, but it was the pump. Could be you need a new one.
2.) I took my 92 suburban on a roadtrip the other day (early 350 tbi). Drove all day with no issues, until I got back to town. Was cutting in and out like it was out of gas but it wasnt. Turns out it was the fuel line the goes into the back of the TBI. It was so loose, it was about a quarter turn from falling out completely. Was loosing about a shot glass of gas ever couple seconds. Could be that fitting is loose, and it's not getting sufficient gas, or maybe the line down the way has a loose fitting or pinhole leak.
3.) A while back my 93 yukon started doing that thing where it would rev super high, and if you shut it off and turn it back on it would sometimes rev super low. That was the throttle position sensor.
That's just things I have experience with.

I'd say dont sell it, it can be a good truck with some more tinkering, and dont bother with a new engine, that one is still healthy.
If it were me personally I would totally swap all that computer crap out for a carb, but thats just me. Might not be the best thing for you. I'd try to fix it first, then see about changing things. But if you do carb swap it, and simplify it, I think it can be done alot cheaper than what your proposing. A junkyard 4bbl dual plane, Rochester, and a HEi, you could be in and out in and hour for 50$.

Just a few thoughts of mine. I'm no expert, but I've worked on chevys my whole life.

The cutting out and popping was fixed finally. A new computer rebuilt throttle body and regulator Distributor pickup coil was the problem it was shot. In the process I replaced the TPS MAP TEMP and IAC. Also I did volt and signal tests TPS vacuum test on map and ohm on temp and an actual test of temp being sent to make sure it was accurate. Wiring checked as best I could going back to computer.
So now the only bad is the shutting off AND the odd rpms. Idle low, so low you think the engine quit it is so quiet with magnaflows even. Then if you shut it off even when warmed it will idle at 2500 or more. You can drive 25 mph without hitting the gas. Sometimes if you shut it down and restart it will go back to low idle. Sometimes not but will eventually settle down. This issue can be a bad new IAC I suppose. Maybe the new TPS went bad real quick. OR maybe shop didn't tighten something when rebuilding the throttle body?
Those things I can go back and double check. But this shutting down and losing power is unnerving. So much that I got off phone with shop and told him I may chuck all the parts I just replaced including the computer. Go to carb. Even drop tanks and convert them and run a single pump and possibly do a new wiring harness as well for a non computer 86. I think Painless has one for 693.
My junk yards in the area do not have older cars or trucks. Most are 2000 and newer. SO I will be buying all new. I may ask or look for threads on suggestions in parts for this from All the helpful folks.
Today though I would like to find this power loss.

Guys once again...Thanks. I appreciate anything you can think of. Wife said she wants to hold off buying new car tomorrow because she likes this truck as well and would rather buy a 6200$ engine and maybe another trans and have a drivetrain for about 10k instead of 25-30k car. Which was the plan for Saturday.
I will check back during the day and see if there are any other ideas that came to mind.
Thanks

If I stumble across it though I will let you know so it is in the knowledge banks.
For the record....it came with those wheels but I have them in the garage and bought a set of factory rallies with new coopers on it. I will say the coopers lose traction easy in the rain taking off. Remember it is only a 305 lol

 

[idahovette]

@eric 87 Can't help with anything,but I gotta say that your wife is a KEEPER!!!!!

 

[eric 87]

@eric 87 Can't help with anything,but I gotta say that your wife is a KEEPER!!!!!

Shhhh....I know....HEY anyone that reads this will have a chuckle. One day she is walking through the grocery parking lot. Notices she is being followed. When close enough to the store front she couldn't take it any longer and turned around. Man says, "Hey will you sell it to me"?
Beg your pardon?
The TRUCK. Would you sell me your truck. Whew.
Another occasion she is pumping gas and a man two fuel islands over yells, "HEY NICE SQUARE BODY".
At first she was a little annoyed because she didn't know the term and she has lost a ton of weight and is now back to her college freshman weight!
Why is he call me a square body. After leaving the gas station she figured it out before calling me. He was talking about the truck!

 

[eric 87]

Mild update. I took it for a spin today. Probably 5 miles and it never died. Have been driving it today with the air cleaner off since I was looking at all plugs to sensors etc. BUT the idle is up and down. It will idle fine for a bit even after warm up. Then on it's own it jumps up to about 1800. Then wait a little and it goes back down.
I shut it off and started it. Had to crank longer than when it was cold before it started which it never does. Finally when it started it idled at the 1800 again. Shutting it off sometimes will change it if I restart but this time it didn't. Took it for a 5 mile jaunt and when I am accelerating fairly hard from a roll just before a kickdown level it really does a good job as if it got a new life. But mid accel it has a hard...not sure what to call it, stall or something. I normally call it a cough....then it continues pulling fine. It did this every time I did the hard accel. If I had to explain I would say a distance of 1/8 mile it coughed at 1/16 of a mile. Not right away but mid track if I were racing.
I must have a short in my brake circuit because I hit brake pedal and it still kills the volt gauge. It is volt and not amp isn't it? Anyway it drops to 8 then comes back up to about 12. Today it was at 13 often though. I suppose the fix to that is not to hit the brakes?
Wife and I decided. I think if I put a carb on it then it will run more consistent. That means swapping intake carb and distributor and fuel delivery. But in the event the 305 lets go down the road I believe all those parts will work on a 350 long block. She has said if we do this then lets not buy a new car as she loves the truck.

