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Off-idle stumble when cold

2K views 17 replies 4 participants last post by  dauntless89 
#1 ·
I did the following stuff to my truck this last weekend:

Pulled injectors and set pop from 4000 to 4200 psi
Advanced timing from 16* to 20*
Replaced head bolts one at a time

It got a stumble, just off idle, right after being fired up when cold. Doesn't take long to go away. Driving around the block will get it to run normally. Also, it gets better when the truck sits in the sun all day (when I start it to go home from work).

Ever since installing my 4ks, the truck has been slightly overfueled just off idle, so I started backing off my governor springs one click at a time. I was initially at 3 clicks after the initial click, and I am now down to one click after the initial. This has improved the stumble, but it still does it, and I'm not making the kind of horsepower that justifies this rough running when it's at least 60* outside.

My next inclination is to switch back to my 3k springs and see what happens.

My question is: Is this kind of behavior to be expected when running timing around 20?
 
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#2 ·
It's not abnormal to have a slight "lope" or "stumble" once you go over 20°
 
#4 ·
It can take a minute to start, are you allowing the truck to warm up before putting it in gear? Though at 20° it is "mildly" advanced, what timing method did you use? Maybe it's more advanced than you think?
 
#5 ·
I start the truck, plug my phone into the charger, stow my piece in the center console, click my stereo faceplate on, make sure my coffee/water bottle is secure, and then put the truck in gear. I've never had a problem with it stumbling before when I was at 16* timing unless it was below 20* outside and hadn't been plugged in. And even then, usually only when I was too impatient to wait for the grids.

In the winter, it's plugged in just long enough to keep the grids from cycling at home. At work, it doesn't get plugged in unless it's single-digit temps. I've never had the need for plugging it in to get it to start on the first try. In such a situation, it did do the same thing as it's doing now. Just that it mainly stuttered/ran rough when it was below 20* outside with no block heater. Now it's doing basically the same thing but at 60* ambient temp. This morning it was 50* and it ran quite rough. It didn't die, but was spewing smoke, shaking the truck, etc. It smoothed out when the grids cycled on, then ran rough again when they switched off. It did this for about 15 to 20 seconds and I did wait for it to smooth out and idle speed to come up to normal before taking off. By the way, I bumped my idle up to 850 last night to see if that would help. Evidently not enough to matter.

I set timing with a dial indicator. Due to my balancer marking methods, a more accurate way to express my timing is 20.5 ±.5 degrees.

I found exact TDC using the drop-valve method in conjunction with a dial indicator, using the ESS wire on the bottom of the engine as a reference. Then when setting my timing, I transferred the TDC mark to line up exactly with the trailing edge of the ESS (so I could see it from the top). That or physically making the initial marks in slightly the wrong spot is the only possible source of error. I checked the timing twice after setting it, and as near as I could tell by both sets of marks, 20.5 was where it was with a possible error of 1/2 degree either way.

I'm increasingly convinced that the only solutions are to wait for it to warm up (perhaps more easily facilitated by installing a remote start) and deal with it, or knock my timing back just enough to get it to quit doing it at summertime morning temps.
 
#7 · (Edited)
I did controlled testing and found that on my truck and with my turbo (WH1C) my EGTs are higher at 35 than at 30 with the same total fuel delivery. Not by a lot, but 35 is overheated air in my case.

As for timing, there's no reason to not run as much timing as you can without running into reliability issues. Also, the gains from timing are felt across the RPM range, and extra boost is only useful when pushing the truck that hard.
 
#8 ·
IDK, but to me it sounds like you will be better off with getting a 3rd gen turbo in and call it a day instead of keep trying to bandaid things.
With remote start in cold days it might need some throttle to get going. Then you will be looking for High idle kit.
BTW, let us know how your MPGs
Doing with 20*. But no BS!

Sent from AutoGuide.com Free App
 
#13 ·
Could you please explain how a turbo has affect on idle? I would be interested to know.
 
#9 ·
I have no intention of staying with this turbo. I'm most likely looking into a HE351VGT, but cost and swap downtime are factors.

Regarding the remote start, perfectly valid points that are causing hesitation on that plan. Remote start/keyless entry would be nice to have, but I have reservations on it's effectiveness in dealing with the rough running issue. A high-idle kit is something I was considering as well.

Hoping to get a decent mileage gain out of the timing, we'll see I guess. I'll probably back it off a degree this weekend to make for smoother running when started.
 
#10 ·
Just throwing these out there..have you checked your fuel prefilter? I was starting to have similar issues, on a stock truck, and it was that starting to get plugged up again..just a thought, I realize it has nothing to do with the work performed.

I'm not familiar with pop pressure, still learning, but could the higher pressure be effecting idle quality?
 
#11 ·
It didn't on my stock injectors. From imbalanced at 3400 psi to perfectly balanced at 4200 psi, the only thing I noticed was cold engine haze decreased slightly and the engine seemed to run smoother all around.

I suppose I could check the prefilter. Fuel pressure has been gradually dropping a slight amount over the last few weeks, but average temperature has also been increasing enough to account for it.
 
#12 ·
Plunger lift at TDC was only .183 when I went to re-set it today. Misreading the dial is the only thing I can think would explain this. Looks like I went full retard. Well, not full retard because .183 would still be before TDC but not by much. My chart only goes down to .203, which is 9.5 degrees on my pump. So .183 would be something like 7 degrees?

Anyway, I set it to .285 (about 20.25*) just now and it fired right up with less haze than usual after it's been taken apart. So I'm happy there.
 
#14 · (Edited)
Glad it got fixed, i was wondering if the timing was even close to right based on your description of the issue.
 
#16 ·
I was just badgering to play devils advocate. I agree, a turbo on a stock 12v opens up worlds of opportunity. Something that I have learned from the old timers on this forum is to direct your response in the exact direction you want it taken.
 
#18 ·
Alright, for the record, after running 20* (in real life) for a few days, there's a pretty stark difference as compared to 16*. EGTs are lower across the board by about 75*, boost is pretty much the same, maybe 1 or 2 psi lower than at 16* at highway cruise conditions (10psi and below in my case). The engine does rattle more than it did, but it vibrates less and is very responsive. I'm still working up the courage to put the boots to it and probably need to make some minor AFC adjustments but I'm sure it'll run even stronger above 2000 rpm.
 
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