Throwing Parts at the Wall

[MidneightReider]

I'm a whole 24 hours into owning my latest square and just cannot fight the urge to "improve" what I am able to.

I've had a couple OBS TBI's but don't have much experience tuning or maintaining the systems so I want to ask those with more experience.

My 90 Suburban with127k mi runs pretty damn good but has a couple things I would like to address to get it closer to "perfect". It's not a primary source of transportation or a house fire that needs put out immediately so I have time to discuss and consider, test and return to base etc.

The "issues" I perceive are mainly these 2 things:

1. It high idles for probably 45 seconds before settling down every time it is started, cold, warm or at running temp. This of course causes a firmer-than-desired shift into gear so I will be waiting for it to settle down every single time. I feel that when it's warm it shouldn't need to high idle that long or at all.

2: When it is at operating temp, say after driving 30 minutes, parked and shut off for a few minutes, restarted and shifted into reverse, it occasionally drops idle low enough to die and need started again. This is unacceptable and has to be addressed.

I suspect the IAC needs cleaned or replaced. Other thoughts are TPS and coolant temp sensor but again I don't have the most experience with troubleshooting a TBI.

I will of course check all vacuum lines for leaks (if you have suggestions on 'usual suspects' I'm all ears), and replace any lines that seem like they need it, but my question is this - Is it a BAD idea to just go ahead and toss all new sensors into it and recalibrate?

I would do this one at a time to be able to observe and log the changes each replacement induces, but I would be concerned with the possibility that aftermarket sensors just wouldn't be as good as the originals they are replacing. If you have sources and suggestions for high quality sensors, please feel free to offer those.

I know that some people consider it, as the title states, "throwing parts at it", but I like to stay ahead of problems, something formerly referred to as 'maintenance'. I'm not concerned about replacing something that wasn't malfunctioning, as long as the new part is of equal or higher quality. I would consider changing a sensor before it fails the equivalent of changing the oil before the engine locks up so the cost alone doesn't bother me.

I have been a 'professional' certified motorcycle tech for over a decade and turned plenty of wrenches on early to late model cars and trucks of my own and for a paycheck but there are many others more qualified than myself so I would like to get a collective opinion from this platform. Let me know what you think. Thanks in advance for your time and consideration.

PIc of the patient currently under examination. Year is 1990, engine is 350.

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[Rickf]

I have that same style paint job & color on my 1990 1/2 ton.

 

[PartsMan79]

I would make sure your injectors are good and clean and you have good fuel before changing any parts.
Those throttle body injected trucks were bad about dirty injectors and that one only has 3700 miles a year on it.

 

[Matt69olds]

The fast idle start is designed into the calibration. GM programmed the PROM that way to ensure the O2 sensor and catalytic converter heats up as quickly as possible.

I’d start with cleaning the IAC bore and pintle. I remember seeing an extra pair of wires (besides the common purple and green pairs of wires) on some neutral safety switches, maybe those are used as an input do the computer knows the vehicle is in gear? I don’t have any service manuals here to verify, might be something to check.

 

[Drauka99]

The fast idle start is designed into the calibration. GM programmed the PROM that way to ensure the O2 sensor and catalytic converter heats up as quickly as possible.

What he said.... I had a Chevy TBI truck in the 90s, pretty well maintained... high idle for around the time you said is what it had always done and did my whole time of ownership.

 

[bucket]

I know it is common for them to high idle that long on a warm start, but none of my tbi engines have ever done that, unless the IAC valve was sticking. When you shut the truck off, the ecm opens the IAC so it is ready for the next startup. If it's sticking or slow moving, it can cause the extended high idle on a warm startup.

 

[gmbellew]

Replacement parts, even AC Delco, often aren't as reliable as a 35yr old OEM part. I'd not replace what isn't broken.

For no 1, I'd start simple with the IAC and do a hard reset. Jumper A and B on the ALDL and turn key to "on" for 30-45 sec. You should eventually hear the stepper motor in the IAC clicking bc it is all the way closed. Now the computer knows where closed is (0 IAC counts). That might fix the issue. Otherwise, I'd suspect a vacuum leak. I think the IAC has a set "decay" function on how it steps down the IAC on a start to idle down. Could be that the leak keeps the idle higher even on a warm start. If those two don't bear fruit, I'd replace the IAC with an AC Delco part.

For No 2, does your voltmeter drop down in reverse? Makes me wonder if there is an issue in the reverse light wiring or maybe the neutrally safety switch that is adding too much load and pulling tbe volts down. That's my thought if it only does it in R and never D. Could also potentially be some sort of transmission issue?

For TBI, I'd get the emissions and drivabality manual (and all the other service manuals while you are there) out of the reference library on this site, get a Bluetooth OBD1 cable, and ALDLDroid app. Troubleshooting will be a lot easier when you can see what the computer is seeing.

