Bad low throttle/RPM bucking/hesitation
08-31-2016, 09:36 AM
#1
Bad low throttle/RPM bucking/hesitation I'm not sure if this is ignition, fuel, or tune/timing related, but its to the point of making the car not fun to drive anymore. Here's the problems:
When low RPM/low throttle, such as city driving and highway, it has a bucking/hesitation to it, but if I open the throttle to get on it, or WOT, it jumps right to life and will pull hard like it should. Howver it goes back to hesitating surging at low RPM/low throttle.
Fuel injectors were just cleaned/tested when I did the AFR head/intake swap too. I had this issue start right before doing the head/intake swap as well, but it seems to be getting more pronounced the more I drive it.
I logged it and the timing seemed to be pretty low for cruising, 28-32*, (knock sensor is disabled, retard set to 0, E85), and the LT BLM's were running between 122-128, and IAC was between 25-40 while cruising.
Any ideas what to check next? I don't want to start throwing parts at it to fix the issue.
When low RPM/low throttle, such as city driving and highway, it has a bucking/hesitation to it, but if I open the throttle to get on it, or WOT, it jumps right to life and will pull hard like it should. Howver it goes back to hesitating surging at low RPM/low throttle.
Fuel injectors were just cleaned/tested when I did the AFR head/intake swap too. I had this issue start right before doing the head/intake swap as well, but it seems to be getting more pronounced the more I drive it.
I logged it and the timing seemed to be pretty low for cruising, 28-32*, (knock sensor is disabled, retard set to 0, E85), and the LT BLM's were running between 122-128, and IAC was between 25-40 while cruising.
Any ideas what to check next? I don't want to start throwing parts at it to fix the issue.
Last edited by kgkern01; 11-16-2016 at 09:20 AM.
08-31-2016, 11:10 AM
#3
It happens after startup in Open Loop too. When the engine is completely cold in the morning it is when it is the very worst. I initially attributed it to running E85 and cold start issues, but it still does it once the engine is warm, but not nearly as bad.
It has an MSD coil, could arcing or a weak spark be the culprit?
It has an MSD coil, could arcing or a weak spark be the culprit?
Last edited by kgkern01; 08-31-2016 at 12:43 PM.
08-31-2016, 01:17 PM
#5
TECH Addict
Arcing can absolutely cause this issue. Mine did this at moderate to low throttle in high gear a bunch of times. First time was the #5 Spark plug wire burnt against the Exhaust Manifold. Another time the Spark plug wire from the coil to the Opti was resting on the Waterpump, arcing; when I replaced the wire the conductor inside turned out to be transformed into dust. Then a Cracked Spark Plug. Some were worse than others of course, but they all led to a lagging/bucking engine at low RPM. Some led to a high RPM miss too (I rev to 6800 RPM though and didn't notice the miss feeling, like valve float almost, until about 6100).
09-20-2016, 08:30 AM
#7
So I saw a random arc on the MSD coil, to the square bracket on the coil, replaced it with a new stock coil (ended up being lower resistance to the MSD coil anyway 8.2k vs 9.3k), I no longer see any arcing at the coil. I can't see any arcing on the wires either, just the corona glow on the plug ceramic. Checked the fuel pressure for any fluctuations, none while idling or driving, stayed consistent at 40psi, 44psi when accelerating.
It is most noticeable in low rpm, part throttle driving, and feels like a random misfire, when accelerating it isn't really noticeable.
Could it be a bad cap/rotor? Anything else to look for in the datalogs? I really don't want to start throwing parts at it.
It is most noticeable in low rpm, part throttle driving, and feels like a random misfire, when accelerating it isn't really noticeable.
Could it be a bad cap/rotor? Anything else to look for in the datalogs? I really don't want to start throwing parts at it.
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09-21-2016, 11:42 PM
#8
Another thing to check is your CCV tube from the valve cover to the TB, if that's cracked or torn and letting unmetered air in, at low rpm with low airflow that can throw things off, and it doesn't show up at WOT at high air flow. That was the cause of my similar issue.
09-22-2016, 11:34 PM
#9
TECH Apprentice
you have alot more variables than i did, i had a similiar bucking/ hesitation problem, ended up being a bad tps sensor on the throttlebody....cheap part u can buy and if its not the issue return, also easy to put on!
