1987 Chevrolet 350 tbi high idle

[gmbellew]

if possible, get readings pushing the accelerator pedal. that is how it is driven...

 

[iamtherealJayy]

Does it have to be pedal? Or can I go to the throttle linkage and move that? I could video the multimeter if I gotta do the pedal or I can just watch it if I can do throttle linkage

 

[gmbellew]

pedal is how you drive it, so that is how I would do the full range. I think anything over 4 or 4.5v the ECM sees as WOT....

you can do both pedal and linkage nd compare results, I guess. you will probably have a little more range with the linkage I bet.

 

[iamtherealJayy]

I will video the multimeter and do a pedal press from nothing to full throttle. Also the throttle cable should be perfectly fine, if I disconnect it and let go of the end of the cable the weight of the pedal on the other end pulls the cable.

 

[iamtherealJayy]

Alright so after getting my slave cylinder fixed, again…, I was leaving somewhere yesterday the truck fired up just fine but after a second of idling it did the exhaust bubbling sound and since it was on a hill I just rolled back in neutral and started to turn but the rpms dropped way off I lost power steering then the truck jumped way up in rpms(1800ish) and never stalled but had a huge dip in speed. I’m thinking I may replace the throttle cable since I did on the k10 and it moves almost too easy in my opinion but I’m now noticing how stiff this trucks pedal is. It’s hard at first like a stiff spot then it’s easy(sucks driving in lower gears because the truck jumps and takes off to get the peda to move)

 

[SirRobyn0]

@iamtherealJayy Jacob, First off I have to preface this by saying I did not read though all the pages. But the GM TBI systems are pretty darn reliable but they can be a little tricky to diagnose. This test that @gmbellew, Glen, is asking you to run with a multi-meter is a great thing to do. But what your doing with the multi-meter you could do 100X easier with a scanner, plus a lot of other stuff. If you intended to keep the truck investing in a cheap scanner that will read the old ALDL data stream is priceless. For me I have an old dedicated handheld scanner one I bought back in the day, but if I didn't have that already I'd probably get this.

EBAY ALDL LAPTOP SCANNER

So you'd need a laptop obviously, most folks already have one, and you'd just boot the laptop plug it in, look at the data stream and push the gas petal. You'd also be able to look at the steps the idle air control valve is at the coolant temp sensor readings, spark advance, fuel trim, the list goes on and on. You'd be able to take screen shots on the laptop and upload them for us to look at and potentially catch an off reading. In time you'd learn what to look for on your own. It would just make your life a little easier, but conversely a lot can be tested one by one with a multimeter on the TBI systems.

If this hasn't been mentioned, if the truck has an EGR valve still it would be worth while to remove it and check for leakage.

If you haven't cleaned the throttle body you should. With the engine off open up the throttle spray some cleaner in and scrub the throttle plate and bore with a tooth brush. Remove the idle air control valve and clean the seat inside and the valve, or replace the valve if it's really nasty.

Have you checked the spray pattern of the fuel injectors? A poor spray pattern can result in a poor idle quality.

 

[iamtherealJayy]

Alright let me catch you up a little, I’ve got an aldl cable and a few programs but could not get the laptop to connect with the truck. I’m unfamiliar with it and couldn’t find out how to get it to work, I’ve replaced the egr and redone gaskets on the throttle body. It sprays fine and is clean as I could get it. The iac is new as well as engine coolant temp sensor map and I have a throttle position sensor. My only problems I’m having currently is the idle staying up a lot of the time, say I pull the truck out of gear rolling to a stop from 2200 rpms, it’s usually still over 1000 by the time I take off again. The idle drops very slowly. I did wd40 the throttle cable the other day and the linkage and return spring area hoping that would make the pedal a little easier to press. The pedal moves easily after a certain point. It’s stiff in the beginning then revs way up which makes it hard to take off from a stop occasionally. I also just got fuel for the first time since may 27, if that shows how much I drive this truck. I got from 95928 to 96081 miles. 153 miles in almost 14 gallons came to just under 11 mpg. Last time I’d refilled I’d gotten just over 13. So something has changed in the last month. The truck does have a weird popping noise if you start if after it’s warm, more often if I shut it off run in to pay for fuel and run back out and let it idle down(usually never sees below 1000) and idle for a minute. As someone suggested removing the cat, it’s going away right here shortly. I’ve got a piece of 3” exhaust pipe going in it’s place. I’m hoping removing it will help something if anything maybe it won’t sound like it’s screaming at 3000 rpms. The 78 k10 with a carb will rev to 4500 no problem but this tbi setup is screaming by 3000 so it rarely gets ran hard.

 

[SirRobyn0]

I'm personally not a fan of cat removal. It does seem likely that you have at least some issues with the cat, so pulling it to test how it runs is fine. Then of course it's up to you to weigh the options as to replacing it or running without. I could also see a fuel pump issue, if you still have problems with it at high RPM.