 

[MidneightReider]

Thanks everyone for these suggestions they have been very helpful and insightful.

I've been consuming all the videos I can on cleaning and resealing the injectors, cleaning and resetting the IAC etc.

Today I'm doing something I overlooked initially which is changing out what looks to be the original fuel filter to start the process. Hate to do all that real work and have it end up being basic maintenance that I should've done anyway. Came off pretty easy and I'm about to install the new one while I wait for the machine shop to press in a new rear ujoint that was really bad.

Things are looking up for this rig already.

Any input on an injector spacer? Not a throttle body spacer, just the 1/4" injector spacer. I already got one to install when the injectors come off for cleaning and O-rings. Seems to make sense for better atomization and I've read lots of info that says it will run better but if anyone has had a bad experience I'd like to know.

 

[MidneightReider]

Just pulled a plug.

Champions with about half an electrode.

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[DoubleDingo]

I was going suggest checking spark plugs for low idle and dying.

 

[MidneightReider]

I was going suggest checking spark plugs for low idle and dying.

4 of them were burned up like the first one I pulled and 2 of them were at least a whole thread from tight. Not surprised at that since they were installed by someone who thinks lawnmower plugs are good enough for a 350.

Ran to O'Reilly's got some CR43TS gapped at .035 and plugged them in. I know it will run better and I think it will probably solve the dying when warm and shifted to reverse. I think the fact that it happened in reverse is simply that upon warm restarts I have always been parked such that it needs to go into reverse to back out of the driveway.

Still waiting on the Ujoint replacement so it'll probably be tomorrow before I can test drive it.

 

[bucket]

Thanks everyone for these suggestions they have been very helpful and insightful.

I've been consuming all the videos I can on cleaning and resealing the injectors, cleaning and resetting the IAC etc.

Today I'm doing something I overlooked initially which is changing out what looks to be the original fuel filter to start the process. Hate to do all that real work and have it end up being basic maintenance that I should've done anyway. Came off pretty easy and I'm about to install the new one while I wait for the machine shop to press in a new rear ujoint that was really bad.

Things are looking up for this rig already.

Any input on an injector spacer? Not a throttle body spacer, just the 1/4" injector spacer. I already got one to install when the injectors come off for cleaning and O-rings. Seems to make sense for better atomization and I've read lots of info that says it will run better but if anyone has had a bad experience I'd like to know.

I've ran the injector spacer on 3 or 4 engines. They all seemed to improve the top end feel and maybe give a slight mpg increase. Well worth the 20 bucks they cost at the time. I don't know what they cost now.

 

[MidneightReider]

I've ran the injector spacer on 3 or 4 engines. They all seemed to improve the top end feel and maybe give a slight mpg increase. Well worth the 20 bucks they cost at the time. I don't know what they cost now.

Thanks for the input. I paid $13 on eBay. I think I'll get a TBI "rebuild" kit and do everything in the area at one time.

Happy to report that the spark plugs and fuel filter made an expected and significant difference. It no longer tries to die when shifted into reverse at operating temp. It takes far less time to drop from high idle when warm, maybe 15 seconds now. It idles smoother and idle to midrange rpm is a lot more pleasant. It takes less throttle input to maintain speed which should translate to better fuel efficiency. It even shifts 1-2 and 2-3 smoother, some of which is probably attributed to less feedback from that old ujoint.

Big step in the right direction and makes it even more of a joy to drive.

Next up, 4 new shocks and some fresh exhaust components. She's a little bouncy and the exhaust from the cat back is in bad shape. I'll continue to use this thread to update on the basic tuneup procedures.

Thanks everyone for the suggestions and by all means continue to give any input on any part of the process.

 

[MidneightReider]

Exhaust parts ordered. The pancake converter that's laying on the crossmember is gonna do the Stray Cat Strut to the scrapyard and get replaced with straight pipe. First person to say "but it needs the back pressure!" or "gonna throw codes!" gets laughed at. I don't make the rules. Should be enough pieces to cobble a tailpipe together too. Anticipate an opportunity to make fun of my welding "skills".

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And order the muffler, a 24" straight through unit. Not the cream of the crop for a muffler but it will guaranteed be better than the rusty piece of junk that's on there. Hopefully "aggressive smooth deep tone" is an accurate description of the sound.

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Just need to snag up some clamps and collector gaskets from the parts store now. Fingers crossed for no broken collector bolts.

 

[MidneightReider]

Exhaust all shipped Monday and showed up today via FedEx. I'm impressed with the high quality of the pipe pieces for under $100 to my door. I will relist the unused pieces on marketplace or ebay and probably get most of my money back. The muffler is heavy but hopefully that means it will last. The offset inlet is not welded perfectly square with the muffler and that bothers me some. It likely won't affect installation but it's worth noting.

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