PS anyone want to buy these ugly wheels on the cheap with the spacers included lol. Taking space in the garage.

 

[gmbellew]

IAC controls idle speed. if idle is bad I would start there if you are sure you don't have any vaccuum leaks.

 

[SirRobyn0]

@eric 87 My only concern. If this is an electrical short of some kind it maybe possible that you convert to carb and still have issues, if the short is bad enough to drop the voltage low enough to kill the spark. I would put on the brake pedal to hold it on (maybe your wife so she can look at the gauges to) and then get under dash and start wiggling stuff. Then do under the hood and the bulk head connector. I know your pretty well set on doing the carb conversion, as you said to me in another thread, and that's fine, I just don't want you to still be chasing an electrical issue.

 

[eric 87]

@eric 87 My only concern. If this is an electrical short of some kind it maybe possible that you convert to carb and still have issues, if the short is bad enough to drop the voltage low enough to kill the spark. I would put on the brake pedal to hold it on (maybe your wife so she can look at the gauges to) and then get under dash and start wiggling stuff. Then do under the hood and the bulk head connector. I know your pretty well set on doing the carb conversion, as you said to me in another thread, and that's fine, I just don't want you to still be chasing an electrical issue.

Oh this electrical issue is first to get solved. Will not do anything until then.

Not so much set on it if this would run right which it is running better when not shutting down. No this is more prophylactic in nature. I figure if I bite the bullet now and do all these conversions I can be done with it. I want as my wife does to be able to go to the garage , start up and know that I am going to point A & B and back. Sure everything breaks down. But when a truck ages and the wiring ages then any single sensor can throw a monkey wrench into how a TBI system runs. Sure I suppose if I want to do some other types work I could invest in a new wiring harness for the truck and then go from there. Then all might be better. Often wiring is overlooked and pieced together over years on rebuilds and restorations.
I can say I have learned TONS on TBI during this process. I have been pricing items and so far I am still under 1000$ It has been awhile since I have done some of this stuff so I am sure I will be forgetting things. I also go down rabbit holes when researching and reading such as to use a port or manifold pressure for vacuum advance and that whole debate.
Either way this thing is getting tiring. This part of car ownership but I prefer the fun part of adding to make it better and nice routine maintenance. Chasing problems not so much.
Wife just left in old Ford farm truck over the Sierra because of no confidence. She is even fine if I spend the money on a GM performance fully dressed with accessories and 330hp if it will make it a new truck. She said 6200 + core is better than 25k for another car. And she likes the truck. part of me says go that way since I have the funds and it would make it new. I might add another grand for a new th400. Why the 400 because I may one day pull a canned ham size trailer if you know those. Think Vintage RV/ camper. And around town and local interstate a 400 is fine with the 2.73 gears I think it has.

As far as the electrical short. You are correct I am going to do some looking and rooting today. And if not it will be in shop Tuesday to see if they can find. I will not do any conversion just yet until I see if that can be remedied. It may run and be fine.

Let me be the first to start another debate....I could save my dimes and wait for GM to produce a crate EV motor to drop in. THen like people that put airbags under I could mod my bed to put the batteries under the bed and have a shallower bed!
Already thinking ahead. Kinda like stealth speed! Ford intro'd a crate E motor at Sema I am told and sold out of their inventory.

 

[eric 87]

IAC controls idle speed. if idle is bad I would start there if you are sure you don't have any vaccuum leaks.

Well that is the suggestion or was from the mechanic as well. But we have since replaced it twice and it still is not doing well. Which is why I am considering the conversion. Just so many problems if you have just one wire or signal bad in the chain for the computer to make adjustments to it and it doesn't run right. I would prefer not to make the conversion but when one of these goes bad like this and it isn't traceable then it is either convert or put in a whole new wiring harness to cover your bases. This part gun cost is going up each day. And replacing sensors multiple times because they are "often bad" off the shelf is no good either.

No only that but I have learned how to test each sensor and back pin things as well to get readings of volts and or ohms. If nothing else I have gained that knowledge.

 

[squarelyfe]

This decal could be a cheap temporary fix.. they would think she's les-bo and when you drive it would put the reverse feelings on her since other women will look at you

Have you tried pulling the plastic bulkhead connections to the panel and cleaning the contacts? I had an issue with 46 year old dielectric grease hardening up and completely covering the contact and going against what it was supposed to do once I cleaned it it made a world of difference. While you're under there unscrew the panel and look at the backside for any wires that may have melted and crossed.
I'd also look around in general under the dash.