Rough idle, occasionally dies while driving, running out of ideas....

[MouthForWar]

Truck: 1992 K1500 Silverado 5.7L TBI

Symptoms: Rough idle when warm/hot, occasional stall. Very rarely, truck will also die while driving. (This occurs without any stumble, miss, or other forewarning. Engine simply cuts; on decel or cruising from what I've noticed. Never under load.) Truck maintains power/electricity [gauges, electronics, etc.] Also of note is that the truck will fire right back up without hesitation in either circumstance, as if nothing happened, and will continue on. So far, neither symptoms occur when engine is cold. This has been occurring for months.

Repairs completed so far (including unrelated repairs & maintenance):

-Intake Manifold Gasket (plus associated gaskets: Thermostat Housing, EGR, Distributor, Valve Covers, etc.)
-Throttle Body/Injector Pod Assembly fully rebuilt
-Oxygen Sensor
-Throttle Position Sensor
-Idle Air Control Valve
-Coolant Temperature Sensor/Switch #1 (for ECM/Thermostat Housing)
-Oil Pressure Sender/Switch
-Spark Plugs/Wires
-Distributor Cap/Ignition Rotor
-Ignition Coil
-EGR Valve & Vacuum Hose
-Base Timing Reset, confirmed via timing light. (Computer advance also confirmed via timing light.)
-Fuel Filter
-Multiple oil/filter changes and open element air filter cleanings
-Seafoamed intake, fuel tank, and crankcase
-PCV Valve & Hose

Additional Diagnostic Measures/Notes:

-MAP tests good for output voltage; voltage also decreases/increases smoothly with vacuum changes.
-Climbed under truck during a stumbling episode and listened to fuel pump...does not cut, stumble, or stall. Very consistent. Pitch of pump never changes.
-Battery tests good and holds charge. Connections are good.
-Visual inspection of old plugs reveals no fouling and no difference in wear between cylinders. Inspection of old wires did not indicate arcing or shorting.
-Injector pattern appears even and good. No drips or cutouts.
-No smoke of any color out exhaust
-No hard or extended starting symptoms, whether hot or cold.
-Accelerates smoothly and handles load without issue. Does not cut or stumble at WOT or on interstate. Also still has plenty of power.
-No codes ever thrown or stored
-No pinging, knocking, ticking, etc. under any circumstance.
-All fluids are clean and levels are good
-No vacuum leaks (carb cleaner test revealed nothing anywhere, and most hoses are new)

Remaining ideas:

-EGR Vacuum Solenoid (had replaced bad valve, but not the solenoid)
-Ignition Module (but that usually dies completely and suddenly...and if not, symptoms also usually present under load/acceleration as well, which is not occurring.)
-MAP Sensor (tests fine, but have heard of them going bad without being off very much or throwing codes)

I don't have the cash to throw those parts at it right now, but would like to see of anyone has any other ideas that I can implement once funds start flowing again.

When it isn't stumbling or dying (e.g. when it's cold or hasn't been driven long), it runs great. I really love this truck, which is why this is driving me so insane. I feel like I've done everything, to no avail.

Any ideas from our more experienced members or techs?

 

[MouthForWar]

Damn; did I stump the whole forum? Lol.

 

[Supercharged111]

What kind of fuel pressure are you making? I had a similar issue and it ended up being a plugged fuel filter. Reason mine did better cold was because they run rich when cold. I see no mention of a new pump and the new regulator may still suck.

 

[MouthForWar]

I've not been able to do an actual pressure test, but for what it's worth, as mentioned, the pump isn't straining, cutting out, etc. Consistent in pitch even during stumble. Also, fuel filter is new and injector pattern appears full and even...and entire system has been SeaFoamed.

I've briefly considered the pump, but that's a fairly expensive and time-consuming shot in the dark. I'd like to exhaust all other options before coming to that, especially since the fuel system seems to be okay based on the above indicators.

Not ruling it out, but it's towards the bottom of the list of things left to try.

As far as the regulator goes, I'll throw that on the list of things to consider. Idk; it just doesn't feel like a fuel issue, but I could definitely be wrong. We're chasing a super gremlin...anything's possible.

