1993 Chevy K3500 - Rough idle and stall

[JWAZ73]

Hello Friends! My son and I are working on a 'project truck' that is his first truck. It was found in someone's backyard and hadn't been regularly driven in almost two years. It wouldn't start but a TBI rebuild kit and new injectors later, we drove it home. When idling, there was a noticeable cough, stutter, miss, not sure what to call it. Now that the weather in Phoenix has gotten hotter the condition is worse. When idling in park you can hear the 'miss' and the RMPs will slow. When driving, after coming to a complete stop, in gear with your foot on the break, it will stall almost every time then starts back up with no issue.

Here is what I have done so far.
* New PCV valve, gasket and hose (valve to TBI)
* New MAP vacuum hose (the old one was cracked)
* New IAC valve
* Inspected remaining vacuum hoses, all are in good shape, not brittle or cracked (evap canister hose & EGR solenoid hose).
* New distributor cap and rotor
* New plugs (.035 gap)

The vehicle passed the emission test with very good numbers. Seems to have good power but I feel the fuel economy could be better (currently about 11 MPG).

Before I shoot the parts cannon at this engine I wanted to ask about it...
Plug wires?
TPS?
MAP Sensor?
Could timing be an issue (haven't checked it)?

Any thoughts?

I appreciate you're input!

-James

 

[PlayingWithTBI]

When you changed the IAC did you calibrate it? This is old but it still applies to our TBI systems.

http://www.gmcmidwestclassics.org/Web pages/Tuning the TBI.pdf

Do you have a scan tool or some way to data log while it's stumbling/stalling?

 

[JWAZ73]

Thanks for your quick reply! The ‘calibration’ process I read was simply to drive the vehicle above 40 MPH for 10 minutes to let the computer adjust. Based on the document you linked to, that wasn’t it. (I think those directions were with the new IAC valve.)

I don’t have a scan tool or data logger for this year vehicle. Do they make USB or serial adapters to do this from a PC?

 

[PlayingWithTBI]

Do they make USB or serial adapters to do this from a PC?

Yes I use my laptop running Windows 10, a USB OBDI cable, 12 or 16 pin depending on what year

http://aldlcable.com/aldl.asp

And TunerPro RT v5

http://www.tunerpro.net/

With some reading

http://www.gearhead-efi.com/Fuel-Injection/forumdisplay.php?27-TunerPro-Tuning-Talk

But start with your calibration - you may ruin the new IAC if it's not adjusted properly.

 

[JWAZ73]

Well, looks like the truck is a driveway ornament for this week. I'll have to get to it next weekend. Thanks for the info! I'll follow up when I have more info.

 

[scott_williams2]

If you engine light is on then you can easily get it scanned for free if you can make it to a parts store or buy a scanner. I’m not sure if they sell obd1 scanners but they do sell obd1 adapters to put on obd2 scanners like they use for modern cars. The scanner should tell u everything u need to know

 

[JWAZ73]

When you changed the IAC did you calibrate it? This is old but it still applies to our TBI systems.

http://www.gmcmidwestclassics.org/Web pages/Tuning the TBI.pdf

Do you have a scan tool or some way to data log while it's stumbling/stalling?

@PlayingWithTBI, I want to make sure I'm using the correct tables for reverence in checking the calibration of the IAC. The document you linked refers to a "Target Idle RPM vs Coolant Temp" table. I understand that every vehicle could be different. The tables I'm seeing in the base .bin for this truck including the following tables...

Idle Speed vs. Temperature (P/N, A/C Off)
Idle Speed vs. Temperature (P/N, A/C On)
Idle Speed vs. Temperature (Drive, A/C Off)
Idle Speed vs. Temperature (Drive, A/C On)

The conclusion I'm drawing from this is that I need to locate the lowest RPM value that could be called in these tables and set the minimum idle speed in relation to that.

Does that make sense?

 

[PlayingWithTBI]

The conclusion I'm drawing from this is that I need to locate the lowest RPM value that could be called in these tables and set the minimum idle speed in relation to that.

Yep, that's it. The reason being you don't want to have the ECM trying to drive the idle lower than your min stop will allow, that will burn up the IAC stepper motor.

 

[Schurkey]

1. What is the fuel pressure? How old is the fuel filter?

2. It's possible to fix OBD1 vehicles without a scan tool...but there's no point. It takes hours--or days--to track down stuff that would be a five-minute deal with a proper scan tool.

3. A "code reader" is not a scan tool. If you don't have access to the data stream, you're just guessing.

4. What is the cranking compression of all eight?

 

[JWAZ73]

Okay, it has been a full day. My ALDL cable arrived yesterday and I was able to connect to the PCM with TunerProRT using the $85 .xdl and .adx files from here: http://www.gearhead-efi.com/Fuel-Injection/showthread.php?317-16147060-PCM-Information-85 (@EagleMark post from 01-02-2012).

I started the day by checking the compression. Five cylinders were 160, one was 165 and two were 170.

Next, I began to work on the IAC calibration @PlayingWithTBI mentioned in his first post in this thread. The idle table for the sample .bin I downloaded showed a minimum idle of 550. I was not able to get the truck to run trying to set the idle to 425 but I know the base idle is set lower than 550. Watching the IAC with TunerPro, it appears to be functioning and stays around 35 to 40 when idling.

The rough idle behavior is still present.

Next, I set out to troubleshoot the EGR, TPS, MAP sensor, and O2 Sensor. I followed the procedures outlined in the Haynes manual for this truck. Here are the results.

Using a hand vacuum pump I tested the EGR and EGR solenoid, they behaved properly according to the Haynes.

Testing the MAP sensor, it had 5.04V at the wiring harness (key on, engine not running. Haynes says it should be 5.0V). Key on, engine not running the sensor value from TP was 4.63V (Haynes says it should be between 4.0V and 5.0V) Engine running at idle the sensor value was 1.1V (Haynes says it should be between 0.5V and 2.0V) MAP sensor seems to be fine according to this test.

Testing the O2 sensor, key on and engine not running the sensor voltage from TP was about 430mV (Haynes says it should be between 400mV and 500 mV). Engine running at idle, sensor value was all over the place, the low I saw was about 300mV and the high was in the low 600mV range (Haynes says this should be between 100mV and 900mV and it should fluctuate). O2 sensor seems to be fine.

Testing the TPS, (key on, engine not running) TP sensor value was .47V in the idle position and 4.31V at WOT. The Haynes manual said these values should be .60V in the idle position and 4.5V at WOT. The difference at idle was better than 20% so I decided to try to correct it. I elongated one of the mounting holes on the TPS and used TP to read the value of the TPS when reinstalling it. It was reinstalled to be .59V in the idle position. Looking at my notes now I realize I forgot to check the voltage of the TPS at WOT after the modification.

The truck still has the same issue as before all this testing today.

I did start data recording with TP on the truck and captured a stall event. I can't upload this file type in the post so here is a link to it in DropBox. I'm going to try to figure out how to see what happened in TP but I've got a lot to learn with the scanner tool.

@Schurkey, to answer your question about fuel, I don't see a sensor in TP for fuel pressure and I don't have a tool to test it. I will have to borrow a tool from the parts store to do that but that's not a bad idea. As far as the fuel pump and filter, they both were replaced by the previous owner when he was trying to get the truck to run before we took possession of it. They have less than 1000 miles on them.

Friends, I am open to suggestions at this point. Other than fuel pressure, I'm not sure what else to look at here.

Thank you all for taking the time to read this post and the responses I've received already!