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Discussion Starter · #1 ·
Having a weird surging and stalling problem. I can be driving, any speed, and suddenly the engine will start to accelerate. Other times it acts like it's trying to die because the whole truck lurches forward like it tried to stall...but of course I was driving so it must have immediately restarted or something to that effect. Anyway, even when idling it will sometimes surge, drop RPM and then catch itself, or die. So far it restarts immediately every time it stalls. Got home and hooked up the computer and below is what I got for results. The pics in this first post are the surging. The reply is when it dies. These were both sitting in my driveway today. Oh, and the cold temps are warm because it was 104 out today.

The APP PIDS on the left are for the accelerator pedal to make sure it wasn't giving some sort of off signal. If you think I should have a few more PIDS in there let me know which ones and I'll pull em up. Oh, also I did check cylinder contribution and nothing showed up.

First pic, KOEO.
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Running while cold in a steady state.

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Just starting to surge.


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Maximum surge



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Discussion Starter · #2 · (Edited)
So now here are the screen shots of when it dies. The First pic I thought I had the monitor running for longer before starting, but apparently I caught just the very beginning of me starting it up, which explains the slight ICP voltage, PSI, and IPR. Didn't even have enough time for the RPM to register yet.


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Here's the pic of just before it's about to start stalling.



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So this next pic is the very first indication that something is about to happen. For 30-40 second prior to this pic everything was stable and the only difference a few hundredths of a second earlier was the ICP voltage which was .91V with the same 628 PSI.


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Now seven hundredths of a second later and you see the PSI finally drop, ICP voltage drop a hair more as it begins whatever it's doing as it decides to stall.



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Next pic is just as it's trying to compensate by raising the IPR.


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This final one is the maximum the IPR reached while trying to keep the engine running...everything after this the IPR is dropping because, I'm assuming, the PCM finally realized it's no longer worth trying to keep it going.


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Discussion Starter · #3 ·
The last time this exact thing happened to me it wound up being a bad coil on an injector, but the odd thing is it cause my PATS system to activate, preventing any starting/diagnosis. It was pure guesswork based on a code that showed up which I decided to try.

This time when I pulled codes the only ones which popped up initially were for the accelerator pedal and ICP being lower than expected. I rechecked every time it surged or stalled and nothing new ever showed up. I had ZERO codes, and I checked after six episodes.
 

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Discussion Starter · #5 ·
Yeah, caught that but it also surges and stalls when the ICP is unplugged. I figured I would check the wiring tomorrow and look for a short. I know an open won't do that....would just show zero all the time.
 

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Discussion Starter · #7 ·
It's less than a year old. Not sure exactly when I changed it, but it was in the fall of last year. Connector looks good, but I'm going to pull back the wrapping to see what I can find. Plus I'm going to probe the sensor to see what resistance there is and will compare it to a known good one.
 
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Discussion Starter · #8 ·
Well, I can't find anything wrong with the wiring or the connector. All of it looks good. No oil in the connector, no oil in sensor, no chafing, breaks, etc in wire. I even did the push/pull test all along the wire while watching sensor values. However, SOMETHING has changed....I now have proper voltage at KOEO. The pics below are before I did anything, then with the ICP unplugged, and next is about 1 second after I plug in the ICP the pressure spikes. I'm guessing that's either abnormal or normal and it's just due to the sudden change in connectivity. Either way I replicates it with another sensor, so not sure. Thoughts? Oh, and the last pic is just showing it going back to normal.



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Discussion Starter · #10 ·
Sure looks like a v-reference short in the ICP wiring
So, to be sure (because I'm still figuring out some of the Vref diagnostics tactics) I should backprobe the IPR connector on pin number 1 (or A depending on which pic you use) to see what the ICP sensor is receiving for Vref, right? And I should probably go ahead and check the wiring from ICP to PCM connector while I'm at if. If I find nothing wrong, my thought is to wait until the truck acts up again and repeat. If I get zero voltage again with FORScan, and the input into the sensor is within spec for Vref, then that would mean bad sensor, yes? And if I see bad voltage into sensor (no matter the circumstance) then I need to find why it is low by back tracking along the circuit, correct?


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Discussion Starter · #13 ·
It's a late model 04. I have the schematic....I was just too lazy to pull it up so I got that pic off the web. Lol I'll double check it before I go probing.
 

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Discussion Starter · #14 ·
Verified manually that 4.98V was on the Vref wire, and .25V was on the signal, both matching what I saw in FORScan. With both the probe hooked up and FORScan running I began manipulating the wiring from one end to the other. I even wiggled it in places I didn't think had anything to do with the ICP. I also pressed it and squeezed it, and pushed it against rub points....of course I had already looked it all over but figured it was still a needed step. The voltages never wavered more than 1/100 of a volt....which it does normally anyway.

So, if your hunch is right about it being a short in the Vref circuit for the ICP then I'm taking the easy route for now and am assuming it's something internal to the sensor. I replaced it with my known good spare and I guess we'll see what happens. If it continues then I'll know I still have some searching to do. If it stays away then I know I got a lemon of a sensor.

Any other ideas?
 

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Discussion Starter · #16 ·
Well, 150 miles gone by without even a hiccup. Doesn't necessarily mean anything, but crossing my fingers.
 
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