 

[Supercharged111]

You need to check the pressure and go from there. Don't throw parts at it and stop guessing. You can't look at the spray pattern, hear th pump running and assume all is well. If the fuel system checks out then logically approach the next theory.

 

[89RCLB]

Agreed on the fuel pressure test, your pump should be pushing 13psi. Specs call for 9-13psi but if it's not making 13 you've got a problem. Had a similar issue that ended up being a split 1.5 inch piece of hose between the fuel pump and the feed line, on the pump itself, inside the tank. I'm not saying that's your problem but in my case it was.

 

[MouthForWar]

Fair enough guys; I'll be ordering a pressure tester/TBI adapter here soon. Something I have needed to pick up for a while anyway; just keep putting it off.

In the meantime, I also did some additional testing on the charging system if anyone's curious.

After sitting overnight, battery tested at 12.3 volts.

Upon startup/cold idle, battery and alternator both put out around 14.5 volts with no/minimal load. Battery dropped to 14.3 to 14.5 under full load (all electronics, lighting, and accessories); alternator continued to put out just under 14.5 volts.

After a couple of highways pulls and some driving around town, getting it nice and hot and getting it to stumble a bit...the battery test at idle dropped to 13.61 under load; alternator output dropped to 13.86 at idle.

I'm losing a full volt just because the engine is hot. Should I be concerned and have the alt actually load tested down at O'Reilly?

Interesting note: I have alternator whine in my stereo when my phone is plugged in to the power cord and im using the aux input from my phone to the head unit. Ive read this is fairly common, and I've experienced it in multiple vehicles. However, the volume/intensity has doubled in the last couple of weeks. Do you guys think that could point to a failed/failing node in the alternator? Or do you think it just might be a bad ground somewhere? Battery, chassis, and block grounds are all solid and clean, as are the alternator connections. I don't know where to start checking beyond that though.

Another interesting note: When the engine was hot and I switched my multimeter to test for A/C current out of the alternator (to see if the alt is failing), the damn thing sparked at me and the engine nearly died. It did NOT do this to me when the engine was cold, and it read like .002 if I recall (below the allowable/concern threshold) at that point. What do y'all make of that?

 

[chipskittles]

Never heard of checking for ac current on the B+ terminal so I can't say what to expect with this. Not saying it's wrong but i've never thought to try it myself. I know how to use a meter though and i've often mistakenly switched to ac volts instead of dc volts and nothing has come of it other than not giving me the reading I expected. If you alternator is putting out over the battery stand alone voltage, it IS working. The alternator will kick on the battery light if voltage drops to battery voltage also, which would also indicate that it's not working of course.

And there's more. The CS-130 voltage regulator does regulate charging by temperature. Once it gets hot it will drop voltage. That's good. You don't want to push full voltage to a hot battery, the battery does not need it. Your alternator is fine. The behavior you have described tells me the alternator is working fine.

You ever check your fuel pressure or ignition module? Both common problems, way more common than some of the other stuff you have thrown at it.

 

[MouthForWar]

Thanks for the extra info on the alt! Appreciate it. It didn't occur to me that it might be temperature regulated.

As mentioned, fuel pressure will be tested as soon as my tester gets in. Ordering one today.

Have not tested/replaced module yet; did replace coil first for a few reasons. Module is high on the list next though.

Also, everyone keep in mind the list of things done to it has not all necessarily been thrown at this specific problem with abandon. Wanted to give a full list of things that have been done to this truck recently just so everyone has a full picture. But some of the things on the list either tested bad, needed done regardless, or solved other problems (of which this truck has many, lol.) I just wanted to give everyone the full picture of everything I've touched on it since I got it a couple of months ago. I value giving as much information and circunstance as possible, if that makes sense.

 

[MouthForWar]

Oh boy, here's a fun new thing. Twice in the last week my brake pedal has gone to the floor, then recovered. The second time happened while I was in park (went to shift into reverse and discovered I had no brakes). Also, idle went way up each time I pumped the pedal. This occured for a few seconds until my pedal gained pressure again.

I have heard of the internal check valve in the brake booster crapping out and causing simultaneous rough idle issues. Has anyone else heard of that? Could the two be related?