 

[iamtherealJayy]

I’m hoping if nothing else removing this ancient cat will help with sound and exhaust flow. I don’t believe there’s a fuel issue since it revs up fine just screams it’s not a good sounding high rpms I don’t know the differences tbi and carb make in the sound, but the two sound very very different. The truck occasionally loads up sorta in my opinion, I’ll be driving aay taking off and I’ll be on the throttle and it’s doing all it can to be moving then something just poof and it takes off, someone has mentioned sticky advance springs in the distributor previously.

 

[SirRobyn0]

I’m hoping if nothing else removing this ancient cat will help with sound and exhaust flow. I don’t believe there’s a fuel issue since it revs up fine just screams it’s not a good sounding high rpms I don’t know the differences tbi and carb make in the sound, but the two sound very very different. The truck occasionally loads up sorta in my opinion, I’ll be driving aay taking off and I’ll be on the throttle and it’s doing all it can to be moving then something just poof and it takes off, someone has mentioned sticky advance springs in the distributor previously.

Ya, I mean sticky advance springs certainly could cause that. Checking them is pretty easy, basically what I do is to turn the rotor a couple times and see how it feels then remove the rotor inspect the springs and the the turn the mechanism and see that weights move alright. If they are sticky, as long as the weight pins aren't so bad that they are dragging and wearing a grove in, most times you can carefully without getting the stuff under it, you can usually carefully clean them up with a tiny bit of WD-40 and working them back and forth. Wipe up the excess WD-40 and apply a little dielectric grease where the weights move, work that in and then your done. Just make sure you don't leave enough in there to get slung around when the engine is running.

For that matter if the weights are sticking in both positions this could be your idle problem too. Regardless it's super easy to check and most times easy to correct.

 

[Redfish]

@SirRobyn0 does the TBI distributor actually have advance springs and weights? The TBI setup looks entirely different from the earlier distributors and if it does have them they are concealed from view.

My new and old distributor for my '87 TBI 350:

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[SirRobyn0]

I think @Redfish is right. I kind of forgot about that. I think the ECM controls the advance. It would only take a few minutes for him to find out for sure, but ya I think you right about that.

 

[iamtherealJayy]

With this engine being half frankenstiened together. I really have no idea what’s actually in the distributor. I’ve had no reaosn yet to actually take the cap off, it looks similar to an hei cap but has plugs all across the top instead of the coil, so I assume it’s the correct one, and no one mentioned it being wrong yet. But if it is all ecm controlled than that shot down the idea of the advance being a problem. @AuroraGirl had mentioned ecm problems and I’m not computer enough on vehicles to dive into that. My oil pressure gauge is pegged all the way to the right (3 o clock position) is this the gauge or the sensor? It’s been like that sense before I got it but I checked the oil and it’s a quart low so I’d like my gauge to work now since I don’t daily this truck much anymore.

 

[Redfish]

With this engine being half frankenstiened together. I really have no idea what’s actually in the distributor. I’ve had no reaosn yet to actually take the cap off, it looks similar to an hei cap but has plugs all across the top instead of the coil, so I assume it’s the correct one, and no one mentioned it being wrong yet. But if it is all ecm controlled than that shot down the idea of the advance being a problem. @AuroraGirl had mentioned ecm problems and I’m not computer enough on vehicles to dive into that. My oil pressure gauge is pegged all the way to the right (3 o clock position) is this the gauge or the sensor? It’s been like that sense before I got it but I checked the oil and it’s a quart low so I’d like my gauge to work now since I don’t daily this truck much anymore.

This truck has more drama than Days of Our Lives and As the World Turns put together.

First, the oil pressure is probably but not definitely the sensor. If you have a multimeter you can test it pretty easily. The sensor should be close to your oil filter. The one that is right under and behind the distributor is a switch and is not attached to your gauge... but in your case, I guess that may not be a definite answer.

This is what mine read when it went bad and pegged out the gauge.

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Next, pulling the distributor cap and checking it and the rotor are pretty basic and as long as you have been fighting this I would have figured you would have done this by now.

You do not however have to pull the distributor to know what type it is. The '87 TBI should NOT have a vacuum advance connection. There should be two sets of wires plugging into the Ignition Control Module. The plugs will be at the base of the distributor cap, pointing at the firewall more or less towards the steering wheel.

You can see part of the ICM and one plug in this pic:

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By the way, that shiny silver thing screwed into the shiny brass thing underneath the distributor is the Oil Pressure Switch...or at least it SHOULD be.


I haven't been following this one very closely since it has gone on for so long but I will share with you a little saying from the motorcycling world. "90 percent of carburetor problems are electrical" That is probably more relevant to the older, carbureted bikes but it may have some value when troubleshooting this truck.

Your ICM is an easy and cheap part to replace. The MAP is supposed to communicate with the ICM via the ECM...

If you need to check your timing you will have to know the super secret location of The Plug. On my '87 it is close to the steering column, below the brake booster. It is a single wire plug and when you unplug it (make sure the engine is NOT running during the plug/unplug) the timing should be at 0 degrees. With it plugged back in and the engine started it will be advanced significantly.

Sorry for the blurry pic:

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[iamtherealJayy]

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.