350 TBI runs rough if at all

Mungo

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Hi,
I have a '90 V3500 crewcab, yes a very late square body, ex-USAF.
The truck hasn't been run much during the last year while I had the long-bed buck off to weld small repairs to the under-floor cross-beams. I got the rust fixing/protecting work done and the buck back on and now I have a fuel problem. The truck will start but is unhappy and won't idle, it dies away. While running it just about keeps going with plenty of throttle action but sounds very sick. I know that it is definately not an ignition problem.
I changed the fuel filter, inside the passenger side chassis rail, near the transfer-case, but this did not fix the issue. I'm guessing that it could be the fuel pump failing.
Does anyone have experience of TBI fuel issues like this?
Any other likely causes? Is there another fuel filter? Anything to check at the throttle body?
Is the fuel pump in the tank as I'm assuming?
Is it worthwhile taking the buck off again for easy assess to the fuel tank on this long frame, long bed?
I appreciate any experience that you can share please.
 

GMFORLIFE

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How about a fuel filter and new fuel?
 

Bextreme04

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Actually troubleshoot the problem before dumping money in it. Do you have any codes? Have you done a compression test? What is the fuel pressure at? You can rent a fuel pressure tester and compression tester at most local auto parts stores.
 

Mungo

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GMFORLIFE
Thank you, I hadn't thought about the fuel, I'll drain it and try fresh. I changed the fuel filter.

Bextreme04
Thank you also, I haven't ever tried to check fault codes on this truck in 20 years of use, until laid up this time, I'll do that. I can do a compression test.
Which location to connect the fuel pressure tester, right at the union on the throttle body?
 

Bextreme04

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GMFORLIFE
Thank you, I hadn't thought about the fuel, I'll drain it and try fresh. I changed the fuel filter.

Bextreme04
Thank you also, I haven't ever tried to check fault codes on this truck in 20 years of use, until laid up this time, I'll do that. I can do a compression test.
Which location to connect the fuel pressure tester, right at the union on the throttle body?

That's what I usually see for the TBI's, right at the back of the throttle body is the usual location. I'd recommend maybe buying the filter and rent the fuel pressure tester at the same time. Check the pressure first, if it's low then change the filter and see if it fixes it. If fuel pressure is good, the filter and pump aren't the issue.

Pressure should be between 9-13psi, if it isn't in that range then I'd be looking at the fuel pump first(see if it's getting voltage and turning on). Then, I'd see if you have fuel flowing but pressure is out of whack(high or low). This would most likely be a failed pressure regulator(also pretty common).

A good way to check a bunch of those things is to remove the air filter and look down into the throttle body while turning the key on. You should see a nice even fan spray of fuel from both injectors as a prime. If that doesn't happen(no spray or not even fan), but fuel pressure is good and pump is running, you might have clogged injectors.
 

RecklessWOT

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Does it throw a check engine code? Are there any other symptoms? Does it backfire or run incredibly rich? After sitting a while my coolant temp sensor went bad which rendered the truck un-drivable, it barely wanted to start unless at full throttle and even then just chugged black smoke and barely stayed running enough to move it around the driveway.

Also, a bad fuel injector would cause really rough running and make it not to want to idle well. Sitting a while can make those 30+ year old seals crack and finally give out. With it running and the air cleaner off, how does the spray pattern look? You should see a "cone" more than a "stream", and big drips of fuel are a no-no also.
 

Bextreme04

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Does it throw a check engine code? Are there any other symptoms? Does it backfire or run incredibly rich? After sitting a while my coolant temp sensor went bad which rendered the truck un-drivable, it barely wanted to start unless at full throttle and even then just chugged black smoke and barely stayed running enough to move it around the driveway. Also, a bad fuel injector would cause really rough running and make it not to want to idle. With it running and the air cleaner off, how does the spray pattern look? You should see a "cone" more than a "stream", and big drips of fuel are a no-no also.

That's a good one I forgot also... full throttle is the "Clear Flood" condition. So if you turn the key on and get a big drippy blob of fuel, you can put your foot on the floor and crank to see if it will start. The CTS has a huge amount of authority for cold starts, so it is well known to make it chug and smoke as reckless said.
 

