1987 Chevrolet 350 tbi high idle

[Bextreme04]

As far as I know on both the carb and TBI engines there are 2 oil ports, one by the filter and one by the distributor. Carbed engines used the one by the distributor, TBI used the one by the oil filter. Some engines used both.

So the way the fuel pump works is though a relay. Just like Andrew said the system is back up not permissive. As I understand it the principle is so if the engine stalls and the key is on the fuel pump is off. I believe the main reason is so if there were an accident and the driver is unable to turn the key off the pump does dump 20 gallons of fuel on the ground. If there were a problem in this system your engine would not run, if I were you I'd scratch it off the list.

This has been discussed a bunch of times. The oil pressure switch on the TBI's is only to keep the fuel pump going if the relay fails while the engine is running. This is why a relay failure will never cause the engine to die while driving. You always get to your destination, shut it off, and then get a no-start on the next attempt. A failed oil pressure switch will only cause the fuel pump to not run IF the relay is also bad.

Its also why running your oil low will still blow up your engine on a TBI motor... zero oil pressure will NOT shut the engine off(at least not until it starts shedding parts)

 

[iamtherealJayy]

I’m working on getting an oil pressure gauge working so I can visibly see I have good oil pressure at idle and higher in rpms. The truck has been running decent other than the idle being funky. Like the other day I put the truck in neutral to coast to a stop and the truck stayed around 2000 rpms until I was already taking off again(it did slowly drop but it never got below 1000)

 

[Bextreme04]

I’m working on getting an oil pressure gauge working so I can visibly see I have good oil pressure at idle and higher in rpms. The truck has been running decent other than the idle being funky. Like the other day I put the truck in neutral to coast to a stop and the truck stayed around 2000 rpms until I was already taking off again(it did slowly drop but it never got below 1000)

I'm still betting this is an intake manifold gasket leak or a vacuum leak somewhere else in the system. A cracked hose could cause these intermittent symptoms, but I think the intake manifold gasket is the likely culprit of your poor fuel mileage and intermittent high idle.

 

[SirRobyn0]

I'm still betting this is an intake manifold gasket leak or a vacuum leak somewhere else in the system. A cracked hose could cause these intermittent symptoms, but I think the intake manifold gasket is the likely culprit of your poor fuel mileage and intermittent high idle.

I could definitely see that. @iamtherealJayy Jacob, Let me relay a quick story to you. This was on my truck. I developed a oil use issue effecting one cylinder only. After eliminating a bunch of stuff I finally pulled the intake to find it had been sucking oil in from the valley. Prior to the oil usage and as long as I'd owned the truck I'd had some issues with idle quality that I had chocked up to old truck basically. Well let me tell you after doing the intake the thing idles as smooth as glass now. If I'd have realized that sooner I would have done the intake gasket years ago. I had done the spray trick looking for a vacuum leak and even smoke tested the intake at work and found no issues. My point is that intake gasket leaks can be tricky and sometimes they can hind. I've got some physical issues that make that sort of work difficult for me, in my case I needed to clear the carbon out of the EGR passage in the intake manifold as well. And replace the water pump. Took me about 6 hours. You really should consider doing the intake. You'll get to change the valve cover gaskets at the same time and really get a good view of the top of the inside of the engine.

 

[Vbb199]

Won't lie, I saw this was from last year, and I read maybe the first 2 pages.

Here's some basics, and forgive me if these have already been answered.

Was the 160° CTS ever addressed?
Coolant temperature readings have such a heavy sat in operation of the tbi system.
SO MUCH.

The ecm will stay in open loop almost forever without the proper PROGRAMMED coolant temp being matched physically with your thermostat.

Stock ecm set temp it wants is like ...195 i believe
The motor would probably be happier at 210 on a stock tune .

The hotter, the better on TBI systems.
They're built and designed to create hot exhaust Temps thru restrictions in the exhaust and high temp thermostats to make the egr system work correctly.
They even have an unheated, 1 wire o2 sensor to boot! If it never reaches operating temps, the o2 never heats properly.

Unless you actually bypass it all (via exhaust, egr delete, new tune with new parameters), running just a cooler thermostat will cause it to stay in the open loop.



How about the throttle shaft bushings?
99.9999999999% of the time TBI bushings being worn cause high idle.

