V8S - hesitates around 2,400 rpm?
Discussion
Hi All,
I have a V8S and it has a hesitation at 2,400 rpm. If I acceleration normally, it pulls sweet as a nut, then a big hesitation around 2,400 rpm, then from 2,500 rpm upwards pulls sweet as a nut.
If I accelerate hard it will muscle straight through the hesitation, but it is very noticeable under normal acceleration. It almost goes away after a good run.
I pulled the plugs and cleaned them. But it is just the same. Any ideas?
Thanks!
I have a V8S and it has a hesitation at 2,400 rpm. If I acceleration normally, it pulls sweet as a nut, then a big hesitation around 2,400 rpm, then from 2,500 rpm upwards pulls sweet as a nut.
If I accelerate hard it will muscle straight through the hesitation, but it is very noticeable under normal acceleration. It almost goes away after a good run.
I pulled the plugs and cleaned them. But it is just the same. Any ideas?
Thanks!
Worn throttle pot would be my first suspect. It's very common for these to wear around the normal range of throttle positions and cause glitching.
It's also possible the vac line to the vac advance cannister is blocked, which is more likely if you still have the vac delay valve fitted, and even more so if it's been put in back to front by mistake - but that wouldn't normally show up so consistently during a pull. It would be quick to disconnect the vac line from the vac advance cannister (and plug the hose to prevent an air leak into the manifold) as a quick check
The sensitivity to RPM and throttle position would fit a lambda sensor problem but since yours is a precat it wouldn't normally be doing anything RPM/throttle position sensitive. It's just remotely possible you have a faulty tune resistor / circuit and it thinks it can run in closed loop mode, but that seems pretty unlikely to me and I think you'd see more widespread problems in that case.
Next time you're under the bonnet just make sure all the HT leads are firmly seated and not too close to the exhaust. It's not unheard of for these to go intermittent with temperature and when they're jostled by engine movements. If you have plug extenders fitted, make a brief test without them to see whether that helps.
It's also possible the vac line to the vac advance cannister is blocked, which is more likely if you still have the vac delay valve fitted, and even more so if it's been put in back to front by mistake - but that wouldn't normally show up so consistently during a pull. It would be quick to disconnect the vac line from the vac advance cannister (and plug the hose to prevent an air leak into the manifold) as a quick check
The sensitivity to RPM and throttle position would fit a lambda sensor problem but since yours is a precat it wouldn't normally be doing anything RPM/throttle position sensitive. It's just remotely possible you have a faulty tune resistor / circuit and it thinks it can run in closed loop mode, but that seems pretty unlikely to me and I think you'd see more widespread problems in that case.
Next time you're under the bonnet just make sure all the HT leads are firmly seated and not too close to the exhaust. It's not unheard of for these to go intermittent with temperature and when they're jostled by engine movements. If you have plug extenders fitted, make a brief test without them to see whether that helps.
Beast Master said:
Thanks for good reply. Is there any way to test the throttle pot? And silly question where is it?
It's a cylindrical plastic component on the end of the throttle spindle with three wires coming from it. Two wires connect to the two ends of a potentiometer and the third connects to a wiper that moves along the potentiometer.To test it you can use an ordinary analog volt meter to measure the resistance between the wiper and either of the other wires as you move the throttle. If you have access to a 'scope, that's even better.
Subtle problems can be quite hard to detect and need to you play around with the throttle trying to get the signal to glitch. If you feel tempted to remove the throttle pot to test or replace it, bear in mind that the position has to be adjusted very carefully to get the correct value at idle - best to leave alone if you can.
Okay maybe I won't go messing with it as I may make things worse.
It is an odd one. It is pretty consistant. Always between 2,400 and 2,500. On the motorway cruising at that RPM you can feel a miss.
And as said, under normal acceleration, it feels like I let off the accelerator for a split second at 2,400 and then put my foot back down. It's very noticeable. But then after a good run, it almost goes away to just a slight miss and sometimes feels completely gone.
It is an odd one. It is pretty consistant. Always between 2,400 and 2,500. On the motorway cruising at that RPM you can feel a miss.
And as said, under normal acceleration, it feels like I let off the accelerator for a split second at 2,400 and then put my foot back down. It's very noticeable. But then after a good run, it almost goes away to just a slight miss and sometimes feels completely gone.
That is a strange one ..
i've thought about the dizzy and the advance / vac advance .. but although that mechanical dizzy advance is pretty much reliable against revs (ie same mechanical advance every time to revisit a certain rev point) I'm struggling to make that only affect 2400rpm ..
not much help i'm afraid sorry.
i've thought about the dizzy and the advance / vac advance .. but although that mechanical dizzy advance is pretty much reliable against revs (ie same mechanical advance every time to revisit a certain rev point) I'm struggling to make that only affect 2400rpm ..
not much help i'm afraid sorry.
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