GMFORLIFE

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Ethanol fuel doesn't last long.
I have 10 vehicles that rarely get started.
Try not to leave fuel in them for this reason.
Add fresh fuel when I need a joy ride.
 

Mungo

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Thank you all, I'll take a look down the TB throat, makes a lot of sense but I wouldn't have thought of it myself. Also I'll read the manual to learn how to read the codes.
The coolant Temp Sensor - is there a resistor value that can be used in place of the sensor when unplugged to prove runs ok?
I'll work on locating a pressure tester and fittings - is there a small filter at the TB fuel inlet?
 

RoryH19

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Was going to mention the coolant temp sensor.
One of the biggest issues but easy to swap for testing.
Not sure if you have dual tanks, if so a good check to isolate to fuel pump is by switching tanks.
Been a while but I'm almost positive there is a filter/screen inside the tank besides the one on the rail.
If it turns out to be the pressure regulator, I would rebuild the TBI and put an adjustable one in.
I did this the last time I rebuilt mine. Under a $100 and you adjust. The factory is fixed and in a range. Sometimes it is set a little low.
Also borrow a noid light, if you don't have one to check the fuel injectors. Autozone has them.
 

Salty Crusty

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^ intake coolant temp sensor, easy test. Buy a new one, you'll need it sooner or later. Unplug the wire from the current one, plug the replacement in and just let it lay on the intake, no reason to replace it until you're sure that's it. When they fail, the engine thinks it's damned cold and add fuel to warm it up. I've had this happen enough that I have a spare in my parts drawer at the shop just for testing.
 

Bextreme04

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Thank you all, I'll take a look down the TB throat, makes a lot of sense but I wouldn't have thought of it myself. Also I'll read the manual to learn how to read the codes.
The coolant Temp Sensor - is there a resistor value that can be used in place of the sensor when unplugged to prove runs ok?
I'll work on locating a pressure tester and fittings - is there a small filter at the TB fuel inlet?

This thread has some info. https://www.thirdgen.org/forums/tbi/613982-tbi-sensor-testing-101-a.html
 

Galane

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It could be the fuel pressure regulator. It's underneath the back side of the throttle body. A problem with these is the spring getting rusty then breaking into two or more pieces. Get a rebuild kit for the throttle body. That will come with new gaskets and some other parts but NOT the fuel pressure regulator spring. You'll have to look it up to see what spring your vehicle needs.
 

Raider L

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I was going to mention the throttle position sensor but a TBI isn't like that? But since you all are talking fuel pressure sensor, etc., it's probably that from what you all are saying it controls with the hard starts, chugging, and poor running, that does sound like fuel something.

I know a carb is somewhat like that. When it's dirty, "injectors", and all clogged up the engine won't run right and doesn't perform like it should, hard starts, chugging, not running at all I would think the carb is ready for a overhaul kit and a general cleaning. Even though parts need to be replaced from time to time the longer it sits up the worse it gets, "sensors", the accelerator pump needs to be replaced, vacuum secondary diaphram, it's very similar.
 

Bextreme04

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I was going to mention the throttle position sensor but a TBI isn't like that? But since you all are talking fuel pressure sensor, etc., it's probably that from what you all are saying it controls with the hard starts, chugging, and poor running, that does sound like fuel something.

I know a carb is somewhat like that. When it's dirty, "injectors", and all clogged up the engine won't run right and doesn't perform like it should, hard starts, chugging, not running at all I would think the carb is ready for a overhaul kit and a general cleaning. Even though parts need to be replaced from time to time the longer it sits up the worse it gets, "sensors", the accelerator pump needs to be replaced, vacuum secondary diaphram, it's very similar.

The TBI is very similar in that regard, if it sits for any amount of time or hasn't been rebuilt in a while you can get all kinds of issues with fueling. A rebuild is a good idea if it has never been done to the OP's knowledge. The TBI does have a TPS, and the expected values are given in the thread I linked. A TPS issue usually shows up as an acceleration issue though.
 

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