It's an uncontrolled air leak when they wear enough to let air squeak by.

Anyone and everyone who's owned a tbi has had issued with slop in the throttle shafts. And if they don't, it's either had new bushings put in, or they don't realize it

I've had several, they all had slop.

The suburban that's been in the family all it's life that I own now....

High idle issues due to slop in the throttle shafts. I can physically push the shaft and linkage back and it'll slow the idle back down.

I have my own machined TBI for my suburban, and that was one of the first things I fixed. Sloppy throttle shaft bushings.

 

[iamtherealJayy]

Intake could be an issue(on both of my trucks) if I don’t get this 357 built soon I’ll replace the intake gasket. The thermostat is standard temp now I’m not sure about the linkage or any play, the truck has a little over 96000 miles hasnt rolled over yet, but it has been totaled before. And engine has been “rebuilt”? Not really but kinda sorta? The block was replaced I doubt much else got touched. Here’s some clips from earlier

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[iamtherealJayy]

I checked the linkage this morning and there’s no movement or change in idle when touching it. The truck started way faster than usual for first start of the day and it was idling weird. Bouncing from 650-1100 repeatedly.

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[SirRobyn0]

I checked the linkage this morning and there’s no movement or change in idle when touching it. The truck started way faster than usual for first start of the day and it was idling weird. Bouncing from 650-1100 repeatedly.

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Ok wait a sec. in the video it looks like the light on the tach is pulsing. Is it actually doing that or is it the camera? If so do all the lights do that? You might have a diode problem in the alternator, and if it's putting AC current into the line or even just pulsing the DC current that can sometimes cause confusion of the computer. Maybe fire it up at night and look at the lights if your not sure. It's kind of a left field thought, but I don't want it to go by without mentioning it.

 

[iamtherealJayy]

It’s just in the video as far as I can tell, I have no idea why that video only uploaded in 240p. Generally the only time my lights dim is turning cranking

 

[iamtherealJayy]

But do you have any ideas about the bubbling sound? I’ve never heard anything like it

 

[AuroraGirl]

I’ve actually done the timing on this truck. Unplugged the brown wire and set to 0°, I will check for the vacuum advance hose to make sure this truck does have the correct distributor since the block has been replaced I have no idea what has what. You know now that I think of it, do carburetor blocks have the oil pressure sensor by the oil filter? This is a later block with tbi heads up. The oil pressure switch is what kicks on the fuel pump correct? Or am I dreaming that up? I’m pretty sure I read somewhere if it doesn’t sense oil pressure it won’t power the fuel pump so you don’t destroy things? Maybe? But what around should the oil sensor be? I’ve changed the oil twice in this truck and I haven’t been able to find it. And I would have taken the cap off by now but this tbi stuff is a little more complex than my 78. So I’ve been trying to ask for help more than just diving into it. I’ve had the distributor out more than once in the 78. Truck does have a lot of problems some way more major than others, I’m actually looking for an 81-87 k20 parts truck to do a complete frame off build of this truck since it has frame damage. Oh and like sand through the hour glass, these are the days of our lives.

the oil pressure sensor is normally at the back of the intake for the gauge, but the qjet choke one I think you are talking about , that Im not sure

 

[AuroraGirl]

But do you have any ideas about the bubbling sound? I’ve never heard anything like it

coolant need purging?

 

[iamtherealJayy]

There’s some kind of sensor in the glove box. I have no idea what it is it’s kinda cone shaped it’s been in there since I got the truck. I haven’t been driving this truck due to slave cylinder problems once again lost all clutch pedal.

 

[BurbanPena]

I would Replace the temperature sensor on the intake manifold and the connector, they tend to both go bad This temp sensor communicates with your computer. Also when setting the timing there is a light brown wire with a black connector that needs to be disconnected it’s next to the brake booster then set timing to 0 then key off reconnect the connector

 

[iamtherealJayy]

Engine temp sensor has been replaced, it helped the idle go from no idling on 87 to being able to run on 87. When I first got the truck it would not idle at all unless I ran 93 I eventually decided it would probably be cheaper to just fix it than deal with it. Luckily it was when 93 was 2.50 a gallon now 87 is 3.42 lol. I haven’t messed with this truck much since my slave cylinder won’t keep pressure long enough to make it a comfortable drive.