Stumbling when put in gear?

[1500z71]

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1992 K1500 5.7 350

Having an issue with my dad’s truck. It seems to stumble, especially when cold, when shifted into gear from park. It gets up and runs and shifts fine, it’s only at idle. It has actually stalled out twice over the course of a almost a year with this issue. It’s a farm truck so it doesn’t get the most attention. I’ve replaced the TPS and that didn’t work. Hasn’t been tuned in awhile, not sure if that would cause this kind of issue though. There was a couple old hoses that looked like they went to the EGR valve but there’s no telling how long they’ve been that way. I know there’s a few directions to go from here, so I wanted to hear what you guys think. Video is attached. Thanks!

 

[Schurkey]

Verify fuel pressure.

Connect scan tool, look at data stream. What is commanded idle speed vs. actual idle speed? Does the Idle Air Control work? What are the short- and long-term fuel trims? MAP sensor readout make sense? Engine coolant temp sensor readout make sense?

As you already know, plugs/wires/cap/rotor are suspect. Verify initial ignition timing, and the timing advance.

Make sure the EGR valve isn't leaking exhaust gas into the intake at idle.

 

[Ivan90]

Mine was similar but wasn't stumbling quite that much. It ran smoother in park then stumbled at idle forward, especially giving it a little gas. I don't have an OBD1 scan tool to diagnose like Schurkey recommended, but I replaced plugs/wires/cap/rotor as he noted and adjusted the timing, which was advanced significantly. It still stumbled. Then the problem went away when I broke the vaccuum line between the EGR solenoid and EGR valve. I replaced the line and the problem came back. I'm guessing the valve is not functioning properly. Maybe its opening and not closing so the engine is choking on exhaust gas. Hopefully I'll know more after I replace the valve and solenoid. I'm not sure its the same issue you're having though.

 

[1500z71]

Mine was similar but wasn't stumbling quite that much. It ran smoother in park then stumbled at idle forward, especially giving it a little gas. I don't have an OBD1 scan tool to diagnose like Schurkey recommended, but I replaced plugs/wires/cap/rotor as he noted and adjusted the timing, which was advanced significantly. It still stumbled. Then the problem went away when I broke the vaccuum line between the EGR solenoid and EGR valve. I replaced the line and the problem came back. I'm guessing the valve is not functioning properly. Maybe its opening and not closing so the engine is choking on exhaust gas. Hopefully I'll know more after I replace the valve and solenoid. I'm not sure its the same issue you're having though.

Thanks. I have a spare EGR I may throw on it to just try and see. Keep us posted for sure!

 

[1500z71]

Verify fuel pressure.

Connect scan tool, look at data stream. What is commanded idle speed vs. actual idle speed? Does the Idle Air Control work? What are the short- and long-term fuel trims? MAP sensor readout make sense? Engine coolant temp sensor readout make sense?

As you already know, plugs/wires/cap/rotor are suspect. Verify initial ignition timing, and the timing advance.

Make sure the EGR valve isn't leaking exhaust gas into the intake at idle.

Do you have any specific OBD1 tools you recommend? Also, how could I check that since it’s not throwing any check engine lights?

 

[Schurkey]

Do you have any specific OBD1 tools you recommend?

I bought a Snap-On MTG2500 used from eBay almost twenty years ago. The MT2500 is similar, less-expensive, and more common, but doesn't do graphing of sensor outputs. Get it with the adapters and software cartridges suitable for the newest and oldest vehicles you'd be using it on. Software starts with '81, goes as new as 2009 because that's when Snappy stopped supporting this tool. My software goes to 2006. The pro-grade software includes ABS, Air Bag, body computer, instrument cluster and other functionality that is vehicle-dependent. Asian software is fairly common, European software is fairly rare.

These tools are now so old that you'd probably be better-off with something from the Snap-On "Solus" line--Solus, Solus Pro, Solus Ultra, Solus Edge. I keep promising myself that I"m going to upgrade from my '2500...but I never actually buy the newer tool.

There are competing tools from OTC, (Genysis, and the newer Pegysis) sometimes with Mac or Matco branding. I've never used them. Again, get the adapters and software suitable for the vehicles you're going to work on. As you might expect, the newer the software, the more expensive it's likely to be.

how could I check that since it’s not throwing any check engine lights?

Looking at the data stream is entirely different from "reading codes". Having or not having the Check Engine light on makes no difference.

 

[1997]

OBD1 sets some codes without turning on the check engine light.
Easy to check, google how to pull OBD1 codes using a paper clip, or get an OBD1 code reader for about 30 bucks.

Might be that easy, might not.

 

[1500z71]

OBD1 sets some codes without turning on the check engine light.
Easy to check, google how to pull OBD1 codes using a paper clip, or get an OBD1 code reader for about 30 bucks.

Might be that easy, might not.

Okay I see. I’ve done this for my ‘93 when it threw a check engine light on the cluster, but you’re saying this will also work even if the light isn’t on?

 

[1997]

yes, you'll get 1 flash 2 flashes =12 (3x) saying everything is good, or after 12, you can get additional codes without check engine light staying on when driving, stupid system alright

 

[Schurkey]

get an OBD1 code reader for about 30 bucks.

Waste of $30. "Reading codes" is NOT looking at the data stream. "Code readers" are consumer-grade crap.

Codes can be useful. The data stream is ESSENTIAL, especially if there are no codes. A REAL scan tool can read codes, AND display live data, AND work with the vehicle's computer systems to turn select items on/off to verify operation.