11-15-2016, 03:24 PM
#10
Ok, fuel pressure stays steady around 40-44 psi while idling, driving, and WOT. I've replaced the coil, plugs, wires, opti, TPS, IAC, adjusted the TPS to get .67 volts at idle and about 30 IAC, idle timing is at 34*. Fuel injectors just came back from being cleaned and flow tested. It idles fine, as it did before, but I am still having the low rpm/throttle bucking/hesitation issue. It is even worse when the engine cold, as in the first time it runs for the day, for the first few minutes, then it gradually gets better as the engine temp goes up. Startups/initial driving after the engine is no longer completely cold are fine. Given that it acted like this before the AFR heads/LE LT4 intake swap as well, leads me to believe that something in the tune is off? Cruising BLM's seem to stay 124-128, so its not overly rich either.
What kind of timing should I be running in low throttle/RPM and cruising for a 243/251 110LSA cam? On the highway yesterday it seemed to have around 35-38* cruising timing. What should it be at for cruising?
I can send datalogs etc if anyone wants to take a look?
I am getting frustrated with this thing, I need help finding this issue, so I can enjoy the car again!
Thanks!
What kind of timing should I be running in low throttle/RPM and cruising for a 243/251 110LSA cam? On the highway yesterday it seemed to have around 35-38* cruising timing. What should it be at for cruising?
I can send datalogs etc if anyone wants to take a look?
I am getting frustrated with this thing, I need help finding this issue, so I can enjoy the car again!
Thanks!
11-15-2016, 04:06 PM
#11
I chased a "stumble" which was evident under load (4th gear at 35-40 mph and give it gas and it stumbled. same on fwy in 6th). Swapped the same parts you did and more.Data loged, sent files to tuner....nada
It turned out to be plug arc. Yeah I looked dozens of times but never saw the arc. While under the car with it running I by chance saw #6 plug arc. Could not see it from up top. I had the factory metal heat shield boots over the plug wires (replaced wires twice chasing the problem). I removed the heat shield and all was/is good. The arc was between the plug boot and inside of heat shield on plug wire. Just about impossible to see.
Not saying this is your issue but if you have not 100% laid eyes on ever part of the plug than you need to
do you have the EGR?....and have you confirmed it is functioning right if you do have it?
It turned out to be plug arc. Yeah I looked dozens of times but never saw the arc. While under the car with it running I by chance saw #6 plug arc. Could not see it from up top. I had the factory metal heat shield boots over the plug wires (replaced wires twice chasing the problem). I removed the heat shield and all was/is good. The arc was between the plug boot and inside of heat shield on plug wire. Just about impossible to see.
Not saying this is your issue but if you have not 100% laid eyes on ever part of the plug than you need to
do you have the EGR?....and have you confirmed it is functioning right if you do have it?
11-25-2016, 01:26 PM
#15
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I had a similar issue when I first got my car but it was intermittent on mine. Ended up being the O2 sensor and knock sensor cables getting too close to the PSGR side headers. Now I'm chasing a similar issues but only at WOT & high rpm.
11-25-2016, 02:12 PM
#16
O2's appear to be reading/switching fine, no knock sensor enabled due to E85. I'm thinking it is in the tune, because I tried to run it in SD mode with the MAF unplugged and disabled, and I can not get it to want to idle at all when it is in SD mode, unless I have put my foot on the throttle. Aftermarket TB, but I have the IAC port closed off so it only goes to the IAC port on the intake via 1/2 tube, and the opening is drilled out to 13/64. I have the Target AFR set to 9.8 (E85 stoich), and using EEHack I have the changed the real-time timing and AFR, and the only way it will stay idling in SD mode on its own is with AFR set to 20 and IAC count set to 160, and it still idles really low. If I plug the MAF back in and re-enable it, it will idle correctly again, although still needs throttle for cold starts.
12-09-2016, 09:44 PM
#17
I'm going to check all of the spark plugs again on it, I feel like at this point that it has to be a random miss even though I just replaced opti, coil, and wires and no change, not sure if I have a bad plug, wrong gap, heat range, etc.
Would the live cylinder deactivation test in EEHack identify the cylinder, and if so, what should I look for in the test?
Would the live cylinder deactivation test in EEHack identify the cylinder, and if so, what should I look for in the test?
12-16-2016, 11:11 AM
#18
Update: Cylinder deactivation test all the cylinders were pretty close, nothing stood out there. Went back to the tune and decided to adjust the BLM MAP table, as it was still stock, bumped it up to 50/70/80, and ran VE Master through it again, and it made a huge difference, it's actually driveable on the highway and cruising now. So I guess my BLM MAP table was the main culprit.