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High Idle "Hic-Up/Fishbite"

JiFaire said:
Question, Dennis - maybe I'm just not reading right, but - if you manually raise the rpm to 1200 and hold it, everything is fine. If you then flip the 1050 switch (keeping the RPM manually at 1200), does it start to fishbite?

Doing this would clearly place the fault on the electrical (switch), rather than rpm, glows, fuel, or anything else.
Click to expand...

That's what I had suggested Jim. To have him run a jumper wire to try and eliminate the possibility of the problem being electrical.

From an electrical standpoint, given everything you have said, it sounds to ME that the problem is something electrical related. Dumb question, but what gauge wire did you run from the PCM to the ground point? Wires will expand and contract just like every other metal at low and high temperatures. Perhaps after a little heat the conductor is expanding again to create a better connection vs when its ice cold?

Just throwing fuel into the fire if you will:rolleyes5:
 
Well Blue, I wasn't even hinting about a jumper wire, more about the whole ECM circuit. If the truck is idling fine above 1200 rpm and he flips the switch, the high idle modification shouldn't affect the IP at all (ECM logic should read the rpm and not engage the high idle). If it STILL fishbites, then the problem is electrical, and has to do with the switches/grounds, splices associated with the mod.

I'm actually sort of thinking about timing right now... wondering about CTS. This only happens with a cold engine; would be nice to have a scanner hooked up when flipping the switch.
 
More good questions for me - I love it! (seriously - no sarcasm there).

Sven, I'm not discounting anything right now. I just am having a hard time wrapping my brain around 2 to 3 minutes worth or return fuel heating up 42 gallons, you know? But, then, I have no idea how much 3 minutes of return fuel equates to in volume, either. But, like I said, I'm not counting anything out. After 2 -3 minutes of idling, yes, engaging the high idle will no longer cause biting.

I'm going to try to answer the new questions while at the same time providing a summary of exactly what is wrong and what has been tried. Bear with me, I'll try to keep it as concise as possible.

-- Biting happens ONLY during warming up a very cold engine with High Idle ("HI") engaged, and even then it only happens for the first 2 to 3 minutes.

-- It does not matter what combination of HI switches are thrown. The result - biting - is the same.

-- Manually elevating the throttle will not cause biting.

-- There are absolutely NO drivability issues.

-- IP (GM Warranty, PMD in stock location) has about 15K on it. Injectors (Kennedy Hi-Pop), Glow plugs (Beru self-regulating) all have about 9K on them, but I have not tested them recently.

-- With the proper glow time, it starts very easily.

-- Lift pump (purchased from Heath) has about 15K on it. I can very easily hear it rumbling away during glow cycle and after engine shut down. But, I have not tested for pressure, yet.

-- HI circuit is wired with 18g (maybe 16g) wire. The ground is solid.

-- I tried running a ground wire directly from battery to the ECM side of the HI switch (thereby eliminating the switch and possible bad ground)

-- I tried heating the ECM with a hair dryer for over 20 minutes

-- After reading the above posts, I just tried pulling the CTS wire. The engine had not been run for about 6-1/2 hours. I would consider the engine "cool". It DID NOT bite... as I was sort of hoping it would. However, the CTS is functional as it the ECM held the RPMs higher when CTS was disconnected and when I plugged it back in, the RPM's immediately dropped back down.

Tomorrow morning I will start the engine and put the HI on. When it starts to bite, I will manually raise the throttle and try to hold it around 1300 (or, at least, enough that it is over the 1050 HI mark).


SCANNER: Would ScanGauge or Care Code be a good choice? I don't have access to a Tech 2.
 
DennisG01 said:
-- After reading the above posts, I just tried pulling the CTS wire. The engine had not been run for about 6-1/2 hours. I would consider the engine "cool". It DID NOT bite... as I was sort of hoping it would. However, the CTS is functional as it the ECM held the RPMs higher when CTS was disconnected and when I plugged it back in, the RPM's immediately dropped back down.
Click to expand...

Now, there's a contradiction. I be confused... When you pulled the wire off the CTS, it sends the signal to the ECM that it's -40* out, and calls for advanced timing and raised idle.

And it didn't fishbite. Did you engage the HI switch with the CTS plug pulled?

When you plugged it back in and the RPMs dropped to normal, did you THEN try the HI switch and get a fishbite?

Inquiring minds want to know...what I'm pondering is, if the CTS isn't telling your engine how cold it really is, then the timing isn't advancing enough, and the HI will cause fishbite . Just a random thought....
 
JiFaire said:
Now, there's a contradiction. I be confused... When you pulled the wire off the CTS, it sends the signal to the ECM that it's -40* out, and calls for advanced timing and raised idle.

And it didn't fishbite. Did you engage the HI switch with the CTS plug pulled?

When you plugged it back in and the RPMs dropped to normal, did you THEN try the HI switch and get a fishbite?

Inquiring minds want to know...what I'm pondering is, if the CTS isn't telling your engine how cold it really is, then the timing isn't advancing enough, and the HI will cause fishbite . Just a random thought....
Click to expand...

Yeah, when you mentioned the CTS, I got all excited thinking I could replicate the problem w/o having to wait overnight!

Here's how it went down:

Started the engine w/everything normal (but again, "cool" engine). Flicked HI on, no bite. Shut off engine. Pulled CTS plug, started engine (it definitely started different - fired off faster and then the idle stayed higher), flicked the HI on, no bite.

After plugging the CTS back in, I did not try the HI again.

So, tricking the truck into thinking it was very cold did not cause it to bite. It appears it must be cold for real.
 
