High Idle "Hic-Up/Fishbite"

[DennisG01]

Here's some more info (still trying to decide what pressure gauge to get, by the way).

I could not duplicate the fish-bite manually. I am 100% positive it only happens when in Park with the high-idle engaged. It also must be a very cold engine.

When this fish-bite occurs, the speedo needle also dances up and down a bit. For example: From 0 up to 15, down to 5, back up to 10, down to 0, etc etc. I have noticed this dancing in cold weather before, but I guess I never had the high-idle engaged (this is my first winter with the high-idle).

I experimented with the (3) high-idle settings by flipping one, then another, then another, etc. Whenever I would flip one on, demanding a higher idle, the engine would sputter/lag for about second before catching up with the demanded RPM setting. After about 2 minutes, everything was fine and it ran as though there was never a problem. Very quickly pushing the go-pedal down with my foot would not result in any problems - the RPM's would behave as expected.

Does this help explain the problem or just make it more confusing? :rolleyes5:

 

[WarWagon]

Have you checked the engine and trans mounts? Also check the frame bushing for the transmission shifter: for wear and transferring a miss like vibration to the steering wheel.

How old is the PMD and located where? Sorry if I missed that.

Have you ohm checked the glow plugs?

 

[schiker]

I experimented with the (3) high-idle settings by flipping one, then another, then another, etc. Whenever I would flip one on, demanding a higher idle, the engine would sputter/lag for about second before catching up with the demanded RPM setting. After about 2 minutes, everything was fine and it ran as though there was never a problem.

Very quickly pushing the go-pedal down with my foot would not result in any problems - the RPM's would behave as expected.

Try cranking it up cold and very slowly apply pedal to ease up to high idle rpm's. As my truck has aged and 60G's (maybe one or more are weakened too) when its cold its a bit more cold natured for the first minute or 2. I did not notice this as much last year and it still starts ok. In fact at first start really cold its appreciably harder to manually ease up the rpm's its pretty cold natured compared to after after glow and a bit warmer.

Thinking guessing just maybe...
18:1 may just be a bit more cold natured than PCM expects for high idle control for 1-2 minutes depending on temps and plug health???? Maybe one weak glow plug and the higher light fuel rate is not burning good until enough heat is generated. When you apply fuel quickly with foot it might put enough in other cylinders to cover up a slight miss. ????

My speedo at times comes off zero when I high idle with pedal or rev engine never tried to corelate when it does it exactly but have noticed it before and read others have too. Guessing its just a coincidence here. Maybe voltage surge as alternator surges to recharge starting drop and afterglow draws ?????

 

[DennisG01]

Try cranking it up cold and very slowly apply pedal to ease up to high idle rpm's. As my truck has aged and 60G's (maybe one or more are weakened too) when its cold its a bit more cold natured for the first minute or 2. I did not notice this as much last year and it still starts ok. In fact at first start really cold its appreciably harder to manually ease up the rpm's its pretty cold natured compared to after after glow and a bit warmer.

Thinking guessing just maybe...
18:1 may just be a bit more cold natured than PCM expects for high idle control for 1-2 minutes depending on temps and plug health???? Maybe one weak glow plug and the higher light fuel rate is not burning good until enough heat is generated. When you apply fuel quickly with foot it might put enough in other cylinders to cover up a slight miss. ????

My speedo at times comes off zero when I high idle with pedal or rev engine never tried to corelate when it does it exactly but have noticed it before and read others have too. Guessing its just a coincidence here. Maybe voltage surge as alternator surges to recharge starting drop and afterglow draws ?????

I have manually "crept" the throttle up slowly. More by accident than anything else - while trying to hold a steady, elevated RPM. There were no problems when I did that.

Kennedy reflashed my ECM, specifically for the 18:1. But, you bring up a good point and I suppose a call to John would be in order to get his thoughts.

The only reason I brought up "snapping" the throttle manually was because when I flipped both switches to go to 1600 RPM's, for example, it bogged. But manually it did not. However, am I correctly understanding you that the flip to 1600 RPMs via the switches doesn't call for as much fuel as doing it manually? So the manual snap actually calls for more fuel than is needed to overcome the bog?

 

[WarWagon]

My speedo at times comes off zero when I high idle with pedal or rev engine never tried to corelate when it does it exactly but have noticed it before and read others have too. Guessing its just a coincidence here. Maybe voltage surge as alternator surges to recharge starting drop and afterglow draws ?????

The vibration from the engine is enough to move the output shaft teeth by the speedometer pickup coil. The teeth vibrating back and forth can generate an AC signal in the same way that they do when spinning. The sensor moving from vibration and the shaft sitting still will generate an AC output and a speed reading.

Bad engine mounts give the drive train more twisting room to vibrate thus making the 'problem' worse. But it is usually normal.

 

[DennisG01]

Have you checked the engine and trans mounts? Also check the frame bushing for the transmission shifter: for wear and transferring a miss like vibration to the steering wheel.

