1990 c1500 5.7tbi Rough idle when warmed up Help Wanted!

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Schurkey

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Connect a scan tool, look for short- and long-term fuel trims, MAP voltage, TPS voltage, Coolant sensor signal, and ALL THE REST of the sensor information.

When warming up the engine runs smoother but has a noticeable roughness in it, once at operating temp it idles rough, even worse in gear with the ac running, it almost seems like a miss at times only at idle!
Worse in closed-loop, somewhat better in open-loop?

The engine currently has no SES codes and starts up easily!
How do you know?
How about pending codes?

Wires are delphi-packard all of them ohmed out good, when removed one at a time they all caused the same change in idle.
Removing plug wires with the engine running creates an open-circuit. That drives the coil secondary voltage sky-high, making life very difficult on the secondary-side insulation. Don't do that. Ground the spark instead to kill an individual cylinder.

Blunt the sharp tips of eight small nails. Apply a little dielectric silicone grease, and slide 'em between the wire and the boot at the distributor cap, until the nail touches the metal end of the plug wires. DON'T puncture the insulation. That way, when you're done there's no damage to the system.

Ground the lead of a test light--or just use a grounded jumper wire--to ground the spark to individual cylinders.

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cap and rotor are new
icm and pickup coil have been replaced
coil has been replaced
Coolant temperature sensor test out good
i swapped the map sensor with a known good one
replaced the 02 sensor with an ac delco 02 sensor i pulled the iacv and cleaned it
How do you know that the sensors are good? How do you know that the sensor wire harness is good?

A scan tool makes that easy.

fuel pump was replaced with a delphi unit 3 years ago, currently has 11psi of pressure before the filter.
What is the fuel pressure at the throttle body? Doesn't matter what the pressure is before the filter, if the filter is plugged.

i checked the vacuum of the engine, it set on 18 but would drop as it would rough idle/miss
I'd expect poor vacuum from an engine that is running rough.
 

hbryant123

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Connect a scan tool, look for short- and long-term fuel trims, MAP voltage, TPS voltage, Coolant sensor signal, and ALL THE REST of the sensor information.


Worse in closed-loop, somewhat better in open-loop?


How do you know?
How about pending codes?


Removing plug wires with the engine running creates an open-circuit. That drives the coil secondary voltage sky-high, making life very difficult on the secondary-side insulation. Don't do that. Ground the spark instead to kill an individual cylinder.

Blunt the sharp tips of eight small nails. Apply a little dielectric silicone grease, and slide 'em between the wire and the boot at the distributor cap, until the nail touches the metal end of the plug wires. DON'T puncture the insulation. That way, when you're done there's no damage to the system.

Ground the lead of a test light--or just use a grounded jumper wire--to ground the spark to individual cylinders.

You must be registered for see images attach



How do you know that the sensors are good? How do you know that the sensor wire harness is good?

A scan tool makes that easy.


What is the fuel pressure at the throttle body? Doesn't matter what the pressure is before the filter, if the filter is plugged.


I'd expect poor vacuum from an engine that is running rough.
Thank you for the reply, the only way i can read codes is the paper clip cross, then count the blinks of the ses light.

I would say the rough idle is worse in closed loop but i can see the engine having a little shake at idle when warming up!

I replaced the fuel filter about three years ago, it is possible it could be clogged again but then origional was very old and not clogged.

How can i hook this truck up tp a scan tool? i have one that does newer stuff with i think what is called obd2 diagnostics, ide be willing to invest some money in a different scan tool if it would help me figure this problem out, i dont care to spend money but i want to do it in a smart way!
 

Schurkey

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eBay has many scan tools for sale. When it was me, $350 got me a Snap-On Solus Pro, shipped to my door with multiple vehicle adapters, Personality Keys, cables, a battery and charger, instruction manuals on DVDs, software for Domestic and Asian vehicles up to 2007, and a suitcase to carry it around it.

Another one very similar, but with software good to 2009 or '10, just sold for $280 plus shipping.

There are other brands, and other models. Craigslist and pawn-shops also may have usable product. Be sure that what you buy has software suitable for the vehicles you use it on.

Some folks buy a vehicle adapter and use free- or low-cost software on a laptop computer. I can't help you with that; I have zero experience there.
 

hbryant123

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eBay has many scan tools for sale. When it was me, $350 got me a Snap-On Solus Pro, shipped to my door with multiple vehicle adapters, Personality Keys, cables, a battery and charger, instruction manuals on DVDs, software for Domestic and Asian vehicles up to 2007, and a suitcase to carry it around it.

Another one very similar, but with software good to 2009 or '10, just sold for $280 plus shipping.

There are other brands, and other models. Craigslist and pawn-shops also may have usable product. Be sure that what you buy has software suitable for the vehicles you use it on.

