Stumble under load, out of ideas

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Markmartin01

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91 RCSB Z71, stock under the hood other than a small Elgin RV cam. Has always ran fine, started it the other day and it has an occasional rough idle and what sounds like a lean condition under low RPM load, haven't tried WOT as that just sounds like a bad idea. Cap, rotor, wires are clean and fine, new ICM, coil is fine, fuel pressure stays at 12psi, no vacuum leaks, distributor has no end play, injector pulse isn't a perfect fog but it never has been. It does idle hunt occasionally now which it's never done. Seems mostly normal dead cold but 2 minute of run time and it's unhappy. Did replace 02 just in case but it didn't help. Any idea of what to check next, i'm running out of ideas. Only code is 43 and it's had that forever off and on so unlikely to be related. Open to any troubleshooting steps i've missed, i'm out of ideas. Really thought it was going to be low fuel pressure but gauge proved that wrong.
 

Schurkey

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Have you tested the fuel pressure gauge to prove that it's accurate?

Have you tapped the intake manifold or cylinder head with a small hammer to see if the engine RPM drops? (Knock sensor system test.)

Have you connected a scan tool to verify all the sensors and computer outputs?

HOW do you know the "coil is fine"?

How old are the spark plugs?

Have you looked at the magnet on the distributor mainshaft to see if it's cracked? Common TBI issue.
 

Markmartin01

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@Schurkey Fuel pressure gauge I tried on my '14 yukon as a control and it was accurate. Knock sensor is currently unhooked because it loves to come unplugged (not a good positive click) but has never caused an issue before....could it suddenly show different behavior out of the blue? Put a known good 2nd coil I have and it still has the same issue. Plugs are maybe 10k miles old. Will check the dist magnet, worth a look. I don't have a scan tool at my disposal that is OBDI. If i can't find the issue that may be the next resort.

It did do something different last night; I changed the rotor and while changing it cleaned the dist plate and checked wire connections. Once back together with the new rotor, no spark. Put the old rotor back on and no spark. Gonna go back into it later today and more thoroughly clean and inspect the connections as I assume one must be not contacting now.
 

Sabinoerc

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I had a fuel pump read ok at idle but pressure would drop when flooring throttle. If you didn’t check that, perhaps something to check.
 

Markmartin01

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I did run it down the road with a gauge on it and it stayed steady at 12 PSI. HOWEVER....this morning I notice a no start and no fuel spray on startup. So it's entirely possible the FP had micro drops that the pressure gauge wasn't sensitive enough to reflect. Need to do some electrical troubleshooting before declaring the fuel pump dead but that may be the case.
 

PWC Repair

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Things I've seen cause that issue.
As previously stated, cracked reluctor on the distributor shaft.
Broken FPR spring in the TB.
Bad temp sensor in the intake even if it DOESN'T throw a code.
Bad TPS...hook a meter to it and move the throttle slowly to make sure the voltage stays consistent....doesn't skip.
Bad MAP sensor......least likely.
 

Markmartin01

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Partially solved. The stumble once warm was the temp sensor. It would ohm fine until it got nearly to operating temp, then it went to 4M ohms. Still has a warm idle hunt, so that's the next part. That also never occurred until recently, seems to coincide with the temp sensor failure.
 

Erik the Awful

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Knock sensor is currently unhooked because it loves to come unplugged (not a good positive click) but has never caused an issue before....could it suddenly show different behavior out of the blue?
A knock sensor is a piezoelectric element. When your engine pings, the shock creates a voltage in the sensor that is sent to the ECU. If everything is running right the ECM doesn't even know if the knock sensor is there. If your sensor was unplugged and your engine started pinging, the ECM would run on as normal because the knock sensor wouldn't be able to signal the ECM to pull timing.
 

Markmartin01

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I've read conflicting opinions on the knock sensor. Some say without continuity the ECM defaults to a low timing default, some say the ECM only intervenes once knock is detected. I don't know enough about these early ECM tables to know which is actually correct.
 

PlayingWithTBI

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FWIU the ECM looks for resistance from the KS, if it doesn't see it code 43 will be set. Keep in mind, the SES light will not always stay on but the code will be stored for 30 - 40 key cycles or when the battery is disconnected.
 
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