How old is the PMD and located where? Sorry if I missed that.

Have you ohm checked the glow plugs?

Haven't checked mounts - but I personally replaced the mounts with the new engine about 9K ago. I asked the installing mechanic to check everything else out very carefully, although I did not specifically ask about the trans mount.

PMD is in original location - IP/PMD replaced about 15K ago (GM warranty).

Have not checked plugs recently.

 

[DennisG01]

The vibration from the engine is enough to move the output shaft teeth by the speedometer pickup coil. The teeth vibrating back and forth can generate an AC signal in the same way that they do when spinning. The sensor moving from vibration and the shaft sitting still will generate an AC output and a speed reading.

Bad engine mounts give the drive train more twisting room to vibrate thus making the 'problem' worse. But it is usually normal.

Very, very interesting! Thanks for the explanation as I never in a million years would of thought of that... I was wondering why you originally asked me about the mounts. :smile5:

 

[schiker]

I don't know but am guessing it goes something like this normal idle is 6 mmm/1000 strokes fuelrate. 1050 rpm is 9mmm/10000 and 1600 is 15mmm/1000.Pretty little change when warm and stable.

What might happen is when you hit the switches it jumps instantaneously up then adjusts accordingly. It might adjust so fast that a cold natured 18:1 might hunt just a bit first. But when you use foot there is no set amount you just add fuel to achieve the rpm. And you can't match the speed of the PCM for adjustments.

Have you tried to wait just a tad longer to engage high idle. Try this idle normally for 15 seconds until you bump up to high idle? What does it do then? Try different amounts of time.

This only happens within the first 1-2 minutes otherwise normal right. Does severity change over the this time?

 

[WarWagon]

If it is the first 1-2 min I would suspect glow plugs or just not getting them hot enough with the low '18' compression. Maybe the engine just has to warm up more with low compression.

Can you start the engine with the highest idle setting on? This would be the fastest way to heat it up and generate the most compression and get oil to rings to get all 8 firing.

Shot injectors would do this 100% of the time, aka hot, or in my case after cold advance/idle kicked off.

 

[Dave01]

Can you start the engine with the highest idle setting on?

That would make me nervous, taking a dead cold diesel and start it to 1600 rpm immediately, no oil flow yet. I idle for around 30 seconds before going to high idle, and I just use the 1050 rpm switch.

 

[BigBlueChevy]

That would make me nervous, taking a dead cold diesel and start it to 1600 rpm immediately, no oil flow yet. I idle for around 30 seconds before going to high idle, and I just use the 1050 rpm switch.

same here. I wait till my truck has been idiling for at least 10 to 20 seconds before engaging high idle, and even then I only have 1050 switch.

I'm going to throw this out there. The PCM sends the truck into "high idle" when pin #.. has its voltage shunted to ground. If the issue with the fishbiting only happens when using high idle, and NOT caused by manual throttle control, the problem is electrical. Could be a faulty switch that isnt engaging properly, could be a slight bit of corrosion under a grounding connection. Where do you have the high idle ground tapped off of? Directly off the battery, or somewhere else? Try running a jumper between the battery - and the point where your high idle is grounded and try again.

Based on the symptons your describing it sounds to me like somewhere between the batterys - terminal and Pin #.. in the PCM harness, there is some kind of interfearance.

 

[Kennedy Diesel]

I would agree that you should let it run for a bit at std idle to put some heat into the heads. That and let the oil warm a bit first.

My 96 would sometimes run a bit odd if I tried to drive before warmed up.

The 1070 RPM speed is all that you should need/use IMHO.

 

[DennisG01]

Shiker: Yes, only happens for the first few minutes. As the time goes one, it happens less and less until it finally stops.

War: If it was the GP's, wouldn't I also have a problem with manual control?


I just tried putting a hair dryer on the ECM for about 20 minutes before starting... the problem was still there.

Currently, the ground wire from the switches are attached to the large, horizontal steel bar that runs from left to right behind the dash (at about knee height).

Tomorrow I'll try running a wire right from the battery directly to the ECM-side of the switch and just hold it on tightly. That way I'll eliminate both a possible grounding problem and switch problem.

 

[Dave01]

Dennis, if you start up, idle for 1 minute at normal idle, then hit the 1070 high idle switch, does it happen?

If you try to hold high idle rpm with the pedal, you are varying rpm as you ramp up, then try to hold steady, even with a good IP its hard to do it perfectly, the pedal is moving. By contrast, when you hit the switch the truck is told "1070 rpm NOW" and holds perfectly on that number, a much faster and less varied way to get there. My guess still is that you take a few moments to build good fuel pressure, either because of weak lift pump, filter restriction, or cold/dirty fuel. The quick high idle starves for fuel. Until you check pressure I'm leaning this way, not electrical.

 

[DennisG01]

Dennis, if you start up, idle for 1 minute at normal idle, then hit the 1070 high idle switch, does it happen?