Some folks buy a vehicle adapter and use free- or low-cost software on a laptop computer. I can't help you with that; I have zero experience there.
Ok i will see what i can find online, is the main thing that it shows live sensor data? i want to make sure to get a tool that is going to help me!

If i can get set up with the proper scan tool what can i compare my readings to verify they are normal?

Ive read codes and replaced a part to fix the code but i have never really messed with live data!
 

Schurkey

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Ok i will see what i can find online, is the main thing that it shows live sensor data? i want to make sure to get a tool that is going to help me!
"Live sensor data" is the beginning of real diagnostics with a scan tool. Older scan tools won't provide graph outputs of sensor data--just text. Most newer tools will graph the results over time. Very handy.

A professional-grade tool will also have bi-directional control--the tool will be able to command the computer to perform certain tests. How extensive the tests are, depends on the vehicle and the scan tool. Among the more-handy abilities is to control the ABS unit so that the accumulator(s) can be bled. ABS "auto-bleed" for my '97 K2500 is NOT available on my Solus Pro with 8.2 (2008, 2nd Quarter, good for 2007 vehicles) software, even though I can get ABS live data and codes. But the ability to auto-bleed was added to the software bundle later on. 14.2 (2014, 2nd Quarter) software has it, I've been told that 10.x has it.

Many tools have pre-programmed, on-board "repair tips", the common solutions to each code or driveability problem. Some have pre-programmed "known good" sensor graphs for you to compare to.

Cheap scan tools will report engine data. Good tools interface with all the on-board systems having computer control. Newer cars may allow you to get info from the engine, transmission, ABS, air bags, instrument cluster, body computer including A/C, power seat motors, etc.

If i can get set up with the proper scan tool what can i compare my readings to verify they are normal?
The hardest part of scan tools for newbies is to figure out what sensor results are normal, and what is "out-of-range". In particular, MAP sensor readings for us old bastages are backwards. Anyone used to reading a vacuum gauge (like me) will be totally confused by MAP sensor results. The MAP sensor is a PRESSURE sensor, a vacuum gauge is a "Lack-of-pressure" sensor. To keep things simple, consider that barometric pressure is 30 inches of Mercury (in reality, it varies with the weather and altitude.) Where a vacuum gauge might show 10 inches of vacuum, a MAP sensor shows 20 inches of pressure. 20 + 10 = 30.

Thus the need for the scan tool operator manual(s), and experience. There's no substitute for connecting the scan tool to a vehicle that DOESN'T have problems, so you can see what the various sensor data is supposed to look like.

Ive read codes and replaced a part to fix the code but i have never really messed with live data!
The "code" might be for TPS out-of-range. With live data, you can watch the TPS sensor results starting from when you turn on the ignition. If the TPS voltage does not increase or decrease smoothly, having one or more sudden, dramatic changes as you slowly depress and release the gas pedal--you know the sensor is bad but the wire harness is probably good. If the TPS voltage doesn't change at all with throttle position, you have to consider the harness may be at fault, or the ECM.

With bi-directional control, you may be able to diagnose a faulty AIR system by running the engine, and sending commands to operate the upstream/downstream control system, while watching for upstream air directly--or by seeing the effect that the air injection has on the O2 sensor(s).
 
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hbryant123

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OK, I have purchased a bosch 1300 scan tool, it does read obd1 live data, but the issue is it seems the scanner itself causes issues with the engine idle speed, it idles higher with the scan tool, at one point in neutral it was idling at 1800 rpm! Is this normal?

I did get the engine to idle close to normal and have its rough idle issue, the issue im having is deciphering the live data, i am not really sure what to look for, i have found the tps seems to increase smoothe, im getting good engine temps. but as far as fuel trims and the 02 readings im not sure what to look for. any info and help is appreciated!
 

PlayingWithTBI

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the issue is it seems the scanner itself causes issues with the engine idle speed, it idles higher with the scan tool, at one point in neutral it was idling at 1800 rpm! Is this normal?
Some scan tools will add a 10K Ohm resistor between pins A & B which will raise the idle. There should be a way to eliminate that resistor in the settings or physically remove a jumper.
 

hbryant123

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ok so i checked the settings on my scan tool, i could not find a way to remove that resisitor, i also could not find a physical resistor anywhere to remove!
 

Sabinoerc

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W.r.t. the ”10k or ALDL mode”. https://www.thirdgen.org/forums/diy-prom/247135-idles-faster-when-aldl.html

when running in that mode ECM doing more than just changing idle speed. Supposedly for some sort of end of line mfg test or fast diagnosis of certain problems - I don’t know.

I‘d think that would be programable, maybe setting options on interface is not specifically saying ”10k” mode, using different name for the function?
 
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