Lacking power, 87 Sierra 5.7l TBI

[Pmcb]

I need some help, let me give you background:


I have a 1987 GMC Sierra v1500, 5.7l TBI. 51,000 original miles. No rust, had accident when new, some signs of it underneath. Only one fuel tank working. It was in a barn for 10 years, someone started getting it running, change hoses, tires, belts, dash…. In the process they cracked intake manifold (over tighten cold temp. Tried to fix with stop leak. When i purchased starting was difficult, but ran good warm, missing some.

Things done: New manifold, replaced CTS, TPS, EGR, Plugs, Wires, Rotor and Cap. Fuel filter, Added TBI spacer, new accelerator cable, Oil, filter…

Each of those fixes were a story in themselves. Had it die a couple times on the road, changed Ignition module seemed good, but then would not start, (engine would start with a little gas) spark good. Bypassed relay, pump working. Noid light, no power to injectors. Replaced fuse, but only one injector working. Replaced injector all good again (i assume injector went bad blew fuse) Driving a few days later and water pump goes, but at same time while driving home slowly and not overheating it died twice. New water pump, but died out again, Changed out new ignition module and it worked fine (note to self, no cheap parts).

Now she runs smooth, water stays inside engine where it belongs, but it doesn’t seem to have power, running smooth, no misses, but just lacking power, noticeably.

My thoughts: Fuel pump weak? Coil?
Has good compression.

 

[squaredeal91]

Is it extra quiet out the pipe? Maybe clogged exhaust? It was sitting in a barn. Are both injectors working properly? Unplug them one at a time to see if engine can stay alive or not. Those are my first thoughts.

 

[Pmcb]

Exhaust good, glass packs. I will try the injectors one at a time though.
Thanks

 

[Redfish]

I would be curious about the exhaust, to add to @squaredeal91 said, is the catalytic convertor still on the truck? Restricted/plugged exhaust is a possibility after all these years.

I would want to know about the timing. Get a timing light, unplug the magic wire (its under the brake booster) and check the timing.

Also the O2 sensor and the MAP (manifold absolute pressure) sensor are worth looking at.

 

[Redfish]

I would think that if you have the breather cover off and the engine running you can see if the injectors are both pulsing.

 

[Pmcb]

No Catalytic converter. I was thinking about squaredeal91”s suggestion, I did have it running with one injector when I was tracking down that issue. Both are pulsing now. I will look into the O2 sensor. Is there one on each side? Timing is another possibility. The wire under the brake booster you mentioned, does that plug into the distributor?

 

[Redfish]

No Catalytic converter. I was thinking about squaredeal91”s suggestion, I did have it running with one injector when I was tracking down that issue. Both are pulsing now. I will look into the O2 sensor. Is there one on each side? Timing is another possibility. The wire under the brake booster you mentioned, does that plug into the distributor?

The Magic Wire has to be unplugged on the TBI trucks to eliminate the spark advance and allow you to check the timing. It is a single wire, the plug is just to the side of the steering column and beneath the brake booster. Unplug it with the engine off. Check/set the timing with the engine off. Then kill the engine and plug it back in. The timing will advance with the wire connected.

There is only one O2 sensor and it is on the driver's side fairly close to the oil filter. It has a wire going to it, it is easy to find.


The Magic Wire:

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[Pmcb]

Can an O2 sensor be tested or just replaced? One other thing I forgot to mention. The truck has an aftermarket Tachometer that plugs into coil, when the lack of power seemed to start the Tach began to jump Around a lot and still does. Even though the engine is not acting up the Tach seems too.

 

[gmbellew]

Can an O2 sensor be tested or just replaced? One other thing I forgot to mention. The truck has an aftermarket Tachometer that plugs into coil, when the lack of power seemed to start the Tach began to jump Around a lot and still does. Even though the engine is not acting up the Tach seems too.

if the tach is jumping around even though the engine seems to be at constant RPM, there could be extra references from a poor distributor (ICM) ground or interference from the plug wires being too close to distributor/coil wires.

 

[squaredeal91]

Can an O2 sensor be tested or just replaced? One other thing I forgot to mention. The truck has an aftermarket Tachometer that plugs into coil, when the lack of power seemed to start the Tach began to jump Around a lot and still does. Even though the engine is not acting up the Tach seems too.

I believe for o2 you unplug with engine off then start it and drive it. Then do the same with it plugged in and see how it acts.

 

[squaredeal91]

What about bad knock sensor retarding timing? Is that possible?

 

[Pmcb]

I am working on replacing O2 sensor but it is rusted in good and hard to get leverage on wrench. But that will be the first change. I made sure that plug wires were not crossed or touching coil wires. Checked all electrical connections.

 

[GTX63]

I need some help, let me give you background:


I have a 1987 GMC Sierra v1500, 5.7l TBI. 51,000 original miles. No rust, had accident when new, some signs of it underneath. Only one fuel tank working. It was in a barn for 10 years, someone started getting it running, change hoses, tires, belts, dash…. In the process they cracked intake manifold (over tighten cold temp. Tried to fix with stop leak. When i purchased starting was difficult, but ran good warm, missing some.

Things done: New manifold, replaced CTS, TPS, EGR, Plugs, Wires, Rotor and Cap. Fuel filter, Added TBI spacer, new accelerator cable, Oil, filter…

Each of those fixes were a story in themselves. Had it die a couple times on the road, changed Ignition module seemed good, but then would not start, (engine would start with a little gas) spark good. Bypassed relay, pump working. Noid light, no power to injectors. Replaced fuse, but only one injector working. Replaced injector all good again (i assume injector went bad blew fuse) Driving a few days later and water pump goes, but at same time while driving home slowly and not overheating it died twice. New water pump, but died out again, Changed out new ignition module and it worked fine (note to self, no cheap parts).

Now she runs smooth, water stays inside engine where it belongs, but it doesn’t seem to have power, running smooth, no misses, but just lacking power, noticeably.

My thoughts: Fuel pump weak? Coil?
Has good compression.

Just trying to keep from jumping too far ahead. Going back to the beginning, you said it doesn't seem to have power. Compared to what? When you first drove it? Other vehicles you have owned?

 

[Redfish]

What about bad knock sensor retarding timing? Is that possible?

That is absolutely a possibility. Mine was doing that and the difference was significant. Glad you remembered that component, I changed/replaced so much on my '87 that I seem to have forgotten half of it!

 

[Pmcb]

Sorry for not clarifying, it seems to have less power than it did just two weeks ago. I notice it going up hills the most.