If you try to hold high idle rpm with the pedal, you are varying rpm as you ramp up, then try to hold steady, even with a good IP its hard to do it perfectly, the pedal is moving. By contrast, when you hit the switch the truck is told "1070 rpm NOW" and holds perfectly on that number, a much faster and less varied way to get there. My guess still is that you take a few moments to build good fuel pressure, either because of weak lift pump, filter restriction, or cold/dirty fuel. The quick high idle starves for fuel. Until you check pressure I'm leaning this way, not electrical.

Dave,

If I let it idle normally, it will be fine. I've never actually timed it for a full minute, but I have let it idle normally for maybe 30 seconds, then flipped the switch. It then starts to bite - then I flip the switch off and it acts fine. Then I flip it again and it will again start to bite. This will continue to happen until a few minutes have gone by and then everything is normal.

I haven't forgotten about the pressure - still working on getting a gauge.


Oh, someone brought up injectors. Injectors have about 9K on them - they are Kennedy's High-Pop.

 

[Chevylover]

Dave,

If I let it idle normally, it will be fine. I've never actually timed it for a full minute, but I have let it idle normally for maybe 30 seconds, then flipped the switch. It then starts to bite - then I flip the switch off and it acts fine. Then I flip it again and it will again start to bite. This will continue to happen until a few minutes have gone by and then everything is normal.

I haven't forgotten about the pressure - still working on getting a gauge.


Oh, someone brought up injectors. Injectors have about 9K on them - they are Kennedy's High-Pop.

Sounds like a fuel delivery prob for me, too. When do you use the high idle ?
If it is cold outside. So I would say, if the temps go down, EVERY fluid gets a bit thicker so the liftpump has to do a bit more work to get the fuel up to the IP. IF the engine is running a few minutes the fuel inside the tanl will get warmer due to the returning fuel via the return lines So after that, it is much easier for the LP to get the fuel pushed up to the IP.

Just my thoughts ! Could be possible or what do you think ? Give it a try. Start the engine, let it idle for about 3-5 minutes and THAN kick in th high idle.



Cu,
Sven

 

[schiker]

PA probably has good winter fuel. And with FPPF it should be flowing easy enough. I am leaning more towards cold natured with slightly degrading glows. What are they (waps can go bad quickly) most quick heats are questionable for me. I like 60G's and or Bosch or other thermally regulated glows. Also very possible bad LP. I'd do the while idling open fuel drain just to varify LP is running. Could be the IP is pulling fuel and needs a min to get adequate flow. Maybe some extra cold is causing a hose to get hard and air leak or something with bad LP.

But I can't see 1-2 minutes warming 15 ish gallons of the fuel in the tank or lines appreciably. I can see the lines draining down or some internal leak down though in combination with bad LP. If it was just LP it wouldn't be so temperature dependant unless some wierd hose temp thing??? 1-2 minutes can make a significant difference in head temperature ring seal and combustion temperature. Points more to cold natured. I can't think of anything else that drops out in just 1-2 minutes either. Or will cold start advance drop out that quick? If that is different than timing offset? Which I think takes ECT 160-170F ish.

Does it sound appreciably different during afterglow cycles more than you remember last winter?

For what its worth...
I can see manually input might mask cold natured hiccup because you can't adjust the fine metering as fast as PCM. Manually you probably are over fuelling a bit too and might have a tad more white smoke (cold combustion and condensation etc). Cold at first start takes me a good bit of easing pedal to get to hi idle better and I try to wait at least 10 ish seconds at normal idle.

 

[DennisG01]

Well, no luck. I started it up w/o high idle and let it run for about 20 seconds. It was fine. I then jumped a wire directly from battery to the 1100RPM wire that goes to the ECM. It started to bite. I unhooked it and it went back to being fine. Hooked up to the 1600RPM wire and it started to bite. Unhooked it and again it went back to being fine.

I then let it run for about 2 or 3 minutes at normal idle. Then I flipped the switch and it idled high perfectly.

Yeah, I know ALL ABOUT the quick heat plugs! :sad:

I have Beru plugs - about 9K on them. With a short, extra glow cycle this morning (mid to upper 20's over night, 30* when I started the truck) it started like it was 70* outside). But, I have not checked them. They were the brand that Ron Schoolcraft had been using and he had got them from Penninsular.

 

[Chevylover]

Summary :

If you let it idle normal for about 2-3 minutes and then kick in the high idle, there's no fishbite. CORRECT ?

I think it IS temp related. Everything is cold and you won't believe how much heat these engine produce right after the startup. While idling the IP gets more fuel than needed, so the most will go unused back in the tank via the return line. That will heat the fuel. Not 10 or 20 degrees, but perhaps that little bit the LP and IP need.

Give it a try :
Startup the cold engine, let it normal idle about3 min. and then kick in the high idle NOT BEFORE ! If no fishbite, think about my words.



Cu,
Sven

 

[JiFaire]

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.