I think its further pointing to cold natured. Unless someone can suggest something else that can change in the first 1-2 minutes of operation. Noted-the severity decreases over the 1-2 minutes as temp raises. Still could be fuel delivery but not leaning this way. Hearing the LP is not conclusive it can make noise and not pump have to test it.


I've read a lot of extending glow time for 18:1 and or 60G's (60G's don't really need it IMO for std compression). What about after glow. Can it be changed or extended automatically Do slower glows (thermally regulated) work as good for after glow. Does the biting only start after done with afterglow does after glow have any affect? I am suggesting with really cold block and ambient the initial glow cycle heat is dissipated pretty quick with cold intake air and cold fuel. Not all of it but a fair amount such that super fast PCM controlled fuel metering can bite. Initially the combustion chamber is warm enough for start from initial glow and the PCM really adds fuel to crank up easy but afterwards PCM fuel metering is faster and probably just different enough to cause issue. I've read this is one reason GM went with such high compression instant drivibility and no real warm up needed for the masses.

Not sure how to easily test this. Good intake heater mabye (more than hairdryer). You could try not adding cetane improver and see if it is slightly worse indicating fuel is having hard time igniting but may not be significant enough difference to tell conclusively.
 
Just thought of this....Possiblity try to correlate.... Are you using your defrost and when the compressor jumps in and out the pcm quickly reacts and you get the hiccup due to cold nature and delayed combustion ???? I keep thinking the PCM is watching plus or minus maybe 3-5rpm resolution with crank sensor and you are watching mabye 25+/-??? on the tach and its again a quick little adjustment thing. Most never see fishbite on the tach its a feel thing.

Might try sawing on your steering wheel and bump stop to see if you can change load on engine and duplicate hiccup fine rpm adjustment???. Or turn off defrost and bump A/C on and off with button to duplicate. Has to be cold engine within 30 seconds and 2 minutes. Warmed any and fuel will combust fast enough for PCM precise control.
 
I've been keeping all accessories off for this - heater, seat heaters, radio - everything.

I haven't taken notice to exactly what the speedo shows in regards to a drop. I would guess about 100RPM. It's as if someone is flicking a switch off and on - taking about 1/10th of second to do so. I'll watch a little closer next time.

OK, on to results from a few minutes ago:

I started the truck, it idled fine. I flipped the switch and it started to bite. I immediately manually advanced the throttle a bit higher (actually did pretty good holding it steady) and the biting stopped.

I then connected a secondary ground wire that I had pre-run from the battery to the ECM connector (I slipped a bare wire directly into the 1050RPM connector hole, along with the original HI wire). I had this new wire and the battery wire sitting next to me so all I had to do was touch them together. Using that route to ground the ECM, it still did the fishbiting.

NOTE: I'm going to do this same thing again tomorrow. It wasn't very cold last night and while the results above where solid, the whole fish-biting scenario was not as aggressive or intense as it normally is. Plus, it didn't last as long. It should be colder tonight.

I'll take note to the after-glow next time, as well.

Later today, I'll try to do that LP test where you drain fuel into a container and use a stopwatch to time it for 30 seconds. What would be a good volume? (I'll search around for that answer, too). EDIT: NEVER MIND - Just found the answer in the Technical Library :smile5:
 
Note I think the ac compressor will jump in and out if the mode selector is in defrost even if the fan is off I think??? Some one else verify this my compressor is not engaging right now at all to test this. Its to help dehumidify the vent air.
 
LIFT PUMP TEST:

According the Technical Library article, it states that 1-cup (8 oz) of fluid within 15 seconds should indicate a healthy LP.

I timed my test for 30 seconds, just to get a better idea. In 30 seconds, it was exactly 3.5 cups.

So, over 15 seconds, my LP put out 1-3/4 cup.
 
Tried again this morning... same results:

With the ECM wired directly to the battery negative post, it still results in biting. So, I guess my initial wiring and switches are OK.

Manually advancing the throttle (while HI is engaged at 1050) to 1300RPM's (that's what my foot happened to settle in at) stopped the biting.
 
ck fuel lines

i had a similar problem and ignored it a while as it got progressively worse i finally found a pin hole in the fuel line. fixed it and problem went away.
 
todd said:
i had a similar problem and ignored it a while as it got progressively worse i finally found a pin hole in the fuel line. fixed it and problem went away.
Click to expand...

Todd, when did you experience the problem? For example, mine is only on initial start-up, using the high idle on a very cold engine.
 
that would be when fuel is thickest requiring the most suction to draw. But I would think it would do it with the throttle pedal too.
 
add me for +1 with the pinhole in the fuel line, last night my truck did the same thing being described here, with the factory high idle (950 or so) got a few fishbites, gave it some throttle to about 1100-1200 still a few, and then it was gone after running a few minutes. I also got a few fishbites tonight after it had been running a little though. And I have a verified rotted fuel line and sending unit.
 
Update... not sure if this really helps, though:

11* out this morning, but had the block heater plugged in for about 3 hours prior to start-up. It high-idled fine - no bites at all.

So, if nothing else, at least that's good.
 
That's good, but no, it doesn't help. If it fishbites at 11* without being plugged in, it tells you it's something temperature-related... but then you already knew that.

Aaarrggh.
 
What about a loose electrical connection that changes as temps change (expansion/contraction). Try wiggling the connectors next time.
 
ak diesel driver said:
What about a loose electrical connection that changes as temps change (expansion/contraction). Try wiggling the connectors next time.
Click to expand...

Good thought. But, unfortunately I already tried that with no luck. I wiggled the ECM plug wires, heated the ECM with a hair dryer, wiggled the grounding wire, even ran a ground wire direct from battery to ECM.
 
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